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Title: Steam, Its Generation and Use
Author: Babcock & Wilcox Company
Language: English
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STEAM
ITS GENERATION AND USE
[Illustration]
THE BABCOCK & WILCOX CO.
NEW YORK
Thirty-fifth Edition
4th Issue
Copyright, 1919, by The Babcock & Wilcox Co.
* * * * *
Bartlett Orr Press
New York
THE BABCOCK & WILCOX CO.
85 LIBERTY STREET, NEW YORK, U. S. A.
_Works_
BAYONNE NEW JERSEY
BARBERTON OHIO
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W. D. HOXIE, _President_
E. H. WELLS, _Chairman of the Board_
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[Illustration: Works of The Babcock & Wilcox Co., at Bayonne, New Jersey]
[Illustration: Works of The Babcock & Wilcox Co., at Barberton, Ohio]
[Illustration: Works of Babcock & Wilcox, Limited, Renfrew, SCOTLAND]
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[Illustration: Fonderies et Ateliers de la Courneuve, Chaudières Babcock
& Wilcox, Paris, France]
FONDERIES ET ATELIERS DE LA COURNEUVE
CHAUDIÈRES
BABCOCK & WILCOX
6 RUE LAFERRIÈRE, PARIS
WORKS: SEINE--LA COURNEUVE
_Directors_
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ETIENNE BESSON IRÉNÉE CHAVANNE
CHARLES A. KNIGHT JULES LEMAIRE
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TELEGRAPHIC ADDRESS: "BABCOCK-PARIS"
[Illustration: Wrought-steel Vertical Header Longitudinal Drum
Babcock & Wilcox Boiler, Equipped with Babcock & Wilcox Superheater and
Babcock & Wilcox Chain Grate Stoker]
THE EARLY HISTORY OF THE GENERATION AND USE OF STEAM
While the time of man's first knowledge and use of the expansive force
of the vapor of water is unknown, records show that such knowledge
existed earlier than 150 B. C. In a treatise of about that time entitled
"Pneumatica", Hero, of Alexander, described not only existing devices of
his predecessors and contemporaries but also an invention of his own
which utilized the expansive force of steam for raising water above its
natural level. He clearly describes three methods in which steam might
be used directly as a motive of power; raising water by its elasticity,
elevating a weight by its expansive power and producing a rotary motion
by its reaction on the atmosphere. The third method, which is known as
"Hero's engine", is described as a hollow sphere supported over a
caldron or boiler by two trunnions, one of which was hollow, and
connected the interior of the sphere with the steam space of the
caldron. Two pipes, open at the ends and bent at right angles, were
inserted at opposite poles of the sphere, forming a connection between
the caldron and the atmosphere. Heat being applied to the caldron, the
steam generated passed through the hollow trunnion to the sphere and
thence into the atmosphere through the two pipes. By the reaction
incidental to its escape through these pipes, the sphere was caused to
rotate and here is the primitive steam reaction turbine.
Hero makes no suggestions as to application of any of the devices he
describes to a useful purpose. From the time of Hero until the late
sixteenth and early seventeenth centuries, there is no record of
progress, though evidence is found that such devices as were described
by Hero were sometimes used for trivial purposes, the blowing of an
organ or the turning of a skillet.
Mathesius, the German author, in 1571; Besson, a philosopher and
mathematician at Orleans; Ramelli, in 1588; Battista Delia Porta, a
Neapolitan mathematician and philosopher, in 1601; Decause, the French
engineer and architect, in 1615; and Branca, an Italian architect, in
1629, all published treatises bearing on the subject of the generation
of steam.
To the next contributor, Edward Somerset, second Marquis of Worcester,
is apparently due the credit of proposing, if not of making, the first
useful steam engine. In the "Century of Scantlings and Inventions",
published in London in 1663, he describes devices showing that he had in
mind the raising of water not only by forcing it from two receivers by
direct steam pressure but also for some sort of reciprocating piston
actuating one end of a lever, the other operating a pump. His
descriptions are rather obscure and no drawings are extant so that it is
difficult to say whether there were any distinctly novel features to his
devices aside from the double action. While there is no direct authentic
record that any of the devices he described were actually constructed,
it is claimed by many that he really built and operated a steam engine
containing pistons.
In 1675, Sir Samuel Moreland was decorated by King Charles II, for a
demonstration of "a certain powerful machine to raise water." Though
there appears to be no record of the design of this machine, the
mathematical dictionary, published in 1822, credits Moreland with the
first account of a steam engine, on which subject he wrote a treatise
that is still preserved in the British Museum.
[Illustration: 397 Horse-power Babcock & Wilcox Boiler in Course of
Erection at the Plant of the Crocker Wheeler Co., Ampere, N. J.]
Dr. Denys Papin, an ingenious Frenchman, invented in 1680 "a steam
digester for extracting marrowy, nourishing juices from bones by
enclosing them in a boiler under heavy pressure," and finding danger
from explosion, added a contrivance which is the first safety valve on
record.
The steam engine first became commercially successful with Thomas
Savery. In 1699, Savery exhibited before the Royal Society of England
(Sir Isaac Newton was President at the time), a model engine which
consisted of two copper receivers alternately connected by a three-way
hand-operated valve, with a boiler and a source of water supply. When
the water in one receiver had been driven out by the steam, cold water
was poured over its outside surface, creating a vacuum through
condensation and causing it to fill again while the water in the other
reservoir was being forced out. A number of machines were built on this
principle and placed in actual use as mine pumps.
The serious difficulty encountered in the use of Savery's engine was the
fact that the height to which it could lift water was limited by the
pressure the boiler and vessels could bear. Before Savery's engine was
entirely displaced by its successor, Newcomen's, it was considerably
improved by Desaguliers, who applied the Papin safety valve to the
boiler and substituted condensation by a jet within the vessel for
Savery's surface condensation.
In 1690, Papin suggested that the condensation of steam should be
employed to make a vacuum beneath a cylinder which had previously been
raised by the expansion of steam. This was the earliest cylinder and
piston steam engine and his plan took practical shape in Newcomen's
atmospheric engine. Papin's first engine was unworkable owing to the
fact that he used the same vessel for both boiler and cylinder. A small
quantity of water was placed in the bottom of the vessel and heat was
applied. When steam formed and raised the piston, the heat was withdrawn
and the piston did work on its down stroke under pressure of the
atmosphere. After hearing of Savery's engine, Papin developed an
improved form. Papin's engine of 1705 consisted of a displacement
chamber in which a floating diaphragm or piston on top of the water kept
the steam and water from direct contact. The water delivered by the
downward movement of the piston under pressure, to a closed tank, flowed
in a continuous stream against the vanes of a water wheel. When the
steam in the displacement chamber had expanded, it was exhausted to the
atmosphere through a valve instead of being condensed. The engine was,
in fact, a non-condensing, single action steam pump with the steam and
pump cylinders in one. A curious feature of this engine was a heater
placed in the diaphragm. This was a mass of heated metal for the purpose
of keeping the steam dry or preventing condensation during expansion.
This device might be called the first superheater.
Among the various inventions attributed to Papin was a boiler with an
internal fire box, the earliest record of such construction.
While Papin had neglected his earlier suggestion of a steam and piston
engine to work on Savery's ideas, Thomas Newcomen, with his assistant,
John Cawley, put into practical form Papin's suggestion of 1690. Steam
admitted from the boiler to a cylinder raised a piston by its expansion,
assisted by a counter-weight on the other end of a beam actuated by the
piston. The steam valve was then shut and the steam condensed by a jet
of cold water. The piston was then forced downward by atmospheric
pressure and did work on the pump. The condensed water in the cylinder
was expelled through an escapement valve by the next entry of steam.
This engine used steam having pressure but little, if any, above that of
the atmosphere.
[Illustration: Two Units of 8128 Horse Power of Babcock & Wilcox Boilers
and Superheaters at the Fisk Street Station of the Commonwealth Edison
Co., Chicago, Ill., 50,400 Horse Power being Installed in this Station.
The Commonwealth Edison Co. Operates in its Various Stations a Total of
86,000 Horse Power of Babcock & Wilcox Boilers, all Fitted with Babcock
& Wilcox Superheaters and Equipped with Babcock & Wilcox Chain Grate
Stokers]
In 1711, this engine was introduced into mines for pumping purposes.
Whether its action was originally automatic or whether dependent upon
the hand operation of the valves is a question of doubt. The story
commonly believed is that a boy, Humphrey Potter, in 1713, whose duty it
was to open and shut such valves of an engine he attended, by suitable
cords and catches attached to the beam, caused the engine to
automatically manipulate these valves. This device was simplified in
1718 by Henry Beighton, who suspended from the bottom, a rod called the
plug-tree, which actuated the valve by tappets. By 1725, this engine was
in common use in the collieries and was changed but little for a matter
of sixty or seventy years. Compared with Savery's engine, from the
aspect of a pumping engine, Newcomen's was a distinct advance, in that
the pressure in the pumps was in no manner dependent upon the steam
pressure. In common with Savery's engine, the losses from the alternate
heating and cooling of the steam cylinder were enormous. Though
obviously this engine might have been modified to serve many purposes,
its use seems to have been limited almost entirely to the pumping of
water.
The rivalry between Savery and Papin appears to have stimulated
attention to the question of fuel saving. Dr. John Allen, in 1730,
called attention to the fact that owing to the short length of time of
the contact between the gases and the heating surfaces of the boiler,
nearly half of the heat of the fire was lost. With a view to overcoming
this loss at least partially, he used an internal furnace with a smoke
flue winding through the water in the form of a worm in a still. In
order that the length of passage of the gases might not act as a damper
on the fire, Dr. Allen recommended the use of a pair of bellows for
forcing the sluggish vapor through the flue. This is probably the first
suggested use of forced draft. In forming an estimate of the quantity of
fuel lost up the stack, Dr. Allen probably made the first boiler test.
Toward the end of the period of use of Newcomen's atmospheric engine,
John Smeaton, who, about 1770, built and installed a number of large
engines of this type, greatly improved the design in its mechanical
details.
[Illustration: Erie County Electric Co., Erie, Pa., Operating 3082 Horse
Power of Babcock & Wilcox Boilers and Superheaters, Equipped with
Babcock & Wilcox Chain Grate Stokers]
The improvement in boiler and engine design of Smeaton, Newcomen and
their contemporaries, were followed by those of the great engineer,
James Watt, an instrument maker of Glasgow. In 1763, while repairing a
model of Newcomen's engine, he was impressed by the great waste of steam
to which the alternating cooling and heating of the engine gave rise.
His remedy was the maintaining of the cylinder as hot as the entering
steam and with this in view he added a vessel separate from the
cylinder, into which the steam should pass from the cylinder and be
there condensed either by the application of cold water outside or by a
jet from within. To preserve a vacuum in his condenser, he added an air
pump which should serve to remove the water of condensation and air
brought in with the injection water or due to leakage. As the cylinder
no longer acted as a condenser, he could maintain it at a high
temperature by covering it with non-conducting material and, in
particular, by the use of a steam jacket. Further and with the same
object in view, he covered the top of the cylinder and introduced steam
above the piston to do the work previously accomplished by atmospheric
pressure. After several trials with an experimental apparatus based on
these ideas, Watt patented his improvements in 1769. Aside from their
historical importance, Watt's improvements, as described in his
specification, are to this day a statement of the principles which guide
the scientific development of the steam engine. His words are:
"My method of lessening the consumption of steam, and
consequently fuel, in fire engines, consists of the following
principles:
"First, That vessel in which the powers of steam are to be
employed to work the engine, which is called the cylinder in
common fire engines, and which I call the steam vessel, must,
during the whole time the engine is at work, be kept as hot as
the steam that enters it; first, by enclosing it in a case of
wood, or any other materials that transmit heat slowly;
secondly, by surrounding it with steam or other heated bodies;
and, thirdly, by suffering neither water nor any other substance
colder than the steam to enter or touch it during that time.
"Secondly, In engines that are to be worked wholly or partially
by condensation of steam, the steam is to be condensed in
vessels distinct from the steam vessels or cylinders, although
occasionally communicating with them; these vessels I call
condensers; and, whilst the engines are working, these
condensers ought at least to be kept as cold as the air in the
neighborhood of the engines, by application of water or other
cold bodies.
"Thirdly, Whatever air or other elastic vapor is not condensed
by the cold of the condenser, and may impede the working of the
engine, is to be drawn out of the steam vessels or condensers by
means of pumps, wrought by the engines themselves, or otherwise.
"Fourthly, I intend in many cases to employ the expansive force
of steam to press on the pistons, or whatever may be used
instead of them, in the same manner in which the pressure of the
atmosphere is now employed in common fire engines. In cases
where cold water cannot be had in plenty, the engines may be
wrought by this force of steam only, by discharging the steam
into the air after it has done its office....
"Sixthly, I intend in some cases to apply a degree of cold not
capable of reducing the steam to water, but of contracting it
considerably, so that the engines shall be worked by the
alternate expansion and contraction of the steam.
"Lastly, Instead of using water to render the pistons and other
parts of the engine air and steam tight, I employ oils, wax,
resinous bodies, fat of animals, quick-silver and other metals
in their fluid state."
The fifth claim was for a rotary engine, and need not be quoted here.
The early efforts of Watt are typical of those of the poor inventor
struggling with insufficient resources to gain recognition and it was
not until he became associated with the wealthy manufacturer, Mattheu
Boulton of Birmingham, that he met with the success upon which his
present fame is based. In partnership with Boulton, the business of the
manufacture and the sale of his engines were highly successful in spite
of vigorous attacks on the validity of his patents.
Though the fourth claim of Watt's patent describes a non-condensing
engine which would require high pressures, his aversion to such practice
was strong. Notwithstanding his entire knowledge of the advantages
through added expansion under high pressure, he continued to use
pressures not above 7 pounds per square inch above the atmosphere. To
overcome such pressures, his boilers were fed through a stand-pipe of
sufficient height to have the column of water offset the pressure within
the boiler. Watt's attitude toward high pressure made his influence felt
long after his patents had expired.
[Illustration: Portion of 9600 Horse-power Installation of Babcock &
Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox Chain
Grate Stokers at the Blue Island, Ill., Plant of the Public Service Co.
of Northern Illinois. This Company Operates 14,580 Horse Power of
Babcock & Wilcox Boilers and Superheaters in its Various Stations]
In 1782, Watt patented two other features which he had invented as early
as 1769. These were the double acting engine, that is, the use of steam
on both sides of the piston and the use of steam expansively, that is,
the shutting off of steam from the cylinder when the piston had made but
a portion of its stroke, the power for the completion of the stroke
being supplied by the expansive force of the steam already admitted.
He further added a throttle valve for the regulation of steam admission,
invented the automatic governor and the steam indicator, a mercury steam
gauge and a glass water column.
It has been the object of this brief history of the early developments
in the use of steam to cover such developments only through the time of
James Watt. The progress of the steam engine from this time through the
stages of higher pressures, combining of cylinders, the application of
steam vehicles and steamboats, the adding of third and fourth cylinders,
to the invention of the turbine with its development and the
accompanying development of the reciprocating engine to hold its place,
is one long attribute to the inventive genius of man.
While little is said in the biographies of Watt as to the improvement of
steam boilers, all the evidence indicates that Boulton and Watt
introduced the first "wagon boiler", so called because of its shape. In
1785, Watt took out a number of patents for variations in furnace
construction, many of which contain the basic principles of some of the
modern smoke preventing furnaces. Until the early part of the nineteenth
century, the low steam pressures used caused but little attention to be
given to the form of the boiler operated in connection with the engines
above described. About 1800, Richard Trevithick, in England, and Oliver
Evans, in America, introduced non-condensing, and for that time, high
pressure steam engines. To the initiative of Evans may be attributed the
general use of high pressure steam in the United States, a feature which
for many years distinguished American from European practice. The demand
for light weight and economy of space following the beginning of steam
navigation and the invention of the locomotive required boilers designed
and constructed to withstand heavier pressures and forced the adoption
of the cylindrical form of boiler. There are in use to-day many examples
of every step in the development of steam boilers from the first plain
cylindrical boiler to the most modern type of multi-tubular locomotive
boiler, which stands as the highest type of fire-tube boiler
construction.
The early attempts to utilize water-tube boilers were few. A brief
history of the development of the boilers, in which this principle was
employed, is given in the following chapter. From this history it will
be clearly indicated that the first commercially successful utilization
of water tubes in a steam generator is properly attributed to George H.
Babcock and Stephen Wilcox.
[Illustration: Copyright by Underwood & Underwood
Woolworth Building, New York City, Operating 2454 Horse Power of
Babcock & Wilcox Boilers]
BRIEF HISTORY OF WATER-TUBE BOILERS[1]
As stated in the previous chapter, the first water-tube boiler was built
by John Blakey and was patented by him in 1766. Several tubes
alternately inclined at opposite angles were arranged in the furnaces,
the adjacent tube ends being connected by small pipes. The first
successful user of water-tube boilers, however, was James Rumsey, an
American inventor, celebrated for his early experiments in steam
navigation, and it is he who may be truly classed as the originator of
the water-tube boiler. In 1788 he patented, in England, several forms of
boilers, some of which were of the water-tube type. One had a fire box
with flat top and sides, with horizontal tubes across the fire box
connecting the water spaces. Another had a cylindrical fire box
surrounded by an annular water space and a coiled tube was placed within
the box connecting at its two ends with the water space. This was the
first of the "coil boilers". Another form in the same patent was the
vertical tubular boiler, practically as made at the present time.
[Illustration: Blakey, 1766]
The first boiler made of a combination of small tubes, connected at one
end to a reservoir, was the invention of another American, John Stevens,
in 1804. This boiler was actually employed to generate steam for running
a steamboat on the Hudson River, but like all the "porcupine" boilers,
of which type it was the first, it did not have the elements of a
continued success.
[Illustration: John Stevens, 1804]
Another form of water tube was patented in 1805 by John Cox Stevens, a
son of John Stevens. This boiler consisted of twenty vertical tubes, 1¼
inches internal diameter and 40½ inches long, arranged in a circle, the
outside diameter of which was approximately 12 inches, connecting a
water chamber at the bottom with a steam chamber at the top. The steam
and water chambers were annular spaces of small cross section and
contained approximately 33 cubic inches. The illustration shows the cap
of the steam chamber secured by bolts. The steam outlet pipe "A" is a
pipe of one inch diameter, the water entering through a similar aperture
at the bottom. One of these boilers was for a long time at the Stevens
Institute of Technology at Hoboken, and is now in the Smithsonian
Institute at Washington.
[Illustration: John Cox Stevens, 1805]
About the same time, Jacob Woolf built a boiler of large horizontal
tubes, extending across the furnace and connected at the ends to a
longitudinal drum above. The first purely sectional water-tube boiler
was built by Julius Griffith, in 1821. In this boiler, a number of
horizontal water tubes were connected to vertical side pipes, the side
pipes were connected to horizontal gathering pipes, and these latter in
turn to a steam drum.
In 1822, Jacob Perkins constructed a flash boiler for carrying what was
then considered a high pressure. A number of cast-iron bars having 1½
inches annular holes through them and connected at their outer ends by a
series of bent pipes, outside of the furnace walls, were arranged in
three tiers over the fire. The water was fed slowly to the upper tier by
a force pump and steam in the superheated state was discharged to the
lower tiers into a chamber from which it was taken to the engine.
[Illustration: Joseph Eve, 1825]
The first sectional water-tube boiler, with a well-defined circulation,
was built by Joseph Eve, in 1825. The sections were composed of small
tubes with a slight double curve, but being practically vertical, fixed
in horizontal headers, which headers were in turn connected to a steam
space above and a water space below formed of larger pipes. The steam
and water spaces were connected by outside pipes to secure a circulation
of the water up through the sections and down through the external
pipes. In the same year, John M'Curdy of New York, built a "Duplex Steam
Generator" of "tubes of wrought or cast iron or other material" arranged
in several horizontal rows, connected together alternately at the front
and rear by return bends. In the tubes below the water line were placed
interior circular vessels closed at the ends in order to expose a thin
sheet of water to the action of the fire.
[Illustration: Gurney, 1826]
In 1826, Goldsworthy Gurney built a number of boilers, which he used on
his steam carriages. A number of small tubes were bent into the shape of
a "U" laid sidewise and the ends were connected with larger horizontal
pipes. These were connected by vertical pipes to permit of circulation
and also to a vertical cylinder which served as a steam and water
reservoir. In 1828, Paul Steenstrup made the first shell boiler with
vertical water tubes in the large flues, similar to the boiler known as
the "Martin" and suggesting the "Galloway".
The first water-tube boiler having fire tubes within water tubes was
built in 1830, by Summers & Ogle. Horizontal connections at the top and
bottom were connected by a series of vertical water tubes, through which
were fire tubes extending through the horizontal connections, the fire
tubes being held in place by nuts, which also served to make the joint.
[Illustration: Stephen Wilcox, 1856]
Stephen Wilcox, in 1856, was the first to use inclined water tubes
connecting water spaces at the front and rear with a steam space above.
The first to make such inclined tubes into a sectional form was Twibill,
in 1865. He used wrought-iron tubes connected at the front and rear with
standpipes through intermediate connections. These standpipes carried
the system to a horizontal cross drum at the top, the entrained water
being carried to the rear.
Clarke, Moore, McDowell, Alban and others worked on the problem of
constructing water-tube boilers, but because of difficulties of
construction involved, met with no practical success.
[Illustration: Twibill, 1865]
It may be asked why water-tube boilers did not come into more general
use at an early date, that is, why the number of water-tube boilers
built was so small in comparison to the number of shell boilers. The
reason for this is found in the difficulties involved in the design and
construction of water-tube boilers, which design and construction
required a high class of engineering and workmanship, while the plain
cylindrical boiler is comparatively easy to build. The greater skill
required to make a water-tube boiler successful is readily shown in the
great number of failures in the attempts to make them.
[Illustration: Partial View of 7000 Horse-power Installation of Babcock
& Wilcox Boilers at the Philadelphia, Pa., Plant of the Baldwin
Locomotive Works. This Company Operates in its Various Plants a Total of
9280 Horse Power of Babcock & Wilcox Boilers]
REQUIREMENTS OF STEAM BOILERS
Since the first appearance in "Steam" of the following "Requirements of
a Perfect Steam Boiler", the list has been copied many times either word
for word or clothed in different language and applied to some specific
type of boiler design or construction. In most cases, although full
compliance with one or more of the requirements was structurally
impossible, the reader was left to infer that the boiler under
consideration possessed all the desirable features. It is noteworthy
that this list of requirements, as prepared by George H. Babcock and
Stephen Wilcox, in 1875, represents the best practice of to-day.
Moreover, coupled with the boiler itself, which is used in the largest
and most important steam generating plants throughout the world, the
list forms a fitting monument to the foresight and genius of the
inventors.
REQUIREMENTS OF A PERFECT STEAM BOILER
1st. Proper workmanship and simple construction, using materials which
experience has shown to be the best, thus avoiding the necessity of
early repairs.
2nd. A mud drum to receive all impurities deposited from the water, and
so placed as to be removed from the action of the fire.
3rd. A steam and water capacity sufficient to prevent any fluctuation in
steam pressure or water level.
4th. A water surface for the disengagement of the steam from the water,
of sufficient extent to prevent foaming.
5th. A constant and thorough circulation of water throughout the boiler,
so as to maintain all parts at the same temperature.
6th. The water space divided into sections so arranged that, should any
section fail, no general explosion can occur and the destructive effects
will be confined to the escape of the contents. Large and free passages
between the different sections to equalize the water line and pressure
in all.
7th. A great excess of strength over any legitimate strain, the boiler
being so constructed as to be free from strains due to unequal
expansion, and, if possible, to avoid joints exposed to the direct
action of the fire.
8th. A combustion chamber so arranged that the combustion of the gases
started in the furnace may be completed before the gases escape to the
chimney.
9th. The heating surface as nearly as possible at right angles to the
currents of heated gases, so as to break up the currents and extract the
entire available heat from the gases.
10th. All parts readily accessible for cleaning and repairs. This is a
point of the greatest importance as regards safety and economy.
11th. Proportioned for the work to be done, and capable of working to
its full rated capacity with the highest economy.
12th. Equipped with the very best gauges, safety valves and other
fixtures.
The exhaustive study made of each one of these requirements is shown by
the following extract from a lecture delivered by Mr. Geo. H. Babcock at
Cornell University in 1890 upon the subject:
THE CIRCULATION OF WATER IN STEAM BOILERS
You have all noticed a kettle of water boiling over the fire, the fluid
rising somewhat tumultuously around the edges of the vessel, and
tumbling toward the center, where it descends. Similar currents are in
action while the water is simply being heated, but they are not
perceptible unless there are floating particles in the liquid. These
currents are caused by the joint action of the added temperature and two
or more qualities which the water possesses.
1st. Water, in common with most other substances, expands when heated; a
statement, however, strictly true only when referred to a temperature
above 39 degrees F. or 4 degrees C., but as in the making of steam we
rarely have to do with temperatures so low as that, we may, for our
present purposes, ignore that exception.
2nd. Water is practically a non-conductor of heat, though not entirely
so. If ice-cold water was kept boiling at the surface the heat would not
penetrate sufficiently to begin melting ice at a depth of 3 inches in
less than about two hours. As, therefore, the heated water cannot impart
its heat to its neighboring particles, it remains expanded and rises by
its levity, while colder portions come to be heated in turn, thus
setting up currents in the fluid.
Now, when all the water has been heated to the boiling point
corresponding to the pressure to which it is subjected, each added unit
of heat converts a portion, about 7 grains in weight, into vapor,
greatly increasing its volume; and the mingled steam and water rises
more rapidly still, producing ebullition such as we have noticed in the
kettle. So long as the quantity of heat added to the contents of the
kettle continues practically constant, the conditions remain similar to
those we noticed at first, a tumultuous lifting of the water around the
edges, flowing toward the center and thence downward; if, however, the
fire be quickened, the upward currents interfere with the downward and
the kettle boils over (Fig. 1).
[Illustration: Fig. 1]
If now we put in the kettle a vessel somewhat smaller (Fig. 2) with a
hole in the bottom and supported at a proper distance from the side so
as to separate the upward from the downward currents, we can force the
fires to a very much greater extent without causing the kettle to boil
over, and when we place a deflecting plate so as to guide the rising
column toward the center it will be almost impossible to produce that
effect. This is the invention of Perkins in 1831 and forms the basis of
very many of the arrangements for producing free circulation of the
water in boilers which have been made since that time. It consists in
dividing the currents so that they will not interfere each with the
other.
[Illustration: Fig. 2]
But what is the object of facilitating the circulation of water in
boilers? Why may we not safely leave this to the unassisted action of
nature as we do in culinary operations? We may, if we do not care for
the three most important aims in steam-boiler construction, namely,
efficiency, durability, and safety, each of which is more or less
dependent upon a proper circulation of the water. As for efficiency, we
have seen one proof in our kettle. When we provided means to preserve
the circulation, we found that we could carry a hotter fire and boil
away the water much more rapidly than before. It is the same in a steam
boiler. And we also noticed that when there was nothing but the
unassisted circulation, the rising steam carried away so much water in
the form of foam that the kettle boiled over, but when the currents were
separated and an unimpeded circuit was established, this ceased, and a
much larger supply of steam was delivered in a comparatively dry state.
Thus, circulation increases the efficiency in two ways: it adds to the
ability to take up the heat, and decreases the liability to waste that
heat by what is technically known as priming. There is yet another way
in which, incidentally, circulation increases efficiency of surface, and
that is by preventing in a greater or less degree the formation of
deposits thereon. Most waters contain some impurity which, when the
water is evaporated, remains to incrust the surface of the vessel. This
incrustation becomes very serious sometimes, so much so as to almost
entirely prevent the transmission of heat from the metal to the water.
It is said that an incrustation of only one-eighth inch will cause a
loss of 25 per cent in efficiency, and this is probably within the truth
in many cases. Circulation of water will not prevent incrustation
altogether, but it lessens the amount in all waters, and almost entirely
so in some, thus adding greatly to the efficiency of the surface.
[Illustration: Fig. 3]
A second advantage to be obtained through circulation is durability of
the boiler. This it secures mainly by keeping all parts at a nearly
uniform temperature. The way to secure the greatest freedom from unequal
strains in a boiler is to provide for such a circulation of the water as
will insure the same temperature in all parts.
3rd. Safety follows in the wake of durability, because a boiler which is
not subject to unequal strains of expansion and contraction is not only
less liable to ordinary repairs, but also to rupture and disastrous
explosion. By far the most prolific cause of explosions is this same
strain from unequal expansions.
[Illustration: Fig. 4]
[Illustration: 386 Horse-power Installation of Babcock & Wilcox Boilers
at B. F. Keith's Theatre, Boston, Mass.]
Having thus briefly looked at the advantages of circulation of water in
steam boilers, let us see what are the best means of securing it under
the most efficient conditions We have seen in our kettle that one
essential point was that the currents should be kept from interfering
with each other. If we could look into an ordinary return tubular boiler
when steaming, we should see a curious commotion of currents rushing
hither and thither, and shifting continually as one or the other
contending force gained a momentary mastery. The principal upward
currents would be found at the two ends, one over the fire and the other
over the first foot or so of the tubes. Between these, the downward
currents struggle against the rising currents of steam and water. At a
sudden demand for steam, or on the lifting of the safety valve, the
pressure being slightly reduced, the water jumps up in jets at every
portion of the surface, being lifted by the sudden generation of steam
throughout the body of water. You have seen the effect of this sudden
generation of steam in the well-known experiment with a Florence flask,
to which a cold application is made while boiling water under pressure
is within. You have also witnessed the geyser-like action when water is
boiled in a test tube held vertically over a lamp (Fig. 3).
[Illustration: Fig. 5]
If now we take a U-tube depending from a vessel of water (Fig. 4) and
apply the lamp to one leg a circulation is at once set up within it, and
no such spasmodic action can be produced. Thus U-tube is the
representative of the true method of circulation within a water-tube
boiler properly constructed. We can, for the purpose of securing more
heating surface, extend the heated leg into a long incline (Fig. 5),
when we have the well-known inclined-tube generator. Now, by adding
other tubes, we may further increase the heating surface (Fig. 6), while
it will still be the U-tube in effect and action. In such a construction
the circulation is a function of the difference in density of the two
columns. Its velocity is measured by the well-known Torricellian
formula, V = (2gh)^{½}, or, approximately V = 8(h)^{½}, h being measured
in terms of the lighter fluid. This velocity will increase until the
rising column becomes all steam, but the quantity or weight circulated
will attain a maximum when the density of the mingled steam and water in
the rising column becomes one-half that of the solid water in the
descending column which is nearly coincident with the condition of half
steam and half water, the weight of the steam being very slight compared
to that of the water.
[Illustration: Fig. 6]
It becomes easy by this rule to determine the circulation in any given
boiler built on this principle, provided the construction is such as to
permit a free flow of the water. Of course, every bend detracts a little
and something is lost in getting up the velocity, but when the boiler is
well arranged and proportioned these retardations are slight.
Let us take for example one of the 240 horse-power Babcock & Wilcox
boilers here in the University. The height of the columns may be taken
as 4½ feet, measuring from the surface of the water to about the center
of the bundle of tubes over the fire, and the head would be equal to
this height at the maximum of circulation. We should, therefore, have a
velocity of 8(4½)^{½} = 16.97, say 17 feet per second. There are in this
boiler fourteen sections, each having a 4-inch tube opening into the
drum, the area of which (inside) is 11 square inches, the fourteen
aggregating 154 square inches, or 1.07 square feet. This multiplied by
the velocity, 16.97 feet, gives 18.16 cubic feet mingled steam and water
discharged per second, one-half of which, or 9.08 cubic feet, is steam.
Assuming this steam to be at 100 pounds gauge pressure, it will weigh
0.258 pound per cubic foot. Hence, 2.34 pounds of steam will be
discharged per second, and 8,433 pounds per hour. Dividing this by 30,
the number of pounds representing a boiler horse power, we get 281.1
horse power, about 17 per cent, in excess of the rated power of the
boiler. The water at the temperature of steam at 100 pounds pressure
weighs 56 pounds per cubic foot, and the steam 0.258 pound, so that the
steam forms but 1/218 part of the mixture by weight, and consequently
each particle of water will make 218 circuits before being evaporated
when working at this capacity, and circulating the maximum weight of
water through the tubes.
[Illustration: A Portion of 9600 Horse-power Installation of Babcock &
Wilcox Boilers and Superheaters Being Erected at the South Boston,
Mass., Station of the Boston Elevated Railway Co. This Company Operates
in its Various Stations a Total of 46,400 Horse Power of Babcock &
Wilcox Boilers]
[Illustration: Fig. 7]
It is evident that at the highest possible velocity of exit from the
generating tubes, nothing but steam will be delivered and there will be
no circulation of water except to supply the place of that evaporated.
Let us see at what rate of steaming this would occur with the boiler
under consideration. We shall have a column of steam, say 4 feet high on
one side and an equal column of water on the other. Assuming, as before,
the steam at 100 pounds and the water at same temperature, we will have
a head of 866 feet of steam and an issuing velocity of 235.5 feet per
second. This multiplied by 1.07 square feet of opening by 3,600 seconds
in an hour, and by 0.258 gives 234,043 pounds of steam, which, though
only one-eighth the weight of mingled steam and water delivered at the
maximum, gives us 7,801 horse power, or 32 times the rated power of the
boiler. Of course, this is far beyond any possibility of attainment, so
that it may be set down as certain that this boiler cannot be forced to
a point where there will not be an efficient circulation of the water.
By the same method of calculation it may be shown that when forced to
double its rated power, a point rarely expected to be reached in
practice, about two-thirds the volume of mixture of steam and water
delivered into the drum will be steam, and that the water will make 110
circuits while being evaporated. Also that when worked at only about
one-quarter its rated capacity, one-fifth of the volume will be steam
and the water will make the rounds 870 times before it becomes steam.
You will thus see that in the proportions adopted in this boiler there
is provision for perfect circulation under all the possible conditions
of practice.
[Illustration: Fig. 8 [Developed to show Circulation]]
In designing boilers of this style it is necessary to guard against
having the uptake at the upper end of the tubes too large, for if
sufficiently large to allow downward currents therein, the whole effect
of the rising column in increasing the circulation in the tubes is
nullified (Fig. 7). This will readily be seen if we consider the uptake
very large when the only head producing circulation in the tubes will be
that due to the inclination of each tube taken by itself. This objection
is only overcome when the uptake is so small as to be entirely filled
with the ascending current of mingled steam and water. It is also
necessary that this uptake should be practically direct, and it should
not be composed of frequent enlargements and contractions. Take, for
instance, a boiler well known in Europe, copied and sold here under
another name. It is made up of inclined tubes secured by pairs into
boxes at the ends, which boxes are made to communicate with each other
by return bends opposite the ends of the tubes. These boxes and return
bends form an irregular uptake, whereby the steam is expected to rise to
a reservoir above. You will notice (Fig. 8) that the upward current of
steam and water in the return bend meets and directly antagonizes the
upward current in the adjoining tube. Only one result can follow. If
their velocities are equal, the momentum of both will be neutralized and
all circulation stopped, or, if one be stronger, it will cause a back
flow in the other by the amount of difference in force, with practically
the same result.
[Illustration: 4880 Horse-power Installation of Babcock & Wilcox Boilers
at the Open Hearth Plant of the Cambria Steel Co., Johnstown, Pa. This
Company Operates a Total of 52,000 Horse Power of Babcock & Wilcox
Boilers]
[Illustration: Fig. 9]
In a well-known boiler, many of which were sold, but of which none are
now made and a very few are still in use, the inventor claimed that the
return bends and small openings against the tubes were for the purpose
of "restricting the circulation" and no doubt they performed well that
office; but excepting for the smallness of the openings they were not as
efficient for that purpose as the arrangement shown in Fig. 8.
[Illustration: Fig. 10]
Another form of boiler, first invented by Clarke or Crawford, and lately
revived, has the uptake made of boxes into which a number, generally
from two to four tubes, are expanded, the boxes being connected together
by nipples (Fig. 9). It is a well-known fact that where a fluid flows
through a conduit which enlarges and then contracts, the velocity is
lost to a greater or less extent at the enlargements, and has to be
gotten up again at the contractions each time, with a corresponding loss
of head. The same thing occurs in the construction shown in Fig. 9. The
enlargements and contractions quite destroy the head and practically
overcome the tendency of the water to circulate.
A horizontal tube stopped at one end, as shown in Fig. 10, can have no
proper circulation within it. If moderately driven, the water may
struggle in against the issuing steam sufficiently to keep the surface
covered, but a slight degree of forcing will cause it to act like the
test tube in Fig. 3, and the more there are of them in a given boiler
the more spasmodic will be its working.
The experiment with our kettle (Fig. 2) gives the clue to the best means
of promoting circulation in ordinary shell boilers. Steenstrup or
"Martin" and "Galloway" water tubes placed in such boilers also assist
in directing the circulation therein, but it is almost impossible to
produce in shell boilers, by any means the circulation of all the water
in one continuous round, such as marks the well-constructed water-tube
boiler.
As I have before remarked, provision for a proper circulation of water
has been almost universally ignored in designing steam boilers,
sometimes to the great damage of the owner, but oftener to the jeopardy
of the lives of those who are employed to run them. The noted case of
the Montana and her sister ship, where some $300,000 was thrown away in
trying an experiment which a proper consideration of this subject would
have avoided, is a case in point; but who shall count the cost of life
and treasure not, perhaps, directly traceable to, but, nevertheless, due
entirely to such neglect in design and construction of the thousands of
boilers in which this necessary element has been ignored?
In the light of the performance of the exacting conditions of present
day power-plant practice, a review of this lecture and of the foregoing
list of requirements reveals the insight of the inventors of the Babcock
& Wilcox boiler into the fundamental principles of steam generator
design and construction.
Since the Babcock & Wilcox boiler became thoroughly established as a
durable and efficient steam generator, many types of water-tube boilers
have appeared on the market. Most of them, failing to meet enough of the
requirements of a perfect boiler, have fallen by the wayside, while a
few failing to meet all of the requirements, have only a limited field
of usefulness. None have been superior, and in the most cases the most
ardent admirers of other boilers have been satisfied in looking up to
the Babcock & Wilcox boiler as a standard and in claiming that the newer
boilers were "just as good."
Records of recent performances under the most severe conditions of
services on land and sea, show that the Babcock & Wilcox boiler can be
run continually and regularly at higher overloads, with higher
efficiency, and lower upkeep cost than any other boiler on the market.
It is especially adapted for power-plant work where it is necessary to
use a boiler in which steam can be raised quickly and the boiler placed
on the line either from a cold state or from a banked fire in the
shortest possible time, and with which the capacity, with clean feed
water, will be largely limited by the amount of coal that can be burned
in the furnace.
The distribution of the circulation through the separate headers and
sections and the action of the headers in forcing a maximum and
continuous circulation in the lower tubes, permit the operation of the
Babcock & Wilcox boiler without objectionable priming, with a higher
degree of concentration of salts in the water than is possible in any
other type of boiler.
Repeated daily performances at overloads have demonstrated beyond a
doubt the correctness of Mr. Babcock's computation regarding the
circulating tube and header area required for most efficient
circulation. They also have proved that enlargement of the area of
headers and circulating tubes beyond a certain point diminishes the head
available for causing circulation and consequently limits the ability of
the boiler to respond to demands for overloads.
In this lecture Mr. Babcock made the prediction that with the
circulating tube area proportioned in accordance with the principles
laid down, the Babcock & Wilcox boiler could be continuously run at
double its nominal rating, which at that time was based on 12 square
feet of heating surface per horse power. This prediction is being
fulfilled daily in all the large and prominent power plants in this
country and abroad, and it has been repeatedly demonstrated that with
clean water and clean tube surfaces it is possible to safely operate at
over 300 per cent of the nominal rating.
In the development of electrical power stations it becomes more and more
apparent that it is economical to run a boiler at high ratings during
the times of peak loads, as by so doing the lay-over losses are
diminished and the economy of the plant as a whole is increased.
The number and importance of the large electric lighting and power
stations constructed during the last ten years that are equipped with
Babcock & Wilcox boilers, is a most gratifying demonstration of the
merit of the apparatus, especially in view of their satisfactory
operation under conditions which are perhaps more exacting than those of
any other service.
Time, the test of all, results with boilers as with other things, in the
survival of the fittest. When judged on this basis the Babcock & Wilcox
boiler stands pre-eminent in its ability to cover the whole field of
steam generation with the highest commercial efficiency obtainable. Year
after year the Babcock & Wilcox boiler has become more firmly
established as the standard of excellence in the boiler making art.
[Illustration: South Boston Station of the Boston Elevated Ry. Co.,
Boston, Mass. 9600 Horse Power of Babcock & Wilcox Boilers and
Superheaters Installed in this Station]
[Illustration: 3600 Horse-power Installation of Babcock & Wilcox Boilers
at the Phipps Power House of the Duquesne Light Company, Pittsburgh,
Pa.]
EVOLUTION OF THE BABCOCK & WILCOX WATER-TUBE BOILER
Quite as much may be learned from the records of failures as from those
of success. Where a device has been once fairly tried and found to be
imperfect or impracticable, the knowledge of that trial is of advantage
in further investigation. Regardless of the lesson taught by failure,
however, it is an almost every-day occurrence that some device or
construction which has been tried and found wanting, if not worthless,
is again introduced as a great improvement upon a device which has shown
by its survival to be the fittest.
The success of the Babcock & Wilcox boiler is due to many years of
constant adherence to one line of research, in which an endeavor has
been made to introduce improvements with the view to producing a boiler
which would most effectively meet the demands of the times. During the
periods that this boiler has been built, other companies have placed on
the market more than thirty water-tube or sectional water-tube boilers,
most of which, though they may have attained some distinction and sale,
have now entirely disappeared. The following incomplete list will serve
to recall the names of some of the boilers that have had a vogue at
various times, but which are now practically unknown: Dimpfel, Howard,
Griffith & Wundrum, Dinsmore, Miller "Fire Box", Miller "American",
Miller "Internal Tube", Miller "Inclined Tube", Phleger, Weigant, the
Lady Verner, the Allen, the Kelly, the Anderson, the Rogers & Black, the
Eclipse or Kilgore, the Moore, the Baker & Smith, the Renshaw, the
Shackleton, the "Duplex", the Pond & Bradford, the Whittingham, the
Bee, the Hazleton or "Common Sense", the Reynolds, the Suplee or Luder,
the Babbit, the Reed, the Smith, the Standard, etc., etc.
It is with the object of protecting our customers and friends from loss
through purchasing discarded ideas that there is given on the following
pages a brief history of the development of the Babcock & Wilcox boiler
as it is built to-day. The illustrations and brief descriptions indicate
clearly the various designs and constructions that have been used and
that have been replaced, as experience has shown in what way improvement
might be made. They serve as a history of the experimental steps in the
development of the present Babcock & Wilcox boiler, the value and
success of which, as a steam generator, is evidenced by the fact that
the largest and most discriminating users continue to purchase them
after years of experience in their operation.
[Illustration: No. 1]
No. 1. The original Babcock & Wilcox boiler was patented in 1867. The
main idea in its design was safety, to which all other features were
sacrificed wherever they conflicted. The boiler consisted of a nest of
horizontal tubes, serving as a steam and water reservoir, placed above
and connected at each end by bolted joints to a second nest of inclined
heating tubes filled with water. The tubes were placed one above the
other in vertical rows, each row and its connecting end forming a single
casting. Hand-holes were placed at each end for cleaning. Internal tubes
were placed within the inclined tubes with a view to aiding circulation.
No. 2. This boiler was the same as No. 1, except that the internal
circulating tubes were omitted as they were found to hinder rather than
help the circulation.
Nos. 1 and 2 were found to be faulty in both material and design, cast
metal proving unfit for heating surfaces placed directly over the fire,
as it cracked as soon as any scale formed.
No. 3. Wrought-iron tubes were substituted for the cast-iron heating
tubes, the ends being brightened, laid in moulds, and the headers cast
on.
The steam and water capacity in this design were insufficient to secure
regularity of action, there being no reserve upon which to draw during
firing or when the water was fed intermittently. The attempt to dry the
steam by superheating it in the nest of tubes forming the steam space
was found to be impracticable. The steam delivered was either wet, dry
or superheated, according to the rate at which it was being drawn from
the boiler. Sediment was found to lodge in the lowermost point of the
boiler at the rear end and the exposed portions cracked off at this
point when subjected to the furnace heat.
[Illustration: No. 4]
No. 4. A plain cylinder, carrying the water line at its center and
leaving the upper half for steam space, was substituted for the nest of
tubes forming the steam and water space in Nos. 1, 2 and 3. The sections
were made as in No. 3 and a mud drum added to the rear end of the
sections at the point that was lowest and farthest removed from the
fire. The gases were made to pass off at one side and did not come into
contact with the mud drum. Dry steam was obtained through the increase
of separating surface and steam space and the added water capacity
furnished a storage for heat to tide over irregularities of firing and
feeding. By the addition of the drum, the boiler became a serviceable
and practical design, retaining all of the features of safety. As the
drum was removed from the direct action of the fire, it was not
subjected to excessive strain due to unequal expansion, and its
diameter, if large in comparison with that of the tubes formerly used,
was small when compared with that of cylindrical boilers. Difficulties
were encountered in this boiler in securing reliable joints between the
wrought-iron tubes and the cast-iron headers.
[Illustration: No. 5]
No. 5. In this design, wrought-iron water legs were substituted for the
cast-iron headers, the tubes being expanded into the inside sheets and a
large cover placed opposite the front end of the tubes for cleaning. The
tubes were staggered one above the other, an arrangement found to be
more efficient in the absorption of heat than where they were placed in
vertical rows. In other respects, the boiler was similar to No. 4,
except that it had lost the important element of safety through the
introduction of the very objectionable feature of flat stayed surfaces.
The large doors for access to the tubes were also a cause of weakness.
An installation of these boilers was made at the plant of the Calvert
Sugar Refinery in Baltimore, and while they were satisfactory in their
operation, were never duplicated.
[Illustration: No. 6]
No. 6. This was a modification of No. 5 in which longer tubes were used
and over which the gases were caused to make three passes with a view of
better economy. In addition, some of the stayed surfaces were omitted
and handholes substituted for the large access doors. A number of
boilers of this design were built but their excessive first cost, the
lack of adjustability of the structure under varying temperatures, and
the inconvenience of transportation, led to No. 7.
[Illustration: No. 7]
No. 7. In this boiler, the headers and water legs were replaced by
T-heads screwed to the ends of the inclined tubes. The faces of these Ts
were milled and the tubes placed one above the other with the milled
faces metal to metal. Long bolts passed through each vertical section of
the T-heads and through connecting boxes on the heads of the drums
holding the whole together. A large number of boilers of this design
were built and many were in successful operation for over twenty years.
In most instances, however, they were altered to later types.
[Illustration: No. 8]
[Illustration: No. 9]
Nos. 8 and 9. These boilers were known as the Griffith & Wundrum type,
the concern which built them being later merged in The Babcock & Wilcox
Co. Experiments were made with this design with four passages of the
gases across the tubes and the downward circulation of the water at the
rear of the boiler was carried to the bottom row of tubes. In No. 9 an
attempt was made to increase the safety and reduce the cost by reducing
the amount of steam and water capacity. A drum at right angles to the
line of tubes was used but as there was no provision made to secure dry
steam, the results were not satisfactory. The next move in the direction
of safety was the employment of several drums of small diameter instead
of a single drum.
[Illustration: No. 10]
This is shown in No. 10. A nest of small horizontal drums, 15 inches in
diameter, was used in place of the single drum of larger diameter. A set
of circulation tubes was placed at an intermediate angle between the
main bank of heating tubes and the horizontal drums forming the steam
reservoir. These circulators were to return to the rear end of the
circulating tubes the water carried up by the circulation, and in this
way were to allow only steam to be delivered to the small drums above.
There was no improvement in the action of this boiler over that of No.
9.
The four passages of the gas over the tubes tried in Nos. 8, 9 and 10
were not found to add to the economy of the boiler.
[Illustration: No. 11]
No. 11. A trial was next made of a box coil system, in which the water
was made to transverse the furnace several times before being delivered
to the drum above. The tendency here, as in all similar boilers, was to
form steam in the middle of the coil and blow the water from each end,
leaving the tubes practically dry until the steam found an outlet and
the water returned. This boiler had, in addition to a defective
circulation, a decidedly geyser-like action and produced wet steam.
[Illustration: No. 12]
All of the types mentioned, with the exception of Nos. 5 and 6, had
between their several parts a large number of bolted joints which were
subjected to the action of the fire. When these boilers were placed in
operation it was demonstrated that as soon as any scale formed on the
heating surfaces, leaks were caused due to unequal expansion.
No. 12. With this boiler, an attempt was made to remove the joints from
the fire and to increase the heating surface in a given space. Water
tubes were expanded into both sides of wrought-iron boxes, openings
being made for the admission of water and the exit of steam. Fire tubes
were placed inside the water tubes to increase the heating surface. This
design was abandoned because of the rapid stopping up of the tubes by
scale and the impossibility of cleaning them.
[Illustration: No. 13]
No. 13. Vertical straight line headers of cast iron, each containing two
rows of tubes, were bolted to a connection leading to the steam and
water drum above.
[Illustration: No. 14]
No. 14. A wrought-iron box was substituted for the double cast-iron
headers. In this design, stays were necessary and were found, as always,
to be an element to be avoided wherever possible. The boiler was an
improvement on No. 6, however. A slanting bridge wall was introduced
underneath the drum to throw a larger portion of its heating surface
into the combustion chamber under the bank of tubes.
This bridge wall was found to be difficult to keep in repair and was of
no particular benefit.
[Illustration: No. 15]
No. 15. Each row of tubes was expanded at each end into a continuous
header, cast of car wheel metal. The headers had a sinuous form so that
they would lie close together and admit of a staggered position of the
tubes when assembled. While other designs of header form were tried
later, experience with Nos. 14 and 15 showed that the style here adopted
was the best for all purposes and it has not been changed materially
since. The drum in this design was supported by girders resting on the
brickwork. Bolted joints were discarded, with the exception of those
connecting the headers to the front and rear ends of the drums and the
bottom of the rear headers to the mud drum. Even such joints, however,
were found objectionable and were superseded in subsequent construction
by short lengths of tubes expanded into bored holes.
[Illustration: No. 16]
No. 16. In this design, headers were tried which were made in the form
of triangular boxes, in each of which there were three tubes expanded.
These boxes were alternately reversed and connected by short lengths of
expanded tubes, being connected to the drum by tubes bent in a manner to
allow them to enter the shell normally. The joints between headers
introduced an element of weakness and the connections to the drum were
insufficient to give adequate circulation.
[Illustration: No. 17]
No. 17. Straight horizontal headers were next tried, alternately shifted
right and left to allow a staggering of tubes. These headers were
connected to each other and to the drums by expanded nipples. The
objections to this boiler were almost the same as those to No. 16.
[Illustration: No. 18]
[Illustration: No. 19]
Nos. 18 and 19. These boilers were designed primarily for fire
protection purposes, the requirements demanding a small, compact boiler
with ability to raise steam quickly. These both served the purpose
admirably but, as in No. 9, the only provision made for the securing of
dry steam was the use of the steam dome, shown in the illustration. This
dome was found inadequate and has since been abandoned in nearly all
forms of boiler construction. No other remedy being suggested at the
time, these boilers were not considered as desirable for general use as
Nos. 21 and 22. In Europe, however, where small size units were more in
demand, No. 18 was modified somewhat and used largely with excellent
results. These experiments, as they may now be called, although many
boilers of some of the designs were built, clearly demonstrated that the
best construction and efficiency required adherence to the following
elements of design:
1st. Sinuous headers for each vertical row of tubes.
2nd. A separate and independent connection with the drum, both front and
rear, for each vertical row of tubes.
[Illustration: No. 20A]
[Illustration: No. 20B]
3rd. All joints between parts of the boiler proper to be made without
bolts or screw plates.
4th. No surfaces to be used which necessitate the use of stays.
5th. The boiler supported independently of the brickwork so as to allow
freedom for expansion and contraction as it is heated or cooled.
6th. Ample diameter of steam and water drums, these not to be less than
30 inches except for small size units.
7th. Every part accessible for cleaning and repairs.
With these points having been determined, No. 20 was designed. This
boiler had all the desirable features just enumerated, together with a
number of improvements as to detail of construction. The general form of
No. 15 was adhered to but the bolted connections between sections and
drum and sections and mud drum were discarded in favor of connections
made by short lengths of boiler tubes expanded into the adjacent parts.
This boiler was suspended from girders, like No. 15, but these in turn
were carried on vertical supports, leaving the pressure parts entirely
free from the brickwork, the mutually deteriorating strains present
where one was supported by the other being in this way overcome.
Hundreds of thousands of horse power of this design were built, giving
great satisfaction. The boiler was known as the "C. I. F." (cast-iron
front) style, an ornamental cast-iron front having been usually
furnished.
[Illustration: No. 21]
The next step, and the one which connects the boilers as described above
to the boiler as it is built to-day, was the design illustrated in No.
21. These boilers were known as the "W. I. F." style, the fronts
furnished as part of the equipment being constructed largely of wrought
iron. The cast-iron drumheads used in No. 20 were replaced by
wrought-steel flanged and "bumped" heads. The drums were made longer and
the sections connected to wrought-steel cross boxes riveted to the
bottom of the drums. The boilers were supported by girders and columns
as in No. 20.
[Illustration: No. 22]
No. 22. This boiler, which is designated as the "Vertical Header" type,
has the same general features of construction as No. 21, except that the
tube sheet side of the headers is "stepped" to allow the headers to be
placed vertically and at right angles to the drum and still maintain the
tubes at the angle used in Nos. 20 and 21.
[Illustration: No. 23]
No. 23, or the cross drum design of boiler, is a development of the
Babcock & Wilcox marine boiler, in which the cross drum is used
exclusively. The experience of the Glasgow Works of The Babcock &
Wilcox, Ltd., with No. 18 proved that proper attention to details of
construction would make it a most desirable form of boiler where
headroom was limited. A large number of this design have been
successfully installed and are giving satisfactory results under widely
varying conditions. The cross drum boiler is also built in a vertical
header design.
Boilers Nos. 21, 22 and 23, with a few modifications, are now the
standard forms. These designs are illustrated, as they are constructed
to-day, on pages 48, 52, 54, 58 and 60.
The last step in the development of the water-tube boiler, beyond which
it seems almost impossible for science and skill to advance, consists in
the making of all pressure parts of the boiler of wrought steel,
including sinuous headers, cross boxes, nozzles, and the like. This
construction was the result of the demands of certain Continental laws
that are coming into general vogue in this country. The Babcock & Wilcox
Co. have at the present time a plant producing steel forgings that have
been pronounced by the _London Engineer_ to be "a perfect triumph
of the forgers' art".
The various designs of this all wrought-steel boiler are fully
illustrated in the following pages.
[Illustration: Wrought-steel Vertical Header Longitudinal Drum Babcock &
Wilcox Boiler, Equipped with Babcock & Wilcox Superheater and Babcock &
Wilcox Chain Grate Stoker]
THE BABCOCK & WILCOX BOILER
The following brief description of the Babcock & Wilcox boiler will
clearly indicate the manner in which it fulfills the requirements of the
perfect steam boiler already enumerated.
The Babcock & Wilcox boiler is built in two general classes, the
longitudinal drum type and the cross drum type. Either of these designs
may be constructed with vertical or inclined headers, and the headers in
turn may be of wrought steel or cast iron dependent upon the working
pressure for which the boiler is constructed. The headers may be of
different lengths, that is, may connect different numbers of tubes, and
it is by a change in the number of tubes in height per section and the
number of sections in width that the size of the boiler is varied.
The longitudinal drum boiler is the generally accepted standard of
Babcock & Wilcox construction. The cross drum boiler, though originally
designed to meet certain conditions of headroom, has become popular for
numerous classes of work where low headroom is not a requirement which
must be met.
LONGITUDINAL DRUM CONSTRUCTION--The heating surface of this type of
boiler is made up of a drum or drums, depending upon the width of the
boiler extending longitudinally over the other pressure parts. To the
drum or drums there are connected through cross boxes at either end the
sections, which are made up of headers and tubes. At the lower end of
the sections there is a mud drum extending entirely across the setting
and connected to all sections. The connections between all parts are by
short lengths of tubes expanded into bored seats.
[Illustration: Forged-steel Drumhead with Manhole Plate in Position]
The drums are of three sheets, of such thickness as to give the required
factor of safety under the maximum pressure for which the boiler is
constructed. The circular seams are ordinarily single lap riveted though
these may be double lap riveted to meet certain requirements of pressure
or of specifications. The longitudinal seams are properly proportioned
butt and strap or lap riveted joints dependent upon the pressure for
which the boilers are built. Where butt strap joints are used the straps
are bent to the proper radius in an hydraulic press. The courses are
built independently to template and are assembled by an hydraulic
forcing press. All riveted holes are punched one-quarter inch smaller
than the size of rivets as driven and are reamed to full size after the
plates are assembled. All rivets are driven by hydraulic pressure and
held until black.
[Illustration: Forged-steel Drumhead Interior]
The drumheads are hydraulic forged at a single heat, the manhole opening
and stiffening ring being forged in position. Flat raised seats for
water column and feed connections are formed in the forging.
All heads are provided with manholes, the edges of which are turned
true. The manhole plates are of forged steel and turned to fit manhole
opening. These plates are held in position by forged-steel guards and
bolts.
The drum nozzles are of forged steel, faced, and fitted with taper
thread stud bolts.
[Illustration: Forged-steel Drum Nozzle]
Cross boxes by means of which the sections are attached to the drums,
are of forged steel, made from a single sheet.
Where two or more drums are used in one boiler they are connected by a
cross pipe having a flanged outlet for the steam connection.
[Illustration: Forged-steel Cross Box]
The sections are built of 4-inch hot finished seamless open-hearth steel
tubes of No. 10 B. W. G. where the boilers are built for working
pressures up to 210 pounds. Where the working pressure is to be above
this and below 260 pounds, No. 9 B. W. G. tubes are supplied.
[Illustration: Inside Handhole Fittings Wrought-steel Vertical Header]
The tubes are expanded into headers of serpentine or sinuous form, which
dispose the tubes in a staggered position when assembled as a complete
boiler. These headers are of wrought steel or of cast iron, the latter
being ordinarily supplied where the working pressure is not to exceed
160 pounds. The headers may be either vertical or inclined as shown in
the various illustrations of assembled boilers.
[Illustration: Wrought-steel Vertical Header]
Opposite each tube end in the headers there is placed a handhole of
sufficient size to permit the cleaning, removal or renewal of a tube.
These openings in the wrought steel vertical headers are elliptical in
shape, machine faced, and milled to a true plane back from the edge a
sufficient distance to make a seat. The openings are closed by inside
fitting forged plates, shouldered to center in the opening, their
flanged seats milled to a true plane. These plates are held in position
by studs and forged-steel binders and nuts. The joints between plates
and headers are made with a thin gasket.
[Illustration: Inside Handhole Fitting Wrought-steel Inclined Header]
In the wrought-steel inclined headers the handhole openings are either
circular or elliptical, the former being ordinarily supplied. The
circular openings have a raised seat milled to a true plane. The
openings are closed on the outside by forged-steel caps, milled and
ground true, held in position by forged-steel safety clamps and secured
by ball-headed bolts to assure correct alignment. With this style of
fitting, joints are made tight, metal to metal, without packing of any
kind.
[Illustration: Wrought-steel Inclined Header]
Where elliptical handholes are furnished they are faced inside, closed
by inside fitting forged-steel plates, held to their seats by studs and
secured by forged-steel binders and nuts.
The joints between plates and header are made with a thin gasket.
[Illustration: Cast-iron Vertical Header]
The vertical cast-iron headers have elliptical handholes with raised
seats milled to a true plane. These are closed on the outside by
cast-iron caps milled true, held in position by forged-steel safety
clamps, which close the openings from the inside and which are secured
by ball-headed bolts to assure proper alignment. All joints are made
tight, metal to metal, without packing of any kind.
The mud drum to which the sections are attached at the lower end of the
rear headers, is a forged-steel box 7¼ inches square, and of such length
as to be connected to all headers by means of wrought nipples expanded
into counterbored seats. The mud drum is furnished with handholes for
cleaning, these being closed from the inside by forged-steel plates with
studs, and secured on a faced seat in the mud drum by forged-steel
binders and nuts. The joints between the plates and the drum are made
with thin gaskets. The mud drum is tapped for blow-off connection.
All connections between drums and sections and between sections and mud
drum are of hot finished seamless open-hearth steel tubes of No. 9
B. W. G.
Boilers of the longitudinal drum type are suspended front and rear from
wrought-steel supporting frames entirely independent of the brickwork.
This allows for expansion and contraction of the pressure parts without
straining either the boiler or the brickwork, and also allows of
brickwork repair or renewal without in any way disturbing the boiler or
its connections.
[Illustration: Babcock & Wilcox Wrought-steel Vertical Header Cross Drum
Boiler]
CROSS DRUM CONSTRUCTION--The cross drum type of boilers differs from the
longitudinal only in drum construction and method of support. The drum
in this type is placed transversely across the rear of the boiler and is
connected to the sections by means of circulating tubes expanded into
bored seats.
The drums for all pressures are of two sheets of sufficient thickness to
give the required factor of safety. The longitudinal seams are double
riveted butt strapped, the straps being bent to the proper radius in an
hydraulic press. The circulating tubes are expanded into the drums at
the seams, the butt straps serving as tube seats.
The drumheads, drum fittings and features of riveting are the same in
the cross drum as in the longitudinal types. The sections and mud drum
are also the same for the two types.
Cross drum boilers are supported at the rear on the mud drum which rests
on cast-iron foundation plates. They are suspended at the front from a
wrought-iron supporting frame, each section being suspended
independently from the cross members by hook suspension bolts. This
method of support is such as to allow for expansion and contraction
without straining either the boiler or the brickwork and permits of
repair or renewal of the latter without in any way disturbing the boiler
or its connections.
The following features of design and of attachments supplied are the
same for all types.
FRONTS--Ornamental fronts are fitted to the front supporting frame.
These have large doors for access to the front headers and panels above
the fire fronts. The fire fronts where furnished have independent frames
for fire doors which are bolted on, and ashpit doors fitted with blast
catches. The lugs on door frames and on doors are cast solid. The faces
of doors and of frames are planed and the lugs milled. The doors and
frames are placed in their final relative position, clamped, and the
holes for hinge pins drilled while thus held. A perfect alignment of
door and frame is thus assured and the method is representative of the
care taken in small details of manufacture.
The front as a whole is so arranged that any stoker may be applied with
but slight modification wherever boilers are set with sufficient furnace
height.
[Illustration: Cross Drum Boiler Front]
In the vertical header boilers large wrought-iron doors, which give
access to the rear headers, are attached to the rear supporting frame.
[Illustration: Wrought-steel Inclined Header Longitudinal Drum Babcock &
Wilcox Boiler, Equipped with Babcock & Wilcox Superheater]
[Illustration: Automatic Drumhead Stop and Check Valve]
FITTINGS--Each boiler is provided with the following fittings as part of
the standard equipment:
Blow-off connections and valves attached to the mud drum.
Safety valves placed on nozzles on the steam drums.
A water column connected to the front of the drum.
A steam gauge attached to the boiler front.
Feed water connection and valves. A flanged stop and check valve of
heavy pattern is attached directly to each drumhead, closing
automatically in case of a rupture in the feed line.
All valves and fittings are substantially built and are of designs which
by their successful service for many years have become standard with The
Babcock & Wilcox Co.
The fixtures that are supplied with the boilers consist of:
Dead plates and supports, the plates arranged for a fire brick lining.
A full set of grate bars and bearers, the latter fitted with expansion
sockets for side walls.
Flame bridge plates with necessary fastenings, and special fire brick
for lining same.
Bridge wall girder for hanging bridge wall with expansion sockets for
side walls.
A full set of access and cleaning doors through which all portions of
the pressure parts may be reached.
A swing damper and frame with damper operating rig.
There are also supplied with each boiler a wrench for handhole nuts, a
water-driven turbine tube cleaner, a set of fire tools and a metal steam
hose and cleaning pipe equipped with a special nozzle for blowing dust
and soot from the tubes.
Aside from the details of design and construction as covered in the
foregoing description, a study of the illustrations will make clear the
features of the boiler as a whole which have led to its success.
The method of supporting the boiler has been described. This allows it
to be hung at any height that may be necessary to properly handle the
fuel to be burned or to accommodate the stoker to be installed. The
height of the nest of tubes which forms the roof of the furnace is thus
the controlling feature in determining the furnace height, or the
distance from the front headers to the floor line. The sides and front
of the furnace are formed by the side and front boiler walls. The rear
wall of the furnace consists of a bridge wall built from the bottom of
the ashpit to the lower row of tubes. The location of this wall may be
adjusted within limits to give the depth of furnace demanded by the fuel
used. Ordinarily the bridge wall is the determining feature in the
locating of the front baffle. Where a great depth of furnace is
necessary, in which case, if the front baffle were placed at the bridge
wall the front pass of the boiler would be relatively too long, a
patented construction is used which maintains the baffle in what may be
considered its normal position, and a connection made between the baffle
and the bridge wall by means of a tile roof. Such furnace construction
is known as a "Webster" furnace.
[Illustration: Longitudinal Drum Boiler--Front View]
A consideration of this furnace will clearly indicate its adaptability,
by reason of its flexibility, for any fuel and any design of stoker. The
boiler lends itself readily to installation with an extension or Dutch
oven furnace if this be demanded by the fuel to be used, and in general
it may be stated that a satisfactory furnace arrangement may be made in
connection with a Babcock & Wilcox boiler for burning any fuel, solid,
liquid or gaseous.
The gases of combustion evolved in the furnace above described are led
over the heating surfaces by two baffles. These are formed of cast-iron
baffle plates lined with special fire brick and held in position by tube
clamps. The front baffle leads the gases through the forward portion of
the tubes to a chamber beneath the drum or drums. It is in this chamber
that a superheater is installed where such an apparatus is desired. The
gases make a turn over the front baffle, are led downward through the
central portion of the tubes, called the second pass, by means of a
hanging bridge wall of brick and the second baffle, around which they
make a second turn upward, pass through the rear portion of the tubes
and are led to the stack or flue through a damper box in the rear wall,
or around the drums to a damper box placed overhead.
The space beneath the tubes between the bridge wall and the rear boiler
wall forms a pocket into which much of the soot from the gases in their
downward passage through the second pass will be deposited and from
which it may be readily cleaned through doors furnished for the purpose.
The gas passages are ample and are so proportioned that the resistance
offered to the gases is only such as will assure the proper abstraction
of heat from the gases without causing undue friction, requiring
excessive draft.
[Illustration: Partial Vertical Section Showing Method of Introducing
Feed Water]
The method in which the feed water is introduced through the front
drumhead of the boiler is clearly seen by reference to the illustration.
From this point of introduction the water passes to the rear of the
drum, downward through the rear circulating tubes to the sections,
upward through the tubes of the sections to the front headers and
through these headers and front circulating tubes again to the drum
where such water as has not been formed into steam retraces its course.
The steam formed in the passage through the tubes is liberated as the
water reaches the front of the drum. The steam so formed is stored in
the steam space above the water line, from which it is drawn through a
so-called "dry pipe." The dry pipe in the Babcock & Wilcox boiler is
misnamed, as in reality it fulfills none of the functions ordinarily
attributed to such a device. This function is usually to restrict the
flow of steam from a boiler with a view to avoid priming. In the Babcock
& Wilcox boiler its function is simply that of a collecting pipe, and as
the aggregate area of the holes in it is greatly in excess of the area
of the steam outlet from the drum, it is plain that there can be no
restriction through this collecting pipe. It extends nearly the length
of the drum, and draws steam evenly from the whole length of the steam
space.
[Illustration: Cast-iron Vertical Header Longitudinal Drum Babcock &
Wilcox Boiler]
[Illustration:
Closed Open
Patented Side Dusting Doors]
The large tube doors through which access is had to the front headers
and the doors giving such access to the rear headers in boilers of the
vertical header type have already been described and are shown clearly
by the illustrations on pages 56 and 74. In boilers of the inclined
header type, access to the rear headers is secured through the chamber
formed by the headers and the rear boiler wall. Large doors in the sides
of the setting give full access to all parts for inspection and for
removal of accumulations of soot. Small dusting doors are supplied for
the side walls through which all of the heating surfaces may be cleaned
by means of a steam dusting lance. These side dusting doors are a
patented feature and the shutters are self closing. In wide boilers
additional cleaning doors are supplied at the top of the setting to
insure ease in reaching all portions of the heating surface.
The drums are accessible for inspection through the manhole openings.
The removal of the handhole plates makes possible the inspection of each
tube for its full length and gives the assurance that no defect can
exist that cannot be actually seen. This is particularly advantageous
when inspecting for the presence of scale.
The materials entering into the construction of the Babcock & Wilcox
boiler are the best obtainable for the special purpose for which they
are used and are subjected to rigid inspection and tests.
The boilers are manufactured by means of the most modern shop equipment
and appliances in the hands of an old and well-tried organization of
skilled mechanics under the supervision of experienced engineers.
[Illustration: Cast-iron Vertical Header Cross Drum Babcock & Wilcox
Boiler]
ADVANTAGES OF THE BABCOCK & WILCOX BOILER
The advantages of the Babcock & Wilcox boiler may perhaps be most
clearly set forth by a consideration, 1st, of water-tube boilers as a
class as compared with shell and fire-tube boilers; and 2nd, of the
Babcock & Wilcox boiler specifically as compared with other designs of
water-tube boilers.
WATER-TUBE _VERSUS_ FIRE-TUBE BOILERS
Safety--The most important requirement of a steam boiler is that it
shall be safe in so far as danger from explosion is concerned. If the
energy in a large shell boiler under pressure is considered, the thought
of the destruction possible in the case of an explosion is appalling.
The late Dr. Robert H. Thurston, Dean of Sibley College, Cornell
University, and past president of the American Society of Mechanical
Engineers, estimated that there is sufficient energy stored in a plain
cylinder boiler under 100 pounds steam pressure to project it in case of
an explosion to a height of over 3½ miles; a locomotive boiler at 125
pounds pressure from one-half to one-third of a mile; and a 60
horse-power return tubular boiler under 75 pounds pressure somewhat over
a mile. To quote: "A cubic foot of heated water under a pressure of from
60 to 70 pounds per square inch has about the same energy as one pound
of gunpowder." From such a consideration, it may be readily appreciated
how the advent of high pressure steam was one of the strongest factors
in forcing the adoption of water-tube boilers. A consideration of the
thickness of material necessary for cylinders of various diameters under
a steam pressure of 200 pounds and assuming an allowable stress of
12,000 pounds per square inch, will perhaps best illustrate this point.
Table 1 gives such thicknesses for various diameters of cylinders not
taking into consideration the weakening effect of any joints which may
be necessary. The rapidity with which the plate thickness increases with
the diameter is apparent and in practice, due to the fact that riveted
joints must be used, the thicknesses as given in the table, with the
exception of the first, must be increased from 30 to 40 per cent.
In a water-tube boiler the drums seldom exceed 48 inches in diameter and
the thickness of plate required, therefore, is never excessive. The
thinner metal can be rolled to a more uniform quality, the seams admit
of better proportioning, and the joints can be more easily and perfectly
fitted than is the case where thicker plates are necessary. All of these
points contribute toward making the drums of water-tube boilers better
able to withstand the stress which they will be called upon to endure.
The essential constructive difference between water-tube and fire-tube
boilers lies in the fact that the former is composed of parts of
relatively small diameter as against the large diameters necessary in
the latter.
The factor of safety of the boiler parts which come in contact with the
most intense heat in water-tube boilers can be made much higher than
would be practicable in a shell boiler. Under the assumptions considered
above in connection with the thickness of plates required, a number 10
gauge tube (0.134 inch), which is standard in Babcock & Wilcox boilers
for pressures up to 210 pounds under the same allowable stress as was
used in computing Table 1, the safe working pressure for the tubes is
870 pounds per square inch, indicating the very large margin of safety
of such tubes as compared with that possible with the shell of a boiler.
TABLE 1
PLATE THICKNESS REQUIRED
FOR VARIOUS CYLINDER
DIAMETERS
ALLOWABLE STRESS,
12000 POUNDS PER
SQUARE INCH,
200 POUNDS GAUGE
PRESSURE, NO JOINTS
+---------+-----------+
|Diameter | Thickness |
|Inches | Inches |
+---------+-----------+
| 4 | 0.033 |
| 36 | 0.300 |
| 48 | 0.400 |
| 60 | 0.500 |
| 72 | 0.600 |
| 108 | 0.900 |
| 120 | 1.000 |
| 144 | 1.200 |
+---------+-----------+
A further advantage in the water-tube boiler as a class is the
elimination of all compressive stresses. Cylinders subjected to external
pressures, such as fire tubes or the internally fired furnaces of
certain types of boilers, will collapse under a pressure much lower than
that which they could withstand if it were applied internally. This is
due to the fact that if there exists any initial distortion from its
true shape, the external pressure will tend to increase such distortion
and collapse the cylinder, while an internal pressure tends to restore
the cylinder to its original shape.
Stresses due to unequal expansion have been a fruitful source of trouble
in fire-tube boilers.
In boilers of the shell type, the riveted joints of the shell, with
their consequent double thickness of metal exposed to the fire, gives
rise to serious difficulties. Upon these points are concentrated all
strains of unequal expansion, giving rise to frequent leaks and
oftentimes to actual ruptures. Moreover, in the case of such rupture,
the whole body of contained water is liberated instantaneously and a
disastrous and usually fatal explosion results.
Further, unequal strains result in shell or fire-tube boilers due to the
difference in temperature of the various parts. This difference in
temperature results from the lack of positive well defined circulation.
While such a circulation does not necessarily accompany all water-tube
designs, in general, the circulation in water-tube boilers is much more
defined than in fire-tube or shell boilers.
A positive and efficient circulation assures that all portions of the
pressure parts will be at approximately the same temperature and in this
way strains resulting from unequal temperatures are obviated.
If a shell or fire-tubular boiler explodes, the apparatus as a whole is
destroyed. In the case of water-tube boilers, the drums are ordinarily
so located that they are protected from intense heat and any rupture is
usually in the case of a tube. Tube failures, resulting from blisters or
burning, are not serious in their nature. Where a tube ruptures because
of a flaw in the metal, the result may be more severe, but there cannot
be the disastrous explosion such as would occur in the case of the
explosion of a shell boiler.
To quote Dr. Thurston, relative to the greater safety of the water-tube
boiler: "The stored available energy is usually less than that of any of
the other stationary boilers and not very far from the amount stored,
pound for pound, in the plain tubular boiler. It is evident that their
admitted safety from destructive explosion does not come from this
relation, however, but from the division of the contents into small
portions and especially from those details of construction which make it
tolerably certain that any rupture shall be local. A violent explosion
can only come from the general disruption of a boiler and the liberation
at once of large masses of steam and water."
Economy--The requirement probably next in importance to safety in a
steam boiler is economy in the use of fuel. To fulfill such a
requirement, the three items, of proper grate for the class of fuel to
be burned, a combustion chamber permitting complete combustion of gases
before their escape to the stack, and the heating surface of such a
character and arrangement that the maximum amount of available heat may
be extracted, must be co-ordinated.
Fire-tube boilers from the nature of their design do not permit the
variety of combinations of grate surface, heating surface, and
combustion space possible in practically any water-tube boiler.
In securing the best results in fuel economy, the draft area in a boiler
is an important consideration. In fire-tube boilers this area is limited
to the cross sectional area of the fire tubes, a condition further
aggravated in a horizontal boiler by the tendency of the hot gases to
pass through the upper rows of tubes instead of through all of the tubes
alike. In water-tube boilers the draft area is that of the space outside
of the tubes and is hence much greater than the cross sectional area of
the tubes.
Capacity--Due to the generally more efficient circulation found in
water-tube than in fire-tube boilers, rates of evaporation are possible
with water-tube boilers that cannot be approached where fire-tube
boilers are employed.
Quick Steaming--Another important result of the better circulation
ordinarily found in water-tube boilers is in their ability to raise
steam rapidly in starting and to meet the sudden demands that may be
thrown on them.
In a properly designed water-tube boiler steam may be raised from a cold
boiler to 200 pounds pressure in less than one-half hour.
For the sake of comparison with the figure above, it may be stated that
in the U. S. Government Service the shortest time allowed for getting up
steam in Scotch marine boilers is 6 hours and the time ordinarily
allowed is 12 hours. In large double-ended Scotch boilers, such as are
generally used in Trans-Atlantic service, the fires are usually started
24 hours before the time set for getting under way. This length of time
is necessary for such boilers in order to eliminate as far as possible
excessive strains resulting from the sudden application of heat to the
surfaces.
Accessibility--In the "Requirements of a Perfect Steam Boiler", as
stated by Mr. Babcock, he demonstrates the necessity for complete
accessibility to all portions of the boiler for cleaning, inspection and
repair.
Cleaning--When the great difference is realized in performance, both as
to economy and capacity of a clean boiler and one in which the heating
surfaces have been allowed to become fouled, it may be appreciated that
the ability to keep heating surfaces clean internally and externally is
a factor of the highest importance.
Such results can be accomplished only by the use of a design in boiler
construction which gives complete accessibility to all portions. In
fire-tube boilers the tubes are frequently nested together with a space
between them often less than 1¼ inches and, as a consequence, nearly the
entire tube surface is inaccessible. When scale forms upon such tubes it
is impossible to remove it completely from the inside of the boiler and
if it is removed by a turbine hammer, there is no way of knowing how
thorough a job has been done. With the formation of such scale there is
danger through overheating and frequent tube renewals are necessary.
[Illustration: Portion of 29,000 Horse-power Installation of Babcock &
Wilcox Boilers in the L Street Station of the Edison Electric
Illuminating Co. of Boston, Mass. This Company Operates in its Various
Stations a Total of 39,000 Horse Power of Babcock & Wilcox Boilers]
In Scotch marine boilers, even with the engines operating condensing,
complete tube renewals at intervals of six or seven years are required,
while large replacements are often necessary in less than one year. In
return tubular boilers operated with bad feed water, complete tube
renewals annually are not uncommon. In this type of boiler much sediment
falls on the bottom sheets where the intense heat to which they are
subjected bakes it to such an excessive hardness that the only method of
removing it is to chisel it out. This can be done only by omitting tubes
enough to leave a space into which a man can crawl and the discomforts
under which he must work are apparent. Unless such a deposit is removed,
a burned and buckled plate will invariably result, and if neglected too
long an explosion will follow.
In vertical fire-tube boilers using a water leg construction, a deposit
of mud in such legs is an active agent in causing corrosion and the
difficulty of removing such deposit through handholes is well known. A
complete removal is practically impossible and as a last resort to
obviate corrosion in certain designs, the bottom of the water legs in
some cases have been made of copper. A thick layer of mud and scale is
also liable to accumulate on the crown sheet of such boilers and may
cause the sheet to crack and lead to an explosion.
The soot and fine coal swept along with the gases by the draft will
settle in fire tubes and unless removed promptly, must be cut out with a
special form of scraper. It is not unusual where soft coal is used to
find tubes half filled with soot, which renders useless a large portion
of the heating surface and so restricts the draft as to make it
difficult to burn sufficient coal to develop the required power from
such heating surface as is not covered by soot.
Water-tube boilers in general are from the nature of their design more
readily accessible for cleaning than are fire-tube boilers.
Inspection--The objections given above in the consideration of the
inability to properly clean fire-tube boilers hold as well for the
inspection of such boilers.
Repairs--The lack of accessibility in fire-tube boilers further leads to
difficulties where repairs are required.
In fire-tube boilers tube renewals are a serious undertaking. The
accumulation of hard deposit on the exterior of the surfaces so enlarges
the tubes that it is oftentimes difficult, if not impossible, to draw
them through the tube sheets and it is usually necessary to cut out such
tubes as will allow access to the one which has failed and remove them
through the manhole.
When a tube sheet blisters, the defective part must be cut out by
hand-tapped holes drilled by ratchets and as it is frequently impossible
to get space in which to drive rivets, a "soft patch" is necessary. This
is but a makeshift at best and usually results in either a reduction of
the safe working pressure or in the necessity for a new plate. If the
latter course is followed, the old plate must be cut out, a new one
scribed to place to locate rivet holes and in order to obtain room for
driving rivets, the boiler will have to be re-tubed.
The setting must, of course, be at least partially torn out and
replaced.
In case of repairs, of such nature in fire-tube boilers, the working
pressure of such repaired boilers will frequently be lowered by the
insurance companies when the boiler is again placed in service.
In the case of a rupture in a water-tube boiler, the loss will
ordinarily be limited to one or two tubes which can be readily replaced.
The fire-tube boiler will be so completely demolished that the question
of repairs will be shifted from the boiler to the surrounding property,
the damage to which will usually exceed many times the cost of a boiler
of a type which would have eliminated the possibility of a disastrous
explosion. In considering the proper repair cost of the two types of
boilers, the fact should not be overlooked that it is poor economy to
invest large sums in equipment that, through a possible accident to the
boiler may be wholly destroyed or so damaged that the cost of repairs,
together with the loss of time while such repairs are being made, would
purchase boilers of absolute safety and leave a large margin beside. The
possibility of loss of human life should also be considered, though this
may seem a far cry from the question of repair costs.
Space Occupied--The space required for the boilers in a plant often
exceeds the requirements for the remainder of the plant equipment. Any
saving of space in a boiler room will be a large factor in reducing the
cost of real estate and of the building. Even when the boiler plant is
comparatively small, the saving in space frequently will amount to a
considerable percentage of the cost of the boilers. Table 2 shows the
difference in floor space occupied by fire-tube boilers and Babcock &
Wilcox boilers of the same capacity, the latter being taken as
representing the water-tube class. This saving in space will increase
with the size of the plant for the reason that large size boiler units
while common in water-tube practice are impracticable in fire-tube
practice.
TABLE 2
COMPARATIVE APPROXIMATE FLOOR
SPACE OCCUPIED BY BABCOCK & WILCOX
AND H. R. T. BOILERS
+------------+----------------+---------------+
|Size of unit|Babcock & Wilcox| H. R. T. |
|Horse Power |Feet and Inches |Feet and Inches|
+------------+----------------+---------------+
| 100 | 7 3 × 19 9 | 10 0 × 20 0 |
| 150 | 7 10 × 19 9 | 10 0 × 22 6 |
| 200 | 9 0 × 19 9 | 11 6 × 23 10 |
| 250 | 9 0 × 19 9 | 11 6 × 23 10 |
| 300 | 10 2 × 19 9 | 12 0 × 25 0 |
+------------+----------------+---------------+
BABCOCK & WILCOX BOILERS AS COMPARED WITH OTHER WATER-TUBE DESIGNS
It must be borne in mind that the simple fact that a boiler is of the
water-tube design does not as a necessity indicate that it is a good or
safe boiler.
Safety--Many of the water-tube boilers on the
market are as lacking as are fire-tube boilers in the positive
circulation which, as has been demonstrated by Mr. Babcock's lecture, is
so necessary in the requirements of the perfect steam boiler. In boilers
using water-leg construction, there is danger of defective circulation,
leaks are common, and unsuspected corrosion may be going on in portions
of the boiler that cannot be inspected. Stresses due to unequal
expansion of the metal cannot be well avoided but they may be minimized
by maintaining at the same temperature all pressure parts of the boiler.
The result is to be secured only by means of a well defined circulation.
The main feature to which the Babcock & Wilcox boiler owes its safety is
the construction made possible by the use of headers, by which the water
in each vertical row of tubes is separated from that in the adjacent
rows. This construction results in the very efficient circulation
produced through the breaking up of the steam and water in the front
headers, the effect of these headers in producing such a positive
circulation having been clearly demonstrated in Mr. Babcock's lecture.
The use of a number of sections, thus composed of headers and tubes, has
a distinct advantage over the use of a common chamber at the outlet ends
of the tubes. In the former case the circulation of water in one
vertical row of tubes cannot interfere with that in the other rows,
while in the latter construction there will be downward as well as
upward currents and such downward currents tend to neutralize any good
effect there might be through the diminution of the density of the water
column by the steam.
Further, the circulation results directly from the design of the boiler
and requires no assistance from "retarders", check valves and the like,
within the boiler. All such mechanical devices in the interior of a
boiler serve only to complicate the design and should not be used.
This positive and efficient circulation assures that all portions of the
pressure parts of the Babcock & Wilcox boiler will be at approximately
the same temperature and in this way strains resulting from unequal
temperatures are obviated.
Where the water throughout the boiler is at the temperature of the steam
contained, a condition to be secured only by proper circulation, danger
from internal pitting is minimized, or at least limited only to effects
of the water fed the boiler. Where the water in any portion of the
boiler is lower than the temperature of the steam corresponding to the
pressure carried, whether the fact that such lower temperatures exist as
a result of lack of circulation, or because of intentional design,
internal pitting or corrosion will almost invariably result.
Dr. Thurston has already been quoted to the effect that the admitted
safety of a water-tube boiler is the result of the division of its
contents into small portions. In boilers using a water-leg construction,
while the danger from explosion will be largely limited to the tubes,
there is the danger, however, that such legs may explode due to the
deterioration of their stays, and such an explosion might be almost as
disastrous as that of a shell boiler. The headers in a Babcock & Wilcox
boiler are practically free from any danger of explosion. Were such an
explosion to occur, it would still be localized to a much larger extent
than in the case of a water-leg boiler and the header construction thus
almost absolutely localizes any danger from such a cause.
Staybolts are admittedly an undesirable element of construction in any
boiler. They are wholly objectionable and the only reason for the
presence of staybolts in a boiler is to enable a cheaper form of
construction to be used than if they were eliminated.
In boilers utilizing in their design flat-stayed surfaces, or staybolt
construction under pressure, corrosion and wear and tear in service
tends to weaken some single part subject to continual strain, the result
being an increased strain on other parts greatly in excess of that for
which an allowance can be made by any reasonable factor of safety. Where
the construction is such that the weakening of a single part will
produce a marked decrease in the safety and reliability of the whole, it
follows of necessity, that there will be a corresponding decrease in the
working pressure which may be safely carried.
In water-leg boilers, the use of such flat-stayed surfaces under
pressure presents difficulties that are practically unsurmountable. Such
surfaces exposed to the heat of the fire are subject to unequal
expansion, distortion, leakage and corrosion, or in general, to many of
the objections that have already been advanced against the fire-tube
boilers in the consideration of water-tube boilers as a class in
comparison with fire-tube boilers.
[Illustration: McAlpin Hotel, New York City, Operating 2360 Horse Power
of Babcock & Wilcox Boilers]
Aside from the difficulties that may arise in actual service due to the
failure of staybolts, or in general, due to the use of flat-stayed
surfaces, constructional features are encountered in the actual
manufacture of such boilers that make it difficult if not impossible to
produce a first-class mechanical job. It is practically impossible in
the building of such a boiler to so design and place the staybolts that
all will be under equal strain. Such unequal strains, resulting from
constructional difficulties, will be greatly multiplied when such a
boiler is placed in service. Much of the riveting in boilers of this
design must of necessity be hand work, which is never the equal of
machine riveting. The use of water-leg construction ordinarily requires
the flanging of large plates, which is difficult, and because of the
number of heats necessary and the continual working of the material, may
lead to the weakening of such plates.
In vertical or semi-vertical water-tube boilers utilizing flat-stayed
surfaces under pressure, these surfaces are ordinarily so located as to
offer a convenient lodging place for flue dust, which fuses into a hard
mass, is difficult of removal and under which corrosion may be going on
with no possibility of detection.
Where stayed surfaces or water legs are features in the design of a
water-tube boiler, the factor of safety of such parts must be most
carefully considered. In such parts too, is the determination of the
factor most difficult, and because of the "rule-of-thumb" determination
frequently necessary, the factor of safety becomes in reality a factor
of ignorance. As opposed to such indeterminate factors of safety, in the
Babcock & Wilcox boiler, when the factor of safety for the drum or drums
has been determined, and such a factor may be determined accurately, the
factors for all other portions of the pressure parts are greatly in
excess of that of the drum. All Babcock & Wilcox boilers are built with
a factor of safety of at least five, and inasmuch as the factor of the
safety of the tubes and headers is greatly in excess of this figure, it
applies specifically to the drum or drums. This factor represents a
greater degree of safety than a considerably higher factor applied to a
boiler in which the shell or any riveted portion is acted upon directly
by the fire, or the same factor applied to a boiler utilizing
flat-stayed surface construction, where the accurate determination of
the limiting factor of safety is difficult, if not impossible.
That the factor of safety of stayed surfaces is questionable may perhaps
be best realized from a consideration of the severe requirements as to
such factor called for by the rules and regulations of the Board of
Supervising Inspectors, U. S. Government.
In view of the above, the absence of any stayed surfaces in the Babcock
& Wilcox boiler is obviously a distinguishing advantage where safety is
a factor. It is of interest to note, in the article on the evolution of
the Babcock & Wilcox boiler, that staybolt construction was used in
several designs, found unsatisfactory and unsafe, and discarded.
Another feature in the design of the Babcock & Wilcox boiler tending
toward added safety is its manner of suspension. This has been indicated
in the previous chapter and is of such nature that all of the pressure
parts are free to expand or contract under variations of temperature
without in any way interfering with any part of the boiler setting. The
sectional nature of the boiler allows a flexibility under varying
temperature changes that practically obviates internal strain.
In boilers utilizing water-leg construction, on the other hand, the
construction is rigid, giving rise to serious internal strains and the
method of support ordinarily made necessary by the boiler design is not
only unmechanical but frequently dangerous, due to the fact that proper
provision is not made for expansion and contraction under temperature
variations.
Boilers utilizing water-leg construction are not ordinarily provided
with mud drums. This is a serious defect in that it allows impurities
and sediment to collect in a portion of the boiler not easily inspected,
and corrosion may result.
Economy--That the water-tube boiler as a class lends itself more readily
than does the fire-tube boiler to a variation in the relation of grate
surface, heating surface and combustion space has been already pointed
out. In economy again, the construction made possible by the use of
headers in Babcock & Wilcox boilers appears as a distinct advantage.
Because of this construction, there is a flexibility possible, in an
unlimited variety of heights and widths that will satisfactorily meet
the special requirements of the fuel to be burned in individual cases.
An extended experience in the design of furnaces best suited for a wide
variety of fuels has made The Babcock & Wilcox Co. leaders in the field
of economy. Furnaces have been built and are in successful operation for
burning anthracite and bituminous coals, lignite, crude oil, gas-house
tar, wood, sawdust and shavings, bagasse, tan bark, natural gas, blast
furnace gas, by-product coke oven gas and for the utilization of waste
heat from commercial processes. The great number of Babcock & Wilcox
boilers now in satisfactory operation under such a wide range of fuel
conditions constitutes an unimpeachable testimonial to the ability to
meet all of the many conditions of service.
The limitations in the draft area of fire-tube boilers as affecting
economy have been pointed out. That a greater draft area is possible in
water-tube boilers does not of necessity indicate that proper advantage
of this fact is taken in all boilers of the water-tube class. In the
Babcock & Wilcox boiler, the large draft area taken in connection with
the effective baffling allows the gases to be brought into intimate
contact with all portions of the heating surfaces and renders such
surfaces highly efficient.
In certain designs of water-tube boilers the baffling is such as to
render ineffective certain portions of the heating surface, due to the
tendency of soot and dirt to collect on or behind baffles, in this way
causing the interposition of a layer of non-conducting material between
the hot gases and the heating surfaces.
In Babcock & Wilcox boilers the standard baffle arrangement is such as
to allow the installation of a superheater without in any way altering
the path of the gases from furnace to stack, or requiring a change in
the boiler design. In certain water-tube boilers the baffle arrangement
is such that if a superheater is to be installed a complete change in
the ordinary baffle design is necessary. Frequently to insure
sufficiently hot gas striking the heating surfaces, a portion is
by-passed directly from the furnace to the superheater chamber without
passing over any of the boiler heating surfaces. Any such arrangement
will lead to a decrease in economy and the use of boilers requiring it
should be avoided.
Capacity--Babcock & Wilcox boilers are run successfully in every-day
practice at higher ratings than any other boilers in practical service.
The capacities thus obtainable are due directly to the efficient
circulation already pointed out. Inasmuch as the construction utilizing
headers has a direct bearing in producing such circulation, it is also
connected with the high capacities obtainable with this apparatus.
Where intelligently handled and kept properly cleaned, Babcock & Wilcox
boilers are operated in many plants at from 200 to 225 per cent of their
rated evaporative capacity and it is not unusual for them to be operated
at 300 per cent of such rated capacity during periods of peak load.
Dry Steam--In the list of the requirements of the perfect steam boiler,
the necessity that dry steam be generated has been pointed out. The
Babcock & Wilcox boiler will deliver dry steam under higher capacities
and poorer conditions of feed water than any other boiler now
manufactured. Certain boilers will, when operated at ordinary ratings,
handle poor feed water and deliver steam in which the moisture content
is not objectionable. When these same boilers are driven at high
overloads, there will be a direct tendency to prime and the percentage
of moisture in the steam delivered will be high. This tendency is the
result of the lack of proper circulation and once more there is seen the
advantage of the headers of the Babcock & Wilcox boiler, resulting as it
does in the securing of a positive circulation.
In the design of the Babcock & Wilcox boiler sufficient space is
provided between the steam outlet and the disengaging point to insure
the steam passing from the boiler in a dry state without entraining or
again picking up any particles of water in its passage even at high
rates of evaporation. Ample time is given for a complete separation of
steam from the water at the disengaging surface before the steam is
carried from the boiler. These two features, which are additional causes
for the ability of the Babcock & Wilcox boiler to deliver dry steam,
result from the proper proportioning of the steam and water space of the
boiler. From the history of the development of the boiler, it is evident
that the cubical capacity per horse power of the steam and water space
has been adopted after numerous experiments.
That the "dry pipe" serves in no way the generally understood function
of such device has been pointed out. As stated, the function of the "dry
pipe" in a Babcock & Wilcox boiler is simply that of a collecting pipe
and this statement holds true regardless of the rate of operation of the
boiler.
In certain boilers, "superheating surface" is provided to "dry the
steam," or to remove the moisture due to priming or foaming. Such
surface is invariably a source of trouble unless the steam is initially
dry and a boiler which will deliver dry steam is obviously to be
preferred to one in which surface must be supplied especially for such
purpose. Where superheaters are installed with Babcock & Wilcox boilers,
they are in every sense of the word superheaters and not driers, the
steam being delivered to them in a dry state.
The question has been raised in connection with the cross drum design of
the Babcock & Wilcox boiler as to its ability to deliver dry steam.
Experience has shown the absolute lack of basis for any such objection.
The Babcock & Wilcox Company at its Bayonne Works some time ago made a
series of experiments to see in what manner the steam generated was
separated from the water either in the drum or in its passage to the
drum. Glass peepholes were installed in each end of a drum in a boiler
of the marine design, at the point midway between that at which the
horizontal circulating tubes entered the drum and the drum baffle plate.
By holding a light at one of these peepholes the action in the drum was
clearly seen through the other. It was found that with the boiler
operated under three-quarter inch ashpit pressure, which, with the fuel
used would be equivalent to approximately 185 per cent of rating for
stationary boiler practice, that each tube was delivering with great
velocity a stream of solid water, which filled the tube for half its
cross sectional area. There was no spray or mist accompanying such
delivery, clearly indicating that the steam had entirely separated from
the water in its passage through the horizontal circulating tubes, which
in the boiler in question were but 50 inches long.
[Illustration: Northwest Station of the Commonwealth Edison Co.,
Chicago, Ill. This Installation Consists of 11,360 Horse Power of
Babcock & Wilcox Boilers and Superheaters, Equipped with Babcock &
Wilcox Chain Grate Stokers]
These experiments proved conclusively that the size of the steam drums
in the cross drum design has no appreciable effect in determining the
amount of liberating surface, and that sufficient liberating surface is
provided in the circulating tubes alone. If further proof of the ability
of this design of boiler to deliver dry steam is required, such proof is
perhaps best seen in the continued use of the Babcock & Wilcox marine
boiler, in which the cross drum is used exclusively, and with which
rates of evaporation are obtained far in excess of those secured in
ordinary practice.
Quick Steaming--The advantages of water-tube boilers as a class over
fire-tube boilers in ability to raise steam quickly have been indicated.
Due to the constant and thorough circulation resulting from the
sectional nature of the Babcock & Wilcox boiler, steam may be raised
more rapidly than in practically any other water-tube design.
In starting up a cold Babcock & Wilcox boiler with either coal or oil
fuel, where a proper furnace arrangement is supplied, steam may be
raised to a pressure of 200 pounds in less than half an hour. With a
Babcock & Wilcox boiler in a test where forced draft was available,
steam was raised from an initial temperature of the boiler and its
contained water of 72 degrees to a pressure of 200 pounds, in 12½
minutes after lighting the fire. The boiler also responds quickly in
starting from banked fires, especially where forced draft is available.
In Babcock & Wilcox boilers the water is divided into many small streams
which circulate without undue frictional resistance in thin envelopes
passing through the hottest part of the furnace, the steam being carried
rapidly to the disengaging surface. There is no part of the boiler
exposed to the heat of the fire that is not in contact with water
internally, and as a result there is no danger of overheating on
starting up quickly nor can leaks occur from unequal expansion such as
might be the case where an attempt is made to raise steam rapidly in
boilers using water leg construction.
Storage Capacity for Steam and Water--Where sufficient steam and water
capacity are not provided in a boiler, its action will be irregular, the
steam pressure varying over wide limits and the water level being
subject to frequent and rapid fluctuation.
Owing to the small relative weight of steam, water capacity is of
greater importance in this respect than steam space. With a gauge
pressure of 180 pounds per square inch, 8 cubic feet of steam, which is
equivalent to one-half cubic foot of water space, are required to supply
one boiler horse power for one minute and if no heat be supplied to the
boiler during such an interval, the pressure will drop to 150 pounds per
square inch. The volume of steam space, therefore, may be over rated,
but if this be too small, the steam passing off will carry water with it
in the form of spray. Too great a water space results in slow steaming
and waste of fuel in starting up; while too much steam space adds to the
radiating surface and increases the losses from that cause.
That the steam and water space of the Babcock & Wilcox boiler are the
result of numerous experiments has previously been pointed out.
Accessibility--Cleaning. That water-tube boilers are more accessible as
a class than are fire-tube boilers has been indicated. All water-tube
boilers, however, are not equally accessible. In certain designs, due to
the arrangement of baffling used it is practically impossible to remove
all deposits of soot and dirt. Frequently, in order to cheapen the
product, sufficient cleaning and access doors are not supplied as part
of the boiler equipment. The tendency of soot to collect on the crown
sheets of certain vertical water-tube boilers has been noted. Such
deposits are difficult to remove and if corrosion goes on beneath such a
covering the sheet may crack and an explosion result.
[Illustration: Rear View--Longitudinal Drum Vertical Header Boiler,
Showing Access Doors to Rear Headers]
It is almost impossible to thoroughly clean water legs internally, and
in such places also is there a tendency to unsuspected corrosion under
deposits that cannot be removed.
In Babcock & Wilcox boilers every portion of the interior of the heating
surfaces can be reached and kept clean, while any soot deposited on the
exterior surfaces can be blown off while the boiler is under pressure.
Inspection--The accessibility which makes possible the thorough cleaning
of all portions of the Babcock & Wilcox boiler also provides a means for
a thorough inspection.
Drums are accessible for internal inspection by the removal of the
manhole plates. Front headers may be inspected through large doors
furnished for the purpose. Rear headers in the inclined header designs
may be inspected from the chamber formed by such headers and the rear
wall of the boiler. In the vertical header designs rear tube doors are
furnished, as has been stated. In certain designs of water-tube boilers
in order to assure accessibility for inspection of the rear ends of the
tubes, the rear portion of the boiler is exposed to the atmosphere with
resulting excessive radiation losses. In other designs the means of
access to the rear ends of the tubes are of a makeshift and
unworkmanlike character.
By the removal of handhole plates, all tubes in a Babcock & Wilcox
boiler may be inspected for their full length either for the presence of
scale or for suspected corrosion.
Repairs--In Babcock & Wilcox boilers the possession of great strength,
the elimination of stresses due to uneven temperatures and of the
resulting danger of leaks and corrosion, the protection of the drums
from the intense heat of the fire, and the decreased liability of the
scale forming matter to lodge on the hottest tube surfaces, all tend to
minimize the necessity for repairs. The tubes of the Babcock & Wilcox
boiler are practically the only part which may need renewal and these
only at infrequent intervals When necessary, such renewals may be made
cheaply and quickly. A small stock of tubes, 4 inches in diameter, of
sufficient length for the boiler used, is all that need be carried to
make renewals.
Repairs in water-leg boilers are difficult at best and frequently
unsatisfactory when completed. When staybolt replacements are necessary,
in order to get at the inner sheet of the water leg, several tubes must
in some cases be cut out. Not infrequently a replacement of an entire
water leg is necessary and this is difficult and requires a lengthy
shutdown. With the Babcock & Wilcox boiler, on the other hand, even if
it is necessary to replace a section, this may be done in a few hours
after the boiler is cool.
In the case of certain staybolt failures the working pressure of a
repaired boiler utilizing such construction will frequently be lowered
by the insurance companies when the boiler is again placed in service.
The sectional nature of the Babcock & Wilcox boiler enables it to
maintain its original working pressure over long periods of time, almost
regardless of the nature of any repair that may be required.
[Illustration: 1456 Horse-power Installation of Babcock & Wilcox Boilers
at the Raritan Woolen Mills, Raritan, N. J. The First of These Boilers
were Installed in 1878 and 1881 and are still Operated at 80 Pounds
Pressure]
Durability--Babcock & Wilcox boilers are being operated in every-day
service with entirely satisfactory results and under the same steam
pressure as that for which they were originally sold that have been
operated from thirty to thirty-five years. It is interesting to note in
considering the life of a boiler that the length of life of a Babcock &
Wilcox boiler must be taken as the criterion of what length of life is
possible. This is due to the fact that there are Babcock & Wilcox
boilers in operation to-day that have been in service from a time that
antedates by a considerable margin that at which the manufacturer of any
other water-tube boiler now on the market was started.
Probably the very best evidence of the value of the Babcock & Wilcox
boiler as a steam generator and of the reliability of the apparatus, is
seen in the sales of the company. Since the company was formed, there
have been sold throughout the world over 9,900,000 horse power.
A feature that cannot be overlooked in the consideration of the
advantages of the Babcock & Wilcox boiler is the fact that as a part of
the organization back of the boiler, there is a body of engineers of
recognized ability, ready at all times to assist its customers in every
possible way.
[Illustration: 2400 Horse-power Installation of Babcock & Wilcox Boilers
in the Union Station Power House of the Pennsylvania Railroad Co.,
Pittsburgh, Pa. This Company has a Total of 28,500 Horse Power of
Babcock & Wilcox Boilers Installed]
HEAT AND ITS MEASUREMENT
The usual conception of heat is that it is a form of energy produced by
the vibratory motion of the minute particles or molecules of a body. All
bodies are assumed to be composed of these molecules, which are held
together by mutual cohesion and yet are in a state of continual
vibration. The hotter a body or the more heat added to it, the more
vigorous will be the vibrations of the molecules.
As is well known, the effect of heat on a body may be to change its
temperature, its volume, or its state, that is, from solid to liquid or
from liquid to gaseous. Where water is melted from ice and evaporated
into steam, the various changes are admirably described in the lecture
by Mr. Babcock on "The Theory of Steam Making", given in the next
chapter.
The change in temperature of a body is ordinarily measured by
thermometers, though for very high temperatures so-called pyrometers are
used. The latter are dealt with under the heading "High Temperature
Measurements" at the end of this chapter.
[Illustration: Fig. 11]
By reason of the uniform expansion of mercury and its great
sensitiveness to heat, it is the fluid most commonly used in the
construction of thermometers. In all thermometers the freezing point and
the boiling point of water, under mean or average atmospheric pressure
at sea level, are assumed as two fixed points, but the division of the
scale between these two points varies in different countries. The
freezing point is determined by the use of melting ice and for this
reason is often called the melting point. There are in use three
thermometer scales known as the Fahrenheit, the Centigrade or Celsius,
and the Réaumur. As shown in Fig. 11, in the Fahrenheit scale, the space
between the two fixed points is divided into 180 parts; the boiling
point is marked 212, and the freezing point is marked 32, and zero is a
temperature which, at the time this thermometer was invented, was
incorrectly imagined to be the lowest temperature attainable. In the
centigrade and the Réaumur scales, the distance between the two fixed
points is divided into 100 and 80 parts, respectively. In each of these
two scales the freezing point is marked zero, and the boiling point is
marked 100 in the centigrade and 80 in the Réaumur. Each of the 180, 100
or 80 divisions in the respective thermometers is called a degree.
Table 3 and appended formulae are useful for converting from one scale
to another.
In the United States the bulbs of high-grade thermometers are usually
made of either Jena 58^{III} borosilicate thermometer glass or Jena
16^{III} glass, the stems being made of ordinary glass. The Jena
16^{III} glass is not suitable for use at temperatures much above 850
degrees Fahrenheit and the harder Jena 59^{III} should be used in
thermometers for temperatures higher than this.
Below the boiling point, the hydrogen-gas thermometer is the almost
universal standard with which mercurial thermometers may be compared,
while above this point the nitrogen-gas thermometer is used. In both of
these standards the change in temperature is measured by the change in
pressure of a constant volume of the gas.
In graduating a mercurial thermometer for the Fahrenheit scale,
ordinarily a degree is represented as 1/180 part of the volume of the
stem between the readings at the melting point of ice and the boiling
point of water. For temperatures above the latter, the scale is extended
in degrees of the same volume. For very accurate work, however, the
thermometer may be graduated to read true-gas-scale temperatures by
comparing it with the gas thermometer and marking the temperatures at 25
or 50 degree intervals. Each degree is then 1/25 or 1/50 of the volume
of the stem in each interval.
Every thermometer, especially if intended for use above the boiling
point, should be suitably annealed before it is used. If this is not
done, the true melting point and also the "fundamental interval", that
is, the interval between the melting and the boiling points, may change
considerably. After continued use at the higher temperatures also, the
melting point will change, so that the thermometer must be calibrated
occasionally to insure accurate readings.
TABLE 3
COMPARISON OF THERMOMETER SCALES
+---------------+----------+----------+----------+
| |Fahrenheit|Centigrade| Réaumur |
+---------------+----------+----------+----------+
|Absolute Zero | -459.64 | -273.13 | -218.50 |
| | 0 | -17.78 | -14.22 |
| | 10 | -12.22 | -9.78 |
| | 20 | -6.67 | -5.33 |
| | 30 | -1.11 | -0.89 |
|Freezing Point | 32 | 0 | 0 |
|Maximum Density| | | |
| of Water | 39.1 | 3.94 | 3.15 |
| | 50 | 10 | 8 |
| | 75 | 23.89 | 19.11 |
| | 100 | 37.78 | 30.22 |
| | 200 | 93.33 | 74.67 |
|Boiling Point | 212 | 100 | 80 |
| | 250 | 121.11 | 96.89 |
| | 300 | 148.89 | 119.11 |
| | 350 | 176.67 | 141.33 |
+---------------+----------+----------+----------+
F = 9/5C+32° = 9/4R+32°
C = 5/9(F-32°) = 5/4R
R = 4/9(F-32°) = 4/5C
As a general rule thermometers are graduated to read correctly for total
immersion, that is, with bulb and stem of the thermometer at the same
temperature, and they should be used in this way when compared with a
standard thermometer. If the stem emerges into space either hotter or
colder than that in which the bulb is placed, a "stem correction" must
be applied to the observed temperature in addition to any correction
that may be found in the comparison with the standard. For instance, for
a particular thermometer, comparison with the standard with both fully
immersed made necessary the following corrections:
_Temperature_ _Correction_
40°F 0.0
100 0.0
200 0.0
300 +2.5
400 -0.5
500 -2.5
When the sign of the correction is positive (+) it must be added to the
observed reading, and when the sign is a negative (-) the correction
must be subtracted. The formula for the stem correction is as follows:
Stem correction = 0.000085 × n (T-t)
in which T is the observed temperature, t is the mean temperature of the
emergent column, n is the number of degrees of mercury column emergent,
and 0.000085 is the difference between the coefficient of expansion of
the mercury and that in the glass in the stem.
Suppose the observed temperature is 400 degrees and the thermometer is
immersed to the 200 degrees mark, so that 200 degrees of the mercury
column project into the air. The mean temperature of the emergent column
may be found by tying another thermometer on the stem with the bulb at
the middle of the emergent mercury column as in Fig. 12. Suppose this
mean temperature is 85 degrees, then
Stem correction = 0.000085 × 200 × (400 - 85) = 5.3 degrees.
As the stem is at a lower temperature than the bulb, the thermometer
will evidently read too low, so that this correction must be added to
the observed reading to find the reading corresponding to total
immersion. The corrected reading will therefore be 405.3 degrees. If
this thermometer is to be corrected in accordance with the calibrated
corrections given above, we note that a further correction of 0.5 must
be applied to the observed reading at this temperature, so that the
correct temperature is 405.3 - 0.5 = 404.8 degrees or 405 degrees.
[Illustration: Fig. 12]
[Illustration: Fig. 13]
Fig. 12 shows how a stem correction can be obtained for the case just
described.
Fig. 13 affords an opportunity for comparing the scale of a thermometer
correct for total immersion with one which will read correctly when
submerged to the 300 degrees mark, the stem being exposed at a mean
temperature of 110 degrees Fahrenheit, a temperature often prevailing
when thermometers are used for measuring temperatures in steam mains.
Absolute Zero--Experiments show that at 32 degrees Fahrenheit a perfect
gas expands 1/491.64 part of its volume if its pressure remains constant
and its temperature is increased one degree. Thus if gas at 32 degrees
Fahrenheit occupies 100 cubic feet and its temperature is increased one
degree, its volume will be increased to 100 + 100/491.64 = 100.203 cubic
feet. For a rise of two degrees the volume would be 100 + (100 × 2) /
491.64 = 100.406 cubic feet. If this rate of expansion per one degree
held good at all temperatures, and experiment shows that it does above
the freezing point, the gas, if its pressure remained the same, would
double its volume, if raised to a temperature of 32 + 491.64 = 523.64
degrees Fahrenheit, while under a diminution of temperature it would
shrink and finally disappear at a temperature of 491.64 - 32 = 459.64
degrees below zero Fahrenheit. While undoubtedly some change in the law
would take place before the lower temperature could be reached, there is
no reason why the law may not be used within the range of temperature
where it is known to hold good. From this explanation it is evident that
under a constant pressure the volume of a gas will vary as the number of
degrees between its temperature and the temperature of -459.64 degrees
Fahrenheit. To simplify the application of the law, a new thermometric
scale is constructed as follows: the point corresponding to -460 degrees
Fahrenheit, is taken as the zero point on the new scale, and the degrees
are identical in magnitude with those on the Fahrenheit scale.
Temperatures referred to this new scale are called absolute temperatures
and the point -460 degrees Fahrenheit (= -273 degrees centigrade) is
called the absolute zero. To convert any temperature Fahrenheit to
absolute temperature, add 460 degrees to the temperature on the
Fahrenheit scale: thus 54 degrees Fahrenheit will be 54 + 460 = 514
degrees absolute temperature; 113 degrees Fahrenheit will likewise be
equal to 113 + 460 = 573 degrees absolute temperature. If one pound of
gas is at a temperature of 54 degrees Fahrenheit and another pound is at
a temperature of 114 degrees Fahrenheit the respective volumes at a
given pressure would be in the ratio of 514 to 573.
[Illustration: Ninety-sixth Street Station of the New York Railways Co.,
New York City, Operating 20,000 Horse Power of Babcock & Wilcox Boilers.
This Company and its Allied Companies Operate a Total of 100,000 Horse
Power of Babcock & Wilcox Boilers]
British Thermal Unit--The quantitative measure of heat is the British
thermal unit, ordinarily written B. t. u. This is the quantity of heat
required to raise the temperature of one pound of pure water one degree
at 62 degrees Fahrenheit; that is, from 62 degrees to 63 degrees. In the
metric system this unit is the _calorie_ and is the heat necessary
to raise the temperature of one kilogram of pure water from 15 degrees
to 16 degrees centigrade. These two definitions lead to a discrepancy of
0.03 of 1 per cent, which is insignificant for engineering purposes, and
in the following the B. t. u. is taken with this discrepancy ignored.
The discrepancy is due to the fact that there is a slight difference in
the specific heat of water at 15 degrees centigrade and 62 degrees
Fahrenheit. The two units may be compared thus:
1 Calorie = 3.968 B. t. u. 1 B. t. u. = 0.252 Calories.
_Unit_ _Water_ _Temperature Rise_
1 B. t. u. 1 Pound 1 Degree Fahrenheit
1 Calorie 1 Kilogram 1 Degree centigrade
But 1 kilogram = 2.2046 pounds and 1 degree centigrade = 9/5 degree
Fahrenheit.
Hence 1 calorie = (2.2046 × 9/5) = 3.968 B. t. u.
The heat values in B. t. u. are ordinarily given per pound, and the heat
values in calories per kilogram, in which case the B. t. u. per pound
are approximately equivalent to 9/5 the calories per kilogram.
As determined by Joule, heat energy has a certain definite relation to
work, one British thermal unit being equivalent from his determinations
to 772 foot pounds. Rowland, a later investigator, found that 778 foot
pounds were a more exact equivalent. Still later investigations indicate
that the correct value for a B. t. u. is 777.52 foot pounds or
approximately 778. The relation of heat energy to work as determined is
a demonstration of the first law of thermo-dynamics, namely, that heat
and mechanical energy are mutually convertible in the ratio of 778 foot
pounds for one British thermal unit. This law, algebraically expressed,
is W = JH; W being the work done in foot pounds, H being the heat in
B. t. u., and J being Joules equivalent. Thus 1000 B. t. u.'s would be
capable of doing 1000 × 778 = 778000 foot pounds of work.
Specific Heat--The specific heat of a substance is the quantity of heat
expressed in thermal units required to raise or lower the temperature of
a unit weight of any substance at a given temperature one degree. This
quantity will vary for different substances For example, it requires
about 16 B. t. u. to raise the temperature of one pound of ice 32
degrees or 0.5 B. t. u. to raise it one degree, while it requires
approximately 180 B. t. u. to raise the temperature of one pound of
water 180 degrees or one B. t. u. for one degree.
If then, a pound of water be considered as a standard, the ratio of the
amount of heat required to raise a similar unit of any other substance
one degree, to the amount required to raise a pound of water one degree
is known as the specific heat of that substance. Thus since one pound of
water required one B. t. u. to raise its temperature one degree, and one
pound of ice requires about 0.5 degrees to raise its temperature one
degree, the ratio is 0.5 which is the specific heat of ice. To be exact,
the specific heat of ice is 0.504, hence 32 degrees × 0.504 = 16.128
B. t. u. would be required to raise the temperature of one pound of ice
from 0 to 32 degrees. For solids, at ordinary temperatures, the specific
heat may be considered a constant for each individual substance,
although it is variable for high temperatures. In the case of gases a
distinction must be made between specific heat at constant volume, and
at constant pressure.
Where specific heat is stated alone, specific heat at ordinary
temperature is implied, and _mean_ specific heat refers to the average
value of this quantity between the temperatures named.
The specific heat of a mixture of gases is obtained by multiplying the
specific heat of each constituent gas by the percentage by weight of
that gas in the mixture, and dividing the sum of the products by 100.
The specific heat of a gas whose composition by weight is CO_{2}, 13 per
cent; CO, 0.4 per cent; O, 8 per cent; N, 78.6 per cent, is found as
follows:
CO_{2} : 13 × 0.217 = 2.821
CO : 0.4 × 0.2479 = 0.09916
O : 8 × 0.2175 = 1.74000
N : 78.6 × 0.2438 = 19.16268
--------
100.0 23.82284
and 23.8228 ÷ 100 = 0.238 = specific heat of the gas.
The specific heats of various solids, liquids and gases are given in
Table 4.
Sensible Heat--The heat utilized in raising the temperature of a body,
as that in raising the temperature of water from 32 degrees up to the
boiling point, is termed sensible heat. In the case of water, the
sensible heat required to raise its temperature from the freezing point
to the boiling point corresponding to the pressure under which
ebullition occurs, is termed the heat of the liquid.
Latent Heat--Latent heat is the heat which apparently disappears in
producing some change in the condition of a body without increasing its
temperature If heat be added to ice at freezing temperature, the ice
will melt but its temperature will not be raised. The heat so utilized
in changing the condition of the ice is the latent heat and in this
particular case is known as the latent heat of fusion. If heat be added
to water at 212 degrees under atmospheric pressure, the water will not
become hotter but will be evaporated into steam, the temperature of
which will also be 212 degrees. The heat so utilized is called the
latent heat of evaporation and is the heat which apparently disappears
in causing the substance to pass from a liquid to a gaseous state.
TABLE 4
SPECIFIC HEATS OF VARIOUS SUBSTANCES
+--------------------------------------------------------------------+
| SOLIDS |
+-------------------------------+----------------+-------------------+
| | Temperature[2]| |
| | Degrees | Specific |
| | Fahrenheit | Heat |
+-------------------------------+----------------+-------------------+
| Copper | 59-460 | .0951 |
| Gold | 32-212 | .0316 |
| Wrought Iron | 59-212 | .1152 |
| Cast Iron | 68-212 | .1189 |
| Steel (soft) | 68-208 | .1175 |
| Steel (hard) | 68-208 | .1165 |
| Zinc | 32-212 | .0935 |
| Brass (yellow) | 32 | .0883 |
| Glass (normal ther. 16^{III}) | 66-212 | .1988 |
| Lead | 59 | .0299 |
| Platinum | 32-212 | .0323 |
| Silver | 32-212 | .0559 |
| Tin | -105-64 | .0518 |
| Ice | | .5040 |
| Sulphur (newly fused) | | .2025 |
+-------------------------------+----------------+-------------------+
| LIQUIDS |
+-------------------------------+----------------+-------------------+
| | Temperature[2]| |
| | Degrees | Specific |
| | Fahrenheit | Heat |
+-------------------------------+----------------+-------------------+
| Water[3] | 59 | 1.0000 |
| Alcohol | 32 | .5475 |
| | 176 | .7694 |
| Mercury | 32 | .03346 |
| Benzol | 50 | .4066 |
| | 122 | .4502 |
| Glycerine | 59-102 | .576 |
| Lead (Melted) | to 360 | .0410 |
| Sulphur (melted) | 246-297 | .2350 |
| Tin (melted) | | .0637 |
| Sea Water (sp. gr. 1.0043) | 64 | .980 |
| Sea Water (sp. gr. 1.0463) | 64 | .903 |
| Oil of Turpentine | 32 | .411 |
| Petroleum | 64-210 | .498 |
| Sulphuric Acid | 68-133 | .3363 |
+-------------------------------+----------------+-------------------+
| GASES |
+--------------------------+---------------+--------------+----------+
| | | Specific | Specific |
| | Temperature[2]| Heat at | Heat at |
| | Degrees | Constant | Constant |
| | Fahrenheit | Pressure | Volume |
+--------------------------+---------------+--------------+----------+
| Air | 32-392 | .2375 | .1693 |
| Oxygen | 44-405 | .2175 | .1553 |
| Nitrogen | 32-392 | .2438 | .1729 |
| Hydrogen | 54-388 | 3.4090 | 2.4141 |
| Superheated Steam | | See table 25 | |
| Carbon Monoxide | 41-208 | .2425 | .1728 |
| Carbon Dioxide | 52-417 | .2169 | .1535 |
| Methane | 64-406 | .5929 | .4505 |
| Blast Fur. Gas (approx.) | ... | .2277 | ... |
| Flue gas (approx.) | ... | .2400 | ... |
+--------------------------+---------------+--------------+----------+
Latent heat is not lost, but reappears whenever the substances pass
through a reverse cycle, from a gaseous to a liquid, or from a liquid to
a solid state. It may, therefore, be defined as stated, as the heat
which apparently disappears, or is lost to thermometric measurement,
when the molecular constitution of a body is being changed. Latent heat
is expended in performing the work of overcoming the molecular cohesion
of the particles of the substance and in overcoming the resistance of
external pressure to change of volume of the heated body. Latent heat of
evaporation, therefore, may be said to consist of internal and external
heat, the former being utilized in overcoming the molecular resistance
of the water in changing to steam, while the latter is expended in
overcoming any resistance to the increase of its volume during
formation. In evaporating a pound of water at 212 degrees to steam at
212 degrees, 897.6 B. t. u. are expended as internal latent heat and
72.8 B. t. u. as external latent heat. For a more detailed description
of the changes brought about in water by sensible and latent heat, the
reader is again referred to the chapter on "The Theory of Steam Making".
Ebullition--The temperature of ebullition of any liquid, or its boiling
point, may be defined as the temperature which exists where the addition
of heat to the liquid no longer increases its temperature, the heat
added being absorbed or utilized in converting the liquid into vapor.
This temperature is dependent upon the pressure under which the liquid
is evaporated, being higher as the pressure is greater.
TABLE 5
BOILING POINTS AT ATMOSPHERIC PRESSURE
+---------------------+--------------+
| | Degrees |
| | Fahrenheit |
+---------------------+--------------+
| Ammonia | 140 |
| Bromine | 145 |
| Alcohol | 173 |
| Benzine | 212 |
| Water | 212 |
| Average Sea Water | 213.2 |
| Saturated Brine | 226 |
| Mercury | 680 |
+---------------------+--------------+
Total Heat of Evaporation--The quantity of heat required to raise a unit
of any liquid from the freezing point to any given temperature, and to
entirely evaporate it at that temperature, is the total heat of
evaporation of the liquid for that temperature. It is the sum of the
heat of the liquid and the latent heat of evaporation.
To recapitulate, the heat added to a body is divided as follows:
Total heat = Heat to change the temperature + heat to overcome the
molecular cohesion + heat to overcome the external pressure
resisting an increase of volume of the body.
Where water is converted into steam, this total heat is divided as
follows:
Total heat = Heat to change the temperature of the water + heat to
separate the molecules of the water + heat to overcome
resistance to increase in volume of the steam,
= Heat of the liquid + internal latent heat + external
latent heat,
= Heat of the liquid + total latent heat of steam,
= Total heat of evaporation.
The steam tables given on pages 122 to 127 give the heat of the liquid
and the total latent heat through a wide range of temperatures.
Gases--When heat is added to gases there is no internal work done; hence
the total heat is that required to change the temperature plus that
required to do the external work. If the gas is not allowed to expand
but is preserved at constant volume, the entire heat added is that
required to change the temperature only.
Linear Expansion of Substances by Heat--To find the increase in the
length of a bar of any material due to an increase of temperature,
multiply the number of degrees of increase in temperature by the
coefficient of expansion for one degree and by the length of the bar.
Where the coefficient of expansion is given for 100 degrees, as in Table
6, the result should be divided by 100. The expansion of metals per one
degree rise of temperature increases slightly as high temperatures are
reached, but for all practical purposes it may be assumed to be constant
for a given metal.
TABLE 6
LINEAL EXPANSION OF SOLIDS AT ORDINARY TEMPERATURES
(Tabular values represent increase per foot per 100 degrees increase
in temperature, Fahrenheit or centigrade)
+-------------------+--------------+----------------+----------------+
| | Temperature | | |
| | Conditions[4]|Coefficient per |Coefficient per |
| Substance | Degrees | 100 Degrees | 100 Degrees |
| | Fahrenheit | Fahrenheit | Centigrade |
+-------------------+--------------+----------------+----------------+
|Brass (cast) | 32 to 212 | .001042 | .001875 |
|Brass (wire) | 32 to 212 | .001072 | .001930 |
|Copper | 32 to 212 | .000926 | .001666 |
|Glass (English | | | |
|flint) | 32 to 212 | .000451 | .000812 |
|Glass (French | | | |
|flint) | 32 to 212 | .000484 | .000872 |
|Gold | 32 to 212 | .000816 | .001470 |
|Granite (average) | 32 to 212 | .000482 | .000868 |
|Iron (cast) | 104 | .000589 | .001061 |
|Iron (soft forged) | 0 to 212 | .000634 | .001141 |
|Iron (wire) | 32 to 212 | .000800 | .001440 |
|Lead | 32 to 212 | .001505 | .002709 |
|Mercury | 32 to 212 | .009984[5] | .017971 |
|Platinum | 104 | .000499 | .000899 |
|Limestone | 32 to 212 | .000139 | .000251 |
|Silver | 104 | .001067 | .001921 |
|Steel (Bessemer | | | |
|rolled, hard) | 0 to 212 | .00056 | .00101 |
|Steel (Bessemer | | | |
|rolled, soft) | 0 to 212 | .00063 | .00117 |
|Steel (cast, | | | |
|French) | 104 | .000734 | .001322 |
|Steel (cast | | | |
|annealed, English) | 104 | .000608 | .001095 |
+-------------------+--------------+----------------+----------------+
High Temperature Measurements--The temperatures to be dealt with in
steam-boiler practice range from those of ordinary air and steam to the
temperatures of burning fuel. The gases of combustion, originally at the
temperature of the furnace, cool as they pass through each successive
bank of tubes in the boiler, to nearly the temperature of the steam,
resulting in a wide range of temperatures through which definite
measurements are sometimes required.
Of the different methods devised for ascertaining these temperatures,
some of the most important are as follows:
1st. Mercurial pyrometers for temperatures up to 1000 degrees
Fahrenheit.
2nd. Expansion pyrometers for temperatures up to 1500 degrees
Fahrenheit.
3rd. Calorimetry for temperatures up to 2000 degrees Fahrenheit.
4th. Thermo-electric pyrometers for temperatures up to 2900
degrees Fahrenheit.
5th. Melting points of metal which flow at various temperatures
up to the melting point of platinum 3227 degrees Fahrenheit.
6th. Radiation pyrometers for temperatures up to 3600 degrees
Fahrenheit.
7th. Optical pyrometers capable of measuring temperatures up to
12,600 degrees Fahrenheit.[6] For ordinary boiler practice
however, their range is 1600 to 3600 degrees Fahrenheit.
[Illustration: 228 Horse-power Babcock & Wilcox Boiler, Installed at the
Wentworth Institute, Boston, Mass.]
Table 7 gives the degree of accuracy of high temperature measurements.
TABLE 7
ACCURACY OF HIGH TEMPERATURE MEASUREMENTS[7]
+------------------------+------------------------+
| Centigrade | Fahrenheit |
+-------------+----------+-------------+----------+
| | Accuracy | | Accuracy |
| Temperature | Plus or | Temperature | Plus or |
| Range | Minus | Range | Minus |
| | Degrees | | Degrees |
+-------------+----------+-------------+----------+
| 200- 500 | 0.5 | 392- 932 | 0.9 |
| 500- 800 | 2 | 932-1472 | 3.6 |
| 800-1100 | 3 | 1472-2012 | 5.4 |
| 1100-1600 | 15 | 2012-2912 | 27 |
| 1600-2000 | 25 | 2912-3632 | 45 |
+-------------+----------+-------------+----------+
Mercurial Pyrometers--At atmospheric pressure mercury boils at 676
degrees Fahrenheit and even at lower temperatures the mercury in
thermometers will be distilled and will collect in the upper part of the
stem. Therefore, for temperatures much above 400 degrees Fahrenheit,
some inert gas, such as nitrogen or carbon dioxide, must be forced under
pressure into the upper part of the thermometer stem. The pressure at
600 degrees Fahrenheit is about 15 pounds, or slightly above that of the
atmosphere, at 850 degrees about 70 pounds, and at 1000 degrees about
300 pounds.
Flue-gas temperatures are nearly always taken with mercurial
thermometers as they are the most accurate and are easy to read and
manipulate. Care must be taken that the bulb of the instrument projects
into the path of the moving gases in order that the temperature may
truly represent the flue gas temperature. No readings should be
considered until the thermometer has been in place long enough to heat
it up to the full temperature of the gases.
Expansion Pyrometers--Brass expands about 50 per cent more than iron and
in both brass and iron the expansion is nearly proportional to the
increase in temperature. This phenomenon is utilized in expansion
pyrometers by enclosing a brass rod in an iron pipe, one end of the rod
being rigidly attached to a cap at the end of the pipe, while the other
is connected by a multiplying gear to a pointer moving around a
graduated dial. The whole length of the expansion piece must be at a
uniform temperature before a correct reading can be obtained. This fact,
together with the lost motion which is likely to exist in the mechanism
connected to the pointer, makes the expansion pyrometer unreliable; it
should be used only when its limitations are thoroughly understood and
it should be carefully calibrated. Unless the brass and iron are known
to be of the same temperature, its action will be anomalous: for
instance, if it be allowed to cool after being exposed to a high
temperature, the needle will rise before it begins to fall. Similarly, a
rise in temperature is first shown by the instrument as a fall. The
explanation is that the iron, being on the outside, heats or cools more
quickly than the brass.
Calorimetry--This method derives its name from the fact that the process
is the same as the determination of the specific heat of a substance by
the water calorimeter, except that in one case the temperature is known
and the specific heat is required, while in the other the specific heat
is known and the temperature is required. The temperature is found as
follows:
A given weight of some substance such as iron, nickel or fire brick, is
heated to the unknown temperature and then plunged into water and the
rise in temperature noted.
If X = temperature to be measured, w = weight of heated body in pounds,
W = weight of water in pounds, T = final temperature of water, t =
difference between initial and final temperatures of water, s = known
specific heat of body. Then X = T + Wt ÷ ws
Any temperatures secured by this method are affected by so many sources
of error that the results are very approximate.
Thermo-electric Pyrometers--When wires of two different metals are
joined at one end and heated, an electromotive force will be set up
between the free ends of the wires. Its amount will depend upon the
composition of the wires and the difference in temperature between the
two. If a delicate galvanometer of high resistance be connected to the
"thermal couple", as it is called, the deflection of the needle, after a
careful calibration, will indicate the temperature very accurately.
In the thermo-electric pyrometer of Le Chatelier, the wires used are
platinum and a 10 per cent alloy of platinum and rhodium, enclosed in
porcelain tubes to protect them from the oxidizing influence of the
furnace gases. The couple with its protecting tubes is called an
"element". The elements are made in different lengths to suit
conditions.
It is not necessary for accuracy to expose the whole length of the
element to the temperature to be measured, as the electromotive force
depends only upon the temperature of the juncture at the closed end of
the protecting tube and that of the cold end of the element. The
galvanometer can be located at any convenient point, since the length of
the wires leading to it simply alter the resistance of the circuit, for
which allowance may be made.
The advantages of the thermo-electric pyrometer are accuracy over a wide
range of temperatures, continuity of readings, and the ease with which
observations can be taken. Its disadvantages are high first cost and, in
some cases, extreme delicacy.
Melting Points of Metals--The approximate temperature of a furnace or
flue may be determined, if so desired, by introducing certain metals of
which the melting points are known. The more common metals form a series
in which the respective melting points differ by 100 to 200 degrees
Fahrenheit, and by using these in order, the temperature can be fixed
between the melting points of some two of them. This method lacks
accuracy, but it suffices for determinations where approximate readings
are satisfactory.
The approximate melting points of certain metals that may be used for
determinations of this nature are given in Table 8.
Radiation Pyrometers--These are similar to thermo-electric pyrometers in
that a thermo-couple is employed. The heat rays given out by the hot
body fall on a concave mirror and are brought to a focus at a point at
which is placed the junction of a thermo-couple. The temperature
readings are obtained from an indicator similar to that used with
thermo-electric pyrometers.
Optical Pyrometers--Of the optical pyrometers the Wanner is perhaps the
most reliable. The principle on which this instrument is constructed is
that of comparing the quantity of light emanating from the heated body
with a constant source of light, in this case a two-volt osmium lamp.
The lamp is placed at one end of an optical tube, while at the other an
eyepiece is provided and a scale. A battery of cells furnishes the
current for the lamp. On looking through the pyrometer, a circle of red
light appears, divided into distinct halves of different intensities.
Adjustment may be made so that the two halves appear alike and a reading
is then taken from the scale. The temperatures are obtained from a table
of temperatures corresponding to scale readings. For standardizing the
osmium lamp, an amylacetate lamp, is provided with a stand for holding
the optical tube.
TABLE 8
APPROXIMATE MELTING POINTS OF METALS[8]
+-----------------+------------------+
| Metal | Temperature |
| |Degrees Fahrenheit|
+-----------------+------------------+
|Wrought Iron | 2737 |
|Pig Iron (gray) | 2190-2327 |
|Cast Iron (white)| 2075 |
|Steel | 2460-2550 |
|Steel (cast) | 2500 |
|Copper | 1981 |
|Zinc | 786 |
|Antimony | 1166 |
|Lead | 621 |
|Bismuth | 498 |
|Tin | 449 |
|Platinum | 3191 |
|Gold | 1946 |
|Silver | 1762 |
|Aluminum | 1216 |
+-----------------+------------------+
Determination of Temperature from Character of Emitted Light--As a
further means of determining approximately the temperature of a furnace,
Table 9, compiled by Messrs. White & Taylor, may be of service. The
color at a given temperature is approximately the same for all kinds of
combustibles under similar conditions.
TABLE 9
CHARACTER OF EMITTED LIGHT AND CORRESPONDING
APPROXIMATE TEMPERATURE[9]
+--------------------------------------+-----------+
| Character of Emitted Light |Temperature|
| | Degrees |
| | Fahrenheit|
+--------------------------------------+-----------+
|Dark red, blood red, low red | 1050 |
|Dark cherry red | 1175 |
|Cherry, full red | 1375 |
|Light cherry, bright cherry, light red| 1550 |
|Orange | 1650 |
|Light orange | 1725 |
|Yellow | 1825 |
|Light yellow | 1975 |
|White | 2200 |
+--------------------------------------+-----------+
THE THEORY OF STEAM MAKING
[Extracts from a Lecture delivered by George H. Babcock, at Cornell
University, 1887[10]]
The chemical compound known as H_{2}O exists in three states or
conditions--ice, water and steam; the only difference between these
states or conditions is in the presence or absence of a quantity of
energy exhibited partly in the form of heat and partly in molecular
activity, which, for want of a better name, we are accustomed to call
"latent heat"; and to transform it from one state to another we have
only to supply or extract heat. For instance, if we take a quantity of
ice, say one pound, at absolute zero[11] and supply heat, the first
effect is to raise its temperature until it arrives at a point 492
Fahrenheit degrees above the starting point. Here it stops growing
warmer, though we keep on adding heat. It, however, changes from ice to
water, and when we have added sufficient heat to have made it, had it
remained ice, 283 degrees hotter or a temperature of 315 degrees
Fahrenheit's thermometer, it has all become water, at the same
temperature at which it commenced to change, namely, 492 degrees above
absolute zero, or 32 degrees by Fahrenheit's scale. Let us still
continue to add heat, and it will now grow warmer again, though at a
slower rate--that is, it now takes about double the quantity of heat to
raise the pound one degree that it did before--until it reaches a
temperature of 212 degrees Fahrenheit, or 672 degrees absolute (assuming
that we are at the level of the sea). Here we find another critical
point. However much more heat we may apply, the water, as water, at that
pressure, cannot be heated any hotter, but changes on the addition of
heat to steam; and it is not until we have added heat enough to have
raised the temperature of the water 966 degrees, or to 1,178 degrees by
Fahrenheit's thermometer (presuming for the moment that its specific
heat has not changed since it became water), that it has all become
steam, which steam, nevertheless, is at the temperature of 212 degrees,
at which the water began to change. Thus over four-fifths of the heat
which has been added to the water has disappeared, or become insensible
in the steam to any of our instruments.
It follows that if we could reduce steam at atmospheric pressure to
water, without loss of heat, the heat stored within it would cause the
water to be red hot; and if we could further change it to a solid, like
ice, without loss of heat, the solid would be white hot, or hotter than
melted steel--it being assumed, of course, that the specific heat of the
water and ice remain normal, or the same as they respectively are at the
freezing point.
After steam has been formed, a further addition of heat increases the
temperature again at a much faster ratio to the quantity of heat added,
which ratio also varies according as we maintain a constant pressure or
a constant volume; and I am not aware that any other critical point
exists where this will cease to be the fact until we arrive at that very
high temperature, known as the point of dissociation, at which it
becomes resolved into its original gases.
The heat which has been absorbed by one pound of water to convert it
into a pound of steam at atmospheric pressure is sufficient to have
melted 3 pounds of steel or 13 pounds of gold. This has been transformed
into something besides heat; stored up to reappear as heat when the
process is reversed. That condition is what we are pleased to call
latent heat, and in it resides mainly the ability of the steam to do
work.
[Graph: Temperature in Fahrenheit Degrees (from Absolute Zero)
against Quantity of Heat in British Thermal Units]
The diagram shows graphically the relation of heat to temperature, the
horizontal scale being quantity of heat in British thermal units, and
the vertical temperature in Fahrenheit degrees, both reckoned from
absolute zero and by the usual scale. The dotted lines for ice and water
show the temperature which would have been obtained if the conditions
had not changed. The lines marked "gold" and "steel" show the relation
to heat and temperature and the melting points of these metals. All the
inclined lines would be slightly curved if attention had been paid to
the changing specific heat, but the curvature would be small. It is
worth noting that, with one or two exceptions, the curves of all
substances lie between the vertical and that for water. That is to say,
that water has a greater capacity for heat than all other substances
except two, hydrogen and bromine.
In order to generate steam, then, only two steps are required: 1st,
procure the heat, and 2nd, transfer it to the water. Now, you have it
laid down as an axiom that when a body has been transferred or
transformed from one place or state into another, the same work has been
done and the same energy expended, whatever may have been the
intermediate steps or conditions, or whatever the apparatus. Therefore,
when a given quantity of water at a given temperature has been made into
steam at a given temperature, a certain definite work has been done, and
a certain amount of energy expended, from whatever the heat may have
been obtained, or whatever boiler may have been employed for the
purpose.
A pound of coal or any other fuel has a definite heat producing
capacity, and is capable of evaporating a definite quantity of water
under given conditions. That is the limit beyond which even perfection
cannot go, and yet I have known, and doubtless you have heard of, cases
where inventors have claimed, and so-called engineers have certified to,
much higher results.
The first step in generating steam is in burning the fuel to the best
advantage. A pound of carbon will generate 14,500 British thermal units,
during combustion into carbonic dioxide, and this will be the same,
whatever the temperature or the rapidity at which the combustion may
take place. If possible, we might oxidize it at as slow a rate as that
with which iron rusts or wood rots in the open air, or we might burn it
with the rapidity of gunpowder, a ton in a second, yet the total heat
generated would be precisely the same. Again, we may keep the
temperature down to the lowest point at which combustion can take place,
by bringing large bodies of air in contact with it, or otherwise, or we
may supply it with just the right quantity of pure oxygen, and burn it
at a temperature approaching that of dissociation, and still the heat
units given off will be neither more nor less. It follows, therefore,
that great latitude in the manner or rapidity of combustion may be taken
without affecting the quantity of heat generated.
But in practice it is found that other considerations limit this
latitude, and that there are certain conditions necessary in order to
get the most available heat from a pound of coal. There are three ways,
and only three, in which the heat developed by the combustion of coal in
a steam boiler furnace may be expended.
1st, and principally. It should be conveyed to the water in the boiler,
and be utilized in the production of steam. To be perfect, a boiler
should so utilize all the heat of combustion, but there are no perfect
boilers.
2nd. A portion of the heat of combustion is conveyed up the chimney in
the waste gases. This is in proportion to the weight of the gases, and
the difference between their temperature and that of the air and coal
before they entered the fire.
3rd. Another portion is dissipated by radiation from the sides of the
furnace. In a stove the heat is all used in these latter two ways,
either it goes off through the chimney or is radiated into the
surrounding space. It is one of the principal problems of boiler
engineering to render the amount of heat thus lost as small as possible.
The loss from radiation is in proportion to the amount of surface, its
nature, its temperature, and the time it is exposed. This loss can be
almost entirely eliminated by thick walls and a smooth white or polished
surface, but its amount is ordinarily so small that these extraordinary
precautions do not pay in practice.
It is evident that the temperature of the escaping gases cannot be
brought below that of the absorbing surfaces, while it may be much
greater even to that of the fire. This is supposing that all of the
escaping gases have passed through the fire. In case air is allowed to
leak into the flues, and mingle with the gases after they have left the
heating surfaces, the temperature may be brought down to almost any
point above that of the atmosphere, but without any reduction in the
amount of heat wasted. It is in this way that those low chimney
temperatures are sometimes attained which pass for proof of economy with
the unobserving. All surplus air admitted to the fire, or to the gases
before they leave the heating surfaces, increases the losses.
We are now prepared to see why and how the temperature and the rapidity
of combustion in the boiler furnace affect the economy, and that though
the amount of heat developed may be the same, the heat available for the
generation of steam may be much less with one rate or temperature of
combustion than another.
Assuming that there is no air passing up the chimney other than that
which has passed through the fire, the higher the temperature of the
fire and the lower that of the escaping gases the better the economy,
for the losses by the chimney gases will bear the same proportion to the
heat generated by the combustion as the temperature of those gases bears
to the temperature of the fire. That is to say, if the temperature of
the fire is 2500 degrees and that of the chimney gases 500 degrees above
that of the atmosphere, the loss by the chimney will be 500/2500 = 20
per cent. Therefore, as the escaping gases cannot be brought below the
temperature of the absorbing surface, which is practically a fixed
quantity, the temperature of the fire must be high in order to secure
good economy.
The losses by radiation being practically proportioned to the time
occupied, the more coal burned in a given furnace in a given time, the
less will be the proportionate loss from that cause.
It therefore follows that we should burn our coal rapidly and at a high
temperature to secure the best available economy.
[Illustration: Portion of 9880 Horse-power Installation of Babcock &
Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox Chain
Grate Stokers at the South Side Elevated Ry. Co., Chicago, Ill.]
PROPERTIES OF WATER
Pure water is a chemical compound of one volume of oxygen and two
volumes of hydrogen, its chemical symbol being H_{2}O.
The weight of water depends upon its temperature. Its weight at four
temperatures, much used in physical calculations, is given in Table 10.
TABLE 10
WEIGHT OF WATER AT TEMPERATURES
USED IN PHYSICAL CALCULATIONS
+---------------------------+----------+----------+
| Temperature Degrees |Weight per|Weight per|
| Fahrenheit |Cubic Foot|Cubic Inch|
| | Pounds | Pounds |
+---------------------------+----------+----------+
|At 32 degrees or freezing | | |
| point at sea level | 62.418 | 0.03612 |
|At 39.2 degrees or point of| | |
| maximum density | 62.427 | 0.03613 |
|At 62 degrees or standard | | |
| temperature | 62.355 | 0.03608 |
|At 212 degrees or boiling | | |
| point at sea level | 59.846 | 0.03469 |
+---------------------------+----------+----------+
While authorities differ as to the weight of water, the range of values
given for 62 degrees Fahrenheit (the standard temperature ordinarily
taken) being from 62.291 pounds to 62.360 pounds per cubic foot, the
value 62.355 is generally accepted as the most accurate.
A United States standard gallon holds 231 cubic inches and weighs, at 62
degrees Fahrenheit, approximately 8-1/3 pounds.
A British Imperial gallon holds 277.42 cubic inches and weighs, at 62
degrees Fahrenheit, 10 pounds.
The above are the true weights corrected for the effect of the buoyancy
of the air, or the weight in vacuo. If water is weighed in air in the
ordinary way, there is a correction of about one-eighth of one per cent
which is usually negligible.
TABLE 11
VOLUME AND WEIGHT OF DISTILLED WATER AT
VARIOUS TEMPERATURES[12]
+-----------+---------------+----------+
|Temperature|Relative Volume|Weight per|
| Degrees | Water at 39.2 |Cubic Foot|
| Fahrenheit| Degrees = 1 | Pounds |
+-----------+---------------+----------+
| 32 | 1.000176 | 62.42 |
| 39.2 | 1.000000 | 62.43 |
| 40 | 1.000004 | 62.43 |
| 50 | 1.00027 | 62.42 |
| 60 | 1.00096 | 62.37 |
| 70 | 1.00201 | 62.30 |
| 80 | 1.00338 | 62.22 |
| 90 | 1.00504 | 62.11 |
| 100 | 1.00698 | 62.00 |
| 110 | 1.00915 | 61.86 |
| 120 | 1.01157 | 61.71 |
| 130 | 1.01420 | 61.55 |
| 140 | 1.01705 | 61.38 |
| 150 | 1.02011 | 61.20 |
| 160 | 1.02337 | 61.00 |
| 170 | 1.02682 | 60.80 |
| 180 | 1.03047 | 60.58 |
| 190 | 1.03431 | 60.36 |
| 200 | 1.03835 | 60.12 |
| 210 | 1.04256 | 59.88 |
| 212 | 1.04343 | 59.83 |
| 220 | 1.0469 | 59.63 |
| 230 | 1.0515 | 59.37 |
| 240 | 1.0562 | 59.11 |
| 250 | 1.0611 | 58.83 |
| 260 | 1.0662 | 58.55 |
| 270 | 1.0715 | 58.26 |
| 280 | 1.0771 | 57.96 |
| 290 | 1.0830 | 57.65 |
| 300 | 1.0890 | 57.33 |
| 310 | 1.0953 | 57.00 |
| 320 | 1.1019 | 56.66 |
| 330 | 1.1088 | 56.30 |
| 340 | 1.1160 | 55.94 |
| 350 | 1.1235 | 55.57 |
| 360 | 1.1313 | 55.18 |
| 370 | 1.1396 | 54.78 |
| 380 | 1.1483 | 54.36 |
| 390 | 1.1573 | 53.94 |
| 400 | 1.167 | 53.5 |
| 410 | 1.177 | 53.0 |
| 420 | 1.187 | 52.6 |
| 430 | 1.197 | 52.2 |
| 440 | 1.208 | 51.7 |
| 450 | 1.220 | 51.2 |
| 460 | 1.232 | 50.7 |
| 470 | 1.244 | 50.2 |
| 480 | 1.256 | 49.7 |
| 490 | 1.269 | 49.2 |
| 500 | 1.283 | 48.7 |
| 510 | 1.297 | 48.1 |
| 520 | 1.312 | 47.6 |
| 530 | 1.329 | 47.0 |
| 540 | 1.35 | 46.3 |
| 550 | 1.37 | 45.6 |
| 560 | 1.39 | 44.9 |
+-----------+---------------+----------+
Water is but slightly compressible and for all practical purposes may be
considered non-compressible. The coefficient of compressibility ranges
from 0.000040 to 0.000051 per atmosphere at ordinary temperatures, this
coefficient decreasing as the temperature increases.
Table 11 gives the weight in vacuo and the relative volume of a cubic
foot of distilled water at various temperatures.
The weight of water at the standard temperature being taken as 62.355
pounds per cubic foot, the pressure exerted by the column of water of
any stated height, and conversely the height of any column required to
produce a stated pressure, may be computed as follows:
The pressure in pounds per square foot = 62.355 × height of column in
feet.
The pressure in pounds per square inch = 0.433 × height of column in
feet.
Height of column in feet = pressure in pounds per square foot ÷ 62.355.
Height of column in feet = pressure in pounds per square inch ÷ 0.433.
Height of column in inches = pressure in pounds per square inch × 27.71.
Height of column in inches = pressure in ounces per square inch × 1.73.
By a change in the weights given above, the pressure exerted and height
of column may be computed for temperatures other than 62 degrees.
A pressure of one pound per square inch is exerted by a column of water
2.3093 feet or 27.71 inches high at 62 degrees Fahrenheit.
Water in its natural state is never found absolutely pure. In solvent
power water has a greater range than any other liquid. For common salt,
this is approximately a constant at all temperatures, while with such
impurities as magnesium and sodium sulphates, this solvent power
increases with an increase in temperature.
TABLE 12
BOILING POINT OF WATER AT VARIOUS ALTITUDES
+--------------+----------------+-------------+---------------+
|Boiling Point | Altitude Above | Atmospheric | Barometer |
| Degrees | Sea Level | Pressure | Reduced |
| Fahrenheit | Feet | Pounds per | to 32 Degrees |
| | | Square Inch | Inches |
+--------------+----------------+-------------+---------------+
| 184 | 15221 | 8.20 | 16.70 |
| 185 | 14649 | 8.38 | 17.06 |
| 186 | 14075 | 8.57 | 17.45 |
| 187 | 13498 | 8.76 | 17.83 |
| 188 | 12934 | 8.95 | 18.22 |
| 189 | 12367 | 9.14 | 18.61 |
| 190 | 11799 | 9.34 | 19.02 |
| 191 | 11243 | 9.54 | 19.43 |
| 192 | 10685 | 9.74 | 19.85 |
| 193 | 10127 | 9.95 | 20.27 |
| 194 | 9579 | 10.17 | 20.71 |
| 195 | 9031 | 10.39 | 21.15 |
| 196 | 8481 | 10.61 | 21.60 |
| 197 | 7932 | 10.83 | 22.05 |
| 198 | 7381 | 11.06 | 22.52 |
| 199 | 6843 | 11.29 | 22.99 |
| 200 | 6304 | 11.52 | 23.47 |
| 201 | 5764 | 11.76 | 23.95 |
| 202 | 5225 | 12.01 | 24.45 |
| 203 | 4697 | 12.26 | 24.96 |
| 204 | 4169 | 12.51 | 25.48 |
| 205 | 3642 | 12.77 | 26.00 |
| 206 | 3115 | 13.03 | 26.53 |
| 207 | 2589 | 13.30 | 27.08 |
| 208 | 2063 | 13.57 | 27.63 |
| 209 | 1539 | 13.85 | 28.19 |
| 210 | 1025 | 14.13 | 28.76 |
| 211 | 512 | 14.41 | 29.33 |
| 212 | Sea Level | 14.70 | 29.92 |
+--------------+----------------+-------------+---------------+
Sea water contains on an average approximately 3.125 per cent of its
weight of solid matter or a thirty-second part of the weight of the
water and salt held in solution. The approximate composition of this
solid matter will be: sodium chloride 76 per cent, magnesium chloride 10
per cent, magnesium sulphate 6 per cent, calcium sulphate 5 per cent,
calcium carbonate 0.5 per cent, other substances 2.5 per cent.
[Illustration: 7200 Horse-power Installation of Babcock & Wilcox Boilers
and Superheaters at the Capital Traction Co., Washington, D. C.]
The boiling point of water decreases as the altitude above sea level
increases. Table 12 gives the variation in the boiling point with the
altitude.
Water has a greater specific heat or heat-absorbing capacity than any
other known substance (bromine and hydrogen excepted) and its specific
heat is the basis for measurement of the capacity of heat absorption of
all other substances. From the definition, the specific heat of water is
the number of British thermal units required to raise one pound of water
one degree. This specific heat varies with the temperature of the water.
The generally accepted values are given in Table 13, which indicates the
values as determined by Messrs. Marks and Davis and Mr. Peabody.
TABLE 13
SPECIFIC HEAT OF WATER AT VARIOUS TEMPERATURES
+----------------------+--------------------------------+
| MARKS AND DAVIS | PEABODY |
| From Values of | From Values of |
| Barnes and Dieterici | Barnes and Regnault |
+-----------+----------+---------------------+----------+
|Temperature| Specific | Temperature | Specific |
+-----------+ Heat +----------+----------+ Heat |
| Degrees | | Degrees | Degrees | |
|Fahrenheit | |Centigrade|Fahrenheit| |
+-----------+----------+----------+----------+----------+
| 30 | 1.0098 | 0 | 32 | 1.0094 |
| 40 | 1.0045 | 5 | 41 | 1.0053 |
| 50 | 1.0012 | 10 | 50 | 1.0023 |
| 55 | 1.0000 | 15 | 59 | 1.0003 |
| 60 | 0.9990 | 16.11 | 61 | 1.0000 |
| 70 | 0.9977 | 20 | 68 | 0.9990 |
| 80 | 0.9970 | 25 | 77 | 0.9981 |
| 90 | 0.9967 | 30 | 86 | 0.9976 |
| 100 | 0.9967 | 35 | 95 | 0.9974 |
| 110 | 0.9970 | 40 | 104 | 0.9974 |
| 120 | 0.9974 | 45 | 113 | 0.9976 |
| 130 | 0.9979 | 50 | 122 | 0.9980 |
| 140 | 0.9986 | 55 | 131 | 0.9985 |
| 150 | 0.9994 | 60 | 140 | 0.9994 |
| 160 | 1.0002 | 65 | 149 | 1.0004 |
| 170 | 1.0010 | 70 | 158 | 1.0015 |
| 180 | 1.0019 | 75 | 167 | 1.0028 |
| 190 | 1.0029 | 80 | 176 | 1.0042 |
| 200 | 1.0039 | 85 | 185 | 1.0056 |
| 210 | 1.0052 | 90 | 194 | 1.0071 |
| 220 | 1.007 | 95 | 203 | 1.0086 |
| 230 | 1.009 | 100 | 212 | 1.0101 |
+-----------+----------+----------+----------+----------+
In consequence of this variation in specific heat, the variation in the
heat of the liquid of the water at different temperatures is not a
constant. Table 22[13] gives the heat of the liquid in a pound of water
at temperatures ranging from 32 to 340 degrees Fahrenheit.
The specific heat of ice at 32 degrees is 0.463. The specific heat of
saturated steam (ice and saturated steam representing the other forms in
which water may exist), is something that is difficult to define in any
way which will not be misleading. When no liquid is present the specific
heat of saturated steam is negative.[14] The use of the value of the
specific heat of steam is practically limited to instances where
superheat is present, and the specific heat of superheated steam is
covered later in the book.
BOILER FEED WATER
All natural waters contain some impurities which, when introduced into a
boiler, may appear as solids. In view of the apparent present-day
tendency toward large size boiler units and high overloads, the
importance of the use of pure water for boiler feed purposes cannot be
over-estimated.
Ordinarily, when water of sufficient purity for such use is not at hand,
the supply available may be rendered suitable by some process of
treatment. Against the cost of such treatment, there are many factors to
be considered. With water in which there is a marked tendency toward
scale formation, the interest and depreciation on the added boiler units
necessary to allow for the systematic cleaning of certain units must be
taken into consideration. Again there is a considerable loss in taking
boilers off for cleaning and replacing them on the line. On the other
hand, the decrease in capacity and efficiency accompanying an increased
incrustation of boilers in use has been too generally discussed to need
repetition here. Many experiments have been made and actual figures
reported as to this decrease, but in general, such figures apply only to
the particular set of conditions found in the plant where the boiler in
question was tested. So many factors enter into the effect of scale on
capacity and economy that it is impossible to give any accurate figures
on such decrease that will serve all cases, but that it is large has
been thoroughly proven.
While it is almost invariably true that practically any cost of
treatment will pay a return on the investment of the apparatus, the fact
must not be overlooked that there are certain waters which should never
be used for boiler feed purposes and which no treatment can render
suitable for such purpose. In such cases, the only remedy is the
securing of other feed supply or the employment of evaporators for
distilling the feed water as in marine service.
TABLE 14
APPROXIMATE CLASSIFICATION OF IMPURITIES FOUND IN FEED WATERS
THEIR EFFECT AND ORDINARY METHODS OF RELIEF
+-----------------------+--------------+-----------------------------+
| Difficulty Resulting | Nature of | Ordinary Method of |
| from Presence of | Difficulty | Overcoming or Relieving |
+-----------------------+--------------+-----------------------------+
| Sediment, Mud, etc. | Incrustation | Settling tanks, filtration, |
| | | blowing down. |
| | | |
| Readily Soluble Salts | Incrustation | Blowing down. |
| | | |
| Bicarbonates of Lime, | Incrustation | Heating feed. Treatment by |
| Magnesia, etc. | | addition of lime or of lime |
| | | and soda. Barium carbonate. |
| | | |
| Sulphate of Lime | Incrustation | Treatment by addition of |
| | | soda. Barium carbonate. |
| | | |
| Chloride and Sulphate | Corrosion | Treatment by addition of |
| of Magnesium | | carbonate of soda. |
| | | |
| Acid | Corrosion | Alkali. |
| | | |
| Dissolved Carbonic | Corrosion | Heating feed. Keeping air |
| Acid and Oxygen | | from feed. Addition of |
| | | caustic soda or slacked |
| | | lime. |
| | | |
| Grease | Corrosion | Filter. Iron alum as |
| | | coagulent. Neutralization |
| | | by carbonate of soda. Use |
| | | of best hydrocarbon oils. |
| | | |
| Organic Matter | Corrosion | Filter. Use of coagulent. |
| | | |
| Organic Matter | Priming | Settling tanks. Filter in |
| (Sewage) | | connection with coagulent. |
| | | |
| Carbonate of Soda in | Priming | Barium carbonate. New feed |
| large quantities | | supply. If from treatment, |
| | | change. |
+-----------------------+--------------+-----------------------------+
It is evident that the whole subject of boiler feed waters and their
treatment is one for the chemist rather than for the engineer. A brief
outline of the difficulties that may be experienced from the use of poor
feed water and a suggestion as to a method of overcoming certain of
these difficulties is all that will be attempted here. Such a brief
outline of the subject, however, will indicate the necessity for a
chemical analysis of any water before a treatment is tried and the
necessity of adapting the treatment in each case to the nature of the
difficulties that may be experienced.
Table 14 gives a list of impurities which may be found in boiler feed
water, grouped according to their effect on boiler operation and giving
the customary method used for overcoming difficulty to which they lead.
Scale--Scale is formed on boiler heating surfaces by the depositing of
impurities in the feed water in the form of a more or less hard adherent
crust. Such deposits are due to the fact that water loses its soluble
power at high temperatures or because the concentration becomes so high,
due to evaporation, that the impurities crystallize and adhere to the
boiler surfaces. The opportunity for formation of scale in a boiler will
be apparent when it is realized that during a month's operation of a 100
horse-power boiler, 300 pounds of solid matter may be deposited from
water containing only 7 grains per gallon, while some spring and well
waters contain sufficient to cause a deposit of as high as 2000 pounds.
The salts usually responsible for such incrustation are the carbonates
and sulphates of lime and magnesia, and boiler feed treatment in general
deals with the getting rid of these salts more or less completely.
TABLE 15
SOLUBILITY OF MINERAL SALTS IN WATER (SPARKS)
IN GRAINS PER U. S. GALLON (58,381 GRAINS), EXCEPT AS NOTED
+------------------------------+------------+-------------+
|Temperature Degrees Fahrenheit| 60 Degrees | 212 Degrees |
+------------------------------+------------+-------------+
|Calcium Carbonate | 2.5 | 1.5 |
|Calcium Sulphate | 140.0 | 125.0 |
|Magnesium Carbonate | 1.0 | 1.8 |
|Magnesium Sulphate | 3.0 pounds | 12.0 pounds |
|Sodium Chloride | 3.5 pounds | 4.0 pounds |
|Sodium Sulphate | 1.1 pounds | 5.0 pounds |
+------------------------------+------------+-------------+
CALCIUM SULPHATE AT TEMPERATURE ABOVE
212 DEGREES (CHRISTIE)
+------------------------------+----+----+-------+----+---+
|Temperature degrees Fahrenheit|284 |329 |347-365| 464|482|
|Corresponding gauge pressure | 38 | 87 |115-149| 469|561|
|Grains per gallon |45.5|32.7| 15.7 |10.5|9.3|
+------------------------------+----+----+-------+----+---+
Table 15 gives the solubility of these mineral salts in water at various
temperatures in grains per U. S. gallon (58,381 grains). It will be seen
from this table that the carbonates of lime and magnesium are not
soluble above 212 degrees, and calcium sulphate while somewhat insoluble
above 212 degrees becomes more greatly so as the temperature increases.
Scale is also formed by the settling of mud and sediment carried in
suspension in water. This may bake or be cemented to a hard scale when
mixed with other scale-forming ingredients.
Corrosion--Corrosion, or a chemical action leading to the actual
destruction of the boiler metal, is due to the solvent or oxidizing
properties of the feed water. It results from the presence of acid,
either free or developed[15] in the feed, the admixture of air with the
feed water, or as a result of a galvanic action. In boilers it takes
several forms:
1st. Pitting, which consists of isolated spots of active corrosion which
does not attack the boiler as a whole.
2nd. General corrosion, produced by naturally acid waters and where the
amount is so even and continuous that no accurate estimate of the metal
eaten away may be made.
3rd. Grooving, which, while largely a mechanical action which may occur
in neutral waters, is intensified by acidity.
Foaming--This phenomenon, which ordinarily occurs with waters
contaminated with sewage or organic growths, is due to the fact that the
suspended particles collect on the surface of the water in the boiler
and render difficult the liberation of steam bubbles arising to that
surface. It sometimes occurs with water containing carbonates in
solution in which a light flocculent precipitate will be formed on the
surface of the water. Again, it is the result of an excess of sodium
carbonate used in treatment for some other difficulty where animal or
vegetable oil finds its way into the boiler.
Priming--Priming, or the passing off of steam from a boiler in belches,
is caused by the concentration of sodium carbonate, sodium sulphate or
sodium chloride in solution. Sodium sulphate is found in many southern
waters and also where calcium or magnesium sulphate is precipitated with
soda ash.
Treatment of Feed Water--For scale formation. The treatment of feed
water, carrying scale-forming ingredients, is along two main lines: 1st,
by chemical means by which such impurities as are carried by the water
are caused to precipitate; and 2nd, by the means of heat, which results
in the reduction of the power of water to hold certain salts in
solution. The latter method alone is sufficient in the case of certain
temporarily hard waters, but the heat treatment, in general, is used in
connection with a chemical treatment to assist the latter.
Before going further into detail as to the treatment of water, it may be
well to define certain terms used.
_Hardness_, which is the most widely known evidence of the presence in
water of scale-forming matter, is that quality, the variation of which
makes it more difficult to obtain a lather or suds from soap in one
water than in another. This action is made use of in the soap test for
hardness described later. Hardness is ordinarily classed as either
temporary or permanent. Temporarily hard waters are those containing
carbonates of lime and magnesium, which may be precipitated by boiling
at 212 degrees and which, if they contain no other scale-forming
ingredients, become "soft" under such treatment. Permanently hard waters
are those containing mainly calcium sulphate, which is only precipitated
at the high temperatures found in the boiler itself, 300 degrees
Fahrenheit or more. The scale of hardness is an arbitrary one, based on
the number of grains of solids per gallon and waters may be classed on
such a basis as follows: 1-10 grain per gallon, soft water; 10-20 grain
per gallon, moderately hard water; above 25 grains per gallon, very hard
water.
_Alkalinity_ is a general term used for waters containing compounds with
the power of neutralizing acids.
_Causticity_, as used in water treatment, is a term coined by A. McGill,
indicating the presence of an excess of lime added during treatment.
Though such presence would also indicate alkalinity, the term is
arbitrarily used to apply to those hydrates whose presence is indicated
by phenolphthalein.
Of the chemical methods of water treatment, there are three general
processes:
1st. Lime Process. The lime process is used for waters containing
bicarbonates of lime and magnesia. Slacked lime in solution, as lime
water, is the reagent used. This combines with the carbonic acid which
is present, either free or as carbonates, to form an insoluble
monocarbonate of lime. The soluble bicarbonates of lime and magnesia,
losing their carbonic acid, thereby become insoluble and precipitate.
2nd. Soda Process. The soda process is used for waters containing
sulphates of lime and magnesia. Carbonate of soda and hydrate of soda
(caustic soda) are used either alone or together as the reagents.
Carbonate of soda, added to water containing little or no carbonic acid
or bicarbonates, decomposes the sulphates to form insoluble carbonate of
lime or magnesia which precipitate, the neutral soda remaining in
solution. If free carbonic acid or bicarbonates are present, bicarbonate
of lime is formed and remains in solution, though under the action of
heat, the carbon dioxide will be driven off and insoluble monocarbonates
will be formed. Caustic soda used in this process causes a more
energetic action, it being presumed that the caustic soda absorbs the
carbonic acid, becomes carbonate of soda and acts as above.
3rd. Lime and Soda Process. This process, which is the combination of
the first two, is by far the most generally used in water purification.
Such a method is used where sulphates of lime and magnesia are contained
in the water, together with such quantity of carbonic acid or
bicarbonates as to impair the action of the soda. Sufficient soda is
used to break down the sulphates of lime and magnesia and as much lime
added as is required to absorb the carbonic acid not taken up in the
soda reaction.
All of the apparatus for effecting such treatment of feed waters is
approximately the same in its chemical action, the numerous systems
differing in the methods of introduction and handling of the reagents.
The methods of testing water treated by an apparatus of this description
follow.
When properly treated, alkalinity, hardness and causticity should be in
the approximate relation of 6, 5 and 4. When too much lime is used in
the treatment, the causticity in the purified water, as indicated by the
acid test, will be nearly equal to the alkalinity. If too little lime is
used, the causticity will fall to approximately half the alkalinity. The
hardness should not be in excess of two points less than the alkalinity.
Where too great a quantity of soda is used, the hardness is lowered and
the alkalinity raised. If too little soda, the hardness is raised and
the alkalinity lowered.
Alkalinity and causticity are tested with a standard solution of
sulphuric acid. A standard soap solution is used for testing for
hardness and a silver nitrate solution may also be used for determining
whether an excess of lime has been used in the treatment.
Alkalinity: To 50 cubic centimeters of treated water, to which there has
been added sufficient methylorange to color it, add the acid solution,
drop by drop, until the mixture is on the point of turning red. As the
acid solution is first added, the red color, which shows quickly,
disappears on shaking the mixture, and this color disappears more slowly
as the critical point is approached. One-tenth cubic centimeter of the
standard acid solution corresponds to one degree of alkalinity.
[Illustration: 2640 Horse-power Installation of Babcock & Wilcox Boilers
at the Botany Worsted Mills, Passaic, N. J.]
Causticity: To 50 cubic centimeters of treated water, to which there has
been added one drop of phenolphthalein dissolved in alcohol to give the
water a pinkish color, add the acid solution, drop by drop, shaking
after each addition, until the color entirely disappears. One-tenth
cubic centimeter of acid solution corresponds to one degree of
causticity.
The alkalinity may be determined from the same sample tested for
causticity by the coloring with methylorange and adding the acid until
the sample is on the point of turning red. The total acid added in
determining both causticity and alkalinity in this case is the measure
of the alkalinity.
Hardness: 100 cubic centimeters of the treated water is used for this
test, one cubic centimeter of the soap solution corresponding to one
degree of hardness. The soap solution is added a very little at a time
and the whole violently shaken. Enough of the solution must be added to
make a permanent lather or foam, that is, the soap bubbles must not
disappear after the shaking is stopped.
Excess of lime as determined by nitrate of silver: If there is an excess
of lime used in the treatment, a sample will become a dark brown by the
addition of a small quantity of silver nitrate, otherwise a milky white
solution will be formed.
Combined Heat and Chemical Treatment: Heat is used in many systems of
feed treatment apparatus as an adjunct to the chemical process. Heat
alone will remove temporary hardness by the precipitation of carbonates
of lime and magnesia and, when used in connection with the chemical
process, leaves only the permanent hardness or the sulphates of lime to
be taken care of by chemical treatment.
TABLE 16
REAGENTS REQUIRED IN LIME AND SODA PROCESS
FOR TREATING 1000 U. S. GALLONS OF WATER
PER GRAIN PER GALLON OF CONTAINED IMPURITIES[16]
+-----------------------+-----------+-----------+
| | Lime[17] | Soda[18] |
| | Pounds | Pounds |
+-----------------------+-----------+-----------+
| Calcium Carbonate | 0.098 | ... |
| Calcium Sulphate | ... | 0.124 |
| Calcium Chloride | ... | 0.151 |
| Calcium Nitrate | ... | 0.104 |
| Magnesium Carbonate | 0.234 | ... |
| Magnesium Sulphate | 0.079 | 0.141 |
| Magnesium Chloride | 0.103 | 0.177 |
| Magnesium Nitrate | 0.067 | 0.115 |
| Ferrous Carbonate | 0.169 | ... |
| Ferrous Sulphate | 0.070 | 0.110 |
| Ferric Sulphate | 0.074 | 0.126 |
| Aluminum Sulphate | 0.087 | 0.147 |
| Free Sulphuric Acid | 0.100 | 0.171 |
| Sodium Carbonate | 0.093 | ... |
| Free Carbon Dioxide | 0.223 | ... |
| Hydrogen Sulphite | 0.288 | ... |
+-----------------------+-----------+-----------+
The chemicals used in the ordinary lime and soda process of feed water
treatment are common lime and soda. The efficiency of such apparatus
will depend wholly upon the amount and character of the impurities in
the water to be treated. Table 16 gives the amount of lime and soda
required per 1000 gallons for each grain per gallon of the various
impurities found in the water. This table is based on lime containing 90
per cent calcium oxide and soda containing 58 per cent sodium oxide,
which correspond to the commercial quality ordinarily purchasable. From
this table and the cost of the lime and soda, the cost of treating any
water per 1000 gallons may be readily computed.
Less Usual Reagents--Barium hydrate is sometimes used to reduce
permanent hardness or the calcium sulphate component. Until recently,
the high cost of barium hydrate has rendered its use prohibitive but at
the present it is obtained as a by-product in cement manufacture and it
may be purchased at a more reasonable figure than heretofore. It acts
directly on the soluble sulphates to form barium sulphate which is
insoluble and may be precipitated. Where this reagent is used, it is
desirable that the reaction be allowed to take place outside of the
boiler, though there are certain cases where its internal use is
permissible.
Barium carbonate is sometimes used in removing calcium sulphate, the
products of the reaction being barium sulphate and calcium carbonate,
both of which are insoluble and may be precipitated. As barium carbonate
in itself is insoluble, it cannot be added to water as a solution and
its use should, therefore, be confined to treatment outside of the
boiler.
Silicate of soda will precipitate calcium carbonate with the formation
of a gelatinous silicate of lime and carbonate of soda. If calcium
sulphate is also present, carbonate of soda is formed in the above
reaction, which in turn will break down the sulphate.
Oxalate of soda is an expensive but efficient reagent which forms a
precipitate of calcium oxalate of a particularly insoluble nature.
Alum and iron alum will act as efficient coagulents where organic matter
is present in the water. Iron alum has not only this property but also
that of reducing oil discharged from surface condensers to a condition
in which it may be readily removed by filtration.
Corrosion--Where there is a corrosive action because of the presence of
acid in the water or of oil containing fatty acids which will decompose
and cause pitting wherever the sludge can find a resting place, it may
be overcome by the neutralization of the water by carbonate of soda.
Such neutralization should be carried to the point where the water will
just turn red litmus paper blue. As a preventative of such action
arising from the presence of the oil, only the highest grades of
hydrocarbon oils should be used.
Acidity will occur where sea water is present in a boiler. There is the
possibility of such an occurrence in marine practice and in stationary
plants using sea water for condensing, due to leaky condenser tubes,
priming in the evaporators, etc. Such acidity is caused through the
dissociation of magnesium chloride into hydrochloride acid and magnesia
under high temperatures. The acid in contact with the metal forms an
iron salt which immediately upon its formation is neutralized by the
free magnesia in the water, thereby precipitating iron oxide and
reforming magnesium chloride. The preventive for corrosion arising from
such acidity is the keeping tight of the condenser. Where it is
unavoidable that some sea water should find its way into a boiler, the
acidity resulting should be neutralized by soda ash. This will convert
the magnesium chloride into magnesium carbonate and sodium chloride,
neither of which is corrosive but both of which are scale-forming.
The presence of air in the feed water which is sucked in by the feed
pump is a well recognized cause of corrosion. Air bubbles form below the
water line and attack the metal of the boiler, the oxygen of the air
causing oxidization of the boiler metal and the formation of rust. The
particle of rust thus formed is swept away by the circulation or is
dislodged by expansion and the minute pit thus left forms an ideal
resting place for other air bubbles and the continuation of the
oxidization process. The prevention is, of course, the removing of the
air from the feed water. In marine practice, where there has been
experienced the most difficulty from this source, it has been found to
be advantageous to pump the water from the hot well to a filter tank
placed above the feed pump suction valves. In this way the air is
liberated from the surface of the tank and a head is assured for the
suction end of the pump. In this same class of work, the corrosive
action of air is reduced by introducing the feed through a spray nozzle
into the steam space above the water line.
Galvanic action, resulting in the eating away of the boiler metal
through electrolysis was formerly considered practically the sole cause
of corrosion. But little is known of such action aside from the fact
that it does take place in certain instances. The means adopted as a
remedy is usually the installation of zinc plates within the boiler,
which must have positive metallic contact with the boiler metal. In this
way, local electrolytic effects are overcome by a still greater
electrolytic action at the expense of the more positive zinc. The
positive contact necessary is difficult to maintain and it is
questionable just what efficacy such plates have except for a short
period after their installation when the contact is known to be
positive. Aside from protection from such electrolytic action, however,
the zinc plates have a distinct use where there is the liability of air
in the feed, as they offer a substance much more readily oxidized by
such air than the metal of the boiler.
Foaming--Where foaming is caused by organic matter in suspension, it may
be largely overcome by filtration or by the use of a coagulent in
connection with filtration, the latter combination having come recently
into considerable favor. Alum, or potash alum, and iron alum, which in
reality contains no alumina and should rather be called potassia-ferric,
are the coagulents generally used in connection with filtration. Such
matter as is not removed by filtration may, under certain conditions, be
handled by surface blowing. In some instances, settling tanks are used
for the removal of matter in suspension, but where large quantities of
water are required, filtration is ordinarily substituted on account of
the time element and the large area necessary in settling tanks.
Where foaming occurs as the result of overtreatment of the feed water,
the obvious remedy is a change in such treatment.
Priming--Where priming is caused by excessive concentration of salts
within a boiler, it may be overcome largely by frequent blowing down.
The degree of concentration allowable before priming will take place
varies widely with conditions of operation and may be definitely
determined only by experience with each individual set of conditions. It
is the presence of the salts that cause priming that may result in the
absolute unfitness of water for boiler feed purposes. Where these salts
exist in such quantities that the amount of blowing down necessary to
keep the degree of concentration below the priming point results in
excessive losses, the only remedy is the securing of another supply of
feed, and the results will warrant the change almost regardless of the
expense. In some few instances, the impurities may be taken care of by
some method of water treatment but such water should be submitted to an
authority on the subject before any treatment apparatus is installed.
[Illustration: 3000 Horse-power Installation of Cross Drum Babcock &
Wilcox Boilers and Superheaters Equipped with Babcock & Wilcox Chain
Grate Stokers at the Washington Terminal Co., Washington, D. C.]
Boiler Compounds--The method of treatment of feed water by far the most
generally used is by the use of some of the so-called boiler compounds.
There are many reliable concerns handling such compounds who
unquestionably secure the promised results, but there is a great
tendency toward looking on the compound as a "cure all" for any water
difficulties and care should be taken to deal only with reputable
concerns.
The composition of these compounds is almost invariably based on soda
with certain tannic substances and in some instances a gelatinous
substance which is presumed to encircle scale particles and prevent
their adhering to the boiler surfaces. The action of these compounds is
ordinarily to reduce the calcium sulphate in the water by means of
carbonate of soda and to precipitate it as a muddy form of calcium
carbonate which may be blown off. The tannic compounds are used in
connection with the soda with the idea of introducing organic matter
into any scale already formed. When it has penetrated to the boiler
metal, decomposition of the scale sets in, causing a disruptive effect
which breaks the scale from the metal sometimes in large slabs. It is
this effect of boiler compounds that is to be most carefully guarded
against or inevitable trouble will result from the presence of loose
scale with the consequent danger of tube losses through burning.
When proper care is taken to suit the compound to the water in use, the
results secured are fairly effective. In general, however, the use of
compounds may only be recommended for the prevention of scale rather
than with the view to removing scale which has already formed, that is,
the compounds should be introduced with the feed water only when the
boiler has been thoroughly cleaned.
FEED WATER HEATING AND METHODS OF FEEDING
Before water fed into a boiler can be converted into steam, it must be
first heated to a temperature corresponding to the pressure within the
boiler. Steam at 160 pounds gauge pressure has a temperature of
approximately 371 degrees Fahrenheit. If water is fed to the boiler at
60 degrees Fahrenheit, each pound must have 311 B. t. u. added to it to
increase its temperature 371 degrees, which increase must take place
before the water can be converted into steam. As it requires 1167.8
B. t. u. to raise one pound of water from 60 to 371 degrees and to
convert it into steam at 160 pounds gauge pressure, the 311 degrees
required simply to raise the temperature of the water from 60 to 371
degrees will be approximately 27 per cent of the total. If, therefore,
the temperature of the water can be increased from 60 to 371 degrees
before it is introduced into a boiler by the utilization of heat from
some source that would otherwise be wasted, there will be a saving in
the fuel required of 311 ÷ 1167.8 = 27 per cent, and there will be a net
saving, provided the cost of maintaining and operating the apparatus for
securing this saving is less than the value of the heat thus saved.
The saving in the fuel due to the heating of feed water by means of heat
that would otherwise be wasted may be computed from the formula:
100 (t - t_{i})
Fuel saving per cent = --------------- (1)
H + 32 - t_{i}
where, t = temperature of feed water after heating, t_{i} = temperature
of feed water before heating, and H = total heat above 32 degrees per
pound of steam at the boiler pressure. Values of H may be found in Table
23. Table 17 has been computed from this formula to show the fuel saving
under the conditions assumed with the boiler operating at 180 pounds
gauge pressure.
TABLE 17
SAVING IN FUEL, IN PER CENT, BY HEATING FEED WATER
GAUGE PRESSURE 180 POUNDS
+-----------+-----------------------------------------+
| Initial | Final Temperature--Degrees Fahrenheit |
|Temperature|-----+-----+-----+-----+-----+-----+-----|
| Fahrenheit| 120 | 140 | 160 | 180 | 200 | 250 | 300 |
+-----------+-----+-----+-----+-----+-----+-----+-----+
| 32 | 7.35| 9.02|10.69|12.36|14.04|18.20|22.38|
| 35 | 7.12| 8.79|10.46|12.14|13.82|18.00|22.18|
| 40 | 6.72| 8.41|10.09|11.77|13.45|17.65|21.86|
| 45 | 6.33| 8.02| 9.71|11.40|13.08|17.30|21.52|
| 50 | 5.93| 7.63| 9.32|11.02|12.72|16.95|21.19|
| 55 | 5.53| 7.24| 8.94|10.64|12.34|16.60|20.86|
| 60 | 5.13| 6.84| 8.55|10.27|11.97|16.24|20.52|
| 65 | 4.72| 6.44| 8.16| 9.87|11.59|15.88|20.18|
| 70 | 4.31| 6.04| 7.77| 9.48|11.21|15.52|19.83|
| 75 | 3.90| 5.64| 7.36| 9.09|10.82|15.16|19.48|
| 80 | 3.48| 5.22| 6.96| 8.70|10.44|14.79|19.13|
| 85 | 3.06| 4.80| 6.55| 8.30|10.05|14.41|18.78|
| 90 | 2.63| 4.39| 6.14| 7.89| 9.65|14.04|18.43|
| 95 | 2.20| 3.97| 5.73| 7.49| 9.25|13.66|18.07|
| 100 | 1.77| 3.54| 5.31| 7.08| 8.85|13.28|17.70|
| 110 | .89| 2.68| 4.47| 6.25| 8.04|12.50|16.97|
| 120 | .00| 1.80| 3.61| 5.41| 7.21|11.71|16.22|
| 130 | | .91| 2.73| 4.55| 6.37|10.91|15.46|
| 140 | | .00| 1.84| 3.67| 5.51|10.09|14.68|
| 150 | | | .93| 2.78| 4.63| 9.26|13.89|
| 160 | | | .00| 1.87| 3.74| 8.41|13.09|
| 170 | | | | .94| 2.83| 7.55|12.27|
| 180 | | | | .00| 1.91| 6.67|11.43|
| 190 | | | | | .96| 5.77|10.58|
| 200 | | | | | .00| 4.86| 9.71|
| 210 | | | | | | 3.92| 8.82|
+-----------+-----+-----+-----+-----+-----+-----+-----+
Besides the saving in fuel effected by the use of feed water heaters,
other advantages are secured. The time required for the conversion of
water into steam is diminished and the steam capacity of the boiler
thereby increased. Further, the feeding of cold water into a boiler has
a tendency toward the setting up of temperature strains, which are
diminished in proportion as the temperature of the feed approaches that
of the steam. An important additional advantage of heating feed water is
that in certain types of heaters a large portion of the scale forming
ingredients are precipitated before entering the boiler, with a
consequent saving in cleaning and losses through decreased efficiency
and capacity.
In general, feed water heaters may be divided into closed heaters, open
heaters and economizers; the first two depend for their heat upon
exhaust, or in some cases live steam, while the last class utilizes the
heat of the waste flue gases to secure the same result. The question of
the type of apparatus to be installed is dependent upon the conditions
attached to each individual case.
In closed heaters the feed water and the exhaust steam do not come into
actual contact with each other. Either the steam or the water passes
through tubes surrounded by the other medium, as the heater is of the
steam-tube or water-tube type. A closed heater is best suited for water
free from scale-forming matter, as such matter soon clogs the passages.
Cleaning such heaters is costly and the efficiency drops off rapidly as
scale forms. A closed heater is not advisable where the engines work
intermittently, as is the case with mine hoisting engines. In this class
of work the frequent coolings between operating periods and the sudden
heatings when operation commences will tend to loosen the tubes or even
pull them apart. For this reason, an open heater, or economizer, will
give more satisfactory service with intermittently operating apparatus.
Open heaters are best suited for waters containing scale-forming matter.
Much of the temporary hardness may be precipitated in the heater and the
sediment easily removed. Such heaters are frequently used with a reagent
for precipitating permanent hardness in the combined heat and chemical
treatment of feed water. The so-called live steam purifiers are open
heaters, the water being raised to the boiling temperature and the
carbonates and a portion of the sulphates being precipitated. The
disadvantage of this class of apparatus is that some of the sulphates
remain in solution to be precipitated as scale when concentrated in the
boiler. Sufficient concentration to have such an effect, however, may
often be prevented by frequent blowing down.
Economizers find their largest field where the design of the boiler is
such that the maximum possible amount of heat is not extracted from the
gases of combustion. The more wasteful the boiler, the greater the
saving effected by the use of the economizer, and it is sometimes
possible to raise the temperature of the feed water to that of high
pressure steam by the installation of such an apparatus, the saving
amounting in some cases to as much as 20 per cent. The fuel used bears
directly on the question of the advisability of an economizer
installation, for when oil is the fuel a boiler efficiency of 80 per
cent or over is frequently realized, an efficiency which would leave a
small opportunity for a commercial gain through the addition of an
economizer.
From the standpoint of space requirements, economizers are at a
disadvantage in that they are bulky and require a considerable increase
over space occupied by a heater of the exhaust type. They also require
additional brickwork or a metal casing, which increases the cost.
Sometimes, too, the frictional resistance of the gases through an
economizer make its adaptability questionable because of the draft
conditions. When figuring the net return on economizer investment, all
of these factors must be considered.
When the feed water is such that scale will quickly encrust the
economizer and throw it out of service for cleaning during an excessive
portion of the time, it will be necessary to purify water before
introducing it into an economizer to make it earn a profit on the
investment.
From the foregoing, it is clearly indicated that it is impossible to
make a definite statement as to the relative saving by heating feed
water in any of the three types. Each case must be worked out
independently and a decision can be reached only after an exhaustive
study of all the conditions affecting the case, including the time the
plant will be in service and probable growth of the plant. When, as a
result of such study, the possible methods for handling the problem have
been determined, the solution of the best apparatus can be made easily
by the balancing of the saving possible by each method against its first
cost, depreciation, maintenance and cost of operation.
Feeding of Water--The choice of methods to be used in introducing feed
water into a boiler lies between an injector and a pump. In most plants,
an injector would not be economical, as the water fed by such means must
be cold, a fact which makes impossible the use of a heater before the
water enters the injector. Such a heater might be installed between the
injector and the boiler but as heat is added to the water in the
injector, the heater could not properly fulfill its function.
TABLE 18
COMPARISON OF PUMPS AND INJECTORS
_________________________________________________________________________
| | | |
| Method of Supplying | | |
| Feed-water to Boiler | Relative amount of | Saving of fuel over|
| Temperature of feed-water as | coal required per | the amount required|
| delivered to the pump or to | unit of time, the | when the boiler is |
| injector, 60 degrees Fahren- | amount for a direct-| fed by a direct- |
| heit. Rate of evaporation of | acting pump, feeding| acting pump without|
| boiler, to pounds of water | water at 60 degrees | heater |
| per pound of coal from and | without a heater, | Per Cent |
| at 212 degrees Fahrenheit | being taken as unity| |
|______________________________|_____________________|____________________|
| | | |
| Direct-acting Pump feeding | | |
| water at 60 degrees without | | |
| a heater | 1.000 | .0 |
| | | |
| Injector feeding water at | | |
| 150 degrees without a heater | .985 | 1.5 |
| Injector feeding through a | | |
| heater in which the water is | | |
| heated from 150 to 200 | | |
| degrees | .938 | 6.2 |
| | | |
| Direct-acting Pump feeding | | |
| water through a heater in | | |
| which it is heated from 60 | | |
| to 200 degrees | .879 | 12.1 |
| | | |
| Geared Pump run from the | | |
| engine, feeding water | | |
| through a heater in which it | | |
| is heated from 60 to 200 | | |
| degrees | .868 | 13.2 |
|______________________________|_____________________|____________________|
The injector, considered only in the light of a combined heater and
pump, is claimed to have a thermal efficiency of 100 per cent, since all
of the heat in the steam used is returned to the boiler with the water.
This claim leads to an erroneous idea. If a pump is used in feeding the
water to a boiler and the heat in the exhaust from the pump is imparted
to the feed water, the pump has as high a thermal efficiency as the
injector. The pump has the further advantage that it uses so much less
steam for the forcing of a given quantity of water into the boiler that
it makes possible a greater saving through the use of the exhaust from
other auxiliaries for heating the feed, which exhaust, if an injector
were used, would be wasted, as has been pointed out.
In locomotive practice, injectors are used because there is no exhaust
steam available for heating the feed, this being utilized in producing a
forced draft, and because of space requirements. In power plant work,
however, pumps are universally used for regular operation, though
injectors are sometimes installed as an auxiliary method of feeding.
Table 18 shows the relative value of injectors, direct-acting steam
pumps and pumps driven from the engine, the data having been obtained
from actual experiment. It will be noted that when feeding cold water
direct to the boilers, the injector has a slightly greater economy but
when feeding through a heater, the pump is by far the more economical.
Auxiliaries--It is the general impression that auxiliaries will take
less steam if the exhaust is turned into the condensers, in this way
reducing the back pressure. As a matter of fact, vacuum is rarely
registered on an indicator card taken from the cylinders of certain
types of auxiliaries unless the exhaust connection is short and without
bends, as long pipes and many angles offset the effect of the condenser.
On the other hand, if the exhaust steam from the auxiliaries can be used
for heating the feed water, all of the latent heat less only the loss
due to radiation is returned to the boiler and is saved instead of being
lost in the condensing water or wasted with the free exhaust. Taking
into consideration the plant as a whole, it would appear that the
auxiliary machinery, under such conditions, is more efficient than the
main engines.
[Illustration: Portion of 4160 Horse-power Installation of Babcock &
Wilcox Boilers at the Prudential Life Insurance Co. Building, Newark,
N. J.]
STEAM
When a given weight of a perfect gas is compressed or expanded at a
constant temperature, the product of the pressure and volume is a
constant. Vapors, which are liquids in aeriform condition, on the other
hand, can exist only at a definite pressure corresponding to each
temperature if in the saturated state, that is, the pressure is a
function of the temperature only. Steam is water vapor, and at a
pressure of, say, 150 pounds absolute per square inch saturated steam
can exist only at a temperature 358 degrees Fahrenheit. Hence if the
pressure of saturated steam be fixed, its temperature is also fixed, and
_vice versa_.
Saturated steam is water vapor in the condition in which it is generated
from water with which it is in contact. Or it is steam which is at the
maximum pressure and density possible at its temperature. If any change
be made in the temperature or pressure of steam, there will be a
corresponding change in its condition. If the pressure be increased or
the temperature decreased, a portion of the steam will be condensed. If
the temperature be increased or the pressure decreased, a portion of the
water with which the steam is in contact will be evaporated into steam.
Steam will remain saturated just so long as it is of the same pressure
and temperature as the water with which it can remain in contact without
a gain or loss of heat. Moreover, saturated steam cannot have its
temperature lowered without a lowering of its pressure, any loss of heat
being made up by the latent heat of such portion as will be condensed.
Nor can the temperature of saturated steam be increased except when
accompanied by a corresponding increase in pressure, any added heat
being expended in the evaporation into steam of a portion of the water
with which it is in contact.
Dry saturated steam contains no water. In some cases, saturated steam is
accompanied by water which is carried along with it, either in the form
of a spray or is blown along the surface of the piping, and the steam is
then said to be wet. The percentage weight of the steam in a mixture of
steam and water is called the quality of the steam. Thus, if in a
mixture of 100 pounds of steam and water there is three-quarters of a
pound of water, the quality of the steam will be 99.25.
Heat may be added to steam not in contact with water, such an addition
of heat resulting in an increase of temperature and pressure if the
volume be kept constant, or an increase in temperature and volume if the
pressure remain constant. Steam whose temperature thus exceeds that of
saturated steam at a corresponding pressure is said to be superheated
and its properties approximate those of a perfect gas.
As pointed out in the chapter on heat, the heat necessary to raise one
pound of water from 32 degrees Fahrenheit to the point of ebullition is
called the _heat of the liquid_. The heat absorbed during ebullition
consists of that necessary to dissociate the molecules, or the _inner
latent heat_, and that necessary to overcome the resistance to the
increase in volume, or the _outer latent heat_. These two make up the
_latent heat of evaporation_ and the sum of this latent heat of
evaporation and the heat of the liquid make the _total heat_ of the
steam. These values for various pressures are given in the steam tables,
pages 122 to 127.
The specific volume of saturated steam at any pressure is the volume in
cubic feet of one pound of steam at that pressure.
The density of saturated steam, that is, its weight per cubic foot, is
obviously the reciprocal of the specific volume. This density varies as
the 16/17 power over the ordinary range of pressures used in steam
boiler work and may be found by the formula, D = .003027p^{.941}, which
is correct within 0.15 per cent up to 250 pounds pressure.
The relative volume of steam is the ratio of the volume of a given
weight to the volume of the same weight of water at 39.2 degrees
Fahrenheit and is equal to the specific volume times 62.427.
As vapors are liquids in their gaseous form and the boiling point is the
point of change in this condition, it is clear that this point is
dependent upon the pressure under which the liquid exists. This fact is
of great practical importance in steam condenser work and in many
operations involving boiling in an open vessel, since in the latter case
its altitude will have considerable influence. The relation between
altitude and boiling point of water is shown in Table 12.
The conditions of feed temperature and steam pressure in boiler tests,
fuel performances and the like, will be found to vary widely in
different trials. In order to secure a means for comparison of different
trials, it is necessary to reduce all results to some common basis. The
method which has been adopted for the reduction to a comparable basis is
to transform the evaporation under actual conditions of steam pressure
and feed temperature which exist in the trial to an equivalent
evaporation under a set of standard conditions. These standard
conditions presuppose a feed water temperature of 212 degrees Fahrenheit
and a steam pressure equal to the normal atmospheric pressure at sea
level, 14.7 pounds absolute. Under such conditions steam would be
generated _at_ a temperature of 212 degrees, the temperature
corresponding to atmospheric pressure at sea level, _from_ water at 212
degrees. The weight of water which _would_ be evaporated under the
assumed standard conditions by exactly the amount of heat absorbed by
the boiler under actual conditions existing in the trial, is, therefore,
called the equivalent evaporation "from and at 212 degrees."
The factor for reducing the weight of water actually converted into
steam from the temperature of the feed, at the steam pressure existing
in the trial, to the equivalent evaporation under standard conditions is
called the _factor of evaporation._ This factor is the ratio of the
total heat added to one pound of steam under the standard conditions to
the heat added to each pound of steam in heating the water from the
temperature of the feed in the trial to the temperature corresponding to
the pressure existing in the trial. This heat added is obviously the
difference between the total heat of evaporation of the steam at the
pressure existing in the trial and the heat of the liquid in the water
at the temperature at which it was fed in the trial. To illustrate by an
example:
In a boiler trial the temperature of the feed water is 60 degrees
Fahrenheit and the pressure under which steam is delivered is 160.3
pounds gauge pressure or 175 pounds absolute pressure. The total heat of
one pound of steam at 175 pounds pressure is 1195.9 B. t. u. measured
above the standard temperature of 32 degrees Fahrenheit. But the water
fed to the boiler contained 28.08 B. t. u. as the heat of the liquid
measured above 32 degrees Fahrenheit. Therefore, to each pound of steam
there has been added 1167.82 B. t. u. To evaporate one pound of water
under standard conditions would, on the other hand, have required but
970.4 B. t. u., which, as described, is the latent heat of evaporation
at 212 degrees Fahrenheit. Expressed differently, the total heat of one
pound of steam at the pressure corresponding to a temperature of 212
degrees is 1150.4 B. t. u. One pound of water at 212 degrees contains
180 B. t. u. of sensible heat above 32 degrees Fahrenheit. Hence, under
standard conditions, 1150.4 - 180 = 970.4 B. t. u. is added in the
changing of one pound of water into steam at atmospheric pressure and a
temperature of 212 degrees. This is in effect the definition of the
latent heat of evaporation.
Hence, if conditions of the trial had been standard, only 970.4 B. t. u.
would be required and the ratio of 1167.82 to 970.4 B. t. u. is the
ratio determining the factor of evaporation. The factor in the assumed
case is 1167.82 ÷ 970.4 = 1.2034 and if the same amount of heat had been
absorbed under standard conditions as was absorbed in the trial
condition, 1.2034 times the amount of steam would have been generated.
Expressed as a formula for use with any set of conditions, the factor
is,
H - h
F = ----- (2)
970.4
Where H = the total heat of steam above 32 degrees Fahrenheit from steam
tables,
h = sensible heat of feed water above 32 degrees Fahrenheit from
Table 22.
In the form above, the factor may be determined with either saturated or
superheated steam, provided that in the latter case values of H are
available for varying degrees of superheat and pressures.
Where such values are not available, the form becomes,
H - h + s(t_{sup} - t_{sat})
F = ---------------------------- (3)
970.4
Where s = mean specific heat of superheated steam at the
pressure existing in the trial from saturated
steam to the temperature existing in the trial,
t_{sup} = final temperature of steam,
t_{sat} = temperature of saturated steam, corresponding to
pressure existing,
(t_{sup} - t_{sat}) = degrees of superheat.
The specific heat of superheated steam will be taken up later.
Table 19 gives factors of evaporation for saturated steam boiler trials
to cover a large range of conditions. Except for the most refined work,
intermediate values may be determined by interpolation.
Steam gauges indicate the pressure above the atmosphere. As has been
pointed out, the atmospheric pressure changes according to the altitude
and the variation in the barometer. Hence, calculations involving the
properties of steam are based on _absolute_ pressures, which are equal
to the gauge pressure plus the atmospheric pressure in pounds to the
square inch. This latter is generally assumed to be 14.7 pounds per
square inch at sea level, but for other levels it must be determined
from the barometric reading at that place.
Vacuum gauges indicate the difference, expressed in inches of mercury,
between atmospheric pressure and the pressure within the vessel to which
the gauge is attached. For approximate purposes, 2.04 inches height of
mercury may be considered equal to a pressure of one pound per square
inch at the ordinary temperatures at which mercury gauges are used.
Hence for any reading of the vacuum gauge in inches, G, the absolute
pressure for any barometer reading in inches, B, will be (B - G) ÷ 2.04.
If the barometer is 30 inches measured at ordinary temperatures and not
corrected to 32 degrees Fahrenheit and the vacuum gauge 24 inches, the
absolute pressure will be (30 - 24) ÷ 2.04 = 2.9 pounds per square inch.
TABLE 19
FACTORS OF EVAPORATION
CALCULATED FROM MARKS AND DAVIS TABLES
______________________________________________________________________
| | |
|Feed | |
|Temp- | |
|erature| |
|Degrees| Steam Pressure by Gauge |
|Fahren-| |
|heit | |
|_______|______________________________________________________________|
| | | | | | | | |
| | 50 | 60 | 70 | 80 | 90 | 100 | 110 |
|_______|________|________|________|________|________|________|________|
| | | | | | | | |
| 32 | 1.2143 | 1.2170 | 1.2194 | 1.2215 | 1.2233 | 1.2233 | 1.2265 |
| 40 | 1.2060 | 1.2087 | 1.2111 | 1.2131 | 1.2150 | 1.2168 | 1.2181 |
| 50 | 1.1957 | 1.1984 | 1.2008 | 1.2028 | 1.2047 | 1.2065 | 1.2079 |
| 60 | 1.1854 | 1.1881 | 1.1905 | 1.1925 | 1.1944 | 1.1961 | 1.1976 |
| 70 | 1.1750 | 1.1778 | 1.1802 | 1.1822 | 1.1841 | 1.1859 | 1.1873 |
| 80 | 1.1649 | 1.1675 | 1.1699 | 1.1720 | 1.1738 | 1.1756 | 1.1770 |
| 90 | 1.1545 | 1.1572 | 1.1596 | 1.1617 | 1.1636 | 1.1653 | 1.1668 |
| 100 | 1.1443 | 1.1470 | 1.1493 | 1.1514 | 1.1533 | 1.1550 | 1.1565 |
| 110 | 1.1340 | 1.1367 | 1.1391 | 1.1411 | 1.1430 | 1.1448 | 1.1462 |
| 120 | 1.1237 | 1.1264 | 1.1288 | 1.1309 | 1.1327 | 1.1345 | 1.1359 |
| 130 | 1.1134 | 1.1161 | 1.1185 | 1.1206 | 1.1225 | 1.1242 | 1.1257 |
| 140 | 1.1031 | 1.1058 | 1.1082 | 1.1103 | 1.1122 | 1.1139 | 1.1154 |
| 150 | 1.0928 | 1.0955 | 1.0979 | 1.1000 | 1.1019 | 1.1036 | 1.1051 |
| 160 | 1.0825 | 1.0852 | 1.0876 | 1.0897 | 1.0916 | 1.0933 | 1.0948 |
| 170 | 1.0722 | 1.0749 | 1.0773 | 1.0794 | 1.0813 | 1.0830 | 1.0845 |
| 180 | 1.0619 | 1.0646 | 1.0670 | 1.0691 | 1.0709 | 1.0727 | 1.0741 |
| 190 | 1.0516 | 1.0543 | 1.0567 | 1.0587 | 1.0606 | 1.0624 | 1.0638 |
| 200 | 1.0412 | 1.0439 | 1.0463 | 1.0484 | 1.0503 | 1.0520 | 1.0535 |
| 210 | 1.0309 | 1.0336 | 1.0360 | 1.0380 | 1.0399 | 1.0417 | 1.0432 |
|_______|________|________|________|________|________|________|________|
______________________________________________________________________
| | |
|Feed | |
|Temp- | |
|erature| |
|Degrees| Steam Pressure by Gauge |
|Fahren-| |
|heit | |
|_______|______________________________________________________________|
| | | | | | | | |
| | 120 | 130 | 140 | 150 | 160 | 170 | 180 |
|_______|________|________|________|________|________|________|________|
| | | | | | | | |
| 32 | 1.2280 | 1.2292 | 1.2304 | 1.2314 | 1.2323 | 1.2333 | 1.2342 |
| 40 | 1.2196 | 1.2209 | 1.2221 | 1.2231 | 1.2241 | 1.2250 | 1.2259 |
| 50 | 1.2093 | 1.2106 | 1.2117 | 1.2128 | 1.2137 | 1.2147 | 1.2156 |
| 60 | 1.1990 | 1.2003 | 1.2014 | 1.2025 | 1.2034 | 1.2044 | 1.2053 |
| 70 | 1.1887 | 1.1900 | 1.1911 | 1.1922 | 1.1931 | 1.1941 | 1.1950 |
| 80 | 1.1785 | 1.1797 | 1.1809 | 1.1819 | 1.1828 | 1.1838 | 1.1847 |
| 90 | 1.1682 | 1.1695 | 1.1706 | 1.1717 | 1.1725 | 1.1735 | 1.1744 |
| 100 | 1.1579 | 1.1592 | 1.1603 | 1.1614 | 1.1623 | 1.1633 | 1.1642 |
| 110 | 1.1477 | 1.1489 | 1.1500 | 1.1511 | 1.1520 | 1.1530 | 1.1539 |
| 120 | 1.1374 | 1.1386 | 1.1398 | 1.1408 | 1.1418 | 1.1427 | 1.1436 |
| 130 | 1.1271 | 1.1284 | 1.1295 | 1.1305 | 1.1315 | 1.1324 | 1.1333 |
| 140 | 1.1168 | 1.1181 | 1.1192 | 1.1203 | 1.1212 | 1.1221 | 1.1230 |
| 150 | 1.1065 | 1.1078 | 1.1089 | 1.1099 | 1.1109 | 1.1118 | 1.1127 |
| 160 | 1.0962 | 1.0975 | 1.0986 | 1.0997 | 1.1006 | 1.1015 | 1.1024 |
| 170 | 1.0859 | 1.0872 | 1.0883 | 1.0893 | 1.0903 | 1.0912 | 1.0921 |
| 180 | 1.0756 | 1.0768 | 1.0780 | 1.0790 | 1.0800 | 1.0809 | 1.0818 |
| 190 | 1.0653 | 1.0665 | 1.0676 | 1.0687 | 1.0696 | 1.0706 | 1.0715 |
| 200 | 1.0549 | 1.0562 | 1.0573 | 1.0584 | 1.0593 | 1.0602 | 1.0611 |
| 210 | 1.0446 | 1.0458 | 1.0469 | 1.0480 | 1.0489 | 1.0499 | 1.0508 |
|_______|________|________|________|________|________|________|________|
______________________________________________________________________
| | |
|Feed | |
|Temp- | |
|erature| |
|Degrees| Steam Pressure by Gauge |
|Fahren-| |
|heit | |
|_______|______________________________________________________________|
| | | | | | | | |
| | 190 | 200 | 210 | 220 | 230 | 240 | 250 |
|_______|________|________|________|________|________|________|________|
| | | | | | | | |
| 32 | 1.2350 | 1.2357 | 1.2364 | 1.2372 | 1.2378 | 1.2384 | 1.2390 |
| 40 | 1.2267 | 1.2274 | 1.2282 | 1.2289 | 1.2295 | 1.2301 | 1.2307 |
| 50 | 1.2164 | 1.2171 | 1.2178 | 1.2186 | 1.2192 | 1.2198 | 1.2204 |
| 60 | 1.2061 | 1.2068 | 1.2075 | 1.2083 | 1.2089 | 1.2095 | 1.2101 |
| 70 | 1.1958 | 1.1965 | 1.1972 | 1.1980 | 1.1986 | 1.1992 | 1.1998 |
| 80 | 1.1855 | 1.1863 | 1.1869 | 1.1877 | 1.1883 | 1.1889 | 1.1895 |
| 90 | 1.1750 | 1.1760 | 1.1766 | 1.1774 | 1.1780 | 1.1786 | 1.1792 |
| 100 | 1.1650 | 1.1657 | 1.1664 | 1.1671 | 1.1678 | 1.1684 | 1.1690 |
| 110 | 1.1547 | 1.1554 | 1.1562 | 1.1569 | 1.1575 | 1.1581 | 1.1587 |
| 120 | 1.1444 | 1.1452 | 1.1459 | 1.1466 | 1.1472 | 1.1478 | 1.1484 |
| 130 | 1.1341 | 1.1349 | 1.1356 | 1.1363 | 1.1369 | 1.1375 | 1.1381 |
| 140 | 1.1239 | 1.1246 | 1.1253 | 1.1260 | 1.1266 | 1.1272 | 1.1278 |
| 150 | 1.1136 | 1.1143 | 1.1150 | 1.1157 | 1.1163 | 1.1169 | 1.1176 |
| 160 | 1.1033 | 1.1040 | 1.1047 | 1.1054 | 1.1060 | 1.1066 | 1.1073 |
| 170 | 1.0930 | 1.0937 | 1.0944 | 1.0951 | 1.0957 | 1.0963 | 1.0969 |
| 180 | 1.0826 | 1.0834 | 1.0841 | 1.0848 | 1.0854 | 1.0860 | 1.0866 |
| 190 | 1.0723 | 1.0730 | 1.0737 | 1.0745 | 1.0751 | 1.0757 | 1.0763 |
| 200 | 1.0620 | 1.0627 | 1.0634 | 1.0641 | 1.0647 | 1.0653 | 1.0660 |
| 210 | 1.0516 | 1.0523 | 1.0530 | 1.0538 | 1.0544 | 1.0550 | 1.0556 |
|_______|________|________|________|________|________|________|________|
The temperature, pressure and other properties of steam for varying
amounts of vacuum and the pressure above vacuum corresponding to each
inch of reading of the vacuum gauge are given in Table 20.
TABLE 20
PROPERTIES OF SATURATED STEAM FOR VARYING AMOUNTS OF VACUUM
CALCULATED FROM MARKS AND DAVIS TABLES
______________________________________________________________________
| | | | | | | |
| | | | Heat of | Latent | Total | |
| | | Temp- | the Liquid| Heat | Heat | |
| | | erature | Above | Above | Above |Density or|
| | Absolute | Degrees | 32 De- | 32 De- | 32 De- |Weight per|
| Vacuum | Pressure | Fahren- | grees | grees | grees |Cubic Foot|
|Ins. Hg.| Pounds | heit | B. t. u. |B. t. u.|B. t. u.| Pounds |
|________|__________|_________|___________|________|________|__________|
| | | | | | | |
| 29.5 | .207 | 54.1 | 22.18 | 1061.0 | 1083.2 | 0.000678 |
| 29 | .452 | 76.6 | 44.64 | 1048.7 | 1093.3 | 0.001415 |
| 28.5 | .698 | 90.1 | 58.09 | 1041.1 | 1099.2 | 0.002137 |
| 28 | .944 | 99.9 | 67.87 | 1035.6 | 1103.5 | 0.002843 |
| 27 | 1.44 | 112.5 | 80.4 | 1028.6 | 1109.0 | 0.00421 |
| 26 | 1.93 | 124.5 | 92.3 | 1022.0 | 1114.3 | 0.00577 |
| 25 | 2.42 | 132.6 | 100.5 | 1017.3 | 1117.8 | 0.00689 |
| 24 | 2.91 | 140.1 | 108.0 | 1013.1 | 1121.1 | 0.00821 |
| 22 | 3.89 | 151.7 | 119.6 | 1006.4 | 1126.0 | 0.01078 |
| 20 | 4.87 | 161.1 | 128.9 | 1001.0 | 1129.9 | 0.01331 |
| 18 | 5.86 | 168.9 | 136.8 | 996.4 | 1133.2 | 0.01581 |
| 16 | 6.84 | 175.8 | 143.6 | 992.4 | 1136.0 | 0.01827 |
| 14 | 7.82 | 181.8 | 149.7 | 988.8 | 1138.5 | 0.02070 |
| 12 | 8.80 | 187.2 | 155.1 | 985.6 | 1140.7 | 0.02312 |
| 10 | 9.79 | 192.2 | 160.1 | 982.6 | 1142.7 | 0.02554 |
| 5 | 12.24 | 202.9 | 170.8 | 976.0 | 1146.8 | 0.03148 |
|________|__________|_________|___________|________|________|__________|
From the steam tables, the condensed Table 21 of the properties of steam
at different pressures may be constructed. From such a table there may
be drawn the following conclusions.
TABLE 21
VARIATION IN PROPERTIES OF
SATURATED STEAM WITH PRESSURE
___________________________________________________
| | | | | |
| Pressure |Temperature | Heat of | Latent | Total |
| Pounds | Degrees | Liquid | Heat | Heat |
| Absolute | Fahrenheit |B. t. u. |B. t. u.|B. t. u.|
|__________|____________|_________|________|________|
| | | | | |
| 14.7 | 212.0 | 180.0 | 970.4 | 1150.4 |
| 20.0 | 228.0 | 196.1 | 960.0 | 1156.2 |
| 100.0 | 327.8 | 298.3 | 888.0 | 1186.3 |
| 300.0 | 417.5 | 392.7 | 811.3 | 1204.1 |
|__________|____________|_________|________|________|
As the pressure and temperature increase, the latent heat decreases.
This decrease, however, is less rapid than the corresponding increase in
the heat of the liquid and hence the total heat increases with an
increase in the pressure and temperature. The percentage increase in the
total heat is small, being 0.5, 3.1, and 4.7 per cent for 20, 100, and
300 pounds absolute pressure respectively above the total heat in one
pound of steam at 14.7 pounds absolute. The temperatures, on the other
hand, increase at the rates of 7.5, 54.6, and 96.9 per cent. The
efficiency of a perfect steam engine is proportional to the expression
(t - t_{1})/t in which t and t_{1} are the absolute temperatures of the
saturated steam at admission and exhaust respectively. While actual
engines only approximate the ideal engine in efficiency, yet they follow
the same general law. Since the exhaust temperature cannot be lowered
beyond present practice, it follows that the only available method of
increasing the efficiency is by an increase in the temperature of the
steam at admission. How this may be accomplished by an increase of
pressure is clearly shown, for the increase of fuel necessary to
increase the pressure is negligible, as shown by the total heat, while
the increase in economy, due to the higher pressure, will result
directly from the rapid increase of the corresponding temperature.
TABLE 22
HEAT UNITS PER POUND AND
WEIGHT PER CUBIC FOOT OF WATER
BETWEEN 32 DEGREES FAHRENHEIT AND
340 DEGREES FAHRENHEIT
_________________________________
| | | |
|Temperature|Heat Units| Weight |
| Degrees | per | per |
| Fahrenheit| Pound |Cubic Foot|
|___________|__________|__________|
| | | |
| 32 | 0.00 | 62.42 |
| 33 | 1.01 | 62.42 |
| 34 | 2.01 | 62.42 |
| 35 | 3.02 | 62.43 |
| 36 | 4.03 | 62.43 |
| 37 | 5.04 | 62.43 |
| 38 | 6.04 | 62.43 |
| 39 | 7.05 | 62.43 |
| 40 | 8.05 | 62.43 |
| 41 | 9.05 | 62.43 |
| 42 | 10.06 | 62.43 |
| 43 | 11.06 | 62.43 |
| 44 | 12.06 | 62.43 |
| 45 | 13.07 | 62.42 |
| 46 | 14.07 | 62.42 |
| 47 | 15.07 | 62.42 |
| 48 | 16.07 | 62.42 |
| 49 | 17.08 | 62.42 |
| 50 | 18.08 | 62.42 |
| 51 | 19.08 | 62.41 |
| 52 | 20.08 | 62.41 |
| 53 | 21.08 | 62.41 |
| 54 | 22.08 | 62.40 |
| 55 | 23.08 | 62.40 |
| 56 | 24.08 | 62.39 |
| 57 | 25.08 | 62.39 |
| 58 | 26.08 | 62.38 |
| 59 | 27.08 | 62.37 |
| 60 | 28.08 | 62.37 |
| 61 | 29.08 | 62.36 |
| 62 | 30.08 | 62.36 |
| 63 | 31.07 | 62.35 |
| 64 | 32.07 | 62.35 |
| 65 | 33.07 | 62.34 |
| 66 | 34.07 | 62.33 |
| 67 | 35.07 | 62.33 |
| 68 | 36.07 | 62.32 |
| 69 | 37.06 | 62.31 |
| 70 | 38.06 | 62.30 |
| 71 | 39.06 | 62.30 |
| 72 | 40.05 | 62.29 |
| 73 | 41.05 | 62.28 |
| 74 | 42.05 | 62.27 |
| 75 | 42.05 | 62.26 |
| 76 | 44.04 | 62.26 |
| 77 | 45.04 | 62.25 |
| 78 | 46.04 | 62.24 |
| 79 | 47.04 | 62.23 |
| 80 | 48.03 | 62.22 |
| 81 | 49.03 | 62.21 |
| 82 | 50.03 | 62.20 |
| 83 | 51.02 | 62.19 |
| 84 | 52.02 | 62.18 |
| 85 | 53.02 | 62.17 |
| 86 | 54.01 | 62.16 |
| 87 | 55.01 | 62.15 |
| 88 | 56.01 | 62.14 |
| 89 | 57.00 | 62.13 |
| 90 | 58.00 | 62.12 |
| 91 | 59.00 | 62.11 |
| 92 | 60.00 | 62.09 |
| 93 | 60.99 | 62.08 |
| 94 | 61.99 | 62.07 |
| 95 | 62.99 | 62.06 |
| 96 | 63.98 | 62.05 |
| 97 | 64.98 | 62.04 |
| 98 | 65.98 | 62.03 |
| 99 | 66.97 | 62.02 |
| 100 | 67.97 | 62.00 |
| 101 | 68.97 | 61.99 |
| 102 | 69.96 | 61.98 |
| 103 | 70.96 | 61.97 |
| 104 | 71.96 | 61.95 |
| 105 | 72.95 | 61.94 |
| 106 | 73.95 | 61.93 |
| 107 | 74.95 | 61.91 |
| 108 | 75.95 | 61.90 |
| 109 | 76.94 | 61.88 |
| 110 | 77.94 | 61.86 |
| 111 | 78.94 | 61.85 |
| 112 | 79.93 | 61.83 |
| 113 | 80.93 | 61.82 |
| 114 | 81.93 | 61.80 |
| 115 | 82.92 | 61.79 |
| 116 | 83.92 | 61.77 |
| 117 | 84.92 | 61.75 |
| 118 | 85.92 | 61.74 |
| 119 | 86.91 | 61.72 |
| 120 | 87.91 | 61.71 |
| 121 | 88.91 | 61.69 |
| 122 | 89.91 | 61.68 |
| 123 | 90.90 | 61.66 |
| 124 | 91.90 | 61.65 |
| 125 | 92.90 | 61.63 |
| 126 | 93.90 | 61.61 |
| 127 | 94.89 | 61.59 |
| 128 | 95.89 | 61.58 |
| 129 | 96.89 | 61.56 |
| 130 | 97.89 | 61.55 |
| 131 | 98.89 | 61.53 |
| 132 | 99.88 | 61.52 |
| 133 | 100.88 | 61.50 |
| 134 | 101.88 | 61.49 |
| 135 | 102.88 | 61.47 |
| 136 | 103.88 | 61.45 |
| 137 | 104.87 | 61.43 |
| 138 | 105.87 | 61.41 |
| 139 | 106.87 | 61.40 |
| 140 | 107.87 | 61.38 |
| 141 | 108.87 | 61.36 |
| 142 | 109.87 | 61.34 |
| 143 | 110.87 | 61.33 |
| 144 | 111.87 | 61.31 |
| 145 | 112.86 | 61.29 |
| 146 | 113.86 | 61.27 |
| 147 | 114.86 | 61.25 |
| 148 | 115.86 | 61.24 |
| 149 | 116.86 | 61.22 |
| 150 | 117.86 | 61.20 |
| 151 | 118.86 | 61.18 |
| 152 | 119.86 | 61.16 |
| 153 | 120.86 | 61.14 |
| 154 | 121.86 | 61.12 |
| 155 | 122.86 | 61.10 |
| 156 | 123.86 | 61.08 |
| 157 | 124.86 | 61.06 |
| 158 | 125.86 | 61.04 |
| 159 | 126.86 | 61.02 |
| 160 | 127.86 | 61.00 |
| 161 | 128.86 | 60.98 |
| 162 | 129.86 | 60.96 |
| 163 | 130.86 | 60.94 |
| 164 | 131.86 | 60.92 |
| 165 | 132.86 | 60.90 |
| 166 | 133.86 | 60.88 |
| 167 | 134.86 | 60.86 |
| 168 | 135.86 | 60.84 |
| 169 | 136.86 | 60.82 |
| 170 | 137.87 | 60.80 |
| 171 | 138.87 | 60.78 |
| 172 | 139.87 | 60.76 |
| 173 | 140.87 | 60.73 |
| 174 | 141.87 | 60.71 |
| 175 | 142.87 | 60.69 |
| 176 | 143.87 | 60.67 |
| 177 | 144.88 | 60.65 |
| 178 | 145.88 | 60.62 |
| 179 | 146.88 | 60.60 |
| 180 | 147.88 | 60.58 |
| 181 | 148.88 | 60.56 |
| 182 | 149.89 | 60.53 |
| 183 | 150.89 | 60.51 |
| 184 | 151.89 | 60.49 |
| 185 | 152.89 | 60.47 |
| 186 | 153.89 | 60.45 |
| 187 | 154.90 | 60.42 |
| 188 | 155.90 | 60.40 |
| 189 | 156.90 | 60.38 |
| 190 | 157,91 | 60.36 |
| 191 | 158.91 | 60.33 |
| 192 | 159.91 | 60.31 |
| 193 | 160.91 | 60.29 |
| 194 | 161.92 | 60.27 |
| 195 | 162.92 | 60.24 |
| 196 | 163.92 | 60.22 |
| 197 | 164.93 | 60.19 |
| 198 | 165.93 | 60.17 |
| 199 | 166.94 | 60.15 |
| 200 | 167.94 | 60.12 |
| 201 | 168.94 | 60.10 |
| 202 | 169.95 | 60.07 |
| 203 | 170.95 | 60.05 |
| 204 | 171.96 | 60.02 |
| 205 | 172.96 | 60.00 |
| 206 | 173.97 | 59.98 |
| 207 | 174.97 | 59.95 |
| 208 | 175.98 | 59.93 |
| 209 | 176.98 | 59.90 |
| 210 | 177.99 | 59.88 |
| 211 | 178.99 | 59.85 |
| 212 | 180.00 | 59.83 |
| 213 | 181.0 | 59.80 |
| 214 | 182.0 | 59.78 |
| 215 | 183.0 | 59.75 |
| 216 | 184.0 | 59.73 |
| 217 | 185.0 | 59.70 |
| 218 | 186.1 | 59.68 |
| 219 | 187.1 | 59.65 |
| 220 | 188.1 | 59.63 |
| 221 | 189.1 | 59.60 |
| 222 | 190.1 | 59.58 |
| 223 | 191.1 | 59.55 |
| 224 | 192.1 | 59.53 |
| 225 | 193.1 | 59.50 |
| 226 | 194.1 | 59.48 |
| 227 | 195.2 | 59.45 |
| 228 | 196.2 | 59.42 |
| 229 | 197.2 | 59.40 |
| 230 | 198.2 | 59.37 |
| 231 | 199.2 | 59.34 |
| 232 | 200.2 | 59.32 |
| 233 | 201.2 | 59.29 |
| 234 | 202.2 | 59.27 |
| 235 | 203.2 | 59.24 |
| 236 | 204.2 | 59.21 |
| 237 | 205.3 | 59.19 |
| 238 | 206.3 | 59.16 |
| 239 | 207.3 | 59.14 |
| 240 | 208.3 | 59.11 |
| 241 | 209.3 | 59.08 |
| 242 | 210.3 | 59.05 |
| 243 | 211.4 | 59.03 |
| 244 | 212.4 | 59.00 |
| 245 | 213.4 | 58.97 |
| 246 | 214.4 | 58.94 |
| 247 | 215.4 | 58.91 |
| 248 | 216.4 | 58.89 |
| 249 | 217.4 | 58.86 |
| 250 | 218.5 | 58.83 |
| 260 | 228.6 | 58.55 |
| 270 | 238.8 | 58.26 |
| 280 | 249.0 | 57.96 |
| 290 | 259.3 | 57.65 |
| 300 | 269.6 | 57.33 |
| 310 | 279.9 | 57.00 |
| 320 | 290.2 | 56.66 |
| 330 | 300.6 | 56.30 |
| 340 | 311.0 | 55.94 |
|___________|__________|__________|
The gain due to superheat cannot be predicted from the formula for the
efficiency of a perfect steam engine given on page 119. This formula is
not applicable in cases where superheat is present since only a
relatively small amount of the heat in the steam is imparted at the
maximum or superheated temperature.
The advantage of the use of high pressure steam may be also indicated by
considering the question from the aspect of volume. With an increase of
pressure comes a decrease in volume, thus one pound of saturated steam
at 100 pounds absolute pressure occupies 4.43 cubic feet, while at 200
pounds pressure it occupies 2.29 cubic feet. If then, in separate
cylinders of the same dimensions, one pound of steam at 100 pounds
absolute pressure and one pound at 200 pounds absolute pressure enter
and are allowed to expand to the full volume of each cylinder, the
high-pressure steam, having more room and a greater range for expansion
than the low-pressure steam, will thus do more work. This increase in
the amount of work, as was the increase in temperature, is large
relative to the additional fuel required as indicated by the total heat.
In general, it may be stated that the fuel required to impart a given
amount of heat to a boiler is practically independent of the steam
pressure, since the temperature of the fire is so high as compared with
the steam temperature that a variation in the steam temperature does not
produce an appreciable effect.
The formulae for the algebraic expression of the relation between
saturated steam pressures, temperatures and steam volumes have been up
to the present time empirical. These relations have, however, been
determined by experiment and, from the experimental data, tables have
been computed which render unnecessary the use of empirical formulae.
Such formulae may be found in any standard work of thermo-dynamics. The
following tables cover all practical cases.
Table 22 gives the heat units contained in water above 32 degrees
Fahrenheit at different temperatures.
Table 23 gives the properties of saturated steam for various pressures.
Table 24 gives the properties of superheated steam at various pressures
and temperatures.
These tables are based on those computed by Lionel S. Marks and Harvey
N. Davis, these being generally accepted as being the most correct.
TABLE 23
PROPERTIES OF SATURATED STEAM
REPRODUCED BY PERMISSION FROM
MARKS AND DAVIS "STEAM TABLES AND DIAGRAMS"
(Copyright, 1909, by Longmans, Green & Co.)
____________________________________________________________________
|Pressure,| Temper- |Specific Vol-|Heat of |Latent Heat|Total Heat|
| Pounds |ature De-| ume Cu. Ft. |the Liquid,| of Evap., |of Steam, |
|Absolute |grees F. | per Pound | B. t. u. | B. t. u. | B. t. u. |
|_________|_________|_____________|___________|___________|__________|
| 1 | 101.83 | 333.0 | 69.8 | 1034.6 | 1104.4 |
| 2 | 126.15 | 173.5 | 94.0 | 1021.0 | 1115.0 |
| 3 | 141.52 | 118.5 | 109.4 | 1012.3 | 1121.6 |
| 4 | 153.01 | 90.5 | 120.9 | 1005.7 | 1126.5 |
| 5 | 162.28 | 73.33 | 130.1 | 1000.3 | 1130.5 |
| 6 | 170.06 | 61.89 | 137.9 | 995.8 | 1133.7 |
| 7 | 176.85 | 53.56 | 144.7 | 991.8 | 1136.5 |
| 8 | 182.86 | 47.27 | 150.8 | 988.2 | 1139.0 |
| 9 | 188.27 | 42.36 | 156.2 | 985.0 | 1141.1 |
| 10 | 193.22 | 38.38 | 161.1 | 982.0 | 1143.1 |
| 11 | 197.75 | 35.10 | 165.7 | 979.2 | 1144.9 |
| 12 | 201.96 | 32.36 | 169.9 | 976.6 | 1146.5 |
| 13 | 205.87 | 30.03 | 173.8 | 974.2 | 1148.0 |
| 14 | 209.55 | 28.02 | 177.5 | 971.9 | 1149.4 |
| 15 | 213.0 | 26.27 | 181.0 | 969.7 | 1150.7 |
| 16 | 216.3 | 24.79 | 184.4 | 967.6 | 1152.0 |
| 17 | 219.4 | 23.38 | 187.5 | 965.6 | 1153.1 |
| 18 | 222.4 | 22.16 | 190.5 | 963.7 | 1154.2 |
| 19 | 225.2 | 21.07 | 193.4 | 961.8 | 1155.2 |
| 20 | 228.0 | 20.08 | 196.1 | 960.0 | 1156.2 |
| 22 | 233.1 | 18.37 | 201.3 | 956.7 | 1158.0 |
| 24 | 237.8 | 16.93 | 206.1 | 953.5 | 1159.6 |
| 26 | 242.2 | 15.72 | 210.6 | 950.6 | 1161.2 |
| 28 | 246.4 | 14.67 | 214.8 | 947.8 | 1162.6 |
| 30 | 250.3 | 13.74 | 218.8 | 945.1 | 1163.9 |
| 32 | 254.1 | 12.93 | 222.6 | 942.5 | 1165.1 |
| 34 | 257.6 | 12.22 | 226.2 | 940.1 | 1166.3 |
| 36 | 261.0 | 11.58 | 229.6 | 937.7 | 1167.3 |
| 38 | 264.2 | 11.01 | 232.9 | 935.5 | 1168.4 |
| 40 | 267.3 | 10.49 | 236.1 | 933.3 | 1169.4 |
| 42 | 270.2 | 10.02 | 239.1 | 931.2 | 1170.3 |
| 44 | 273.1 | 9.59 | 242.0 | 929.2 | 1171.2 |
| 46 | 275.8 | 9.20 | 244.8 | 927.2 | 1172.0 |
| 48 | 278.5 | 8.84 | 247.5 | 925.3 | 1172.8 |
| 50 | 281.0 | 8.51 | 250.1 | 923.5 | 1173.6 |
| 52 | 283.5 | 8.20 | 252.6 | 921.7 | 1174.3 |
| 54 | 285.9 | 7.91 | 255.1 | 919.9 | 1175.0 |
| 56 | 288.2 | 7.65 | 257.5 | 918.2 | 1175.7 |
| 58 | 290.5 | 7.40 | 259.8 | 916.5 | 1176.4 |
| 60 | 292.7 | 7.17 | 262.1 | 914.9 | 1177.0 |
| 62 | 294.9 | 6.95 | 264.3 | 913.3 | 1177.6 |
| 64 | 297.0 | 6.75 | 266.4 | 911.8 | 1178.2 |
| 66 | 299.0 | 6.56 | 268.5 | 910.2 | 1178.8 |
| 68 | 301.0 | 6.38 | 270.6 | 908.7 | 1179.3 |
| 70 | 302.9 | 6.20 | 272.6 | 907.2 | 1179.8 |
| 72 | 304.8 | 6.04 | 274.5 | 905.8 | 1180.4 |
| 74 | 306.7 | 5.89 | 276.5 | 904.4 | 1180.9 |
| 76 | 308.5 | 5.74 | 278.3 | 903.0 | 1181.4 |
| 78 | 310.3 | 5.60 | 280.2 | 901.7 | 1181.8 |
| 80 | 312.0 | 5.47 | 282.0 | 900.3 | 1182.3 |
| 82 | 313.8 | 5.34 | 283.8 | 899.0 | 1182.8 |
| 84 | 315.4 | 5.22 | 285.5 | 897.7 | 1183.2 |
| 86 | 317.1 | 5.10 | 287.2 | 896.4 | 1183.6 |
| 88 | 318.7 | 5.00 | 288.9 | 895.2 | 1184.0 |
| 90 | 320.3 | 4.89 | 290.5 | 893.9 | 1184.4 |
| 92 | 321.8 | 4.79 | 292.1 | 892.7 | 1184.8 |
| 94 | 323.4 | 4.69 | 293.7 | 891.5 | 1185.2 |
| 96 | 324.9 | 4.60 | 295.3 | 890.3 | 1185.6 |
| 98 | 326.4 | 4.51 | 296.8 | 889.2 | 1186.0 |
| 100 | 327.8 | 4.429 | 298.3 | 888.0 | 1186.3 |
| 105 | 331.4 | 4.230 | 302.0 | 885.2 | 1187.2 |
| 110 | 334.8 | 4.047 | 305.5 | 882.5 | 1188.0 |
| 115 | 338.1 | 3.880 | 309.0 | 879.8 | 1188.8 |
| 120 | 341.3 | 3.726 | 312.3 | 877.2 | 1189.6 |
| 125 | 344.4 | 3.583 | 315.5 | 874.7 | 1190.3 |
| 130 | 347.4 | 3.452 | 318.6 | 872.3 | 1191.0 |
| 135 | 350.3 | 3.331 | 321.7 | 869.9 | 1191.6 |
| 140 | 353.1 | 3.219 | 324.6 | 867.6 | 1192.2 |
| 145 | 355.8 | 3.112 | 327.4 | 865.4 | 1192.8 |
| 150 | 358.5 | 3.012 | 330.2 | 863.2 | 1193.4 |
| 155 | 361.0 | 2.920 | 332.9 | 861.0 | 1194.0 |
| 160 | 363.6 | 2.834 | 335.6 | 858.8 | 1194.5 |
| 165 | 366.0 | 2.753 | 338.2 | 856.8 | 1195.0 |
| 170 | 368.5 | 2.675 | 340.7 | 854.7 | 1195.4 |
| 175 | 370.8 | 2.602 | 343.2 | 852.7 | 1195.9 |
| 180 | 373.1 | 2.533 | 345.6 | 850.8 | 1196.4 |
| 185 | 375.4 | 2.468 | 348.0 | 848.8 | 1196.8 |
| 190 | 377.6 | 2.406 | 350.4 | 846.9 | 1197.3 |
| 195 | 379.8 | 2.346 | 352.7 | 845.0 | 1197.7 |
| 200 | 381.9 | 2.290 | 354.9 | 843.2 | 1198.1 |
| 205 | 384.0 | 2.237 | 357.1 | 841.4 | 1198.5 |
| 210 | 386.0 | 2.187 | 359.2 | 839.6 | 1198.8 |
| 215 | 388.0 | 2.138 | 361.4 | 837.9 | 1199.2 |
| 220 | 389.9 | 2.091 | 363.4 | 836.2 | 1199.6 |
| 225 | 391.9 | 2.046 | 365.5 | 834.4 | 1199.9 |
| 230 | 393.8 | 2.004 | 367.5 | 832.8 | 1200.2 |
| 235 | 395.6 | 1.964 | 369.4 | 831.1 | 1200.6 |
| 240 | 397.4 | 1.924 | 371.4 | 829.5 | 1200.9 |
| 245 | 399.3 | 1.887 | 373.3 | 827.9 | 1201.2 |
| 250 | 401.1 | 1.850 | 375.2 | 826.3 | 1201.5 |
|_________|_________|_____________|___________|___________|__________|
[Illustration: Portion of 6100 Horse-power Installation of Babcock &
Wilcox Boilers Equipped with Babcock & Wilcox Chain Grate Stokers at the
Campbell Street Plant of the Louisville Railway Co., Louisville, Ky.
This Company Operates a Total of 14,000 Horse Power of Babcock & Wilcox
Boilers]
TABLE 24
PROPERTIES OF SUPERHEATED STEAM
REPRODUCED BY PERMISSION FROM
MARKS AND DAVIS "STEAM TABLES AND DIAGRAMS"
(Copyright, 1909, by Longmans, Green & Co.)
__________________________________________________________________
| | | |
| | | Degrees of Superheat |
|Pressure| |_______________________________________________|
| Pounds |Saturated| | | | | | |
|Absolute| Steam | 50 | 100 | 150 | 200 | 250 | 300 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 162.3 | 212.3 | 262.3 | 312.3 | 362.3 | 412.3 | 462.3 |
| 5 v| 73.3 | 79.7 | 85.7 | 91.8 | 97.8 | 103.8 | 109.8 |
| h| 1130.5 |1153.5 |1176.4 |1199.5 |1222.5 |1245.6 |1268.7 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 193.2 | 243.2 | 293.2 | 343.2 | 393.2 | 443.2 | 493.2 |
| 10 v| 38.4 | 41.5 | 44.6 | 47.7 | 50.7 | 53.7 | 56.7 |
| h| 1143.1 |1166.3 |1189.5 |1212.7 |1236.0 |1259.3 |1282.5 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 213.0 | 263.0 | 313.0 | 363.0 | 413.0 | 463.0 | 513.0 |
| 15 v| 26.27 | 28.40| 30.46| 32.50| 34.53| 36.56| 38.58|
| h| 1150.7 |1174.2 |1197.6 |1221.0 |1244.4 |1267.7 |1291.1 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 228.0 | 278.0 | 328.0 | 378.0 | 428.0 | 478.0 | 528.0 |
| 20 v| 20.08 | 21.69| 23.25| 24.80| 26.33| 27.85| 29.37|
| h| 1156.2 |1179.9 |1203.5 |1227.1 |1250.6 |1274.1 |1297.6 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 240.1 | 290.1 | 340.1 | 390.1 | 440.1 | 490.1 | 540.1 |
| 25 v| 16.30 | 17.60| 18.86| 20.10| 21.32| 22.55| 23.77|
| h| 1160.4 |1184.4 |1208.2 |1231.9 |1255.6 |1279.2 |1302.8 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 250.4 | 300.4 | 350.4 | 400.4 | 450.4 | 500.4 | 550.4 |
| 30 v| 13.74 | 14.83| 15.89| 16.93| 17.97| 18.99| 20.00|
| h| 1163.9 |1188.1 |1212.1 |1236.0 |1259.7 |1283.4 |1307.1 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 259.3 | 309.3 | 359.3 | 409.3 | 459.3 | 509.3 | 559.3 |
| 35 v| 11.89 | 12.85| 13.75| 14.65| 15.54| 16.42| 17.30|
| h| 1166.8 |1191.3 |1215.4 |1239.4 |1263.3 |1287.1 |1310.8 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 267.3 | 317.3 | 367.3 | 417.3 | 467.3 | 517.3 | 567.3 |
| 40 v| 10.49 | 11.33| 12.13| 12.93| 13.70| 14.48| 15.25|
| h| 1169.4 |1194.0 |1218.4 |1242.4 |1266.4 |1290.3 |1314.1 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 274.5 | 324.5 | 374.5 | 424.5 | 474.5 | 524.5 | 574.5 |
| 45 v| 9.39 | 10.14| 10.86| 11.57| 12.27| 12.96| 13.65|
| h| 1171.6 |1196.6 |1221.0 |1245.2 |1269.3 |1293.2 |1317.0 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 281.0 | 331.0 | 381.0 | 431.0 | 481.0 | 531.0 | 581.0 |
| 50 v| 8.51 | 9.19| 9.84| 10.48| 11.11| 11.74| 12.36|
| h| 1173.6 |1198.8 |1223.4 |1247.7 |1271.8 |1295.8 |1319.7 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 287.1 | 337.1 | 387.1 | 437.1 | 487.1 | 537.1 | 587.1 |
| 55 v| 7.78 | 8.40| 9.00| 9.59| 10.16| 10.73| 11.30|
| h| 1175.4 |1200.8 |1225.6 |1250.0 |1274.2 |1298.1 |1322.0 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 292.7 | 342.7 | 392.7 | 442.7 | 492.7 | 542.7 | 592.7 |
| 60 v| 7.17 | 7.75| 8.30| 8.84| 9.36| 9.89| 10.41|
| h| 1177.0 |1202.6 |1227.6 |1252.1 |1276.4 |1300.4 |1324.3 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 298.0 | 348.0 | 398.0 | 448.0 | 498.0 | 548.0 | 598.0 |
| 65 v| 6.65 | 7.20| 7.70| 8.20| 8.69| 9.17| 9.65|
| h| 1178.5 |1204.4 |1229.5 |1254.0 |1278.4 |1302.4 |1326.4 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 302.9 | 352.9 | 402.9 | 452.9 | 502.9 | 552.9 | 602.9 |
| 70 v| 6.20 | 6.71| 7.18| 7.65| 8.11| 8.56| 9.01|
| h| 1179.8 |1205.9 |1231.2 |1255.8 |1280.2 |1304.3 |1328.3 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 307.6 | 357.6 | 407.6 | 457.6 | 507.6 | 557.6 | 607.6 |
| 75 v| 5.81 | 6.28| 6.73| 7.17| 7.60| 8.02| 8.44|
| h| 1181.1 |1207.5 |1232.8 |1257.5 |1282.0 |1306.1 |1330.1 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 312.0 | 362.0 | 412.0 | 462.0 | 512.0 | 562.0 | 612.0 |
| 80 v| 5.47 | 5.92| 6.34| 6.75| 7.17| 7.56| 7.95|
| h| 1182.3 |1208.8 |1234.3 |1259.0 |1283.6 |1307.8 |1331.9 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 316.3 | 366.3 | 416.3 | 466.3 | 516.3 | 566.3 | 616.3 |
| 85 v| 5.16 | 5.59| 6.99| 6.38| 6.76| 7.14| 7.51|
| h| 1183.4 |1210.2 |1235.8 |1260.6 |1285.2 |1309.4 |1333.5 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 320.3 | 370.3 | 420.3 | 470.3 | 520.3 | 570.3 | 620.3 |
| 90 v| 4.89 | 5.29| 5.67| 6.04| 6.40| 6.76| 7.11|
| h| 1184.4 |1211.4 |1237.2 |1262.0 |1286.6 |1310.8 |1334.9 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 324.1 | 374.1 | 424.1 | 474.1 | 524.1 | 574.1 | 624.1 |
| 95 v| 4.65 | 5.03| 5.39| 5.74| 6.09| 6.43| 6.76|
| h| 1185.4 |1212.6 |1238.4 |1263.4 |1288.1 |1312.3 |1336.4 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 327.8 | 377.8 | 427.8 | 477.8 | 527.8 | 577.8 | 627.8 |
| 100 v| 4.43 | 4.79| 5.14| 5.47| 5.80| 6.12| 6.44|
| h| 1186.3 |1213.8 |1239.7 |1264.7 |1289.4 |1313.6 |1337.8 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 331.4 | 381.4 | 431.4 | 481.4 | 531.4 | 581.4 | 631.4 |
| 105 v| 4.23 | 4.58| 4.91| 5.23| 5.54| 5.85| 6.15|
| h| 1187.2 |1214.9 |1240.8 |1265.9 |1290.6 |1314.9 |1339.1 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 334.8 | 384.8 | 434.8 | 484.8 | 534.8 | 584.8 | 634.8 |
| 110 v| 4.05 | 4.38| 4.70| 5.01| 5.31| 5.61| 5.90|
| h| 1188.0 |1215.9 |1242.0 |1267.1 |1291.9 |1316.2 |1340.4 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 338.1 | 388.1 | 438.1 | 488.1 | 538.1 | 588.1 | 638.1 |
| 115 v| 3.88 | 4.20| 4.51| 4.81| 5.09| 5.38| 5.66|
| h| 1188.8 |1216.9 |1243.1 |1268.2 |1293.0 |1317.3 |1341.5 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 341.3 | 391.3 | 441.3 | 491.3 | 541.3 | 591.3 | 641.3 |
| 120 v| 3.73 | 4.04| 4.33| 4.62| 4.89| 5.17| 5.44|
| h| 1189.6 |1217.9 |1244.1 |1269.3 |1294.1 |1318.4 |1342.7 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 344.4 | 394.4 | 444.4 | 494.4 | 544.4 | 594.4 | 644.4 |
| 125 v| 3.58 | 3.88| 4.17| 4.45| 4.71| 4.97| 5.23|
| h| 1190.3 |1218.8 |1245.1 |1270.4 |1295.2 |1319.5 |1343.8 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 347.4 | 397.4 | 447.4 | 497.4 | 547.4 | 597.4 | 647.4 |
| 130 v| 3.45 | 3.74| 4.02| 4.28| 4.54| 4.80| 5.05|
| h| 1191.0 |1219.7 |1246.1 |1271.4 |1296.2 |1320.6 |1344.9 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 350.3 | 400.3 | 450.3 | 500.3 | 550.3 | 600.3 | 650.3 |
| 135 v| 3.33 | 3.61| 3.88| 4.14| 4.38| 4.63| 4.87|
| h| 1191.6 |1220.6 |1247.0 |1272.3 |1297.2 |1321.6 |1345.9 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 353.1 | 403.1 | 453.1 | 503.1 | 553.1 | 603.1 | 653.1 |
| 140 v| 3.22 | 3.49| 3.75| 4.00| 4.24| 4.48| 4.71|
| h| 1192.2 |1221.4 |1248.0 |1273.3 |1298.2 |1322.6 |1346.9 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 355.8 | 405.8 | 455.8 | 505.8 | 555.8 | 605.8 | 655.8 |
| 145 v| 3.12 | 3.38| 3.63| 3.87| 4.10| 4.33| 4.56|
| h| 1192.8 |1222.2 |1248.8 |1274.2 |1299.1 |1323.6 |1347.9 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 358.5 | 408.5 | 458.5 | 508.5 | 558.5 | 608.5 | 658.5 |
| 150 v| 3.01 | 3.27| 3.50| 3.75| 3.97| 4.19| 4.41|
| h| 1193.4 |1223.0 |1249.6 |1275.1 |1300.0 |1324.5 |1348.8 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 361.0 | 411.0 | 461.0 | 511.0 | 561.0 | 611.0 | 661.0 |
| 155 v| 2.92 | 3.17| 3.41| 3.63| 3.85| 4.06| 4.28|
| h| 1194.0 |1223.6 |1250.5 |1276.0 |1300.8 |1325.3 |1349.7 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 363.6 | 413.6 | 463.6 | 513.6 | 563.6 | 613.6 | 663.6 |
| 160 v| 2.83 | 3.07| 3.30| 3.53| 3.74| 3.95| 4.15|
| h| 1194.5 |1224.5 |1251.3 |1276.8 |1301.7 |1326.2 |1350.6 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 366.0 | 416.0 | 466.0 | 516.0 | 566.0 | 616.0 | 666.0 |
| 165 v| 2.75 | 2.99| 3.21| 3.43| 3.64| 3.84| 4.04|
| h| 1195.0 |1225.2 |1252.0 |1277.6 |1302.5 |1327.1 |1351.5 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 368.5 | 418.5 | 468.5 | 518.5 | 568.5 | 618.5 | 668.5 |
| 170 v| 2.68 | 2.91| 3.12| 3.34| 3.54| 3.73| 3.92|
| h| 1195.4 |1225.9 |1252.8 |1278.4 |1303.3 |1327.9 |1352.3 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 370.8 | 420.8 | 470.8 | 520.8 | 570.8 | 620.8 | 670.8 |
| 175 v| 2.60 | 2.83| 3.04| 3.24| 3.44| 3.63| 3.82|
| h| 1195.9 |1226.6 |1253.6 |1279.1 |1304.1 |1328.7 |1353.2 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 373.1 | 423.1 | 473.1 | 523.1 | 573.1 | 623.1 | 673.1 |
| 180 v| 2.53 | 2.75| 2.96| 3.16| 3.35| 3.54| 3.72|
| h| 1196.4 |1227.2 |1254.3 |1279.9 |1304.8 |1329.5 |1353.9 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 375.4 | 425.4 | 475.4 | 525.4 | 575.4 | 625.4 | 675.4 |
| 185 v| 2.47 | 2.68| 2.89| 3.08| 3.27| 3.45| 3.63|
| h| 1196.8 |1227.9 |1255.0 |1280.6 |1305.6 |1330.2 |1354.7 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 377.6 | 427.6 | 477.6 | 527.6 | 577.6 | 627.6 | 677.6 |
| 190 v| 2.41 | 2.62| 2.81| 3.00| 3.19| 3.37| 3.55|
| h| 1197.3 |1228.6 |1255.7 |1281.3 |1306.3 |1330.9 |1355.5 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 379.8 | 429.8 | 479.8 | 529.8 | 579.8 | 629.8 | 679.8 |
| 195 v| 2.35 | 2.55| 2.75| 2.93| 3.11| 3.29| 3.46|
| h| 1197.7 |1229.2 |1256.4 |1282.0 |1307.0 |1331.6 |1356.2 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 381.9 | 431.9 | 481.9 | 531.9 | 581.9 | 631.9 | 681.9 |
| 200 v| 2.29 | 2.49| 2.68| 2.86| 3.04| 3.21| 3.38|
| h| 1198.1 |1229.8 |1257.1 |1282.6 |1307.7 |1332.4 |1357.0 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 384.0 | 434.0 | 484.0 | 534.0 | 584.0 | 634.0 | 684.0 |
| 205 v| 2.24 | 2.44| 2.62| 2.80| 2.97| 3.14| 3.30|
| h| 1198.5 |1230.4 |1257.7 |1283.3 |1308.3 |1333.0 |1357.7 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 386.0 | 436.0 | 486.0 | 536.0 | 586.0 | 636.0 | 686.0 |
| 210 v| 2.19 | 2.38| 2.56| 2.74| 2.91| 3.07| 3.23|
| h| 1198.8 |1231.0 |1258.4 |1284.0 |1309.0 |1333.7 |1358.4 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 388.0 | 438.0 | 488.0 | 538.0 | 588.0 | 638.0 | 688.0 |
| 215 v| 2.14 | 2.33| 2.51| 2.68| 2.84| 3.00| 3.16|
| h| 1199.2 |1231.6 |1259.0 |1284.6 |1309.7 |1334.4 |1359.1 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 389.9 | 439.9 | 489.9 | 539.9 | 589.9 | 639.9 | 689.9 |
| 220 v| 2.09 | 2.28| 2.45| 2.62| 2.78| 2.94| 3.10|
| h| 1199.6 |1232.2 |1259.6 |1285.2 |1310.3 |1335.1 |1359.8 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 391.9 | 441.9 | 491.9 | 541.9 | 591.9 | 641.9 | 691.9 |
| 225 v| 2.05 | 2.23| 2.40| 2.57| 2.72| 2.88| 3.03|
| h| 1199.9 |1232.7 |1260.2 |1285.9 |1310.9 |1335.7 |1360.3 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 393.8 | 443.8 | 493.8 | 543.8 | 593.8 | 643.8 | 693.8 |
| 230 v| 2.00 | 2.18| 2.35| 2.51| 2.67| 2.82| 2.97|
| h| 1200.2 |1233.2 |1260.7 |1286.5 |1311.6 |1336.3 |1361.0 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 395.6 | 445.6 | 495.6 | 545.6 | 595.6 | 645.6 | 695.6 |
| 235 v| 1.96 | 2.14| 2.30| 2.46| 2.62| 2.77| 2.91|
| h| 1200.6 |1233.8 |1261.4 |1287.1 |1312.2 |1337.0 |1361.7 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 397.4 | 447.4 | 497.4 | 547.4 | 597.4 | 647.4 | 697.4 |
| 240 v| 1.92 | 2.09| 2.26| 2.42| 2.57| 2.71| 2.85|
| h| 1200.9 |1234.3 |1261.9 |1287.6 |1312.8 |1337.6 |1362.3 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 399.3 | 449.3 | 499.3 | 549.3 | 599.3 | 649.3 | 699.3 |
| 245 v| 1.89 | 2.05| 2.22| 2.37| 2.52| 2.66| 2.80|
| h| 1201.2 |1234.8 |1262.5 |1288.2 |1313.3 |1338.2 |1362.9 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 401.0 | 451.0 | 501.0 | 551.0 | 601.0 | 651.0 | 701.0 |
| 250 v| 1.85 | 2.02| 2.17| 2.33| 2.47| 2.61| 2.75|
| h| 1201.5 |1235.4 |1263.0 |1288.8 |1313.9 |1338.8 |1363.5 |
|________|_________|_______|_______|_______|_______|_______|_______|
| t| 402.8 | 452.8 | 502.8 | 552.8 | 602.8 | 652.8 | 702.8 |
| 255 v| 1.81 | 1.98| 2.14| 2.28| 2.43| 2.56| 2.70|
| h| 1201.8 |1235.9 |1263.6 |1289.3 |1314.5 |1339.3 |1364.1 |
|________|_________|_______|_______|_______|_______|_______|_______|
t = Temperature, degrees Fahrenheit.
v = Specific volume, in cubic feet, per pound.
h = Total heat from water at 32 degrees, B. t. u.
[Graph: Temperature of Steam--Degrees Fahr.
against Temperature in Calorimeter--Degrees Fahr.
Fig. 15. Graphic Method of Determining Moisture Contained in Steam from
Calorimeter Readings]
MOISTURE IN STEAM
The presence of moisture in steam causes a loss, not only in the
practical waste of the heat utilized to raise this moisture from the
temperature of the feed water to the temperature of the steam, but also
through the increased initial condensation in an engine cylinder and
through friction and other actions in a steam turbine. The presence of
such moisture also interferes with proper cylinder lubrication, causes a
knocking in the engine and a water hammer in the steam pipes. In steam
turbines it will cause erosion of the blades.
The percentage by weight of steam in a mixture of steam and water is
called the _quality of the steam_.
The apparatus used to determine the moisture content of steam is called
a calorimeter though since it may not measure the heat in the steam, the
name is not descriptive of the function of the apparatus. The first form
used was the "barrel calorimeter", but the liability of error was so
great that its use was abandoned. Modern calorimeters are in general of
either the throttling or separator type.
Throttling Calorimeter--Fig. 14 shows a typical form of throttling
calorimeter. Steam is drawn from a vertical main through the sampling
nipple, passes around the first thermometer cup, then through a
one-eighth inch orifice in a disk between two flanges, and lastly around
the second thermometer cup and to the atmosphere. Thermometers are
inserted in the wells, which should be filled with mercury or heavy
cylinder oil.
[Illustration: Fig. 14. Throttling Calorimeter and Sampling Nozzle]
The instrument and all pipes and fittings leading to it should be
thoroughly insulated to diminish radiation losses. Care must be taken to
prevent the orifice from becoming choked with dirt and to see that no
leaks occur. The exhaust pipe should be short to prevent back pressure
below the disk.
When steam passes through an orifice from a higher to a lower pressure,
as is the case with the throttling calorimeter, no external work has to
be done in overcoming a resistance. Hence, if there is no loss from
radiation, the quantity of heat in the steam will be exactly the same
after passing the orifice as before passing. If the higher steam
pressure is 160 pounds gauge and the lower pressure that of the
atmosphere, the total heat in a pound of dry steam at the former
pressure is 1195.9 B. t. u. and at the latter pressure 1150.4 B. t. u.,
a difference of 45.4 B. t. u. As this heat will still exist in the steam
at the lower pressure, since there is no external work done, its effect
must be to superheat the steam. Assuming the specific heat of
superheated steam to be 0.47, each pound passing through will be
superheated 45.4/0.47 = 96.6 degrees. If, however, the steam had
contained one per cent of moisture, it would have contained less heat
units per pound than if it were dry. Since the latent heat of steam at
160 pounds gauge pressure is 852.8 B. t. u., it follows that the one per
cent of moisture would have required 8.5 B. t. u. to evaporate it,
leaving only 45.4 - 8.5 = 36.9 B. t. u. available for superheating;
hence, the superheat would be 36.9/0.47 = 78.5 degrees, as against 96.6
degrees for dry steam. In a similar manner, the degree of superheat for
other percentages of moisture may be determined. The action of the
throttling calorimeter is based upon the foregoing facts, as shown
below.
Let H = total heat of one pound of steam at boiler pressure,
L = latent heat of steam at boiler pressure,
h = total heat of steam at reduced pressure after passing
orifice,
t_{1} = temperature of saturated steam at the reduced pressure,
t_{2} = temperature of steam after expanding through the orifice
in the disc,
0.47 = the specific heat of saturated steam at atmospheric pressure,
x = proportion by weight of moisture in steam.
The difference in B. t. u. in a pound of steam at the boiler pressure
and after passing the orifice is the heat available for evaporating the
moisture content and superheating the steam. Therefore,
H - h = xL + 0.47(t_{2} - t_{1})
H - h - 0.47(t_{2} - t_{1})
or x = --------------------------- (4)
L
Almost invariably the lower pressure is taken as that of the atmosphere.
Under such conditions, h = 1150.4 and t_{1} = 212 degrees. The formula
thus becomes:
H - 1150.4 - 0.47(t_{2} - 212)
x = ------------------------------ (5)
L
For practical work it is more convenient to dispense with the upper
thermometer in the calorimeter and to measure the pressure in the steam
main by an accurate steam pressure gauge.
A chart may be used for determining the value of x for approximate work
without the necessity for computation. Such a chart is shown in Fig. 15
and its use is as follows: Assume a gauge pressure of 180 pounds and a
thermometer reading of 295 degrees. The intersection of the vertical
line from the scale of temperatures as shown by the calorimeter
thermometer and the horizontal line from the scale of gauge pressures
will indicate directly the per cent of moisture in the steam as read
from the diagonal scale. In the present instance, this per cent is 1.0.
Sources of Error in the Apparatus--A slight error may arise from the
value, 0.47, used as the specific heat of superheated steam at
atmospheric pressure. This value, however is very nearly correct and any
error resulting from its use will be negligible.
There is ordinarily a larger source of error due to the fact that the
stem of the thermometer is not heated to its full length, to an initial
error in the thermometer and to radiation losses.
With an ordinary thermometer immersed in the well to the 100 degrees
mark, the error when registering 300 degrees would be about 3 degrees
and the true temperature be 303 degrees.[19]
The steam is evidently losing heat through radiation from the moment it
enters the sampling nipple. The heat available for evaporating moisture
and superheating steam after it has passed through the orifice into the
lower pressure will be diminished by just the amount lost through
radiation and the value of t_{2}, as shown by the calorimeter
thermometer, will, therefore, be lower than if there were no such loss.
The method of correcting for the thermometer and radiation error
recommended by the Power Test Committee of the American Society of
Mechanical Engineers is by referring the readings as found on the boiler
trial to a "normal" reading of the thermometer. This normal reading is
the reading of the lower calorimeter thermometer for dry saturated
steam, and should be determined by attaching the instrument to a
horizontal steam pipe in such a way that the sampling nozzle projects
upward to near the top of the pipe, there being no perforations in the
nozzle and the steam taken only through its open upper end. The test
should be made with the steam in a quiescent state and with the steam
pressure maintained as nearly as possible at the pressure observed in
the main trial, the calorimeter thermometer to be the same as was used
on the trial or one exactly similar.
With a normal reading thus obtained for a pressure approximately the
same as existed in the trial, the true percentage of moisture in the
steam, that is, with the proper correction made for radiation, may be
calculated as follows:
Let T denote the normal reading for the conditions existing in the
trial. The effect of radiation from the instrument as pointed out will
be to lower the temperature of the steam at the lower pressure. Let
x_{1} represent the proportion of water in the steam which will lower
its temperature an amount equal to the loss by radiation. Then,
H - h - 0.47(T - t_{1})
x_{1} = -----------------------
L
This amount of moisture, x_{1} was not in the steam originally but is
the result of condensation in the instrument through radiation. Hence,
the true amount of moisture in the steam represented by X is the
difference between the amount as determined in the trial and that
resulting from condensation, or,
X = x - x_{1}
H - h - 0.47(t_{2} - t_{1}) H - h - 0.47(T - t_{1})
= --------------------------- - -----------------------
L L
0.47(T - t_{2})
= --------------- (6)
L
As T and t_{2} are taken with the same thermometer under the same set of
conditions, any error in the reading of the thermometers will be
approximately the same for the temperatures T and t_{2} and the above
method therefore corrects for both the radiation and thermometer errors.
The theoretical readings for dry steam, where there are no losses due to
radiation, are obtainable from formula (5) by letting x = 0 and solving
for t_{2}. The difference between the theoretical reading and the normal
reading for no moisture will be the thermometer and radiation correction
to be applied in order that the correct reading of t_{2} may be
obtained.
For any calorimeter within the range of its ordinary use, such a
thermometer and radiation correction taken from one normal reading is
approximately correct for any conditions with the same or a duplicate
thermometer.
The percentage of moisture in the steam, corrected for thermometer error
and radiation and the correction to be applied to the particular
calorimeter used, would be determined as follows: Assume a gauge
pressure in the trial to be 180 pounds and the thermometer reading to be
295 degrees. A normal reading, taken in the manner described, gives a
value of T = 303 degrees; then, the percentage of moisture corrected for
thermometer error and radiation is,
0.47(303 - 295)
x = ----------------
845.0
= 0.45 per cent.
The theoretical reading for dry steam will be,
1197.7 - 1150.4 - 0.47(t_{2} - 212)
0 = ------------------------------------
845.0
t_{2} = 313 degrees.
The thermometer and radiation correction to be applied to the instrument
used, therefore over the ordinary range of pressure is
Correction = 313 - 303 = 10 degrees
The chart may be used in the determination of the correct reading of
moisture percentage and the permanent radiation correction for the
instrument used without computation as follows: Assume the same trial
pressure, feed temperature and normal reading as above. If the normal
reading is found to be 303 degrees, the correction for thermometer and
radiation will be the theoretical reading for dry steam as found from
the chart, less this normal reading, or 10 degrees correction. The
correct temperature for the trial in question is, therefore, 305
degrees. The moisture corresponding to this temperature and 180 pounds
gauge pressure will be found from the chart to be 0.45 per cent.
[Illustration: Fig. 16. Compact Throttling Calorimeter]
There are many forms of throttling calorimeter, all of which work upon
the same principle. The simplest one is probably that shown in Fig. 14.
An extremely convenient and compact design is shown in Fig. 16. This
calorimeter consists of two concentric metal cylinders screwed to a cap
containing a thermometer well. The steam pressure is measured by a gauge
placed in the supply pipe or other convenient location. Steam passes
through the orifice A and expands to atmospheric pressure, its
temperature at this pressure being measured by a thermometer placed in
the cup C. To prevent as far as possible radiation losses, the annular
space between the two cylinders is used as a jacket, steam being
supplied to this space through the hole B.
The limits of moisture within which the throttling calorimeter will work
are, at sea level, from 2.88 per cent at 50 pounds gauge pressure and
7.17 per cent moisture at 250 pounds pressure.
Separating Calorimeter--The separating calorimeter mechanically
separates the entrained water from the steam and collects it in a
reservoir, where its amount is either indicated by a gauge glass or is
drained off and weighed. Fig. 17 shows a calorimeter of this type. The
steam passes out of the calorimeter through an orifice of known size so
that its total amount can be calculated or it can be weighed. A gauge is
ordinarily provided with this type of calorimeter, which shows the
pressure in its inner chamber and the flow of steam for a given period,
this latter scale being graduated by trial.
The instrument, like a throttling calorimeter, should be well insulated
to prevent losses from radiation.
While theoretically the separating calorimeter is not limited in
capacity, it is well in cases where the percentage of moisture present
in the steam is known to be high, to attach a throttling calorimeter to
its exhaust. This, in effect, is the using of the separating calorimeter
as a small separator between the sampling nozzle and the throttling
instrument, and is necessary to insure the determination of the full
percentage of moisture in the steam. The sum of the percentages shown by
the two instruments is the moisture content of the steam.
The steam passing through a separating calorimeter may be calculated by
Napier's formula, the size of the orifice being known. There are
objections to such a calculation, however, in that it is difficult to
accurately determine the areas of such small orifices. Further, small
orifices have a tendency to become partly closed by sediment that may be
carried by the steam. The more accurate method of determining the amount
of steam passing through the instrument is as follows:
[Illustration: Fig. 17. Separating Calorimeter]
A hose should be attached to the separator outlet leading to a vessel of
water on a platform scale graduated to 1/100 of a pound. The steam
outlet should be connected to another vessel of water resting on a
second scale. In each case, the weight of each vessel and its contents
should be noted. When ready for an observation, the instrument should be
blown out thoroughly so that there will be no water within the
separator. The separator drip should then be closed and the steam hose
inserted into the vessel of water at the same instant. When the
separator has accumulated a sufficient quantity of water, the valve of
the instrument should be closed and the hose removed from the vessel of
water. The separator should be emptied into the vessel on its scale. The
final weight of each vessel and its contents are to be noted and the
differences between the final and original weights will represent the
weight of moisture collected by the separator and the weight of steam
from which the moisture has been taken. The proportion of moisture can
then be calculated from the following formula:
100 w
x = ----- (7)
W - w
Where x = per cent moisture in steam,
W = weight of steam condensed,
w = weight of moisture as taken out by the separating
calorimeter.
Sampling Nipple--The principle source of error in steam calorimeter
determinations is the failure to obtain an average sample of the steam
delivered by the boiler and it is extremely doubtful whether such a
sample is ever obtained. The two governing features in the obtaining of
such a sample are the type of sampling nozzle used and its location.
The American Society of Mechanical Engineers recommends a sampling
nozzle made of one-half inch iron pipe closed at the inner end and the
interior portion perforated with not less than twenty one-eighth inch
holes equally distributed from end to end and preferably drilled in
irregular or spiral rows, with the first hole not less than one-half
inch from the wall of the pipe. Many engineers object to the use of a
perforated sampling nipple because it ordinarily indicates a higher
percentage of moisture than is actually present in the steam. This is
due to the fact that if the perforations come close to the inner surface
of the pipe, the moisture, which in many instances clings to this
surface, will flow into the calorimeter and cause a large error. Where a
perforated nipple is used, in general it may be said that the
perforations should be at least one inch from the inner pipe surface.
A sampling nipple, open at the inner end and unperforated, undoubtedly
gives as accurate a measure as can be obtained of the moisture in the
steam passing that end. It would appear that a satisfactory method of
obtaining an average sample of the steam would result from the use of an
open end unperforated nipple passing through a stuffing box which would
allow the end to be placed at any point across the diameter of the steam
pipe.
Incidental to a test of a 15,000 K. W. steam engine turbine unit, Mr.
H. G. Stott and Mr. R. G. S. Pigott, finding no experimental data
bearing on the subject of low pressure steam quality determinations,
made a investigation of the subject and the sampling nozzle illustrated
in Fig. 18 was developed. In speaking of sampling nozzles in the
determination of the moisture content of low pressure steam, Mr. Pigott
says, "the ordinary standard perforated pipe sampler is absolutely
worthless in giving a true sample and it is vital that the sample be
abstracted from the main without changing its direction or velocity
until it is safely within the sample pipe and entirely isolated from the
rest of the steam."
[Illustration: Fig. 18. Stott and Pigott Sampling Nozzle]
It would appear that the nozzle illustrated is undoubtedly the best that
has been developed for use in the determination of the moisture content
of steam, not only in the case of low, but also in high pressure steam.
Location of Sampling Nozzle--The calorimeter should be located as near
as possible to the point from which the steam is taken and the sampling
nipple should be placed in a section of the main pipe near the boiler
and where there is no chance of moisture pocketing in the pipe. The
American Society of Mechanical Engineers recommends that a sampling
nipple, of which a description has been given, should be located in a
vertical main, rising from the boiler with its closed end extending
nearly across the pipe. Where non-return valves are used, or where there
are horizontal connections leading from the boiler to a vertical outlet,
water may collect at the lower end of the uptake pipe and be blown
upward in a spray which will not be carried away by the steam owing to a
lack of velocity. A sample taken from the lower part of this pipe will
show a greater amount of moisture than a true sample. With goose-neck
connections a small amount of water may collect on the bottom of the
pipe near the upper end where the inclination is such that the tendency
to flow backward is ordinarily counterbalanced by the flow of steam
forward over its surface; but when the velocity momentarily decreases
the water flows back to the lower end of the goose-neck and increases
the moisture at that point, making it an undesirable location for
sampling. In any case, it should be borne in mind that with low
velocities the tendency is for drops of entrained water to settle to the
bottom of the pipe, and to be temporarily broken up into spray whenever
an abrupt bend or other disturbance is met.
[Illustration: Fig. 19. Illustrating the Manner in which Erroneous
Calorimeter Readings may be Obtained due to Improper Location of Sampling
Nozzle
Case 1--Horizontal pipe. Water flows at bottom. If perforations
in nozzle are too near bottom of pipe, water piles against
nozzle, flows into calorimeter and gives false reading.
Case 2--If nozzle located too near junction of two horizontal
runs, as at a, condensation from vertical pipe which collects at
this point will be thrown against the nozzle by the velocity of
the steam, resulting in a false reading. Nozzle should be
located far enough above junction to be removed from water kept
in motion by the steam velocity, as at b. Case 3--Condensation
in bend will be held by velocity of the steam as shown. When
velocity is diminished during firing intervals and the like
moisture flows back against nozzle, a, and false reading is
obtained. A true reading will be obtained at b provided
condensation is not blown over on nozzle. Case 4--Where
non-return valve is placed before a bend, condensation will
collect on steam line side and water will be swept by steam
velocity against nozzle and false readings result.]
Fig. 19 indicates certain locations of sampling nozzles from which
erroneous results will be obtained, the reasons being obvious from a
study of the cuts.
Before taking any calorimeter reading, steam should be allowed to flow
through the instrument freely until it is thoroughly heated. The method
of using a throttling calorimeter is evident from the description of the
instrument given and the principle upon which it works.
[Illustration: Babcock & Wilcox Superheater]
SUPERHEATED STEAM
Superheated steam, as already stated, is steam the temperature of which
exceeds that of saturated steam at the same pressure. It is produced by
the addition of heat to saturated steam which has been removed from
contact with the water from which it was generated. The properties of
superheated steam approximate those of a perfect gas rather than of a
vapor. Saturated steam cannot be superheated when it is in contact with
water which is also heated, neither can superheated steam condense
without first being reduced to the temperature of saturated steam. Just
so long as its temperature is above that of saturated steam at a
corresponding pressure it is superheated, and before condensation can
take place that superheat must first be lost through radiation or some
other means. Table 24[20] gives such properties of superheated steam for
varying pressures as are necessary for use in ordinary engineering
practice.
Specific Heat of Superheated Steam--The specific heat of superheated
steam at atmospheric pressure and near saturation point was determined
by Regnault, in 1862, who gives it the value of 0.48. Regnault's value
was based on four series of experiments, all at atmospheric pressure and
with about the same temperature range, the maximum of which was 231.1
degrees centigrade. For fifty years after Regnault's determination, this
value was accepted and applied to higher pressures and temperatures as
well as to the range of his experiments. More recent investigations have
shown that the specific heat is not a constant and varies with both
pressure and the temperature. A number of experiments have been made by
various investigators and, up to the present, the most reliable appear
to be those of Knoblauch and Jacob. Messrs. Marks and Davis have used
the values as determined by Knoblauch and Jacob with slight
modifications. The first consists in a varying of the curves at low
pressures close to saturation because of thermodynamic evidence and in
view of Regnault's determination at atmospheric pressure. The second
modification is at high degrees of superheat to follow Holborn's and
Henning's curve, which is accepted as authentic.
For the sake of convenience, the mean specific heat of superheated steam
at various pressures and temperatures is given in tabulated form in
Table 25. These values have been calculated from Marks and Davis Steam
Tables by deducting from the total heat of one pound of steam at any
pressure for any degree of superheat the total heat of one pound of
saturated steam at the same pressure and dividing the difference by the
number of degrees of superheat and, therefore, represent the average
specific heat starting from that at saturation to the value at the
particular pressure and temperature.[21] Expressed as a formula this
calculation is represented by
H_{sup} - H_{sat}
Sp. Ht. = ----------------- (8)
S_{sup} - S_{sat}
Where H_{sup} = total heat of one pound of superheated steam at any
pressure and temperature,
H_{sat} = total heat of one pound of saturated steam at same
pressure,
S_{sup} = temperature of superheated steam taken,
S_{sat} = temperature of saturated steam corresponding to the
pressure taken.
TABLE 25
MEAN SPECIFIC HEAT OF SUPERHEATED STEAM
CALCULATED FROM MARKS AND DAVIS TABLES
_______________________________________________________________
|Gauge | |
|Pressure | Degree of Superheat |
| |_____________________________________________________|
| | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 |
|_________|_____|_____|_____|_____|_____|_____|_____|_____|_____|
| 50 | .518| .517| .514| .513| .511| .510| .508| .507| .505|
| 60 | .528| .525| .523| .521| .519| .517| .515| .513| .512|
| 70 | .536| .534| .531| .529| .527| .524| .522| .520| .518|
| 80 | .544| .542| .539| .535| .532| .530| .528| .526| .524|
| 90 | .553| .550| .546| .543| .539| .536| .534| .532| .529|
| 100 | .562| .557| .553| .549| .544| .542| .539| .536| .533|
| 110 | .570| .565| .560| .556| .552| .548| .545| .542| .539|
| 120 | .578| .573| .567| .561| .557| .554| .550| .546| .543|
| 130 | .586| .580| .574| .569| .564| .560| .555| .552| .548|
| 140 | .594| .588| .581| .575| .570| .565| .561| .557| .553|
| 150 | .604| .595| .587| .581| .576| .570| .566| .561| .557|
| 160 | .612| .603| .596| .589| .582| .576| .571| .566| .562|
| 170 | .620| .612| .603| .595| .588| .582| .576| .571| .566|
| 180 | .628| .618| .610| .601| .593| .587| .581| .575| .570|
| 190 | .638| .627| .617| .608| .599| .592| .585| .579| .574|
| 200 | .648| .635| .624| .614| .605| .597| .590| .584| .578|
| 210 | .656| .643| .631| .620| .611| .602| .595| .588| .583|
| 220 | .664| .650| .637| .626| .616| .607| .600| .592| .586|
| 230 | .672| .658| .644| .633| .622| .613| .605| .597| .591|
| 240 | .684| .668| .653| .640| .629| .619| .610| .602| .595|
| 250 | .692| .675| .659| .645| .633| .623| .614| .606| .599|
|_________|_____|_____|_____|_____|_____|_____|_____|_____|_____|
|Gauge | |
|Pressure | Degree of Superheat |
| |-----------------------------------------------------|
| | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 225 | 250 |
|---------+-----+-----+-----+-----+-----+-----+-----+-----+-----|
| 50 | .504| .503| .502| .501| .500| .500| .499| .497| .496|
| 60 | .511| .509| .508| .507| .506| .504| .504| .502| .500|
| 70 | .516| .515| .513| .512| .511| .510| .509| .506| .504|
| 80 | .522| .520| .518| .516| .515| .514| .513| .511| .508|
| 90 | .527| .525| .523| .521| .519| .518| .517| .514| .510|
| 100 | .531| .529| .527| .525| .523| .522| .521| .517| .513|
| 110 | .536| .534| .532| .529| .528| .526| .525| .520| .517|
| 120 | .540| .537| .535| .533| .531| .529| .528| .523| .519|
| 130 | .545| .542| .539| .537| .535| .533| .531| .527| .523|
| 140 | .550| .547| .544| .541| .539| .536| .534| .530| .526|
| 150 | .554| .550| .547| .544| .542| .539| .537| .533| .529|
| 160 | .558| .554| .551| .548| .545| .543| .541| .536| .531|
| 170 | .562| .558| .555| .552| .549| .546| .544| .538| .533|
| 180 | .566| .561| .558| .555| .552| .549| .546| .540| .536|
| 190 | .569| .565| .562| .558| .555| .552| .549| .543| .538|
| 200 | .574| .569| .566| .562| .558| .555| .552| .546| .541|
| 210 | .578| .573| .569| .565| .561| .558| .555| .549| .543|
| 220 | .581| .577| .572| .568| .564| .561| .558| .551| .545|
| 230 | .585| .580| .575| .572| .567| .564| .561| .554| .548|
| 240 | .589| .584| .579| .575| .571| .567| .564| .556| .550|
| 250 | .593| .587| .582| .577| .574| .570| .567| .559| .553|
|_________|_____|_____|_____|_____|_____|_____|_____|_____|_____|
Factor of Evaporation with Superheated Steam--When superheat is present
in the steam during a boiler trial, where superheated steam tables are
available, the formula for determining the factor of evaporation is that
already given, (2),[22] namely,
H - h
Factor of evaporation = -----
L
Here H = total heat in one pound of superheated steam from the table,
h and L having the same values as in (2).
Where no such tables are available but the specific heat of superheat is
known, the formula becomes:
H - h + Sp. Ht.(T - t)
Factor of evaporation = ----------------------
L
Where H = total heat in one pound of saturated steam at pressure
existing in trial,
h = sensible heat above 32 degrees in one pound of water at the
temperature entering the boiler,
T = temperature of superheated steam as determined in the trial,
t = temperature of saturated steam corresponding to the boiler
pressure,
Sp. Ht. = mean specific heat of superheated steam at the pressure and
temperature as found in the trial,
L = latent heat of one pound of saturated steam at atmospheric
pressure.
Advantages of the Use of Superheated Steam--In considering the saving
possible by the use of superheated steam, it is too often assumed that
there is only a saving in the prime movers, a saving which is at least
partially offset by an increase in the fuel consumption of the boilers
generating steam. This misconception is due to the fact that the fuel
consumption of the boiler is only considered in connection with a
definite weight of steam. It is true that where such a definite weight
is to be superheated, an added amount of fuel must be burned. With a
properly designed superheater where the combined efficiency of the
boiler and superheater will be at least as high as of a boiler alone,
the approximate increase in coal consumption for producing a given
weight of steam will be as follows:
_Superheat_ _Added Fuel_
_Degrees_ _Per Cent_
25 1.59
50 3.07
75 4.38
100 5.69
150 8.19
200 10.58
These figures represent the added fuel necessary for superheating a
definite weight of steam to the number of degrees as given. The standard
basis, however, of boiler evaporation is one of heat units and,
considered from such a standpoint, again providing the efficiency of the
boiler and superheater is as high, as of a boiler alone, there is no
additional fuel required to generate steam containing a definite number
of heat units whether such units be due to superheat or saturation. That
is, if 6 per cent more fuel is required to generate and superheat to 100
degrees, a definite weight of steam, over what would be required to
produce the same weight of saturated steam, that steam when superheated,
will contain 6 per cent more heat units above the fuel water temperature
than if saturated. This holds true if the efficiency of the boiler and
superheater combined is the same as of the boiler alone. As a matter of
fact, the efficiency of a boiler and superheater, where the latter is
properly designed and located, will be slightly higher for the same set
of furnace conditions than would the efficiency of a boiler in which no
superheater were installed. A superheater, properly placed within the
boiler setting in such way that products of combustion for generating
saturated steam are utilized as well for superheating that steam, will
not in any way alter furnace conditions. With a given set of such
furnace conditions for a given amount of coal burned, the fact that
additional surface, whether as boiler heating or superheating surface,
is placed in such a manner that the gases must sweep over it, will tend
to lower the temperature of the exit gases. It is such a lowering of
exit gas temperatures that is the ultimate indication of added
efficiency. Though the amount of this added efficiency is difficult to
determine by test, that there is an increase is unquestionable.
Where a properly designed superheater is installed in a boiler the
heating surface of the boiler proper, in the generation of a definite
number of heat units, is relieved of a portion of the work which would
be required were these heat units delivered in saturated steam. Such a
superheater needs practically no attention, is not subject to a large
upkeep cost or depreciation, and performs its function without in any
way interfering with the operation of the boiler. Its use, therefore
from the standpoint of the boiler room, results in a saving in wear and
tear due to the lower ratings at which the boiler may be run, or its use
will lead to the possibility of obtaining the same number of boiler
horse power from a smaller number of boilers, with the boiler heating
surface doing exactly the same amount of work as if the superheaters
were not installed. The saving due to the added boiler efficiency that
will be obtained is obvious.
Following the course of the steam in a plant, the next advantage of the
use of superheated steam is the absence of water in the steam pipes. The
thermal conductivity of superheated steam, that is, its power to give up
its heat to surrounding bodies, is much lower than that of saturated
steam and its heat, therefore, will not be transmitted so rapidly to the
walls of the pipes as when saturated steam is flowing through the pipes.
The loss of heat radiated from a steam pipe, assuming no loss in
pressure, represents the equivalent condensation when the pipe is
carrying saturated steam. In well-covered steam mains, the heat lost by
radiation when carrying superheated steam is accompanied only by a
reduction of the superheat which, if it be sufficiently high at the
boiler, will enable a considerable amount of heat to be radiated and
still deliver dry or superheated steam to the prime movers.
It is in the prime movers that the advantages of the use of superheated
steam are most clearly seen.
In an engine, steam is admitted into a space that has been cooled by the
steam exhausted during the previous stroke. The heat necessary to warm
the cylinder walls from the temperature of the exhaust to that of the
entering steam can be supplied only by the entering steam. If this steam
be saturated, such an adding of heat to the walls at the expense of the
heat of the entering steam results in the condensation of a portion.
This initial condensation is seldom less than from 20 to 30 per cent of
the total weight of steam entering the cylinder. It is obvious that if
the steam entering be superheated, it must be reduced to the temperature
of saturated steam at the corresponding pressure before any condensation
can take place. If the steam be superheated sufficiently to allow a
reduction in temperature equivalent to the quantity of heat that must be
imparted to the cylinder walls and still remain superheated, it is clear
that initial condensation is avoided. For example: assume one pound of
saturated steam at 200 pounds gauge pressure to enter a cylinder which
has been cooled by the exhaust. Assume the initial condensation to be 20
per cent. The latent heat of the steam is given up in condensation;
hence, .20 × 838 = 167.6 B. t. u. are given up by the steam. If one
pound of superheated steam enters the same cylinder, it would have to be
superheated to a point where its total heat is 1199 + 168 = 1367
B. t. u. or, at 200 pounds gauge pressure, superheated approximately 325
degrees if the heat given up to the cylinder walls were the same as for
the saturated steam. As superheated steam conducts heat less rapidly
than saturated steam, the amount of heat imparted will be less than for
the saturated steam and consequently the amount of superheat required to
prevent condensation will be less than the above figure. This, of
course, is the extreme case of a simple engine with the range of
temperature change a maximum. As cylinders are added, the range in each
is decreased and the condensation is proportionate.
The true economy of the use of superheated steam is best shown in a
comparison of the "heat consumption" of an engine. This is the number of
heat units required in developing one indicated horse power and the
measure of the relative performance of two engines is based on a
comparison of their heat consumption as the measure of a boiler is based
on its evaporation from and at 212 degrees. The water consumption of an
engine in pounds per indicated horse power is in no sense a true
indication of its efficiency. The initial pressures and corresponding
temperatures may differ widely and thus make a difference in the
temperature of the exhaust and hence in the temperature of the condensed
steam returned to the boiler. For example: suppose a certain weight of
steam at 150 pounds absolute pressure and 358 degrees be expanded to
atmospheric pressure, the temperature then being 212 degrees. If the
same weight of steam be expanded from an initial pressure of 125 pounds
absolute and 344 degrees, to enable it to do the same amount of work,
that is, to give up the same amount of heat, expansion then must be
carried to a point below atmospheric pressure to, say, 13 pounds
absolute, the final temperature of the steam then being 206 degrees. In
actual practice, it has been observed that the water consumption of a
compound piston engine running on 26-inch vacuum and returning the
condensed steam at 140 degrees was approximately the same as when
running on 28-inch vacuum and returning water at 90 degrees. With an
equal water consumption for the two sets of conditions, the economy in
the former case would be greater than in the latter, since it would be
necessary to add less heat to the water returned to the boiler to raise
it to the steam temperature.
The lower the heat consumption of an engine per indicated horse power,
the higher its economy and the less the number of heat units must be
imparted to the steam generated. This in turn leads to the lowering of
the amount of fuel that must be burned per indicated horse power.
With the saving in fuel by the reduction of heat consumption of an
engine indicated, it remains to be shown the effect of the use of
superheated steam on such heat consumption. As already explained, the
use of superheated steam reduces condensation not only in the mains but
especially in the steam cylinder, leaving a greater quantity of steam
available to do the work. Furthermore, a portion of the saturated steam
introduced into a cylinder will condense during adiabatic expansion,
this condensation increasing as expansion progresses. Since superheated
steam cannot condense until it becomes saturated, not only is initial
condensation prevented by its use but also such condensation as would
occur during expansion. When superheated sufficiently, steam delivered
by the exhaust will still be dry. In the avoidance of such condensation,
there is a direct saving in the heat consumption of an engine, the heat
given up being utilized in the developing of power and not in changing
the condition of the working fluid. That is, while the number of heat
units lost in overcoming condensation effects would be the same in
either case, when saturated steam is condensed the water of condensation
has no power to do work while the superheated steam, even after it has
lost a like number of heat units, still has the power of expansion. The
saving through the use of superheated steam in the heat consumption of
an engine decreases demands on the boiler and hence the fuel consumption
per unit of power.
Superheated Steam for Steam Turbines--Experience in using superheated
steam in connection with steam turbines has shown that it leads to
economy and that it undoubtedly pays to use superheated steam in place
of saturated steam. This is so well established that it is standard
practice to use superheated steam in connection with steam turbines.
Aside from the economy secured through using superheated steam, there is
an important advantage arising through the fact that it materially
reduces the erosion of the turbine blades by the action of water that
would be carried by saturated steam. In using saturated steam in a steam
turbine or piston engine, the work done on expanding the steam causes
condensation of a portion of the steam, so that even were the steam dry
on entering the turbine, it would contain water on leaving the turbine.
By superheating the steam the water that exists in the low pressure
stages of the turbine may be reduced to an amount that will not cause
trouble.
Again, if saturated steam contains moisture, the effect of this moisture
on the economy of a steam turbine is to reduce the economy to a greater
extent than the proportion by weight of water, one per cent of water
causing approximately a falling off of 2 per cent in the economy.
The water rate of a large economical steam turbine with superheated
steam is reduced about one per cent, for every 12 degrees of superheat
up to 200 degrees Fahrenheit of superheat. To superheat one pound of
steam 12 degrees requires about 7 B. t. u. and if 1050 B. t. u. are
required at the boiler to evaporate one pound of the saturated steam
from the temperature of the feed water, the heat required for the
superheated steam would be 1057 degrees. One per cent of saving,
therefore, in the water consumption would correspond to a net saving of
about one-third of one per cent in the coal consumption. On this basis
100 degrees of superheat with an economical steam turbine would result
in somewhat over 3 per cent of saving in the coal for equal boiler
efficiencies. As a boiler with a properly designed superheater placed
within the setting is more economical for a given capacity than a boiler
without a superheater, the minimum gain in the coal consumption would
be, say, 4 or 5 per cent as compared to a plant with the same boilers
without superheaters.
The above estimates are on the basis of a thoroughly dry saturated steam
or steam just at the point of being superheated or containing a few
degrees of superheat. If the saturated steam is moist, the saving due to
superheat is more and ordinarily the gain in economy due to superheated
steam, for equal boiler efficiencies, as compared with commercially dry
steam is, say, 5 per cent for each 100 degrees of superheat. Aside from
this gain, as already stated, superheated steam prevents erosion of the
turbine buckets that would be caused by water in the steam, and for the
reasons enumerated it is standard practice to use superheated steam for
turbine work. The less economical the steam motor, the more the gain due
to superheated steam, and where there are a number of auxiliaries that
are run with superheated steam, the percentage of gain will be greater
than the figures given above, which are the minimum and are for the most
economical type of large steam turbines.
An example from actual practice will perhaps best illustrate and
emphasize the foregoing facts. In October 1909, a series of comparable
tests were conducted by The Babcock & Wilcox Co. on the steam yacht
"Idalia" to determine the steam consumption both with saturated and
superheated steam of the main engine on that yacht, including as well
the feed pump, circulating pump and air pump. These tests are more
representative than are most tests of like character in that the saving
in the steam consumption of the auxiliaries, which were much more
wasteful than the main engine, formed an important factor. A résumé of
these tests was published in the Journal of the Society of Naval
Engineers, November 1909.
The main engines of the "Idalia" are four cylinder, triple expansion,
11-1/2 × 19 inches by 22-11/16 × 18 inches stroke. Steam is supplied by
a Babcock & Wilcox marine boiler having 2500 square feet of boiler
heating surface, 340 square feet of superheating surface and 65 square
feet of grate surface.
The auxiliaries consist of a feed pump 6 × 4 × 6 inches, an independent
air pump 6 × 12 × 8 inches, and a centrifugal pump driven by a
reciprocating engine 5-7/16 × 5 inches. Under ordinary operating
conditions the superheat existing is about 100 degrees Fahrenheit.
Tests were made with various degrees of superheat, the amount being
varied by by-passing the gases and in the tests with the lower amounts
of superheat by passing a portion of the steam from the boiler to the
steam main without passing it through the superheater. Steam temperature
readings were taken at the engine throttle. In the tests with saturated
steam, the superheater was completely cut out of the system. Careful
calorimeter measurements were taken, showing that the saturated steam
delivered to the superheater was dry.
The weight of steam used was determined from the weight of the condensed
steam discharge from the surface condenser, the water being pumped from
the hot well into a tank mounted on platform scales. The same
indicators, thermometers and gauges were used in all the tests, so that
the results are directly comparable. The indicators used were of the
outside spring type so that there was no effect of the temperature of
the steam. All tests were of sufficient duration to show a uniformity of
results by hours. A summary of the results secured is given in Table 26,
which shows the water rate per indicated horse power and the heat
consumption. The latter figures are computed on the basis of the heat
imparted to the steam above the actual temperature of the feed water
and, as stated, these are the results that are directly comparable.
TABLE 26
RESULTS OF "IDALIA" TESTS
_______________________________________________________________________
| | | | | | |
|Date 1909 |Oct. 11|Oct. 14|Oct. 14|Oct. 12|Oct. 13|
|_______________________________|_______|_______|_______|_______|_______|
|Degrees of superheat Fahrenheit| 0 | 57 | 88 | 96 | 105 |
|Pressures, pounds per} Throttle| 190 | 196 | 201 | 198 | 203 |
|square inch above } First | | | | | |
|Atmospheric Pressure } Receiver| 68.4 | 66.0 | 64.3 | 61.9 | 63.0 |
| } Second | | | | | |
| } Receiver| 9.7 | 9.2 | 8.7 | 7.8 | 8.4 |
|Vacuum, inches | 25.5 | 25.9 | 25.9 | 25.4 | 25.2 |
|Temperature, Degrees Fahrenheit| | | | | |
| } Feed | 201 | 206 | 205 | 202 | 200 |
| } Hot Well | 116 | 109.5 | 115 | 111.5 | 111 |
| | | | | | |
|Revolutions per minute | | | | | |
| {Air Pump | 57 | 56 | 53 | 54 | 45 |
| {Circulating Pump| 196 | 198 | 196 | 198 | 197 |
| {Main Engine | 194.3 | 191.5 | 195.1 | 191.5 | 193.1 |
|Indicated Horse Power, | | | | | |
| Main Engine | 512.3 | 495.2 | 521.1 | 498.3 | 502.2 |
|Water per hour, total pounds |9397 |8430 |8234 |7902 |7790 |
|Water per indicated | | | | | |
| Horse Power, pounds | 18.3 | 17.0 | 15.8 | 15.8 | 15.5 |
|B. t. u. per minute per | | | | | |
| indicated Horse Power | 314 | 300 | 284 | 286 | 283 |
|Per cent Saving of Steam | ... | 7.1 | 13.7 | 13.7 | 15.3 |
|Percent Saving of Fuel | | | | | |
| (computed) | ... | 4.4 | 9.5 | 8.9 | 9.9 |
|_______________________________|_______|_______|_______|_______|_______|
The table shows that the saving in steam consumption with 105 degrees of
superheat was 15.3 per cent and in heat consumption about 10 per cent.
This may be safely stated to be a conservative representation of the
saving that may be accomplished by the use of superheated steam in a
plant as a whole, where superheated steam is furnished not only to the
main engine but also to the auxiliaries. The figures may be taken as
conservative for the reason that in addition to the saving as shown in
the table, there would be in an ordinary plant a saving much greater
than is generally realized in the drips, where the loss with saturated
steam is greatly in excess of that with superheated steam.
The most conclusive and most practical evidence that a saving is
possible through the use of superheated steam is in the fact that in the
largest and most economical plants it is used almost without exception.
Regardless of any such evidence, however, there is a deep rooted
conviction in the minds of certain engineers that the use of superheated
steam will involve operating difficulties which, with additional first
cost, will more than offset any fuel saving. There are, of course,
conditions under which the installation of superheaters would in no way
be advisable. With a poorly designed superheater, no gain would result.
In general, it may be stated that in a new plant, properly designed,
with a boiler and superheater which will have an efficiency at least as
high as a boiler without a superheater, a gain is certain.
Such a gain is dependent upon the class of engine and the power plant
equipment in general. In determining the advisability of making a
superheater installation, all of the factors entering into each
individual case should be considered and balanced, with a view to
determining the saving in relation to cost, maintenance, depreciation
etc.
In highly economical plants, where the water consumption for an
indicated horse power is low, the gain will be less than would result
from the use of superheated steam in less economical plants where the
water consumption is higher. It is impossible to make an accurate
statement as to the saving possible but, broadly, it may vary from 3 to
5 per cent for 100 degrees of superheat in the large and economical
plants using turbines or steam engines, in which there is a large ratio
of expansion, to from 10 to 25 per cent for 100 degrees of superheat for
the less economical steam motors.
Though a properly designed superheater will tend to raise rather than to
decrease the boiler efficiency, it does not follow that all superheaters
are efficient, for if the gases in passing over the superheater do not
follow the path they would ordinarily take in passing over the boiler
heating surface, a loss may result. This is noticeably true where part of
the gases are passed over the superheater and are allowed to pass over
only a part or in some cases none of the boiler heating surface.
With moderate degrees of superheat, from 100 to 200 degrees, where the
piping is properly installed, there will be no greater operating
difficulties than with saturated steam. Engine and turbine builders
guarantee satisfactory operation with superheated steam. With high
degrees of superheat, say, over 250 degrees, apparatus of a special
nature must be used and it is questionable whether the additional care
and liability to operating difficulties will offset any fuel saving
accomplished. It is well established, however, that the operating
difficulties, with the degrees of superheat to which this article is
limited, have been entirely overcome.
The use of cast-iron fittings with superheated steam has been widely
discussed. It is an undoubted fact that while in some instances
superheated steam has caused deterioration of such fittings, in others
cast-iron fittings have been used with 150 degrees of superheat without
the least difficulty. The quality of the cast iron used in such fittings
has doubtless a large bearing on the life of such fittings for this
service. The difficulties that have been encountered are an increase in
the size of the fittings and eventually a deterioration great enough to
lead to serious breakage, the development of cracks, and when flanges
are drawn up too tightly, the breaking of a flange from the body of the
fitting. The latter difficulty is undoubtedly due, in certain instances,
to the form of flange in which the strain of the connecting bolts tended
to distort the metal.
The Babcock & Wilcox Co. have used steel castings in superheated steam
work over a long period and experience has shown that this metal is
suitable for the service. There seems to be a general tendency toward
the use of steel fittings. In European practice, until recently, cast
iron was used with apparently satisfactory results. The claim of
European engineers was to the effect that their cast iron was of better
quality than that found in this country and thus explained the results
secured. Recently, however, certain difficulties have been encountered
with such fittings and European engineers are leaning toward the use of
steel for this work.
The degree of superheat produced by a superheater placed within the
boiler setting will vary according to the class of fuel used, the form
of furnace, the condition of the fire and the rate at which the boiler
is being operated. This is necessarily true of any superheater swept by
the main body of the products of combustion and is a fact that should be
appreciated by the prospective user of superheated steam. With a
properly designed superheater, however, such fluctuations would not be
excessive, provided the boilers are properly operated. As a matter of
fact the point to be guarded against in the use of superheated steam is
that a maximum should not be exceeded. While, as stated, there may be a
considerable fluctuation in the temperature of the steam as delivered
from individual superheaters, where there are a number of boilers on a
line the temperature of the combined flow of steam in the main will be
found to be practically a constant, resulting from the offsetting of
various furnace conditions of one boiler by another.
[Illustration: 8400 Horse-power Installation of Babcock & Wilcox Boilers
and Superheaters at the Butler Street Plant of the Georgia Railway and
Power Co., Atlanta, Ga. This Company Operates a Total of 15,200 Horse
Power of Babcock & Wilcox Boilers]
PROPERTIES OF AIR
Pure air is a mechanical mixture of oxygen and nitrogen. While different
authorities give slightly varying values for the proportion of oxygen
and nitrogen contained, the generally accepted values are:
By volume, oxygen 20.91 per cent, nitrogen 79.09 per cent.
By weight, oxygen 23.15 per cent, nitrogen 76.85 per cent.
Air in nature always contains other constituents in varying amounts,
such as dust, carbon dioxide, ozone and water vapor.
Being perfectly elastic, the density or weight per unit of volume
decreases in geometric progression with the altitude. This fact has a
direct bearing in the proportioning of furnaces, flues and stacks at
high altitudes, as will be shown later in the discussion of these
subjects. The atmospheric pressures corresponding to various altitudes
are given in Table 12.
The weight and volume of air depend upon the pressure and the
temperature, as expressed by the formula:
Pv = 53.33 T (9)
Where P = the absolute pressure in pounds per square foot,
v = the volume in cubic feet of one pound of air,
T = the absolute temperature of the air in degrees Fahrenheit,
53.33 = a constant for air derived from the ratio of pressure, volume
and temperature of a perfect gas.
The weight of one cubic foot of air will obviously be the reciprocal of
its volume, that is, 1/v pounds.
TABLE 27
VOLUME AND WEIGHT OF AIR
AT ATMOSPHERIC PRESSURE
AT VARIOUS TEMPERATURES
_______________________________________
| | | |
| | Volume | |
| Temperature | One Pound | Weight One |
| Degrees | in | Cubic Foot |
| Fahrenheit | Cubic Feet | in Pounds |
|_____________|____________|____________|
| | | |
| 32 | 12.390 | .080710 |
| 50 | 12.843 | .077863 |
| 55 | 12.969 | .077107 |
| 60 | 13.095 | .076365 |
| 65 | 13.221 | .075637 |
| 70 | 13.347 | .074923 |
| 75 | 13.473 | .074223 |
| 80 | 13.599 | .073535 |
| 85 | 13.725 | .072860 |
| 90 | 13.851 | .072197 |
| 95 | 13.977 | .071546 |
| 100 | 14.103 | .070907 |
| 110 | 14.355 | .069662 |
| 120 | 14.607 | .068460 |
| 130 | 14.859 | .067299 |
| 140 | 15.111 | .066177 |
| 150 | 15.363 | .065092 |
| 160 | 15.615 | .064041 |
| 170 | 15.867 | .063024 |
| 180 | 16.119 | .062039 |
| 190 | 16.371 | .061084 |
| 200 | 16.623 | .060158 |
| 210 | 16.875 | .059259 |
| 212 | 16.925 | .059084 |
| 220 | 17.127 | .058388 |
| 230 | 17.379 | .057541 |
| 240 | 17.631 | .056718 |
| 250 | 17.883 | .055919 |
| 260 | 18.135 | .055142 |
| 270 | 18.387 | .054386 |
| 280 | 18.639 | .053651 |
| 290 | 18.891 | .052935 |
| 300 | 19.143 | .052238 |
| 320 | 19.647 | .050898 |
| 340 | 20.151 | .049625 |
| 360 | 20.655 | .048414 |
| 380 | 21.159 | .047261 |
| 400 | 21.663 | .046162 |
| 425 | 22.293 | .044857 |
| 450 | 22.923 | .043624 |
| 475 | 23.554 | .042456 |
| 500 | 24.184 | .041350 |
| 525 | 24.814 | .040300 |
| 550 | 25.444 | .039302 |
| 575 | 26.074 | .038352 |
| 600 | 26.704 | .037448 |
| 650 | 27.964 | .035760 |
| 700 | 29.224 | .034219 |
| 750 | 30.484 | .032804 |
| 800 | 31.744 | .031502 |
| 850 | 33.004 | .030299 |
|_____________|____________|____________|
Example: Required the volume of air in cubic feet under 60.3 pounds
gauge pressure per square inch at 115 degrees Fahrenheit.
P = 144 (14.7 + 60.3) = 10,800.
T = 115 + 460 = 575 degrees.
53.33 × 575
Hence v = ----------- = 2.84 cubic feet, and
10,800
1 1
Weight per cubic foot = - = ---- = 0.352 pounds.
v 2.84
Table 27 gives the weights and volumes of air under atmospheric pressure
at varying temperatures.
Formula (9) holds good for other gases with the change in the value of
the constant as follows:
For oxygen 48.24, nitrogen 54.97, hydrogen 765.71.
The specific heat of air at constant pressure varies with its
temperature. A number of determinations of this value have been made and
certain of those ordinarily accepted as most authentic are given in
Table 28.
TABLE 28
SPECIFIC HEAT OF AIR
AT CONSTANT PRESSURE AND VARIOUS TEMPERATURES
______________________________________________________________
| | | |
| Temperature Range | | |
|_________________________|_______________|____________________|
| | | | |
| Degrees | Degrees | Specific Heat | Authority |
| Centigrade | Fahrenheit | | |
|____________|____________|_______________|____________________|
| | | | |
| -30- 10 | -22- 50 | 0.2377 | Regnault |
| 0-100 | 32- 212 | 0.2374 | Regnault |
| 0-200 | 32- 392 | 0.2375 | Regnault |
| 20-440 | 68- 824 | 0.2366 | Holborn and Curtis |
| 20-630 | 68-1166 | 0.2429 | Holborn and Curtis |
| 20-800 | 68-1472 | 0.2430 | Holborn and Curtis |
| 0-200 | 32- 392 | 0.2389 | Wiedemann |
|____________|____________|_______________|____________________|
This value is of particular importance in waste heat work and it is
regrettable that there is such a variation in the different experiments.
Mallard and Le Chatelier determined values considerably higher than any
given in Table 28. All things considered in view of the discrepancy of
the values given, there appears to be as much ground for the use of a
constant value for the specific heat of air at any temperature as for a
variable value. Where this value is used throughout this book, it has
been taken as 0.24.
Air may carry a considerable quantity of water vapor, which is
frequently 3 per cent of the total weight. This fact is of importance in
problems relating to heating drying and the compressing of air. Table 29
gives the amount of vapor required to saturate air at different
temperatures, its weight, expansive force, etc., and contains sufficient
information for solving practically all problems of this sort that may
arise.
TABLE 29
WEIGHTS OF AIR, VAPOR OF WATER, AND SATURATED MIXTURES OF AIR AND VAPOR
AT DIFFERENT TEMPERATURES,
UNDER THE ORDINARY ATMOSPHERIC PRESSURE OF 29.921 INCHES OF MERCURY
Column Headings:
1: Temperature Degrees Fahrenheit
2: Volume of Dry Air at Different Temperatures, the Volume at 32 Degrees
being 1.000
3: Weight of Cubic Foot of Dry Air at the Different Temperatures Pounds
4: Elastic Force of Vapor in Inches of Mercury (Regnault)
5: Elastic Force of the Air in the Mixture of Air and Vapor in Inches of
Mercury
6: Weight of the Air in Pounds
7: Weight of the Vapor in Pounds
8: Total Weight of Mixture in Pounds
9: Weight of Vapor Mixed with One Pound of Air, in Pounds
10: Weight of Dry Air Mixed with One Pound of Vapor, in Pounds
11: Cubic Feet of Vapor from One Pound of Water at its own Pressure in
Column 4
____________________________________________________________________________
| | | | | | |
| | | | | Mixtures of Air Saturated | |
| | | | | with Vapor | |
|___|_____|_____|______|______________________________________________|______|
| | | | | |Weight of Cubic Foot | | | |
| | | | | | of the Mixture of | | | |
| | | | | | Air and Vapor | | | |
| | | | | |_____________________| | | |
| | | | | | | | | | | |
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
|___|_____|_____|______|______|_____|_______|_______|________|________|______|
| | | | | | | | | | | |
| 0| .935|.0864| .044|29.877|.0863|.000079|.086379| .00092|1092.4 | |
| 12| .960|.0842| .074|29.849|.0840|.000130|.084130| .00155| 646.1 | |
| 22| .980|.0824| .118|29.803|.0821|.000202|.082302| .00245| 406.4 | |
| 32|1.000|.0807| .181|29.740|.0802|.000304|.080504| .00379| 263.81 |3289 |
| 42|1.020|.0791| .267|29.654|.0784|.000440|.078840| .00561| 178.18 |2252 |
| | | | | | | | | | | |
| 52|1.041|.0776| .388|29.533|.0766|.000627|.077227| .00810| 122.17 |1595 |
| 62|1.061|.0761| .556|29.365|.0747|.000881|.075581| .01179| 84.79 |1135 |
| 72|1.082|.0747| .785|29.136|.0727|.001221|.073921| .01680| 59.54 | 819 |
| 82|1.102|.0733| 1.092|28.829|.0706|.001667|.072267| .02361| 42.35 | 600 |
| 92|1.122|.0720| 1.501|28.420|.0684|.002250|.070717| .03289| 30.40 | 444 |
| | | | | | | | | | | |
|102|1.143|.0707| 2.036|27.885|.0659|.002997|.068897| .04547| 21.98 | 334 |
|112|1.163|.0694| 2.731|27.190|.0631|.003946|.067046| .06253| 15.99 | 253 |
|122|1.184|.0682| 3.621|26.300|.0599|.005142|.065042| .08584| 11.65 | 194 |
|132|1.204|.0671| 4.752|25.169|.0564|.006639|.063039| .11771| 8.49 | 151 |
|142|1.224|.0660| 6.165|23.756|.0524|.008473|.060873| .16170| 6.18 | 118 |
| | | | | | | | | | | |
|152|1.245|.0649| 7.930|21.991|.0477|.010716|.058416| .22465| 4.45 | 93.3|
|162|1.265|.0638|10.099|19.822|.0423|.013415|.055715| .31713| 3.15 | 74.5|
|172|1.285|.0628|12.758|17.163|.0360|.016682|.052682| .46338| 2.16 | 59.2|
|182|1.306|.0618|15.960|13.961|.0288|.020536|.049336| .71300| 1.402| 48.6|
|192|1.326|.0609|19.828|10.093|.0205|.025142|.045642| 1.22643| .815| 39.8|
| | | | | | | | | | | |
|202|1.347|.0600|24.450| 5.471|.0109|.030545|.041445| 2.80230| .357| 32.7|
|212|1.367|.0591|29.921| 0.000|.0000|.036820|.036820|Infinite| .000| 27.1|
|___|_____|_____|______|______|_____|_______|_______|________|________|______|
Column 5 = barometer pressure of 29.921, minus the proportion of this
due to vapor pressure from column 4.
COMBUSTION
Combustion may be defined as the rapid chemical combination of oxygen
with carbon, hydrogen and sulphur, accompanied by the diffusion of heat
and light. That portion of the substance thus combined with the oxygen
is called combustible. As used in steam engineering practice, however,
the term combustible is applied to that portion of the fuel which is dry
and free from ash, thus including both oxygen and nitrogen which may be
constituents of the fuel, though not in the true sense of the term
combustible.
Combustion is perfect when the combustible unites with the greatest
possible amount of oxygen, as when one atom of carbon unites with two
atoms of oxygen to form carbon dioxide, CO_{2}. The combustion is
imperfect when complete oxidation of the combustible does not occur, or
where the combustible does not unite with the maximum amount of oxygen,
as when one atom of carbon unites with one atom of oxygen to form carbon
monoxide, CO, which may be further burned to carbon dioxide.
Kindling Point--Before a combustible can unite with oxygen and
combustion takes place, its temperature must first be raised to the
ignition or kindling point, and a sufficient time must be allowed for
the completion of the combustion before the temperature of the gases is
lowered below that point. Table 30, by Stromeyer, gives the approximate
kindling temperatures of different fuels.
TABLE 30
KINDLING TEMPERATURE OF VARIOUS FUELS
____________________________________
| | |
| | Degrees |
| | Fahrenheit |
|_________________|__________________|
| | |
| Lignite Dust | 300 |
| Dried Peat | 435 |
| Sulphur | 470 |
| Anthracite Dust | 570 |
| Coal | 600 |
| Coke | Red Heat |
| Anthracite | Red Heat, 750 |
| Carbon Monoxide | Red Heat, 1211 |
| Hydrogen | 1030 or 1290 |
|_________________|__________________|
Combustibles--The principal combustibles in coal and other fuels are
carbon, hydrogen and sulphur, occurring in varying proportions and
combinations.
Carbon is by far the most abundant as is indicated in the chapters on
fuels.
Hydrogen in a free state occurs in small quantities in some fuels, but
is usually found in combination with carbon, in the form of
hydrocarbons. The density of hydrogen is 0.0696 (Air = 1) and its weight
per cubic foot, at 32 degrees Fahrenheit and under atmospheric pressure,
is 0.005621 pounds.
Sulphur is found in most coals and some oils. It is usually present in
combined form, either as sulphide of iron or sulphate of lime; in the
latter form it has no heat value. Its presence in fuel is objectionable
because of its tendency to aid in the formation of clinkers, and the
gases from its combustion, when in the presence of moisture, may cause
corrosion.
Nitrogen is drawn into the furnace with the air. Its density is 0.9673
(Air = 1); its weight, at 32 degrees Fahrenheit and under atmospheric
pressure, is 0.07829 pounds per cubic foot; each pound of air at
atmospheric pressure contains 0.7685 pounds of nitrogen, and one pound
of nitrogen is contained in 1.301 pounds of air.
Nitrogen performs no useful office in combustion and passes through the
furnace without change. It dilutes the air, absorbs heat, reduces the
temperature of the products of combustion, and is the chief source of
heat losses in furnaces.
Calorific Value--Each combustible element of gas will combine with
oxygen in certain definite proportions and will generate a definite
amount of heat, measured in B. t. u. This definite amount of heat per
pound liberated by perfect combustion is termed the calorific value of
that substance. Table 31, gives certain data on the reactions and
results of combustion for elementary combustibles and several compounds.
TABLE 31
OXYGEN AND AIR REQUIRED FOR COMBUSTION
AT 32 DEGREES AND 29.92 INCHES
Column headings:
1: Oxidizable Substance or Combustible
2: Chemical Symbol
3: Atomic or Combining Weight
4: Chemical Reaction
5: Product of Combustion
6: Oxygen per Pound of Column 1 Pounds
7: Nitrogen per Pound of Column 1. 3.32[23] × O Pounds
8: Air per Pound of Column 1. 4.32[24] × O Pounds
9: Gaseous Product per Pound of Column 1[25] + Column 8 Pounds
10: Heat Value per Pound of Column 1 B. t. u.
11: Volumes of Column 1 Entering Combination Volume
12: Volumes of Oxygen Combining with Column 11 Volume
13: Volumes of Product Formed Volume
14: Volume per Pound of Column 1 in Gaseous Form Cubic Feet
15: Volume of Oxygen per Pound of Column 1 Cubic Feet
16: Volume of Products of Combustion per Pound of Column 1 Cubic Feet
17: Volume of Nitrogen per Pound of Column 1 3.782[26] × Column 15 Cubic
Feet
18: Volume of Gas per pound of Column 1 = Column 10 ÷ Column 17 Cubic
Feet
BY WEIGHT
________________________________________________________________________
| | | | | | |
| 1 | 2 | 3 | 4 | 5 | 6 |
|________________|_______|____|________________|_________________|_______|
| | | | | | |
| Carbon | C | 12 | C+2O = CO_{2} | Carbon Dioxide | 2.667 |
| Carbon | C | 12 | C+O = CO | Carbon Monoxide | 1.333 |
| Carbon Monoxide| CO | 28 | CO+O = CO_{2} | Carbon Dioxide | .571 |
| Hydrogen | H | 1 | 2H+O = H_{2}O | Water | 8 |
| | | / CH_{4}+4O = | Carbon Dioxide \ |
| Methane | CH_{4}| 16 | | | 4 |
| | | \ CO_{2}+2H_{2}O | and Water / |
| Sulphur | S | 32 | S+2O = SO_{2} | Sulphur Dioxide | 1 |
|________________|_______|____|________________|_________________|_______|
________________________________________________________
| | | | | | |
| 1 | 2 | 7 | 8 | 9 | 10 |
|________________|_______|_______|_______|_______|_______|
| | | | | | |
| Carbon | C | 8.85 | 11.52 | 12.52 | 14600 |
| Carbon | C | 4.43 | 5.76 | 6.76 | 4450 |
| Carbon Monoxide| CO | 1.90 | 2.47 | 3.47 | 10150 |
| Hydrogen | H | 26.56 | 34.56 | 35.56 | 62000 |
| | | | | | |
| Methane | CH_{4}| 13.28 | 17.28 | 18.28 | 23550 |
| | | | | | |
| Sulphur | S | 3.32 | 4.32 | 5.32 | 4050 |
|________________|_______|_______|_______|_______|_______|
BY VOLUME
________________________________________________________________
| | | | | | |
| 1 | 2 | 11 | 12 | 13 | 14 |
|_________________|________|______|____|________________|________|
| | | | | | |
| Carbon | C | 1C | 2 | 2CO_{2} | 14.95 |
| Carbon | C | 1C | 1 | 2CO | 14.95 |
| Carbon Monoxide | CO | 2CO | 1 | 2CO_{2} | 12.80 |
| Hydrogen | H | 2H | 1 | 2H_{2}O | 179.32 |
| Methane | CH_{4} | 1C4H | 4 | 1CO_{2} 2H_{2}O| 22.41 |
| Sulphur | S | 1S | 2 | 1SO_{2} | 5.60 |
|_________________|________|______|____|________________|________|
_____________________________________________________________
| | | | | | |
| 1 | 2 | 15 | 16 | 17 | 18 |
|_________________|________|_______|________|________|________|
| | | | | | |
| Carbon | C | 29.89 | 29.89 | 112.98 | 142.87 |
| Carbon | C | 14.95 | 29.89 | 56.49 | 86.38 |
| Carbon Monoxide | CO | 6.40 | 12.80 | 24.20 | 37.00 |
| Hydrogen | H | 89.66 | 179.32 | 339.09 | 518.41 |
| Methane | CH_{4} | 44.83 | 67.34 | 169.55 | 236.89 |
| Sulphur | S | 11.21 | 11.21 | 42.39 | 53.60 |
|_________________|________|_______|________|________|________|
It will be seen from this table that a pound of carbon will unite with
2-2/3 pounds of oxygen to form carbon dioxide, and will evolve 14,600
B. t. u. As an intermediate step, a pound of carbon may unite with 1-1/3
pounds of oxygen to form carbon monoxide and evolve 4450 B. t. u., but
in its further conversion to CO_{2} it would unite with an additional
1-1/3 times its weight of oxygen and evolve the remaining 10,150
B. t. u. When a pound of CO burns to CO_{2}, however, only 4350 B. t. u.
are evolved since the pound of CO contains but 3/7 pound carbon.
Air Required for Combustion--It has already been shown that each
combustible element in fuel will unite with a definite amount of oxygen.
With the ultimate analysis of the fuel known, in connection with Table
31, the theoretical amount of air required for combustion may be readily
calculated.
Let the ultimate analysis be as follows:
_Per Cent_
Carbon 74.79
Hydrogen 4.98
Oxygen 6.42
Nitrogen 1.20
Sulphur 3.24
Water 1.55
Ash 7.82
------
100.00
When complete combustion takes place, as already pointed out, the carbon
in the fuel unites with a definite amount of oxygen to form CO_{2}. The
hydrogen, either in a free or combined state, will unite with oxygen to
form water vapor, H_{2}O. Not all of the hydrogen shown in a fuel
analysis, however, is available for the production of heat, as a portion
of it is already united with the oxygen shown by the analysis in the
form of water, H_{2}O. Since the atomic weights of H and O are
respectively 1 and 16, the weight of the combined hydrogen will be 1/8
of the weight of the oxygen, and the hydrogen available for combustion
will be H - 1/8 O. In complete combustion of the sulphur, sulphur
dioxide SO_{2} is formed, which in solution in water forms sulphuric
acid.
Expressed numerically, the theoretical amount of air for the above
analysis is as follows:
0.7479 C × 2-2/3 = 1.9944 O needed
( 0.0642 )
( 0.0498 - -------) H × 8 = 0.3262 O needed
( 8 )
0.0324 S × 1 = 0.0324 O needed
------
Total 2.3530 O needed
One pound of oxygen is contained in 4.32 pounds of air.
The total air needed per pound of coal, therefore, will be 2.353 × 4.32
= 10.165.
The weight of combustible per pound of fuel is .7479 + .0418[27] + .0324
+ .012 = .83 pounds, and the air theoretically required per pound of
combustible is 10.165 ÷ .83 = 12.2 pounds.
The above is equivalent to computing the theoretical amount of air
required per pound of fuel by the formula:
( O)
Weight per pound = 11.52 C + 34.56 (H - -) + 4.32 S (10)
( 8)
where C, H, O and S are proportional parts by weight of carbon,
hydrogen, oxygen and sulphur by ultimate analysis.
In practice it is impossible to obtain perfect combustion with the
theoretical amount of air, and an excess may be required, amounting to
sometimes double the theoretical supply, depending upon the nature of
the fuel to be burned and the method of burning it. The reason for this
is that it is impossible to bring each particle of oxygen in the air
into intimate contact with the particles in the fuel that are to be
oxidized, due not only to the dilution of the oxygen in the air by
nitrogen, but because of such factors as the irregular thickness of the
fire, the varying resistance to the passage of the air through the fire
in separate parts on account of ash, clinker, etc. Where the
difficulties of drawing air uniformly through a fuel bed are eliminated,
as in the case of burning oil fuel or gas, the air supply may be
materially less than would be required for coal. Experiment has shown
that coal will usually require 50 per cent more than the theoretical net
calculated amount of air, or about 18 pounds per pound of fuel either
under natural or forced draft, though this amount may vary widely with
the type of furnace, the nature of the coal, and the method of firing.
If less than this amount of air is supplied, the carbon burns to
monoxide instead of dioxide and its full heat value is not developed.
An excess of air is also a source of waste, as the products of
combustion will be diluted and carry off an excessive amount of heat in
the chimney gases, or the air will so lower the temperature of the
furnace gases as to delay the combustion to an extent that will cause
carbon monoxide to pass off unburned from the furnace. A sufficient
amount of carbon monoxide in the gases may cause the action known as
secondary combustion, by igniting or mingling with air after leaving the
furnace or in the flues or stack. Such secondary combustion which takes
place either within the setting after leaving the furnace or in the
flues or stack always leads to a loss of efficiency and, in some
instances, leads to overheating of the flues and stack.
Table 32 gives the theoretical amount of air required for various fuels
calculated from formula (10) assuming the analyses of the fuels given in
the table.
The process of combustion of different fuels and the effect of variation
in the air supply for their combustion is treated in detail in the
chapters dealing with the various fuels.
TABLE 32
CALCULATED THEORETICAL AMOUNT OF AIR
REQUIRED PER POUND OF VARIOUS FUELS
____________________________________________________________
| |Weight of Constituents in One |Air Required|
| Fuel |Pound Dry Fuel |per Pound |
| |______________________________|of Fuel |
| | Carbon | Hydrogen| Oxygen |Pounds |
| | Per Cent| Per Cent| Per Cent | |
|________________|_________|_________|__________|____________|
|Coke | 94.0 | . | . | 10.8 |
|Anthracite Coal | 91.5 | 3.5 | 2.6 | 11.7 |
|Bituminous Coal | 87.0 | 5.0 | 4.0 | 11.6 |
|Lignite | 70.0 | 5.0 | 20.0 | 8.9 |
|Wood | 50.0 | 6.0 | 43.5 | 6.0 |
|Oil | 85.0 | 13.0 | 1.0 | 14.3 |
|________________|_________|_________|__________|____________|
[Illustration: 4064 HORSE-POWER Installation of Babcock & Wilcox Boilers
and Superheaters, Equipped with Babcock & Wilcox Chain Grate Stokers, at
the Cosmopolitan Electric Co., Chicago, Ill.]
ANALYSIS OF FLUE GASES
The object of a flue gas analysis is the determination of the
completeness of the combustion of the carbon in the fuel, and the amount
and distribution of the heat losses due to incomplete combustion. The
quantities actually determined by an analysis are the relative
proportions by volume, of carbon dioxide (CO_{2}), oxygen (O), and
carbon monoxide (CO), the determinations being made in this order.
The variations of the percentages of these gases in an analysis is best
illustrated in the consideration of the complete combustion of pure
carbon, a pound of which requires 2.67 pounds of oxygen,[28] or 32 cubic
feet at 60 degrees Fahrenheit. The gaseous product of such combustion
will occupy, when cooled, the same volume as the oxygen, namely, 32
cubic feet. The air supplied for the combustion is made up of 20.91 per
cent oxygen and 79.09 per cent nitrogen by volume. The carbon united
with the oxygen in the form of carbon dioxide will have the same volume
as the oxygen in the air originally supplied. The volume of the nitrogen
when cooled will be the same as in the air supplied, as it undergoes no
change. Hence for complete combustion of one pound of carbon, where no
excess of air is supplied, an analysis of the products of combustion
will show the following percentages by volume:
_Actual Volume_
_for One Pound Carbon_ _Per Cent_
_Cubic Feet_ _by Volume_
Carbon Dioxide 32 = 20.91
Oxygen 0 = 0.00
Nitrogen 121 = 79.09
--- ------
Air required for one pound Carbon 153 = 100.00
For 50 per cent excess air the volume will be as follows:
153 × 1½ = 229.5 cubic feet of air per pound of carbon.
_Actual Volume_
_for One Pound Carbon_ _Per Cent_
_Cubic Feet_ _by Volume_
Carbon Dioxide 32 = 13.91 }
Oxygen 16 = 7.00 } = 20.91 per cent
Nitrogen 181.5 = 79.09
----- ------
229.5 = 100.00
For 100 per cent excess air the volume will be as follows:
153 × 2 = 306 cubic feet of air per pound of carbon.
_Actual Volume_
_for One Pound Carbon_ _Per Cent_
_Cubic Feet_ _by Volume_
Carbon Dioxide 32 = 10.45 }
Oxygen 32 = 10.45 } = 20.91 per cent
Nitrogen 242 = 79.09
--- ------
306 = 100.00
In each case the volume of oxygen which combines with the carbon is
equal to (cubic feet of air × 20.91 per cent)--32 cubic feet.
It will be seen that no matter what the excess of air supplied, the
actual amount of carbon dioxide per pound of carbon remains the same,
while the percentage by volume decreases as the excess of air increases.
The actual volume of oxygen and the percentage by volume increases with
the excess of air, and the percentage of oxygen is, therefore, an
indication of the amount of excess air. In each case the sum of the
percentages of CO_{2} and O is the same, 20.9. Although the volume of
nitrogen increases with the excess of air, its percentage by volume
remains the same as it undergoes no change while combustion takes place;
its percentage for any amount of air excess, therefore, will be the same
after combustion as before, if cooled to the same temperature. It must
be borne in mind that the above conditions hold only for the perfect
combustion of a pound of pure carbon.
Carbon monoxide (CO) produced by the imperfect combustion of carbon,
will occupy twice the volume of the oxygen entering into its composition
and will increase the volume of the flue gases over that of the air
supplied for combustion in the proportion of
100 + ½ the per cent CO
1 to -----------------------
100
When pure carbon is the fuel, the sum of the percentages by volume of
carbon dioxide, oxygen and one-half of the carbon monoxide, must be in
the same ratio to the nitrogen in the flue gases as is the oxygen to the
nitrogen in the air supplied, that is, 20.91 to 79.09. When burning
coal, however, the percentage of nitrogen is obtained by subtracting the
sum of the percentages by volume of the other gases from 100. Thus if an
analysis shows 12.5 per cent CO_{2}, 6.5 per cent O, and 0.6 per cent
CO, the percentage of nitrogen which ordinarily is the only other
constituent of the gas which need be considered, is found as follows:
100 - (12.5 + 6.5 + 0.6) = 80.4 per cent.
The action of the hydrogen in the volatile constituents of the fuel is
to increase the apparent percentage of the nitrogen in the flue gases.
This is due to the fact that the water vapor formed by the combustion of
the hydrogen will condense at a temperature at which the analysis is
made, while the nitrogen which accompanied the oxygen with which the
hydrogen originally combined maintains its gaseous form and passes into
the sampling apparatus with the other gases. For this reason coals
containing high percentages of volatile matter will produce a larger
quantity of water vapor, and thus increase the apparent percentage of
nitrogen.
Air Required and Supplied--When the ultimate analysis of a fuel is
known, the air required for complete combustion with no excess can be
found as shown in the chapter on combustion, or from the following
approximate formula:
Pounds of air required per pound of fuel =
(C O S)
34.56 (- + (H - -) + -)[29] (11)
(3 8 8)
where C, H and O equal the percentage by weight of carbon, hydrogen and
oxygen in the fuel divided by 100.
When the flue gas analysis is known, the total, amount of air supplied
is:
Pounds of air supplied per pound of fuel =
N
3.036 (-----------) × C[30] (12)
CO_{2} + CO
where N, CO_{2} and CO are the percentages by volume of nitrogen, carbon
dioxide and carbon monoxide in the flue gases, and C the percentage by
weight of carbon which is burned from the fuel and passes up the stack
as flue gas. This percentage of C which is burned must be distinguished
from the percentage of C as found by an ultimate analysis of the fuel.
To find the percentage of C which is burned, deduct from the total
percentage of carbon as found in the ultimate analysis, the percentage
of unconsumed carbon found in the ash. This latter quantity is the
difference between the percentage of ash found by an analysis and that
as determined by a boiler test. It is usually assumed that the entire
combustible element in the ash is carbon, which assumption is
practically correct. Thus if the ash in a boiler test were 16 per cent
and by an analysis contained 25 per cent of carbon, the percentage of
unconsumed carbon would be 16 × .25 = 4 per cent of the total coal
burned. If the coal contained by ultimate analysis 80 per cent of carbon
the percentage burned, and of which the products of combustion pass up
the chimney would be 80 - 4 = 76 per cent, which is the correct figure
to use in calculating the total amount of air supplied by formula (12).
The weight of flue gases resulting from the combustion of a pound of dry
coal will be the sum of the weights of the air per pound of coal and the
combustible per pound of coal, the latter being equal to one minus the
percentage of ash as found in the boiler test. The weight of flue gases
per pound of dry fuel may, however, be computed directly from the
analyses, as shown later, and the direct computation is that ordinarily
used.
The ratio of the air actually supplied per pound of fuel to that
theoretically required to burn it is:
N
3.036(---------)×C
CO_{2}+CO
------------------ (13)
C O
34.56(- + H - -)
3 8
in which the letters have the same significance as in formulae (11) and
(12).
The ratio of the air supplied per pound of combustible to the amount
theoretically required is:
N
------------------ (14)
N - 3.782(O - ½CO)
which is derived as follows:
The N in the flue gas is the content of nitrogen in the whole amount of
air supplied. The oxygen in the flue gas is that contained in the air
supplied and which was not utilized in combustion. This oxygen was
accompanied by 3.782 times its volume of nitrogen. The total amount of
excess oxygen in the flue gases is (O - ½CO); hence N - 3.782(O - ½CO)
represents the nitrogen content in the air actually required for
combustion and N ÷ (N - 3.782[O - ½CO]) is the ratio of the air supplied
to that required. This ratio minus one will be the proportion of excess
air.
The heat lost in the flue gases is L = 0.24 W (T - t) (15)
Where L = B. t. u. lost per pound of fuel,
W = weight of flue gases in pounds per pound of dry coal,
T = temperature of flue gases,
t = temperature of atmosphere,
0.24 = specific heat of the flue gases.
The weight of flue gases, W, per pound of carbon can be computed
directly from the flue gas analysis from the formula:
11 CO_{2} + 8 O + 7 (CO + N)
---------------------------- (16)
3 (CO_{2} + CO)
where CO_{2}, O, CO, and N are the percentages by volume as determined
by the flue gas analysis of carbon dioxide, oxygen, carbon monoxide and
nitrogen.
The weight of flue gas per pound of dry coal will be the weight
determined by this formula multiplied by the percentage of carbon in the
coal from an ultimate analysis.
[Graph: Temperature of Escaping Gases--Deg. Fahr.
against Heat carried away by Chimney Gases--In B.t.u.
per pound of Carbon burned.[31]
Fig. 20. Loss Due to Heat Carried Away by Chimney Gases for Varying
Percentages of Carbon Dioxide. Based on Boiler Room Temperature = 80
Degrees Fahrenheit. Nitrogen in Flue Gas = 80.5 Per Cent. Carbon
Monoxide in Flue Gas = 0. Per Cent]
Fig. 20 represents graphically the loss due to heat carried away by dry
chimney gases for varying percentages of CO_{2}, and different
temperatures of exit gases.
The heat lost, due to the fact that the carbon in the fuel is not
completely burned and carbon monoxide is present in the flue gases, in
B. t. u. per pound of fuel burned is:
( CO )
L' = 10,150 × (-----------) (17)
(CO + CO_{2})
where, as before, CO and CO_{2} are the percentages by volume in the
flue gases and C is the proportion by weight of carbon which is burned
and passes up the stack.
Fig. 21 represents graphically the loss due to such carbon in the fuel
as is not completely burned but escapes up the stack in the form of
carbon monoxide.
[Graph: Loss in B.T.U. per Pound of Carbon Burned[32]
against Per Cent CO_{2} in Flue Gas
Fig. 21. Loss Due to Unconsumed Carbon Contained in the
CO in the Flue Gases]
Apparatus for Flue Gas Analysis--The Orsat apparatus, illustrated in
Fig. 22, is generally used for analyzing flue gases. The burette A is
graduated in cubic centimeters up to 100, and is surrounded by a water
jacket to prevent any change in temperature from affecting the density
of the gas being analyzed.
For accurate work it is advisable to use four pipettes, B, C, D, E, the
first containing a solution of caustic potash for the absorption of
carbon dioxide, the second an alkaline solution of pyrogallol for the
absorption of oxygen, and the remaining two an acid solution of cuprous
chloride for absorbing the carbon monoxide. Each pipette contains a
number of glass tubes, to which some of the solution clings, thus
facilitating the absorption of the gas. In the pipettes D and E, copper
wire is placed in these tubes to re-energize the solution as it becomes
weakened. The rear half of each pipette is fitted with a rubber bag, one
of which is shown at K, to protect the solution from the action of the
air. The solution in each pipette should be drawn up to the mark on the
capillary tube.
The gas is drawn into the burette through the U-tube H, which is filled
with spun glass, or similar material, to clean the gas. To discharge any
air or gas in the apparatus, the cock G is opened to the air and the
bottle F is raised until the water in the burette reaches the 100 cubic
centimeters mark. The cock G is then turned so as to close the air
opening and allow gas to be drawn through H, the bottle F being lowered
for this purpose. The gas is drawn into the burette to a point below the
zero mark, the cock G then being opened to the air and the excess gas
expelled until the level of the water in F and in A are at the zero
mark. This operation is necessary in order to obtain the zero reading at
atmospheric pressure.
The apparatus should be carefully tested for leakage as well as all
connections leading thereto. Simple tests can be made; for example: If
after the cock G is closed, the bottle F is placed on top of the frame
for a short time and again brought to the zero mark, the level of the
water in A is above the zero mark, a leak is indicated.
[Illustration: Fig. 22. Orsat Apparatus]
Before taking a final sample for analysis, the burette A should be
filled with gas and emptied once or twice, to make sure that all the
apparatus is filled with the new gas. The cock G is then closed and the
cock I in the pipette B is opened and the gas driven over into B by
raising the bottle F. The gas is drawn back into A by lowering F and
when the solution in B has reached the mark in the capillary tube, the
cock I is closed and a reading is taken on the burette, the level of the
water in the bottle F being brought to the same level as the water in A.
The operation is repeated until a constant reading is obtained, the
number of cubic centimeters being the percentage of CO_{2} in the flue
gases.
The gas is then driven over into the pipette C and a similar operation
is carried out. The difference between the resulting reading and the
first reading gives the percentage of oxygen in the flue gases.
The next operation is to drive the gas into the pipette D, the gas being
given a final wash in E, and then passed into the pipette C to
neutralize any hydrochloric acid fumes which may have been given off by
the cuprous chloride solution, which, especially if it be old, may give
off such fumes, thus increasing the volume of the gases and making the
reading on the burette less than the true amount.
The process must be carried out in the order named, as the pyrogallol
solution will also absorb carbon dioxide, while the cuprous chloride
solution will also absorb oxygen.
As the pressure of the gases in the flue is less than the atmospheric
pressure, they will not of themselves flow through the pipe connecting
the flue to the apparatus. The gas may be drawn into the pipe in the way
already described for filling the apparatus, but this is a tedious
method. For rapid work a rubber bulb aspirator connected to the air
outlet of the cock G will enable a new supply of gas to be drawn into
the pipe, the apparatus then being filled as already described. Another
form of aspirator draws the gas from the flue in a constant stream, thus
insuring a fresh supply for each sample.
The analysis made by the Orsat apparatus is volumetric; if the analysis
by weight is required, it can be found from the volumetric analysis as
follows:
Multiply the percentages by volume by either the densities or the
molecular weight of each gas, and divide the products by the sum of all
the products; the quotients will be the percentages by weight. For most
work sufficient accuracy is secured by using the even values of the
molecular weights.
The even values of the molecular weights of the gases appearing in an
analysis by an Orsat are:
Carbon Dioxide 44
Carbon Monoxide 28
Oxygen 32
Nitrogen 28
Table 33 indicates the method of converting a volumetric flue gas
analysis into an analysis by weight.
TABLE 33
CONVERSION OF A FLUE GAS ANALYSIS BY VOLUME TO ONE BY WEIGHT
Column Headings:
A: Analysis by Volume Per Cent
B: Molecular Weight
C: Volume times Molecular Weight
D: Analysis by Weight Per Cent
_____________________________________________________________________
| | | | | |
| Gas | A | B | C | D |
|________________________|_______|___________|________|_______________|
| | | | | |
| | | | | |
| | | | | 536.8 |
| Carbon Dioxide CO_{2} | 12.2 | 12+(2×16) | 536.8 | ------ = 17.7 |
| | | | | 3022.8 |
| | | | | |
| | | | | 11.2 |
| Carbon Monoxide CO | .4 | 12+16 | 11.2 | ------ = .4 |
| | | | | 3022.8 |
| | | | | |
| | | | | 220.8 |
| Oxygen O | 6.9 | 2×16 | 220.8 | ------ = 7.3 |
| | | | | 3022.8 |
| | | | | |
| | | | | 2254.0 |
| Nitrogen N | 80.5 | 2×14 | 2254.0 | ------ = 74.6 |
| | | | | 3022.8 |
|________________________|_______|___________|________|_______________|
| | | | | |
| Total | 100.0 | | 3022.8 | 100.0 |
|________________________|_______|___________|________|_______________|
Application of Formulae and Rules--Pocahontas coal is burned in the
furnace, a partial ultimate analysis being:
_Per Cent_
Carbon 82.1
Hydrogen 4.25
Oxygen 2.6
Sulphur 1.6
Ash 6.0
B. t. u., per pound dry 14500
The flue gas analysis shows:
_Per Cent_
CO_{2} 10.7
O 9.0
CO 0.0
N (by difference) 80.3
Determine: The flue gas analysis by weight (see Table 33), the amount of
air required for perfect combustion, the actual weight of air per pound
of fuel, the weight of flue gas per pound of coal, the heat lost in the
chimney gases if the temperature of these is 500 degrees Fahrenheit, and
the ratio of the air supplied to that theoretically required.
Solution: The theoretical weight of air required for perfect combustion,
per pound of fuel, from formula (11) will be,
(.821 .026 .016)
W = 34.56 (---- + (.0425 - ----) + ----) = 10.88 pounds.
( 3 8 8 )
If the amount of carbon which is burned and passes away as flue gas is
80 per cent, which would allow for 2.1 per cent of unburned carbon in
terms of the total weight of dry fuel burned, the weight of dry gas per
pound of carbon burned will be from formula (16):
11 × 10.7 + 8 × 9.0 + 7(0 + 80.3)
W = --------------------------------- = 23.42 pounds
3(10.7 + 0)
and the weight of flue gas per pound of coal burned will be .80 × 23.42
= 18.74 pounds.
The heat lost in the flue gases per pound of coal burned will be from
formula (15) and the value 18.74 just determined.
Loss = .24 × 18.74 × (500 - 60) = 1979 B. t. u.
The percentage of heat lost in the flue gases will be 1979 ÷ 14500 =
13.6 per cent.
The ratio of air supplied per pound of coal to that theoretically
required will be 18.74 ÷ 10.88 = 1.72 per cent.
The ratio of air supplied per pound of combustible to that required will
be from formula (14):
.803
------------------------- = 1.73
.803 - 3.782(.09 - ½ × 0)
The ratio based on combustible will be greater than the ratio based on
fuel if there is unconsumed carbon in the ash.
Unreliability of CO_{2} Readings Taken Alone--It is generally assumed
that high CO_{2} readings are indicative of good combustion and hence of
high efficiency. This is true only in the sense that such high readings
do indicate the small amount of excess air that usually accompanies good
combustion, and for this reason high CO_{2} readings alone are not
considered entirely reliable. Wherever an automatic CO_{2} recorder is
used, it should be checked from time to time and the analysis carried
further with a view to ascertaining whether there is CO present. As the
percentage of CO_{2} in these gases increases, there is a tendency
toward the presence of CO, which, of course, cannot be shown by a CO_{2}
recorder, and which is often difficult to detect with an Orsat
apparatus. The greatest care should be taken in preparing the cuprous
chloride solution in making analyses and it must be known to be fresh
and capable of absorbing CO. In one instance that came to our attention,
in using an Orsat apparatus where the cuprous chloride solution was
believed to be fresh, no CO was indicated in the flue gases but on
passing the same sample into a Hempel apparatus, a considerable
percentage was found. It is not safe, therefore, to assume without
question from a high CO_{2} reading that the combustion is
correspondingly good, and the question of excess air alone should be
distinguished from that of good combustion. The effect of a small
quantity of CO, say one per cent, present in the flue gases will have a
negligible influence on the quantity of excess air, but the presence of
such an amount would mean a loss due to the incomplete combustion of the
carbon in the fuel of possibly 4.5 per cent of the total heat in the
fuel burned. When this is considered, the importance of a complete flue
gas analysis is apparent.
Table 34 gives the densities of various gases together with other data
that will be of service in gas analysis work.
TABLE 34
DENSITY OF GASES AT 32 DEGREES FAHRENHEIT AND ATMOSPHERIC PRESSURE
ADAPTED FROM SMITHSONIAN TABLES
+----------+----------+--------+---------+----------+---------------+
| | | | | | Relative |
| | | | Weight | | Density, |
| | | | of | Volume | Hydrogen = 1 |
| | |Specific|One Cubic| of +-------+-------+
| Gas | Chemical |Gravity | Foot |One Pound | |Approx-|
| | Symbol | Air=1 | Pounds |Cubic Feet| Exact | imate |
+----------+----------+--------+---------+----------+-------+-------+
|Oxygen | O | 1.053 | .08922 | 11.208 | 15.87 | 16 |
|Nitrogen | N | 0.9673 | .07829 | 12.773 | 13.92 | 14 |
|Hydrogen | H | 0.0696 | .005621 | 177.90 | 1.00 | 1 |
|Carbon | | | | | | |
| Dioxide | CO_{2} | 1.5291 | .12269 | 8.151 | 21.83 | 22 |
|Carbon | | | | | | |
| Monoxide | CO | 0.9672 | .07807 | 12.809 | 13.89 | 14 |
|Methane | CH_{4} | 0.5576 | .04470 | 22.371 | 7.95 | 8 |
|Ethane |C_{2}H_{6}| 1.075 | .08379 | 11.935 | 14.91 | 15 |
|Acetylene |C_{2}H_{2}| 0.920 | .07254 | 13.785 | 12.91 | 13 |
|Sulphur | | | | | | |
| Dioxide | SO_{2} | 2.2639 | .17862 | 5.598 | 31.96 | 32 |
|Air | ... | 1.0000 | .08071 | 12.390 | ... | ... |
+----------+----------+--------+---------+----------+-------+-------+
[Illustration: 1942 Horse-power Installation of Babcock & Wilcox Boilers
and Superheaters in the Singer Building, New York City]
CLASSIFICATION OF FUELS
(WITH PARTICULAR REFERENCE TO COAL)
Fuels for steam boilers may be classified as solid, liquid or gaseous.
Of the solid fuels, anthracite and bituminous coals are the most common,
but in this class must also be included lignite, peat, wood, bagasse and
the refuse from certain industrial processes such as sawdust, shavings,
tan bark and the like. Straw, corn and coffee husks are utilized in
isolated cases.
The class of liquid fuels is represented chiefly by petroleum, though
coal tar and water-gas tar are used to a limited extent.
Gaseous fuels are limited to natural gas, blast furnace gas and coke
oven gas, the first being a natural product and the two latter
by-products from industrial processes. Though waste gases from certain
processes may be considered as gaseous fuels, inasmuch as the question
of combustion does not enter, the methods of utilizing them differ from
that for combustible gaseous fuel, and the question will be dealt with
separately.
Since coal is by far the most generally used of all fuels, this chapter
will be devoted entirely to the formation, composition and distribution
of the various grades, from anthracite to peat. The other fuels will be
discussed in succeeding chapters and their combustion dealt with in
connection with their composition.
Formation of Coal--All coals are of vegetable origin and are the remains
of prehistoric forests. Destructive distillation due to great pressures
and temperatures, has resolved the organic matter into its invariable
ultimate constituents, carbon, hydrogen, oxygen and other substances, in
varying proportions. The factors of time, depth of beds, disturbance of
beds and the intrusion of mineral matter resulting from such
disturbances have produced the variation in the degree of evolution from
vegetable fiber to hard coal. This variation is shown chiefly in the
content of carbon, and Table 35 shows the steps of such variation.
TABLE 35
APPROXIMATE CHEMICAL CHANGES FROM WOOD
FIBER TO ANTHRACITE COAL
+----------------------+-------+--------+-------+
|Substance |Carbon |Hydrogen|Oxygen |
+----------------------+-------+--------+-------+
|Wood Fiber | 52.65 | 5.25 | 42.10 |
|Peat | 59.57 | 5.96 | 34.47 |
|Lignite | 66.04 | 5.27 | 28.69 |
|Earthy Brown Coal | 73.18 | 5.68 | 21.14 |
|Bituminous Coal | 75.06 | 5.84 | 19.10 |
|Semi-bituminous Coal | 89.29 | 5.05 | 5.66 |
|Anthracite Coal | 91.58 | 3.96 | 4.46 |
+----------------------+-------+--------+-------+
Composition of Coal--The uncombined carbon in coal is known as fixed
carbon. Some of the carbon constituent is combined with hydrogen and
this, together with other gaseous substances driven off by the
application of heat, form that portion of the coal known as volatile
matter. The fixed carbon and the volatile matter constitute the
combustible. The oxygen and nitrogen contained in the volatile matter
are not combustible, but custom has applied this term to that portion of
the coal which is dry and free from ash, thus including the oxygen and
nitrogen.
The other important substances entering into the composition of coal are
moisture and the refractory earths which form the ash. The ash varies in
different coals from 3 to 30 per cent and the moisture from 0.75 to 45
per cent of the total weight of the coal, depending upon the grade and
the locality in which it is mined. A large percentage of ash is
undesirable as it not only reduces the calorific value of the fuel, but
chokes up the air passages in the furnace and through the fuel bed, thus
preventing the rapid combustion necessary to high efficiency. If the
coal contains an excessive quantity of sulphur, trouble will result from
its harmful action on the metal of the boiler where moisture is present,
and because it unites with the ash to form a fusible slag or clinker
which will choke up the grate bars and form a solid mass in which large
quantities of unconsumed carbon may be imbedded.
Moisture in coal may be more detrimental than ash in reducing the
temperature of a furnace, as it is non-combustible, absorbs heat both in
being evaporated and superheated to the temperature of the furnace
gases. In some instances, however, a certain amount of moisture in a
bituminous coal produces a mechanical action that assists in the
combustion and makes it possible to develop higher capacities than with
dry coal.
Classification of Coal--Custom has classified coals in accordance with
the varying content of carbon and volatile matter in the combustible.
Table 36 gives the approximate percentages of these constituents for the
general classes of coals with the corresponding heat values per pound of
combustible.
TABLE 36
APPROXIMATE COMPOSITION AND CALORIFIC VALUE
OF GENERAL GRADES OF COAL ON BASIS OF COMBUSTIBLE
+-------------------+----------------------------+--------------+
| Kind of Coal | Per Cent of Combustible | B. t. u. |
| +------------+---------------+ Per Pound of |
| |Fixed Carbon|Volatile Matter| Combustible |
+-------------------+------------+---------------+--------------+
|Anthracite |97.0 to 92.5| 3.0 to 7.5 |14600 to 14800|
|Semi-anthracite |92.5 to 87.5| 7.5 to 12.5 |14700 to 15500|
|Semi-bituminous |87.5 to 75.0| 12.5 to 25.0 |15500 to 16000|
|Bituminous--Eastern|75.0 to 60.0| 25.0 to 40.0 |14800 to 15300|
|Bituminous--Western|65.0 to 50.0| 35.0 to 50.0 |13500 to 14800|
|Lignite | Under 50 | Over 50 |11000 to 13500|
+-------------------+------------+---------------+--------------+
Anthracite--The name anthracite, or hard coal, is applied to those dry
coals containing from 3 to 7 per cent volatile matter and which do not
swell when burned. True anthracite is hard, compact, lustrous and
sometimes iridescent, and is characterized by few joints and clefts. Its
specific gravity varies from 1.4 to 1.8. In burning, it kindles slowly
and with difficulty, is hard to keep alight, and burns with a short,
almost colorless flame, without smoke.
Semi-anthracite coal has less density, hardness and luster than true
anthracite, and can be distinguished from it by the fact that when newly
fractured it will soot the hands. Its specific gravity is ordinarily
about 1.4. It kindles quite readily and burns more freely than the true
anthracites.
Semi-bituminous coal is softer than anthracite, contains more volatile
hydrocarbons, kindles more easily and burns more rapidly. It is
ordinarily free burning, has a high calorific value and is of the
highest order for steam generating purposes.
Bituminous coals are still softer than those described and contain still
more volatile hydrocarbons. The difference between the semi-bituminous
and the bituminous coals is an important one, economically. The former
have an average heating value per pound of combustible about 6 per cent
higher than the latter, and they burn with much less smoke in ordinary
furnaces. The distinctive characteristic of the bituminous coals is the
emission of yellow flame and smoke when burning. In color they range
from pitch black to dark brown, having a resinous luster in the most
compact specimens, and a silky luster in such specimens as show traces
of vegetable fiber. The specific gravity is ordinarily about 1.3.
Bituminous coals are either of the caking or non-caking class. The
former, when heated, fuse and swell in size; the latter burn freely, do
not fuse, and are commonly known as free burning coals. Caking coals are
rich in volatile hydrocarbons and are valuable in gas manufacture.
Bituminous coals absorb moisture from the atmosphere. The surface
moisture can be removed by ordinary drying, but a portion of the water
can be removed only by heating the coal to a temperature of about 250
degrees Fahrenheit.
Cannel coal is a variety of bituminous coal, rich in hydrogen and
hydrocarbons, and is exceedingly valuable as a gas coal. It has a dull
resinous luster and burns with a bright flame without fusing. Cannel
coal is seldom used for steam coal, though it is sometimes mixed with
semi-bituminous coal where an increased economy at high rates of
combustion is desired. The composition of cannel coal is approximately
as follows: fixed carbon, 26 to 55 per cent; volatile matter, 42 to 64
per cent; earthy matter, 2 to 14 per cent. Its specific gravity is
approximately 1.24.
Lignite is organic matter in the earlier stages of its conversion into
coal, and includes all varieties which are intermediate between peat and
coal of the older formation. Its specific gravity is low, being 1.2 to
1.23, and when freshly mined it may contain as high as 50 per cent of
moisture. Its appearance varies from a light brown, showing a distinctly
woody structure, in the poorer varieties, to a black, with a pitchy
luster resembling hard coal, in the best varieties. It is non-caking and
burns with a bright but slightly smoky flame with moderate heat. It is
easily broken, will not stand much handling in transportation, and if
exposed to the weather will rapidly disintegrate, which will increase
the difficulty of burning it.
Its composition varies over wide limits. The ash may run as low as one
per cent and as high as 50 per cent. Its high content of moisture and
the large quantity of air necessary for its combustion cause large stack
losses. It is distinctly a low-grade fuel and is used almost entirely in
the districts where mined, due to its cheapness.
Peat is organic matter in the first stages of its conversion into coal
and is found in bogs and similar places. Its moisture content when cut
is extremely high, averaging 75 or 80 per cent. It is unsuitable for
fuel until dried and even then will contain as much as 30 per cent
moisture. Its ash content when dry varies from 3 to 12 per cent. In this
country, though large deposits of peat have been found, it has not as
yet been found practicable to utilize it for steam generating purposes
in competition with coal. In some European countries, however, the peat
industry is common.
Distribution--The anthracite coals are, with some unimportant
exceptions, confined to five small fields in Eastern Pennsylvania, as
shown in the following list. These fields are given in the order of
their hardness.
Lehigh or Eastern Middle Field
Green Mountain District
Black Creek District
Hazelton District
Beaver Meadow District
Panther Creek District[33]
Mahanoy or Western Field[34]
East Mahanoy District
West Mahanoy District
Wyoming or Northern Field
Carbondale District
Scranton District
Pittston District
Wilkesbarre District
Plymouth District
Schuylkill or Southern Field
East Schuylkill District
West Schuylkill District
Louberry District
Lykens Valley or Southwestern Field
Lykens Valley District
Shamokin District[35]
Anthracite is also found in Pulaski and Wythe Counties, Virginia; along
the border of Little Walker Mountain, and in Gunnison County, Colorado.
The areas in Virginia are limited, however, while in Colorado the
quality varies greatly in neighboring beds and even in the same bed. An
anthracite bed in New Mexico was described in 1870 by Dr. R. W. Raymond,
formerly United States Mining Commissioner.
Semi-anthracite coals are found in a few small areas in the western part
of the anthracite field. The largest of these beds is the Bernice in
Sullivan County, Pennsylvania. Mr. William Kent, in his "Steam Boiler
Economy", describes this as follows: "The Bernice semi-anthracite coal
basin lies between Beech Creek on the north and Loyalsock Creek on the
south. It is six miles long, east and west, and hardly a third of a mile
across. An 8-foot vein of coal lies in a bed of 12 feet of coal and
slate. The coal of this bed is the dividing line between anthracite and
semi-anthracite, and is similar to the coal of the Lykens Valley
District. Mine analyses give a range as follows: moisture, 0.65 to 1.97;
volatile matter, 3.56 to 9.40; fixed carbon, 82.52 to 89.39; ash, 3.27
to 9.34; sulphur, 0.24 to 1.04."
Semi-bituminous coals are found on the eastern edge of the great
Appalachian Field. Starting with Tioga and Bradford Counties of northern
Pennsylvania, the bed runs southwest through Lycoming, Clearfield,
Centre, Huntingdon, Cambria, Somerset and Fulton Counties, Pennsylvania;
Allegheny County, Maryland; Buchannan, Dickinson, Lee, Russell, Scott,
Tazewell and Wise Counties, Virginia; Mercer, McDowell, Fayette, Raleigh
and Mineral Counties, West Virginia; and ending in northeastern
Tennessee, where a small amount of semi-bituminous is mined.
The largest of the bituminous fields is the Appalachian. Beginning near
the northern boundary of Pennsylvania, in the western portion of the
State, it extends southwestward through West Virginia, touching Maryland
and Virginia on their western borders, passing through southeastern
Ohio, eastern Kentucky and central Tennessee, and ending in western
Alabama, 900 miles from its northern extremity.
The next bituminous coal producing region to the west is the Northern
Field, in north central Michigan. Still further to the west, and second
in importance to the Appalachian Field, is the Eastern Interior Field.
This covers, with the exception of the upper northern portion, nearly
the entire State of Illinois, southwest Indiana and the western portion
of Kentucky.
The Western Field extends through central and southern Iowa, western
Missouri, southwestern Kansas, eastern Oklahoma and the west central
portion of Arkansas. The Southwestern Field is confined entirely to the
north central portion of Texas, in which State there are also two small
isolated fields along the Rio Grande River.
The remaining bituminous fields are scattered through what may be termed
the Rocky Mountain Region, extending from Montana to New Orleans. A
partial list of these fields and their location follows:
Judith Basin Central Montana
Bull Mountain Field Central Montana
Yellowstone Region Southwestern Montana
Big Horn Basin Region Southern Montana
Big Horn Basin Region Northern Wyoming
Black Hills Region Northeastern Wyoming
Hanna Field Southern Wyoming
Green River Region Southwestern Wyoming
Yampa Field Northwestern Colorado
North Park Field Northern Colorado
Denver Region North Central Colorado
Uinta Region Western Colorado
Uinta Region Eastern Utah
Southwestern Region Southwestern Utah
Raton Mountain Region Southern Colorado
Raton Mountain Region Northern New Mexico
San Juan River Region Northwestern New Mexico
Capitan Field Southern New Mexico
Along the Pacific Coast a few small fields are scattered in western
California, southwestern Oregon, western and northwestern Washington.
Most of the coals in the above fields are on the border line between
bituminous and lignite. They are really a low grade of bituminous coal
and are known as sub-bituminous or black lignites.
Lignites--These resemble the brown coals of Europe and are found in the
western states, Wyoming, New Mexico, Arizona, Utah, Montana, North
Dakota, Nevada, California, Oregon and Washington. Many of the fields
given as those containing bituminous coals in the western states also
contain true lignite. Lignite is also found in the eastern part of Texas
and in Oklahoma.
Alaska Coals--Coal has been found in Alaska and undoubtedly is of great
value, though the extent and character of the fields have probably been
exaggerated. Great quantities of lignite are known to exist, and in
quality the coal ranges in character from lignite to anthracite. There
are at present, however, only two fields of high-grade coals known,
these being the Bering River Field, near Controllers Bay, and the
Matanuska Field, at the head of Cooks Inlet. Both of these fields are
known to contain both anthracite and high-grade bituminous coals, though
as yet they cannot be said to have been opened up.
Weathering of Coal--The storage of coal has become within the last few
years to a certain extent a necessity due to market conditions, danger
of labor difficulties at the mines and in the railroads, and the
crowding of transportation facilities. The first cause is probably the
most important, and this is particularly true of anthracite coals where
a sliding scale of prices is used according to the season of the year.
While market conditions serve as one of the principal reasons for coal
storage, most power plants and manufacturing plants feel compelled to
protect their coal supply from the danger of strikes, car shortages and
the like, and it is customary for large power plants, railroads and coal
companies themselves, to store bituminous coal. Naval coaling stations
are also an example of what is done along these lines.
Anthracite is the nearest approach to the ideal coal for storing. It is
not subject to spontaneous ignition, and for this reason is unlimited in
the amount that may be stored in one pile. With bituminous coals,
however, the case is different. Most bituminous coals will ignite if
placed in large enough piles and all suffer more or less from
disintegration. Coal producers only store such coals as are least liable
to ignite, and which will stand rehandling for shipment.
The changes which take place in stored coal are of two kinds: 1st, the
oxidization of the inorganic matter such as pyrites; and 2nd, the direct
oxidization of the organic matter of the actual coal.
The first change will result in an increased volume of the coal, and
sometimes in an increased weight, and a marked disintegration. The
changes due to direct oxidization of the coal substances usually cannot
be detected by the eye, but as they involve the oxidization of the
carbon and available hydrogen and the absorption of the oxygen by
unsaturated hydrocarbons, they are the chief cause of the weathering
losses in heat value. Numerous experiments have led to the conclusion
that this is also the cause for spontaneous combustion.
Experiments to show loss in calorific heat values due to weathering
indicate that such loss may be as high as 10 per cent when the coal is
stored in the air, and 8.75 per cent when stored under water. It would
appear that the higher the volatile content of the coal, the greater
will be the loss in calorific value and the more subject to spontaneous
ignition.
Some experiments made by Messrs. S. W. Parr and W. F. Wheeler, published
in 1909 by the Experiment Station of the University of Illinois,
indicate that coals of the nature found in Illinois and neighboring
states are not affected seriously during storage from the standpoint of
weight and heating value, the latter loss averaging about 3½ per cent
for the first year of storage. They found that the losses due to
disintegration and to spontaneous ignition were of greater importance.
Their conclusions agree with those deduced from the other experiments,
viz., that the storing of a larger size coal than that which is to be
used, will overcome to a certain extent the objection to disintegration,
and that the larger sizes, besides being advantageous in respect to
disintegration, are less liable to spontaneous ignition. Storage under
water will, of course, entirely prevent any fire loss and, to a great
extent, will stop disintegration and reduce the calorific losses to a
minimum.
To minimize the danger of spontaneous ignition in storing coal, the
piles should be thoroughly ventilated.
Pulverized Fuels--Considerable experimental work has been done with
pulverized coal, utilizing either coal dust or pulverizing such coal as
is too small to be burned in other ways. If satisfactorily fed to the
furnace, it would appear to have several advantages. The dust burned in
suspension would be more completely consumed than is the case with the
solid coals, the production of smoke would be minimized, and the process
would admit of an adjustment of the air supply to a point very close to
the amount theoretically required. This is due to the fact that in
burning there is an intimate mixture of the air and fuel. The principal
objections have been in the inability to introduce the pulverized fuel
into the furnace uniformly, the difficulty of reducing the fuel to the
same degree of fineness, liability of explosion in the furnace due to
improper mixture with the air, and the decreased capacity and efficiency
resulting from the difficulty of keeping tube surfaces clean.
Pressed Fuels--In this class are those composed of the dust of some
suitable combustible, pressed and cemented together by a substance
possessing binding and in most cases inflammable properties. Such fuels,
known as briquettes, are extensively used in foreign countries and
consist of carbon or soft coal, too small to be burned in the ordinary
way, mixed usually with pitch or coal tar. Much experimenting has been
done in this country in briquetting fuels, the government having taken
an active interest in the question, but as yet this class of fuel has
not come into common use as the cost and difficulty of manufacture and
handling have made it impossible to place it in the market at a price to
successfully compete with coal.
Coke is a porous product consisting almost entirely of carbon remaining
after certain manufacturing processes have distilled off the hydrocarbon
gases of the fuel used. It is produced, first, from gas coal distilled
in gas retorts; second, from gas or ordinary bituminous coals burned in
special furnaces called coke ovens; and third, from petroleum by
carrying the distillation of the residuum to a red heat.
Coke is a smokeless fuel. It readily absorbs moisture from the
atmosphere and if not kept under cover its moisture content may be as
much as 20 per cent of its own weight.
Gas-house coke is generally softer and more porous than oven coke,
ignites more readily, and requires less draft for its combustion.
[Illustration: 16,000 Horse-power Installation of Babcock & Wilcox
Boilers and Superheaters at the Brunot's Island Plant of the Duquesne
Light Co., Pittsburgh, Pa.]
THE DETERMINATION OF HEATING VALUES OF FUELS
The heating value of a fuel may be determined either by a calculation
from a chemical analysis or by burning a sample in a calorimeter.
In the former method the calculation should be based on an ultimate
analysis, which reduces the fuel to its elementary constituents of
carbon, hydrogen, oxygen, nitrogen, sulphur, ash and moisture, to secure
a reasonable degree of accuracy. A proximate analysis, which determines
only the percentage of moisture, fixed carbon, volatile matter and ash,
without determining the ultimate composition of the volatile matter,
cannot be used for computing the heat of combustion with the same degree
of accuracy as an ultimate analysis, but estimates may be based on the
ultimate analysis that are fairly correct.
An ultimate analysis requires the services of a competent chemist, and
the methods to be employed in such a determination will be found in any
standard book on engineering chemistry. An ultimate analysis, while
resolving the fuel into its elementary constituents, does not reveal how
these may have been combined in the fuel. The manner of their
combination undoubtedly has a direct effect upon their calorific value,
as fuels having almost identical ultimate analyses show a difference in
heating value when tested in a calorimeter. Such a difference, however,
is slight, and very close approximations may be computed from the
ultimate analysis.
Ultimate analyses are given on both a moist and a dry fuel basis.
Inasmuch as the latter is the basis generally accepted for the
comparison of data, it would appear that it is the best basis on which
to report such an analysis. When an analysis is given on a moist fuel
basis it may be readily converted to a dry basis by dividing the
percentages of the various constituents by one minus the percentage of
moisture, reporting the moisture content separately.
_Moist Fuel_ _Dry Fuel_
C 83.95 84.45
H 4.23 4.25
O 3.02 3.04
N 1.27 1.28
S .91 .91
Ash 6.03 6.07
------
100.00
Moisture .59 .59
------
100.00
Calculations from an Ultimate Analysis--The first formula for the
calculation of heating values from the composition of a fuel as
determined from an ultimate analysis is due to Dulong, and this formula,
slightly modified, is the most commonly used to-day. Other formulae have
been proposed, some of which are more accurate for certain specific
classes of fuel, but all have their basis in Dulong's formula, the
accepted modified form of which is:
Heat units in B. t. u. per pound of dry fuel =
O
14,600 C + 62,000(H - -) + 4000 S (18)
8
where C, H, O and S are the proportionate parts by weight of carbon,
hydrogen, oxygen and sulphur.
Assume a coal of the composition given. Substituting in this formula
(18),
Heating value per pound of dry coal
( .0304)
= 14,600 × .8445 + 62,000 (.0425 - -----) + 4000 × .0091 = 14,765 B. t. u.
( 8 )
This coal, by a calorimetric test, showed 14,843 B. t. u., and from a
comparison the degree of accuracy of the formula will be noted.
The investigation of Lord and Haas in this country, Mabler in France,
and Bunte in Germany, all show that Dulong's formula gives results
nearly identical with those obtained from calorimetric tests and may be
safely applied to all solid fuels except cannel coal, lignite, turf and
wood, provided the ultimate analysis is correct. This practically limits
its use to coal. The limiting features are the presence of hydrogen and
carbon united in the form of hydrocarbons. Such hydrocarbons are present
in coals in small quantities, but they have positive and negative heats
of combination, and in coals these appear to offset each other,
certainly sufficiently to apply the formula to such fuels.
High and Low Heat Value of Fuels--In any fuel containing hydrogen the
calorific value as found by the calorimeter is higher than that
obtainable under most working conditions in boiler practice by an amount
equal to the latent heat of the volatilization of water. This heat would
reappear when the vapor was condensed, though in ordinary practice the
vapor passes away uncondensed. This fact gives rise to a distinction in
heat values into the so-called "higher" and "lower" calorific values.
The higher value, _i. e._, the one determined by the calorimeter, is the
only scientific unit, is the value which should be used in boiler
testing work, and is the one recommended by the American Society of
Mechanical Engineers.
There is no absolute measure of the lower heat of combustion, and in
view of the wide difference in opinion among physicists as to the
deductions to be made from the higher or absolute unit in this
determination, the lower value must be considered an artificial unit.
The lower value entails the use of an ultimate analysis and involves
assumptions that would make the employment of such a unit impracticable
for commercial work. The use of the low value may also lead to error and
is in no way to be recommended for boiler practice.
An example of its illogical use may be shown by the consideration of a
boiler operated in connection with a special economizer where the vapor
produced by hydrogen is partially condensed by the economizer. If the
low value were used in computing the boiler efficiency, it is obvious
that the total efficiency of the combined boiler and economizer must be
in error through crediting the combination with the heat imparted in
condensing the vapor and not charging such heat to the heat value of the
coal.
Heating Value of Gaseous Fuels--The method of computing calorific values
from an ultimate analysis is particularly adapted to solid fuels, with
the exceptions already noted. The heating value of gaseous fuels may be
calculated by Dulong's formula provided another term is added to provide
for any carbon monoxide present. Such a method, however, involves the
separating of the constituent gases into their elementary gases, which
is oftentimes difficult and liable to simple arithmetical error. As the
combustible portion of gaseous fuels is ordinarily composed of hydrogen,
carbon monoxide and certain hydrocarbons, a determination of the
calorific value is much more readily obtained by a separation into their
constituent gases and a computation of the calorific value from a table
of such values of the constituents. Table 37 gives the calorific value
of the more common combustible gases, together with the theoretical
amount of air required for their combustion.
TABLE 37
WEIGHT AND CALORIFIC VALUE OF VARIOUS GASES
AT 32 DEGREES FAHRENHEIT AND ATMOSPHERIC PRESSURE
WITH THEORETICAL AMOUNT OF AIR REQUIRED FOR COMBUSTION
+---------------+----------+------+-----+------+----------+-----------+
| Gas | Symbol |Cubic |B.t.u|B.t.u.|Cubic Feet|Cubic Feet |
| | | Feet | per | per | of Air | of Air |
| | |of Gas|Pound|Cubic | Required | Required |
| | | per | | Foot |per Pound | Per Cubic |
| | |Pound | | | of Gas |Foot of Gas|
+---------------+----------+------+-----+------+----------+-----------+
|Hydrogen | H |177.90|62000| 349 | 428.25 | 2.41 |
|Carbon Monoxide| CO | 12.81| 4450| 347 | 30.60 | 2.39 |
|Methane |CH_{4} | 22.37|23550| 1053 | 214.00 | 9.57 |
|Acetylene |C_{2}H_{2}| 13.79|21465| 1556 | 164.87 | 11.93 |
|Olefiant Gas |C_{2}H_{4}| 12.80|21440| 1675 | 183.60 | 14.33 |
|Ethane |C_{2}H_{6}| 11.94|22230| 1862 | 199.88 | 16.74 |
+---------------+----------+------+-----+------+----------+-----------+
In applying this table, as gas analyses may be reported either by weight
or volume, there is given in Table 33[36] a method of changing from
volumetric analysis to analysis by weight.
Examples:
1st. Assume a blast furnace gas, the analysis of which in percentages by
weight is, oxygen = 2.7, carbon monoxide = 19.5, carbon dioxide = 18.7,
nitrogen = 59.1. Here the only combustible gas is the carbon monoxide,
and the heat value will be,
0.195 × 4450 = 867.75 B. t. u. per pound.
The _net_ volume of air required to burn one pound of this gas will be,
0.195 × 30.6 = 5.967 cubic feet.
2nd. Assume a natural gas, the analysis of which in percentages by
volume is oxygen = 0.40, carbon monoxide = 0.95, carbon dioxide = 0.34,
olefiant gas (C_{2}H_{4}) = 0.66, ethane (C_{2}H_{6}) = 3.55, marsh gas
(CH_{4}) = 72.15 and hydrogen = 21.95. All but the oxygen and the carbon
dioxide are combustibles, and the heat per cubic foot will be,
From CO = 0.0095 × 347 = 3.30
C_{2}H_{4} = 0.0066 × 1675 = 11.05
C_{2}H_{6} = 0.0355 × 1862 = 66.10
CH_{4} = 0.7215 × 1050 = 757.58
H = 0.2195 × 349 = 76.61
------
B. t. u. per cubic foot 914.64
The _net_ air required for combustion of one cubic foot of the gas will
be,
CO = 0.0095 × 2.39 = 0.02
C_{2}H_{4} = 0.0066 × 14.33 = 0.09
C_{2}H_{6} = 0.0355 × 16.74 = 0.59
CH_{4} = 0.7215 × 9.57 = 6.90
H = 0.2195 × 2.41 = 0.53
----
Total net air per cubic foot 8.13
Proximate Analysis--The proximate analysis of a fuel gives its
proportions by weight of fixed carbon, volatile combustible matter,
moisture and ash. A method of making such an analysis which has been
found to give eminently satisfactory results is described below.
From the coal sample obtained on the boiler trial, an average sample of
approximately 40 grams is broken up and weighed. A good means of
reducing such a sample is passing it through an ordinary coffee mill.
This sample should be placed in a double-walled air bath, which should
be kept at an approximately constant temperature of 105 degrees
centigrade, the sample being weighed at intervals until a minimum is
reached. The percentage of moisture can be calculated from the loss in
such a drying.
For the determination of the remainder of the analysis, and the heating
value of the fuel, a portion of this dried sample should be thoroughly
pulverized, and if it is to be kept, should be placed in an air-tight
receptacle. One gram of the pulverized sample should be weighed into a
porcelain crucible equipped with a well fitting lid. This crucible
should be supported on a platinum triangle and heated for seven minutes
over the full flame of a Bunsen burner. At the end of such time the
sample should be placed in a desiccator containing calcium chloride, and
when cooled should be weighed. From the loss the percentage of volatile
combustible matter may be readily calculated.
The same sample from which the volatile matter has been driven should be
used in the determination of the percentage of ash. This percentage is
obtained by burning the fixed carbon over a Bunsen burner or in a muffle
furnace. The burning should be kept up until a constant weight is
secured, and it may be assisted by stirring with a platinum rod. The
weight of the residue determines the percentage of ash, and the
percentage of fixed carbon is easily calculated from the loss during the
determination of ash after the volatile matter has been driven off.
Proximate analyses may be made and reported on a moist or dry basis. The
dry basis is that ordinarily accepted, and this is the basis adopted
throughout this book. The method of converting from a moist to a dry
basis is the same as described in the case of an ultimate analysis. A
proximate analysis is easily made, gives information as to the general
characteristics of a fuel and of its _relative_ heating value.
Table 38 gives the proximate analysis and calorific value of a number of
representative coals found in the United States.
TABLE 38
APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF CERTAIN TYPICAL AMERICAN COALS
____________________________________________________________________________
| | | | | |
| | | | | |
No. | State | County | Field, Bed | Mine | Size |
| | | or Vein | | |
| | | | | |
| | | | | |
____|_______|________________|________________|_______________|_____________|
| | | |
| | ANTHRACITES | |
____|_______|_________________________________________________|_____________|
| | | | | |
1 | Pa. | Carbon | Lehigh | Beaver Meadow | |
2 | Pa. | Dauphin | Schuylkill | | Buckwheat |
3 | Pa. | Lackawanna | Wyoming | Belleview | No. 2 Buck. |
4 | Pa. | Lackawanna | Wyoming | Johnson | Culm. |
5 | Pa. | Luzerne | Wyoming | Pittston | No. 2 Buck. |
6 | Pa. | Luzerne | Wyoming | Mammoth | Large |
7 | Pa. | Luzerne | Wyoming | Exeter | Rice |
8 | Pa. | Northumberland | Schuylkill | Treverton | |
9 | Pa. | Schuylkill | Schuylkill | Buck Mountain | |
10 | Pa. | Schuylkill | | York Farm | Buckwheat |
11 | Pa. | | | Victoria | Buckwheat |
12 | Pa. | Carbon | Lehigh | Lehigh & | Buck. & Pea |
| | | | Wilkes C. Co. | |
13 | Pa. | Carbon | Lehigh | | Buckwheat |
14 | Pa. | Lackawanna | |Del. & Hud. Co.| No. 1 Buck. |
____|_______|________________|________________|_______________|_____________|
| | | |
| | SEMI-ANTHRACITES | |
____|_______|_________________________________________________|_____________|
| | | | | |
15 | Pa. | Lycoming | Loyalsock | | |
16 | Pa. | Sullivan | | Lopez | |
17 | Pa. | Sullivan | Bernice | | |
____|_______|________________|________________|_______________|_____________|
| | | |
| | SEMI-BITUMINOUS | |
____|_______|_________________________________________________|_____________|
| | | | | |
18 | Md. | Alleghany | Big Vein, | | |
| | | George's Crk. | | |
19 | Md. | Alleghany | George's Creek | | |
20 | Md. | Alleghany | George's Creek | | |
21 | Md. | Alleghany | George's Creek | Ocean No. 7 | Mine run |
22 | Md. | Alleghany | Cumberland | | |
23 | Md. | Garrett | | Washington | Mine run |
| | | | No. 3 | |
24 | Pa. | Bradford | | Long Valley | |
25 | Pa. | Tioga | | Antrim | |
26 | Pa. | Cambria | "B" or Miller | Soriman Shaft | |
| | | | C. Co. | |
27 | Pa. | Cambria | "B" or Miller | Henrietta | |
28 | Pa. | Cambria | "B" or Miller | Penker | |
29 | Pa. | Cambria | "B" or Miller | Lancashire | |
30 | Pa. | Cambria | Lower | Penn. C. & C. | Mine run |
| | | Kittanning | Co. No. 3 | |
31 | Pa. | Cambria | Upper | Valley | Mine run |
| | | Kittanning | | |
32 | Pa. | Clearfield | Lower | Eureka | Mine run |
| | | Kittanning | | |
33 | Pa. | Clearfield | | Ghem | Mine run |
34 | Pa. | Clearfield | | Osceola | |
35 | Pa. | Clearfield | Reynoldsville | | |
36 | Pa. | Clearfield | Atlantic- | | Mine run |
| | | Clearfield | | |
37 | Pa. | Huntington | Barnet & Fulton| Carbon | Mine run |
38 | Pa. | Huntington | | Rock Hill | Mine run |
39 | Pa. | Somerset | Lower | Kimmelton | Mine run |
| | | Kittanning | | |
40 | Pa. | Somerset | "C" Prime Vein | Jenner | Mine run |
____|_______|________________|________________|_______________|_____________|
_____________________________________________________________________
| | | |
| Proximate Analysis (Dry Coal) |B. t. u.| |
No. |________________________________________| Per | |
| | | | | Pound | Authority |
| Moisture | Volatile | Fixed | Ash | Dry | |
| | Matter | Carbon | | Coal | |
____|__________|__________|________|_________|________|______________|
| | | | | | |
| | | | | | |
____|__________|__________|________|_________|________|______________|
| | | | | | |
1 | 1.50 | 2.41 | 90.30 | 7.29 | | Gale |
2 | 2.15 | 12.88 | 78.23 | 8.89 | 13137 | Whitham |
3 | 8.29 | 7.81 | 77.19 | 15.00 | 12341 | Sadtler |
4 | 13.90 | 11.16 | 65.96 | 22.88 | 10591 | B. & W. Co. |
5 | 3.66 | 4.40 | 78.96 | 16.64 | 12865 | B. & W. Co. |
6 | 4.00 | 3.44 | 90.59 | 5.97 | 13720 | Carpenter |
7 | 0.25 | 8.18 | 79.61 | 12.21 | 12400 | B. & W. Co. |
8 | 0.84 | 6.73 | 86.39 | 6.88 | | Isherwood |
9 | | 3.17 | 92.41 | 4.42 | 14220 | Carpenter |
10 | 0.81 | 5.51 | 75.90 | 18.59 | 11430 | |
11 | 4.30 | 0.55 | 86.73 | 12.72 | 12642 | B. & W. Co. |
12 | 1.57 | 6.27 | 66.53 | 27.20 | 12848 | B. & W. Co. |
| | | | | | |
13 | | 5.00 | 81.00 | 14.00 | 11800 | Carpenter |
14 | 6.20 | | | 11.60 | 12100 | Denton |
____|__________|__________|________|_________|________|______________|
| | | | | | |
| | | | | | |
____|__________|__________|________|_________|________|______________|
| | | | | | |
15 | 1.30 | 8.72 | 84.44 | 6.84 | | |
16 | 5.48 | 7.53 | 81.00 | 11.47 | 13547 | B. & W. Co. |
17 | 1.29 | 8.21 | 84.43 | 7.36 | | |
____|__________|__________|________|_________|________|______________|
| | | | | | |
| | | | | | |
____|__________|__________|________|_________|________|______________|
| | | | | | |
18 | 3.50 | 21.33 | 72.47 | 6.20 | 14682 | B. & W. Co. |
| | | | | | |
19 | 3.63 | 16.27 | 76.93 | 6.80 | 14695 | B. & W. Co. |
20 | 2.28 | 19.43 | 77.44 | 6.13 | 14793 | B. & W. Co. |
21 | 1.13 | | | | 14451 | B. & W. Co. |
22 | 1.50 | 17.26 | 76.65 | 6.09 | 14700 | |
23 | 2.33 | 14.38 | 74.93 | 10.49 | 14033 | U. S. Geo. S.|
| | | | | | [37] |
24 | 1.55 | 20.33 | 68.38 | 11.29 | 12965 | |
25 | 2.19 | 18.43 | 71.87 | 9.70 | 13500 | |
26 | 3.40 | 20.70 | 71.84 | 7.46 | 14484 | N. Y. Ed. Co.|
| | | | | | |
27 | 1.23 | 18.37 | 75.28 | 6.45 | 14770 | So. Eng. Co. |
28 | 3.64 | 21.34 | 70.48 | 8.18 | 14401 | B. & W. Co. |
29 | 4.38 | 21.20 | 70.27 | 8.53 | 14453 | B. & W. Co. |
30 | 3.51 | 17.43 | 75.69 | 6.88 | 14279 | U. S. Geo. S.|
| | | | | | |
31 | 3.40 | 14.89 | 75.03 | 10.08 | 14152 | B. & W. Co. |
| | | | | | |
32 | 5.90 | 16.71 | 77.22 | 6.07 | 14843 | U. S. Geo. S.|
| | | | | | |
33 | 3.43 | 17.53 | 69.67 | 12.80 | 13744 | B. & W. Co. |
34 | 1.24 | 25.43 | 68.56 | 6.01 | 13589 | B. & W. Co. |
35 | 2.91 | 21.55 | 69.03 | 9.42 | 14685 | B. & W. Co. |
36 | 1.55 | 23.36 | 71.15 | 5.94 | 13963 | Whitham |
| | | | | | |
37 | 4.50 | 18.34 | 73.06 | 8.60 | 13770 | B. & W. Co. |
38 | 5.91 | 17.58 | 73.44 | 8.99 | 14105 | B. & W. Co. |
39 | 3.09 | 17.84 | 70.47 | 11.69 | 13424 | U. S. Geo. S.|
| | | | | | |
40 | 9.37 | 16.47 | 75.76 | 7.77 | 14507 | P. R. R. |
____|__________|__________|________|_________|________|______________|
APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF CERTAIN TYPICAL AMERICAN
COALS--Continued
____________________________________________________________________________
| | | | | |
| | | | | |
No. | State | County | Field, Bed | Mine | Size |
| | | or Vein | | |
| | | | | |
| | | | | |
____|_______|________________|________________|_______________|_____________|
| | | | | |
41 | W. Va.| Fayette | New River | Rush Run | Mine run |
42 | W. Va.| Fayette | New River | Loup Creek | |
43 | W. Va.| Fayette | New River | | Slack |
44 | W. Va.| Fayette | New River | | Mine run |
45 | W. Va.| Fayette | New River | Rush Run | Mine run |
46 | W. Va.| McDowell | Pocahontas | Zenith | Mine run |
| | | No. 3 | | |
47 | W. Va.| McDowell | Tug River | Big Sandy | Mine run |
48 | W. Va.| Mercer | Pocahontas | Mora | Lump |
49 | W. Va.| Mineral | Elk Garden | | |
50 | W. Va.| McDowell | Pocahontas | Flat Top | Mine run |
51 | W. Va.| McDowell | Pocahontas | Flat Top | Slack |
52 | W. Va.| McDowell | Pocahontas | Flat Top | Lump |
____|_______|________________|________________|_______________|_____________|
| | | |
| | BITUMINOUS | |
____|_______|_________________________________________________|_____________|
| | | | | |
53 | Ala. | Bibb | Cahaba | Hill Creek | Mine run |
54 | Ala. | Jefferson | Pratt | Pratt No. 13 | |
55 | Ala. | Jefferson | Pratt | Warner | Mine run |
56 | Ala. | Jefferson | | Coalburg | Mine run |
57 | Ala. | Walker | Horse Creek | Ivy C. & I. | Nut |
| | | | Co. No. 8 | |
58 | Ala. | Walker | Jagger | Galloway C. | Mine run |
| | | | Co. No. 5 | |
59 | Ark. | Franklin | Denning | Western No. 4 | Nut |
60 | Ark. | Sebastian | Jenny Lind | Mine No. 12 | Lump |
61 | Ark. | Sebastian | Huntington | Cherokee | Mine run |
62 | Col. | Boulder | South Platte | Lafayette | Mine run |
63 | Col. | Boulder | Laramie | Simson | Mine run |
64 | Col. | Fremont | Canon City | Chandler | Nut and |
| | | | | Slack |
65 | Col. | Las Animas | Trinidad | Hastings | Nut |
66 | Col. | Las Animas | Trinidad | Moreley | Slack |
67 | Col. | Routt | Yampa | Oak Creek | |
68 | Ill. | Christian | Pana | Penwell Col. | Lump |
69 | Ill. | Franklin | No. 6 | Benton | Egg |
70 | Ill. | Franklin | Big Muddy | Zeigler | ¾ inch |
71 | Ill. | Jackson | Big Muddy | | |
72 | Ill. | La Salle | Streator | | |
73 | Ill. | La Salle | Streator | Marseilles | Mine run |
74 | Ill. | Macoupin | Nilwood | Mine No. 2 | Screenings |
75 | Ill. | Macoupin | Mt. Olive | Mine No. 2 | Mine run |
76 | Ill. | Madison | Belleville | Donk Bros. | Lump |
77 | Ill. | Madison | Glen Carbon | | Mine run |
78 | Ill. | Marion | | Odin | Lump |
79 | Ill. | Mercer | Gilchrist | | Screenings |
80 | Ill. | Montgomery | Pana or No. 5 | Coffeen | Mine run |
81 | Ill. | Peoria | No. 5 | Empire | |
82 | Ill. | Perry | Du Quoin | Number 1 | Screenings |
____|_______|________________|________________|_______________|_____________|
APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF CERTAIN TYPICAL AMERICAN
COALS--Continued
_____________________________________________________________________
| | | |
| Proximate Analysis (Dry Coal) |B. t. u.| |
No. |________________________________________| Per | |
| | | | | Pound | Authority |
| Moisture | Volatile | Fixed | Ash | Dry | |
| | Matter | Carbon | | Coal | |
____|__________|__________|________|_________|________|______________|
| | | | | | |
41 | 2.14 | 22.87 | 71.56 | 5.57 | 14959 | U. S. Geo. S.|
42 | 0.55 | 19.36 | 78.48 | 2.16 | 14975 | Hill |
43 | 6.66 | 20.94 | 73.16 | 5.90 | 14412 | B. & W. Co. |
44 | 2.16 | 17.82 | 75.66 | 6.52 | 14786 | B. & W. Co. |
45 | 0.94 | 22.16 | 75.85 | 1.99 | 15007 | B. & W. Co. |
46 | 4.85 | 17.14 | 76.54 | 6.32 | 14480 | U. S. Geo. S.|
| | | | | | |
47 | 1.58 | 18.55 | 76.44 | 4.91 | 15170 | U. S. Geo. S.|
48 | 1.74 | 18.55 | 75.15 | 6.30 | 15015 | U. S. Geo. S.|
49 | 2.10 | 15.70 | 75.40 | 8.90 | 14195 | B. & W. Co. |
50 | 0.52 | 24.02 | 74.59 | 1.39 | 14490 | B. & W. Co. |
51 | 3.24 | 15.33 | 77.60 | 7.07 | 14653 | B. & W. Co. |
52 | 3.63 | 16.03 | 78.04 | 5.93 | 14956 | B. & W. Co. |
____|__________|__________|________|_________|________|______________|
| | | | | | |
| | | | | | |
____|__________|__________|________|_________|________|______________|
| | | | | | |
53 | 6.19 | 28.58 | 55.60 | 15.82 | 12576 | B. & W. Co. |
54 | 4.29 | 25.78 | 67.68 | 6.54 | 14482 | B. & W. Co. |
55 | 2.51 | 27.80 | 61.50 | 10.70 | 13628 | U. S. Geo. S.|
56 | 0.94 | 31.34 | 65.65 | 3.01 | 14513 | B. & W. Co. |
57 | 2.56 | 31.82 | 53.89 | 14.29 | 12937 | U. S. Geo. S.|
| | | | | | |
58 | 4.83 | 34.65 | 51.12 | 14.03 | 12976 | U. S. Geo. S.|
| | | | | | |
59 | 2.22 | 12.83 | 75.35 | 11.82 | | U. S. Geo. S.|
60 | 1.07 | 17.04 | 74.45 | 8.51 | 14252 | U. S. Geo. S.|
61 | 0.97 | 19.87 | 70.30 | 9.83 | 14159 | U. S. Geo. S.|
62 | 19.48 | 38.80 | 49.00 | 12.20 | 11939 | B. & W. Co. |
63 | 19.78 | 44.69 | 48.62 | 6.69 | 12577 | U. S. Geo. S.|
64 | 9.37 | 38.10 | 51.75 | 10.15 | 11850 | B. & W. Co. |
| | | | | | |
65 | 2.15 | 31.07 | 53.40 | 15.53 | 12547 | B. & W. Co. |
66 | 1.88 | 28.47 | 55.58 | 15.95 | 12703 | B. & W. Co. |
67 | 6.67 | 42.91 | 55.64 | 1.45 | | Hill |
68 | 8.05 | 43.67 | 49.97 | 6.36 | 10900 | Jones |
69 | 8.31 | 34.52 | 54.05 | 11.43 | 11727 | U. S. Geo. S.|
70 | 13.28 | 31.97 | 57.37 | 10.66 | 12857 | U. S. Geo. S.|
71 | 4.85 | 31.55 | 62.19 | 6.26 | 11466 | Breckenridge |
72 | 8.40 | 41.76 | 51.42 | 6.82 | 11727 | Breckenridge |
73 | 12.98 | 43.73 | 49.13 | 7.14 | 10899 | B. & W. Co. |
74 | 13.34 | 34.75 | 44.55 | 20.70 | 10781 | B. & W. Co. |
75 | 13.54 | 41.28 | 46.30 | 12.42 | 10807 | U. S. Geo. S.|
76 | 13.47 | 38.69 | 48.07 | 13.24 | 12427 | U. S. Geo. S.|
77 | 9.78 | 38.18 | 51.52 | 10.30 | 11672 | Bryan |
78 | 6.20 | 42.91 | 49.06 | 8.03 | 11880 | Breckenridge |
79 | 8.50 | 36.17 | 41.64 | 22.19 | 10497 | Breckenridge |
80 | 11.93 | 34.05 | 49.85 | 16.10 | 10303 | U. S. Geo. S.|
81 | 17.64 | 31.91 | 46.17 | 21.92 | 10705 | B. & W. Co. |
82 | 9.81 | 33.67 | 48.36 | 17.97 | 11229 | B. & W. Co. |
____|__________|__________|________|_________|________|______________|
APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF CERTAIN TYPICAL AMERICAN
COALS--Continued
____________________________________________________________________________
| | | | | |
| | | | | |
No. | State | County | Field, Bed | Mine | Size |
| | | or Vein | | |
| | | | | |
| | | | | |
|_______|________________|________________|_______________|_____________|
| | | | | |
83 | Ill. | Perry | Du Quoin | Willis | Mine run |
84 | Ill. | Sangamon | | Pawnee | Slack |
85 | Ill. | St. Clair | Standard | Nigger Hollow | Mine run |
86 | Ill. | St. Clair | Standard | Maryville | Mine run |
87 | Ill. | Williamson | Big Muddy | Daws | Mine run |
88 | Ill. | Williamson | Carterville | Carterville | |
| | | or No. 7 | | |
89 | Ill. | Williamson | Carterville | Burr | Nut, Pea |
| | | or No. 7 | | and Slack |
90 | Ind. | Brazil | Brazil | Gartside | Block |
91 | Ind. | Clay | | Louise | Block |
92 | Ind. | Green | Island City | | Mine run |
93 | Ind. | Knox | Vein No. 5 | Tecumseh | Mine run |
94 | Ind. | Parke | Vein No. 6 | Parke Coal Co.| Lump |
95 | Ind. | Sullivan | Sullivan No. 6 | Mildred | Washed |
96 | Ind. | Vigo | Number 6 | Fontanet | Mine run |
97 | Ind. | Vigo | Number 7 | Red Bird | Mine run |
98 | Iowa | Appanoose | Mystic | Mine No. 3 | Lump |
99 | Iowa | Lucas | Lucas | Inland No. 1 | Mine run |
100 | Iowa | Marion | Big Vein | Liberty No. 5 | Mine run |
101 | Iowa | Polk | Third Seam | Altoona No. 4 | Lump |
102 | Iowa | Wapello | Wapello | | Lump |
103 | Kan. | Cherokee | Weir Pittsburgh| Southwestern | Lump |
| | | | Dev. Co. | |
104 | Kan. | Cherokee | Cherokee | | Screenings |
105 | Kan. | Cherokee | Cherokee | | Lump |
106 | Kan. | Linn | Boicourt | | Lump |
107 | Ky. | Bell | Straight Creek | Str. Ck. C. & | Mine run |
| | | | C. Co. | |
108 | Ky. | Hopkins | Bed No. 9 | Earlington | Lump |
109 | Ky. | Hopkins | Bed No. 9 | Barnsley | Mine run |
110 | Ky. | Hopkins | Vein No. 14 | Nebo |Pea and Slack|
111 | Ky. | Johnson | Vein No. 1 | Miller's Creek| Mine run |
112 | Ky. | Mulenburg | Bed No. 9 | Pierce |Pea and Slack|
113 | Ky. | Pulaski | | Greensburg | |
114 | Ky. | Webster | Bed No. 9 | |Pea and Slack|
115 | Ky. | Whitley | | Jellico |Nut and Slack|
116 | Mo. | Adair | | Danforth | Mine run |
117 | Mo. | Bates | Rich Hill | New Home | Mine run |
118 | Mo. | Clay | Lexington | Mo. City Coal | |
| | | | Co. | |
119 | Mo. | Lafayette | Waverly | Buckthorn | |
120 | Mo. | Lafayette | Waverly | Higbee | |
121 | Mo. | Linn | Bevier | Marceline | |
122 | Mo. | Macon | Bevier | Northwest | |
| | | | Coal Co. | |
123 | Mo. | Morgan | Morgan Co. | Morgan Co. | Mine run |
| | | | Coal Co. | |
124 | Mo. | Putnam | Mendotta | Mendotta No. 8| |
125 | N.Mex.| McKinley | Gallup | Gibson |Pea and Slack|
____|_______|________________|________________|_______________|_____________|
______________________________________________________________________
| | | |
| Proximate Analysis (Dry Coal) |B. t. u.| |
No. |________________________________________| Per | |
| | | | | Pound | Authority |
| Moisture | Volatile | Fixed | Ash | Dry | |
| | Matter | Carbon | | Coal | |
____|__________|__________|________|_________|________|______________|
| | | | | | |
83 | 7.22 | 33.06 | 53.97 | 12.97 | 11352 | U. S. Geo. S.|
84 | 4.81 | 41.53 | 39.62 | 18.85 | 10220 | Jones |
85 | 14.39 | 32.90 | 44.84 | 22.26 | 11059 | B. & W. Co. |
86 | 15.71 | 38.10 | 41.10 | 20.80 | 10999 | B. & W. Co. |
87 | 8.17 | 34.33 | 52.50 | 13.17 | 12643 | U. S. Geo. S.|
88 | 4.66 | 35.65 | 56.86 | 7.49 | 12286 | Univ. of Ill.|
| | | | | | |
89 | 11.91 | 33.70 | 55.90 | 10.40 | 12932 | B. & W. Co. |
| | | | | | |
90 | 2.83 | 40.03 | 51.97 | 8.00 | 13375 | Stillman |
91 | 0.83 | 39.70 | 52.28 | 8.02 | 13248 | Jones |
92 | 6.17 | 35.42 | 53.55 | 11.03 | 11916 | Dearborn |
93 | 10.73 | 35.75 | 54.46 | 9.79 | 12911 | B. & W. Co. |
94 | 10.72 | 44.02 | 46.33 | 9.65 | 11767 | U. S. Geo. S.|
95 | 16.59 | 42.17 | 48.44 | 9.59 | 13377 | U. S. Geo. S.|
96 | 2.28 | 34.95 | 50.50 | 14.55 | 11920 | Dearborn |
97 | 11.62 | 41.17 | 46.76 | 12.07 | 12740 | U. S. Geo. S.|
98 | 13.48 | 39.40 | 43.09 | 17.51 | 11678 | U. S. Geo. S.|
99 | 16.01 | 37.82 | 46.24 | 15.94 | 11963 | U. S. Geo. S.|
100 | 14.88 | 41.53 | 39.63 | 18.84 | 11443 | U. S. Geo. S.|
101 | 12.44 | 41.27 | 40.86 | 17.87 | 11671 | U. S. Geo. S.|
102 | 8.69 | 36.23 | 43.68 | 20.09 | 11443 | U. S. Geo. S.|
103 | 4.31 | 33.88 | 53.67 | 12.45 | 13144 | U. S. Geo. S.|
| | | | | | |
104 | 6.16 | 35.56 | 46.90 | 17.54 | 10175 | Jones |
105 | 1.81 | 34.77 | 52.77 | 12.46 | 12557 | Jones |
106 | 4.74 | 36.59 | 47.07 | 16.34 | 10392 | Jones |
107 | 2.89 | 36.67 | 57.24 | 6.09 | 14362 | U. S. Geo. S.|
| | | | | | |
108 | 6.89 | 40.30 | 55.16 | 4.54 | 13381 | St. Col. Ky. |
109 | 7.92 | 40.53 | 48.70 | 10.77 | 13036 | U. S. Geo. S.|
110 | 8.02 | 31.91 | 54.02 | 14.07 | 12448 | B. & W. Co. |
111 | 5.12 | 38.46 | 58.63 | 2.91 | 13743 | U. S. Geo. S.|
112 | 9.22 | 33.94 | 52.18 | 13.88 | 12229 | B. & W. Co. |
113 | 2.80 | 26.54 | 63.58 | 9.88 | 14095 | N. Y. Ed. Co.|
114 | 7.30 | 31.08 | 60.72 | 8.20 | 13600 | B. & W. Co. |
115 | 3.82 | 31.82 | 58.78 | 9.40 | 13175 | B. & W. Co. |
116 | 9.00 | 30.55 | 46.26 | 23.19 | 9889 | B. & W. Co. |
117 | 7.28 | 37.62 | 43.83 | 18.55 | 12109 | U. S. Geo. S.|
118 | 12.45 | 39.39 | 48.47 | 12.14 | 12875 | Univ. of Mo. |
| | | | | | |
119 | 8.58 | 41.78 | 45.99 | 12.23 | 12735 | Univ. of Mo. |
120 | 10.84 | 31.72 | 55.29 | 12.99 | 12500 | Univ. of Mo. |
121 | 9.45 | 36.72 | 52.20 | 11.08 | 13180 | Univ. of Mo. |
122 | 13.09 | 37.83 | 42.95 | 19.22 | 11500 | U. S. Geo. S.|
| | | | | | |
123 | 12.24 | 45.69 | 47.98 | 6.33 | 14197 | U. S. Geo. S.|
| | | | | | |
124 | 20.78 | 39.36 | 50.00 | 10.64 | 12602 | U. S. Geo. S.|
125 | 12.17 | 36.31 | 51.17 | 12.52 | 12126 | B. & W. Co. |
____|__________|__________|________|_________|________|______________|
APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF CERTAIN TYPICAL AMERICAN
COALS--Continued
____________________________________________________________________________
| | | | | |
| | | | | |
No. | State | County | Field, Bed | Mine | Size |
| | | or Vein | | |
| | | | | |
| | | | | |
|_______|________________|________________|_______________|_____________|
| | | | | |
126 | Ohio | Athens | Hocking Valley | Sunday Creek | Slack |
127 | Ohio | Belmont | Pittsburgh | Neff Coal Co. | Mine run |
| | | No. 8 | | |
128 | Ohio | Columbiana | Middle | Palestine | |
| | | Kittanning | | |
129 | Ohio | Coshocton | Middle | Morgan Run | Mine run |
| | | Kittanning | | |
130 | Ohio | Guernsey | Vein No. 7 | Little Kate | |
131 | Ohio | Hocking | Hocking Valley | | Lump |
132 | Ohio | Hocking | Hocking Valley | | |
133 | Ohio | Jackson | Brookville | Superior | Mine run |
| | | | Coal Co. | |
134 | Ohio | Jackson | Lower | Superior | Mine run |
| | | Kittanning | Coal Co. | |
135 | Ohio | Jackson | Quakertown | Wellston | |
136 | Ohio | Jefferson | Pittsburgh | Crow Hollow | ¾ inch |
| | | or No. 8 | | |
137 | Ohio | Jefferson | Pittsburgh | Rush Run No. 1| ¾ inch |
| | | or No. 8 | | |
138 | Ohio | Perry | Hocking | Congo | |
139 | Ohio | Stark | Massillon | | Slack |
140 | Ohio | Vinton | Brookville | Clarion | Nut and |
| | | or No. 4 | | Slack |
141 | Okla. | Choctaw | McAlester | Edwards No. 1 | Mine run |
142 | Okla. | Choctaw | McAlester | Adamson | Slack |
143 | Okla. | Creek | | Henrietta | Lump and |
| | | | | Slack |
144 | Pa. | Allegheny | Pittsburgh | | Slack |
| | | 3rd Pool | | |
145 | Pa. | Allegheny | Monongahela | Turtle Creek | |
146 | Pa. | Allegheny | Pittsburgh | Bertha | ¾ inch |
147 | Pa. | Cambria | | Beach Creek | Slack |
148 | Pa. | Cambria | Miller | Lincoln | Mine run |
149 | Pa. | Clarion | Lower Freeport | | |
150 | Pa. | Fayette | Connellsville | | Slack |
151 | Pa. | Greene | Youghiogheny | | Lump |
152 | Pa. | Greene | Westmoreland | | Screenings |
153 | Pa. | Indiana | | Iselin | Mine run |
154 | Pa. | Jefferson | | Punxsutawney | Mine run |
155 | Pa. | Lawrence | Middle | | |
| | | Kittanning | | |
156 | Pa. | Mercer | Taylor | | |
157 | Pa. | Washington | Pittsburgh | Ellsworth | |
158 | Pa. | Washington | Youghiogheny | Anderson | ¾ inch |
159 | Pa. | Westmoreland | Pittsburgh | Scott Haven | Lump |
160 | Tenn. | Campbell | Jellico | | |
161 | Tenn. | Claiborne | Mingo | | |
162 | Tenn. | Marion | | Etna | |
163 | Tenn. | Morgan | Brushy Mt. | | |
164 | Tenn. | Scott | Glen Mary No. 4| Glen Mary | |
165 | Tex. | Maverick | | Eagle Pass | |
166 | Tex. | Paolo Pinto | | Thurber | Mine run |
167 | Tex. | Paolo Pinto | | Strawn | Mine run |
168 | Va. | Henrico | | Gayton | |
____|_______|________________|________________|_______________|_____________|
_____________________________________________________________________
| | | |
| Proximate Analysis (Dry Coal) |B. t. u.| |
No. |________________________________________| Per | |
| | | | | Pound | Authority |
| Moisture | Volatile | Fixed | Ash | Dry | |
| | Matter | Carbon | | Coal | |
____|__________|__________|________|_________|________|______________|
| | | | | | |
126 | 12.16 | 34.64 | 53.10 | 12.26 | 12214 | |
127 | 5.31 | 38.78 | 52.22 | 9.00 | 12843 | U. S. Geo. S.|
| | | | | | |
128 | 2.15 | 37.57 | 51.80 | 10.63 | 13370 | Lord & Haas |
| | | | | | |
129 | | 41.76 | 45.24 | 13.00 | 13239 | B. & W. Co. |
| | | | | | |
130 | 6.19 | 33.02 | 59.96 | 7.02 | 13634 | B. & W. Co. |
131 | 6.45 | 39.12 | 50.08 | 10.80 | 12700 | Lord & Haas |
132 | 2.60 | 40.80 | 47.60 | 11.60 | 12175 | Jones |
133 | 7.59 | 38.45 | 43.99 | 17.56 | 11704 | U. S. Geo. S.|
| | | | | | |
134 | 8.99 | 41.43 | 50.06 | 8.51 | 13113 | U. S. Geo. S.|
| | | | | | |
135 | 3.38 | 35.26 | 54.18 | 7.56 | 12506 | Hill |
136 | 4.04 | 40.08 | 52.27 | 9.65 | 13374 | U. S. Geo. S.|
| | | | | | |
137 | 4.74 | 36.08 | 54.81 | 9.11 | 13532 | U. S. Geo. S.|
| | | | | | |
138 | 6 41 | 38.33 | 46.71 | 14.96 | 12284 | B. & W. Co. |
139 | 6.67 | 40.02 | 46.46 | 13.52 | 11860 | B. & W. Co. |
140 | 2.47 | 42.38 | 50.39 | 6.23 | 13421 | U. S. Geo. S.|
| | | | | | |
141 | 4.79 | 39.18 | 49.97 | 10.85 | 13005 | U. S. Geo. S.|
142 | 4.72 | 28.54 | 58.17 | 13.29 | 12105 | B. & W. Co. |
143 | 7.65 | 36.77 | 50.14 | 13.09 | 12834 | U. S. Geo. S.|
| | | | | | |
144 | 1.77 | 32.06 | 57.11 | 10.83 | 13205 | Carpenter |
| | | | | | |
145 | 1.75 | 36.85 | 53.94 | 9.21 | 13480 | Lord & Haas |
146 | 2.61 | 35.86 | 57.81 | 6.33 | 13997 | U. S. Geo. S.|
147 | 3.01 | 32.87 | 55.86 | 11.27 | 13755 | B. & W. Co. |
148 | 5.39 | 30.83 | 61.05 | 8.12 | 13600 | B. & W. Co. |
149 | 0.54 | 35.93 | 57.66 | 6.41 | 13547 | |
150 | 1.85 | 28.73 | 63.22 | 7.95 | 13775 | Whitham |
151 | 1.25 | 32.60 | 54.70 | 12.70 | 13100 | B. & W. Co. |
152 | 11.12 | 31.67 | 55.61 | 12.72 | 13100 | P. R. R. |
153 | 2.70 | 29.33 | 63.56 | 7.11 | 14220 | B. & W. Co. |
154 | 3.38 | 29.33 | 64.93 | 5.73 | 14781 | B. & W. Co. |
155 | 0.70 | 37.06 | 56.24 | 6.70 | 13840 | Lord & Haas |
| | | | | | |
156 | 4.18 | 32.19 | 55.55 | 12.26 | 12820 | B. & W. Co. |
157 | 2.46 | 35.35 | 58.46 | 6.19 | 14013 | U. S. Geo. S.|
158 | 1.00 | 39.29 | 54.80 | 5.91 | 13729 | Jones |
159 | 4.06 | 32.91 | 59.78 | 7.31 | 13934 | B. & W. Co. |
160 | 1.80 | 37.76 | 62.12 | 1.12 | 13846 | U. S. Navy |
161 | 4.40 | 34.31 | 59.22 | 6.47 | | U. S. Geo. S.|
162 | 3.16 | 32.98 | 56.59 | 10.43 | | |
163 | 1.77 | 33.46 | 54.73 | 11.87 | 13824 | B. & W. Co. |
164 | 1.53 | 40.80 | 56.78 | 2.42 | 14625 |Ky. State Col.|
165 | 5.42 | 33.73 | 44.89 | 21.38 | 10945 | B. & W. Co. |
166 | 1.90 | 36.01 | 49.09 | 14.90 | 12760 | B. & W. Co. |
167 | 4.19 | 35.40 | 52.98 | 11.62 | 13202 | B. & W. Co. |
168 | 0.82 | 17.14 | 74.92 | 7.94 | 14363 | B. & W. Co. |
____|__________|__________|________|_________|________|______________|
APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF CERTAIN TYPICAL AMERICAN
COALS--Continued
____________________________________________________________________________
| | | | | |
| | | | | |
No. | State | County | Field, Bed | Mine | Size |
| | | or Vein | | |
| | | | | |
| | | | | |
|_______|________________|________________|_______________|_____________|
| | | | | |
169 | Va. | Lee | Darby | Darby | 1½ inch |
170 | Va. | Lee | McConnel | Wilson | Mine run |
171 | Va. | Wise | Upper Banner | Coburn | 3½ inch |
172 | Va. | Rockingham | | Clover Hill | |
173 | Va. | Russel | Clinchfield | | |
174 | Va. | | Monongahela | Bernmont | |
175 | W. Va.| Harrison | Pittsburgh | Ocean | Mine run |
176 | W. Va.| Harrison | | Girard | Nut, Pea |
| | | | | and Slack |
177 | W. Va.| Kanawha | Winifrede | Winifrede | |
178 | W. Va.| Kanawha | Keystone | Keystone | Mine run |
179 | W. Va.| Logan | Island Creek | |Nut and Slack|
180 | W. Va.| Marion | Fairmont | Kingmont | |
181 | W. Va.| Mingo | Thacker | Maritime | |
182 | W. Va.| Mingo | Glen Alum | Glen Alum | Mine run |
183 | W. Va.| Preston | Bakerstown | | |
184 | W. Va.| Putnam | Pittsburgh | Black Betsy | Bug dust |
185 | W. Va.| Randolph | Upper Freeport | Coalton | Lump and Nut|
____|_______|________________|________________|_______________|_____________|
| | | |
| | LIGNITES AND LIGNITIC COALS | |
____|_______|_________________________________________________|_____________|
| | | | | |
186 | Col. | Boulder | | Rex | |
187 | Col. | El Paso | | Curtis | |
188 | Col. | El Paso | | Pike View | |
189 | Col. | Gunnison | South Platte | Mt. Carbon | |
190 | Col. | Las Animas | | Acme | |
191 | Col. | | Lehigh | | |
192 |N. Dak.| McLean | | Eckland | Mine run |
193 |N. Dak.| McLean | | Wilton | Lump |
194 |N. Dak.| McLean | | Casino | |
195 |N. Dak.| Stark | Lehigh | Lehigh | Mine run |
196 |N. Dak.| William | Williston | | Mine run |
197 |N. Dak.| William | Williston | | Mine run |
198 | Tex. | Bastrop | Bastrop | Glenham | |
199 | Tex. | Houston | Crockett | | |
200 | Tex. | Houston | | Houston C. & | |
| | | | C. Co. | |
201 | Tex. | Milam | Rockdale | Worley | |
202 | Tex. | Robertson | Calvert | Coaling No. 1 | |
203 | Tex. | Wood | Hoyt | Consumer's | |
| | | | Lig. Co. | |
204 | Tex. | Wood | Hoyt | | |
205 | Wash. | King | | Black Diamond | |
206 | Wyo. | Carbon | Hanna | | Mine run |
207 | Wyo. | Crook | Black Hills | Stilwell Coal | |
| | | | Co. | |
208 | Wyo. | Sheridan | Sheridan | Monarch | |
209 | Wyo. | Sweetwater | Rock Spring | | Screenings |
210 | Wyo. | Uinta | Adaville | Lazeart | |
____|_______|________________|________________|_______________|_____________|
_____________________________________________________________________
| | | |
| Proximate Analysis (Dry Coal) |B. t. u.| |
No. |________________________________________| Per | |
| | | | | Pound | Authority |
| Moisture | Volatile | Fixed | Ash | Dry | |
| | Matter | Carbon | | Coal | |
____|__________|__________|________|_________|________|______________|
| | | | | | |
169 | 4.35 | 38.46 | 56.91 | 4.63 | 13939 | U. S. Geo. S.|
170 | 3.35 | 36.35 | 57.88 | 5.77 | 13931 | U. S. Geo. S.|
171 | 3.05 | 32.65 | 62.73 | 4.62 | 14470 | U. S. Geo. S.|
172 | | 31.77 | 57.98 | 10.25 | 13103 | |
173 | 2.00 | 35.72 | 56.12 | 8.16 | 14200 | |
174 | | 32.00 | 59.90 | 8.10 | 13424 | Carpenter |
175 | 2.47 | 39.35 | 52.78 | 7.87 | 14202 | U. S. Geo. S.|
176 | | 36.66 | 57.49 | 5.85 | 14548 | B. & W. Co. |
| | | | | | |
177 | 1.05 | 32.74 | 64.38 | 2.88 | 14111 | Hill |
178 | 2.21 | 33.29 | 58.61 | 8.10 | 14202 | U. S. Geo. S.|
179 | 1.12 | 38.61 | 55.91 | 5.48 | 14273 | Hill |
180 | 1.90 | 35.31 | 57.34 | 7.35 | 14198 | U. S. Geo. S.|
181 | 0.68 | 31.89 | 63.48 | 4.63 | 14126 | Hill |
182 | 3.02 | 33.81 | 59.45 | 6.74 | 14414 | U. S. Geo. S.|
183 | 4.14 | 29.09 | 63.50 | 7.41 | 14546 | U. S. Geo. S.|
184 | 7.41 | 32.84 | 53.96 | 13.20 | 12568 | B. & W. Co. |
185 | 2.11 | 29.57 | 59.93 | 10.50 | 13854 | U. S. Geo. S.|
____|__________|__________|________|_________|________|______________|
| | | | | | |
| | | | | | |
____|__________|__________|________|_________|________|______________|
| | | | | | |
186 | 16.05 | 42.12 | 47.97 | 9.91 | 10678 | B. & W. Co. |
187 | 23.25 | 42.11 | 49.38 | 8.51 | 11090 | B. & W. Co. |
188 | 23.77 | 48.70 | 41.47 | 9.83 | 10629 | B. & W. Co. |
189 | 20.38 | 46.38 | 47.50 | 6.12 | | |
190 | 16.74 | 47.90 | 44.60 | 7.50 | |Col. Sc. of M.|
191 | 18.30 | 45.29 | 44.67 | 10.04 | | |
192 | 29.65 | 45.56 | 47.05 | 7.39 | 10553 | Lord |
193 | 35.96 | 49.84 | 38.05 | 12.11 | 11036 | U. S. Geo. S.|
194 | 29.65 | 46.56 | 38.70 | 14.74 | | Lord |
195 | 35.84 | 43.84 | 39.59 | 16.57 | 10121 | U. S. Geo. S.|
196 | 41.76 | 39.37 | 48.09 | 12.54 | 10121 | B. & W. Co. |
197 | 42.74 | 40.83 | 47.79 | 11.38 | 10271 | B. & W. Co. |
198 | 32.77 | 42.76 | 36.88 | 20.36 | 8958 | B. & W. Co. |
199 | 23.27 | 40.95 | 38.37 | 20.68 | 10886 | U. S. Geo. S.|
200 | 31.48 | 46.93 | 34.40 | 18.87 | 10176 | B. & W. Co. |
| | | | | | |
201 | 32.48 | 43.04 | 41.14 | 15.82 | 10021 | B. & W. Co. |
202 | 32.01 | 43.70 | 43.08 | 13.22 | 10753 | B. & W. Co. |
203 | 33.98 | 46.97 | 41.40 | 11.63 | 10600 | U. S. Geo. S.|
| | | | | | |
204 | 30.25 | 43.27 | 41.46 | 15.27 | 10597 | |
205 | 3.71 | 48.72 | 46.56 | 4.72 | | Gale |
206 | 6.44 | 51.32 | 43.00 | 5.68 | 11607 | B. & W. Co. |
207 | 19.08 | 45.21 | 46.42 | 8.37 | 12641 | U. S. Geo. S.|
| | | | | | |
208 | 21.18 | 51.87 | 40.43 | 7.70 | 12316 | U. S. Geo. S.|
209 | 7.70 | 38.57 | 56.99 | 4.44 | 12534 | B. & W. Co. |
210 | 19.15 | 45.50 | 48.11 | 6.39 | 9868 | U. S. Geo. S.|
____|__________|__________|________|_________|________|______________|
[Illustration: Portion of 12,080 Horse-power Installation of Babcock &
Wilcox Boilers and Superheaters at the Potomac Electric Co., Washington,
D. C.]
TABLE 39
SHOWING RELATION BETWEEN PROXIMATE AND ULTIMATE ANALYSES OF COAL
=========================================================================
| | | | Common in |
| | | |Proximate &|
| | Proximate | | Ultimate |
| | Analysis | Ultimate Analysis | Analysis |
|--------------------|-----------|--------------------------|-----------|
| | | | V | | | H | | N | | | M |
| | | | o | | | y | | i | S | | o |
| | | | l M | C | C | d | O | t | u | | i |
| S | | | a a | F a | a | r | x | r | l | | s |
| t | | | t t | i r | r | o | y | o | p | | t |
| a | Field | | i t | x b | b | g | g | g | h | A | u |
| t | or | | l e | e o | o | e | e | e | e | s | r |
| e | Bed | Mine | e r | d n | n | n | n | n | r | h | e |
|---|-------|--------|-----|-----|-----|----|-----|----|----|-----|-----|
| | |Icy Coal| | | | | | | | | |
| | | & Iron | | | | | | | | | |
| | Horse | Co. | | | | | | | | | |
|Ala| Creek | No. 8 |31.81|53.90|72.02|4.78| 6.45|1.66| .80|14.29| 2.56|
|---|----------------|-----|-----|-----|----|-----|----|----|-----|-----|
| | |Central | | | | | | | | | |
| | |C. & C. | | | | | | | | | |
| | Hunt- | Co. | | | | | | | | | |
|Ark|ington | No. 3 |18.99|67.71|76.37|3.90| 3.71|1.49|1.23|13.30| 1.99|
|---|-------|--------|-----|-----|-----|----|-----|----|----|-----|-----|
| | Pana | Clover | | | | | | | | | |
| | or | Leaf, | | | | | | | | | |
|Ill| No. 5 | No. 1 |37.22|45.64|63.04|4.49|10.04|1.28|4.01|17.14|13.19|
|---|-------|--------|-----|-----|-----|----|-----|----|----|-----|-----|
| |No. 5, | | | | | | | | | | |
| |Warrick| | | | | | | | | | |
|Ind| Co. |Electric|41.85|44.45|68.08|4.78| 7.56|1.35|4.53|13.70| 9.11|
|---|-------|--------|-----|-----|-----|----|-----|----|----|-----|-----|
| |No. 11,| St. | | | | | | | | | |
| |Hopkins|Bernard,| | | | | | | | | |
|Ky | Co. | No. 11 |41.10|49.60|72.22|5.06| 8.44|1.33|3.65| 9.30| 7.76|
|---|-------|--------|-----|-----|-----|----|-----|----|----|-----|-----|
| |"B" or | | | | | | | | | | |
| |Lower | | | | | | | | | | |
| |Kittan-| Eureka,| | | | | | | | | |
|Pa | ning | No. 31 |16.71|77.22|84.45|4.25| 3.04|1.28| .91| 6.07| .56|
|---|-------|--------|-----|-----|-----|----|-----|----|----|-----|-----|
| |Indiana| | | | | | | | | | |
|Pa | Co. | |29.55|62.64|79.86|5.02| 4.27|1.86|1.18| 7.81| 2.90|
|---|-------|--------|-----|-----|-----|----|-----|----|----|-----|-----|
|W. | Fire | Rush | | | | | | | | | |
|Va | Creek | Run |22.87|71.56|83.71|4.64| 3.67|1.70| .71| 5.57| 2.14|
=========================================================================
Table 39 gives for comparison the ultimate and proximate analyses of
certain of the coals with which tests were made in the coal testing
plant of the United States Geological Survey at the Louisiana Purchase
Exposition at St. Louis.
The heating value of a fuel cannot be directly computed from a proximate
analysis, due to the fact that the volatile content varies widely in
different fuels in composition and in heating value.
Some methods have been advanced for estimating the calorific value of
coals from the proximate analysis. William Kent[38] deducted from
Mahler's tests of European coals the approximate heating value dependent
upon the content of fixed carbon in the combustible. The relation as
deduced by Kent between the heat and value per pound of combustible and
the per cent of fixed carbon referred to combustible is represented
graphically by Fig. 23.
Goutal gives another method of determining the heat value from a
proximate analysis, in which the carbon is given a fixed value and the
heating value of the volatile matter is considered as a function of its
percentage referred to combustible. Goutal's method checks closely with
Kent's determinations.
All the formulae, however, for computing the calorific value of coals
from a proximate analysis are ordinarily limited to certain classes of
fuels. Mr. Kent, for instance, states that his deductions are correct
within a close limit for fuels containing more than 60 per cent of fixed
carbon in the combustible, while for those containing a lower
percentage, the error may be as great as 4 per cent, either high or low.
While the use of such computations will serve where approximate results
only are required, that they are approximate should be thoroughly
understood.
Calorimetry--An ultimate or a proximate analysis of a fuel is useful in
determining its general characteristics, and as described on page 183,
may be used in the calculation of the approximate heating value. Where
the efficiency of a boiler is to be computed, however, this heating
value should in all instances be determined accurately by means of a
fuel calorimeter.
[Graph: B.T.U. per Pound of Combustible
against Per Cent of Fixed Carbon in Combustible
Fig. 23. Graphic Representation of Relation between Heat Value Per Pound
of Combustible and Fixed Carbon in Combustible as Deduced by Wm. Kent.]
In such an apparatus the fuel is completely burned and the heat
generated by such combustion is absorbed by water, the amount of heat
being calculated from the elevation in the temperature of the water. A
calorimeter which has been accepted as the best for such work is one in
which the fuel is burned in a steel bomb filled with compressed oxygen.
The function of the oxygen, which is ordinarily under a pressure of
about 25 atmospheres, is to cause the rapid and complete combustion of
the fuel sample. The fuel is ignited by means of an electric current,
allowance being made for the heat produced by such current, and by the
burning of the fuse wire.
A calorimeter of this type which will be found to give satisfactory
results is that of M. Pierre Mahler, illustrated in Fig. 24 and
consisting of the following parts:
A water jacket A, which maintains constant conditions outside of the
calorimeter proper, and thus makes possible a more accurate computation
of radiation losses.
The porcelain lined steel bomb B, in which the combustion of the fuel
takes place in compressed oxygen.
[Illustration: Fig. 24. Mahler Bomb Calorimeter]
The platinum pan C, for holding the fuel.
The calorimeter proper D, surrounding the bomb and containing a definite
weighed amount of water.
An electrode E, connecting with the fuse wire F, for igniting the fuel
placed in the pan C.
A support G, for a water agitator.
A thermometer I, for temperature determination of the water in the
calorimeter. The thermometer is best supported by a stand independent of
the calorimeter, so that it may not be moved by tremors in the parts of
the calorimeter, which would render the making of readings difficult. To
obtain accuracy of readings, they should be made through a telescope or
eyeglass.
A spring and screw device for revolving the agitator.
A lever L, by the movement of which the agitator is revolved.
A pressure gauge M, for noting the amount of oxygen admitted to the
bomb. Between 20 and 25 atmospheres are ordinarily employed.
An oxygen tank O.
A battery or batteries P, the current from which heats the fuse wire
used to ignite the fuel.
This or a similar calorimeter is used in the determination of the heat
of combustion of solid or liquid fuels. Whatever the fuel to be tested,
too much importance cannot be given to the securing of an average
sample. Where coal is to be tested, tests should be made from a portion
of the dried and pulverized laboratory sample, the methods of obtaining
which have been described. In considering the methods of calorimeter
determination, the remarks applied to coal are equally applicable to any
solid fuel, and such changes in methods as are necessary for liquid
fuels will be self-evident from the same description.
Approximately one gram of the pulverized dried coal sample should be
placed directly in the pan of the calorimeter. There is some danger in
the using of a pulverized sample from the fact that some of it may be
blown out of the pan when oxygen is admitted. This may be at least
partially overcome by forming about two grams into a briquette by the
use of a cylinder equipped with a plunger and a screw press. Such a
briquette should be broken and approximately one gram used. If a
pulverized sample is used, care should be taken to admit oxygen slowly
to prevent blowing the coal out of the pan. The weight of the sample is
limited to approximately one gram since the calorimeter is proportioned
for the combustion of about this weight when under an oxygen pressure of
about 25 atmospheres.
A piece of fine iron wire is connected to the lower end of the plunger
to form a fuse for igniting the sample. The weight of iron wire used is
determined, and if after combustion a portion has not been burned, the
weight of such portion is determined. In placing the sample in the pan,
and in adjusting the fuse, the top of the calorimeter is removed. It is
then replaced and carefully screwed into place on the bomb by means of a
long handled wrench furnished for the purpose.
The bomb is then placed in the calorimeter, which has been filled with a
definite amount of water. This weight is the "water equivalent" of the
apparatus, _i. e._, the weight of water, the temperature of which would
be increased one degree for an equivalent increase in the temperature of
the combined apparatus. It may be determined by calculation from the
weights and specific heats of the various parts of the apparatus. Such a
determination is liable to error, however, as the weight of the bomb
lining can only be approximated, and a considerable portion of the
apparatus is not submerged. Another method of making such a
determination is by the adding of definite weights of warm water to
definite amounts of cooler water in the calorimeter and taking an
average of a number of experiments. The best method for the making of
such a determination is probably the burning of a definite amount of
resublimed naphthaline whose heat of combustion is known.
The temperature of the water in the water jacket of the calorimeter
should be approximately that of the surrounding atmosphere. The
temperature of the weighed amount of water in the calorimeter is made by
some experimenters slightly greater than that of the surrounding air in
order that the initial correction for radiation will be in the same
direction as the final correction. Other experimenters start from a
temperature the same or slightly lower than the temperature of the room,
on the basis that the temperature after combustion will be slightly
higher than the room temperature and the radiation correction be either
a minimum or entirely eliminated.
While no experiments have been made to show conclusively which of these
methods is the better, the latter is generally used.
After the bomb has been placed in the calorimeter, it is filled with
oxygen from a tank until the pressure reaches from 20 to 25 atmospheres.
The lower pressure will be sufficient in all but exceptional cases.
Connection is then made to a current from the dry batteries in series so
arranged as to allow completion of the circuit with a switch. The
current from a lighting system should not be used for ignition, as there
is danger from sparking in burning the fuse, which may effect the
results. The apparatus is then ready for the test.
Unquestionably the best method of taking data is by the use of
co-ordinate paper and a plotting of the data with temperatures and time
intervals as ordinates and abscissae. Such a graphic representation is
shown in Fig. 25.
[Graph: Temperature--° C. against Time--Hours and Minutes
Fig. 25. Graphic Method of Recording Bomb Calorimeter Results]
After the bomb is placed in the calorimeter, and before the coal is
ignited, readings of the temperature of the water should be taken at one
minute intervals for a period long enough to insure a constant rate of
change, and in this way determine the initial radiation. The coal is
then ignited by completing the circuit, the temperature at the instant
the circuit is closed being considered the temperature at the beginning
of the combustion. After ignition the readings should be taken at
one-half minute intervals, though because of the rapidity of the
mercury's rise approximate readings only may be possible for at least a
minute after the firing, such readings, however, being sufficiently
accurate for this period. The one-half minute readings should be taken
after ignition for five minutes, and for, say, five minutes longer at
minute intervals to determine accurately the final rate of radiation.
Fig. 25 shows the results of such readings, plotted in accordance with
the method suggested. It now remains to compute the results from this
plotted data.
The radiation correction is first applied. Probably the most accurate
manner of making such correction is by the use of Pfaundler's method,
which is a modification of that of Regnault. This assumes that in
starting with an initial rate of radiation, as represented by the
inclination of the line AB, Fig. 25, and ending with a final radiation
represented by the inclination of the line CD, Fig. 25, that the rate of
radiation for the intermediate temperatures between the points B and C
are proportional to the initial and final rates. That is, the rate of
radiation at a point midway between B and C will be the mean between the
initial and final rates; the rate of radiation at a point three-quarters
of the distance between B and C would be the rate at B plus
three-quarters of the difference in rates at B and C, etc. This method
differs from Regnault's in that the radiation was assumed by Regnault to
be in each case proportional to the difference in temperatures between
the water of the calorimeter and the surrounding air plus a constant
found for each experiment. Pfaundler's method is more simple than that
of Regnault, and the results by the two methods are in practical
agreement.
Expressed as a formula, Pfaundler's method is, though not in form given
by him:
_ _
| R' - R |
C = N|R + ------ (T" - T)| (19)
|_ T' - T _|
Where C = correction in degree centigrade,
N = number of intervals over which correction is made,
R = initial radiation in degrees per interval,
R' = final radiation in degrees per interval,
T = average temperature for period through which initial radiation
is computed,
T" = average temperature over period of combustion[39],
T' = average temperature over period through which final radiation
is computed.[39]
The application of this formula to Fig. 25 is as follows:
As already stated, the temperature at the beginning of combustion is the
reading just before the current is turned on, or B in Fig. 25. The point
C or the temperature at which combustion is presumably completed, should
be taken at a point which falls well within the established final rate
of radiation, and not at the maximum temperature that the thermometer
indicates in the test, unless it lies on the straight line determining
the final radiation. This is due to the fact that in certain instances
local conditions will cause the thermometer to read higher than it
should during the time that the bomb is transmitting heat to the water
rapidly, and at other times the maximum temperature might be lower than
that which would be indicated were readings to be taken at intervals of
less than one-half minute, _i. e._, the point of maximum temperature
will fall below the line determined by the final rate of radiation. With
this understanding AB, Fig. 25, represents the time of initial
radiation, BC the time of combustion, and CD the time of final
radiation. Therefore to apply Pfaundler's correction, formula (19), to
the data as represented by Fig. 25.
N = 6, R = 0, R' = .01, T = 20.29, T' = 22.83,
20.29 + 22.54 + 22.84 + 22.88 + 22.87 + 22.86
T" = --------------------------------------------- = 22.36
6
_ _
| .01 - 0 |
C = 6|0 + -------------(22.36 - 20.29)|
|_ 22.85 - 20.29 _|
= 6 × .008 = .048
Pfaundler's formula while simple is rather long. Mr. E. H. Peabody has
devised a simpler formula with which, under proper conditions, the
variation from correction as found by Pfaundler's method is negligible.
It was noted throughout an extended series of calorimeter tests that the
maximum temperature was reached by the thermometer slightly over one
minute after the time of firing. If this period between the time of
firing and the maximum temperature reported was exactly one minute, the
radiation through this period would equal the radiation per one-half
minute _before firing_ plus the radiation per one-half minute _after the
maximum temperature is reached_; or, the radiation through the one
minute interval would be the average of the radiation per minute before
firing and the radiation per minute after the maximum. A plotted chart
of temperatures would take the form of a curve of three straight lines
(B, C', D) in Fig. 25. Under such conditions, using the notation as in
formula (19) the correction would become,
2R + 2R'
C = ------- + (N - 2)R', or R + (N - 1)R' (20)
2
This formula may be generalized for conditions where the maximum
temperature is reached after a period of more than one minute as
follows:
Let M = the number of intervals between the time of firing and the
maximum temperature. Then the radiation through this period will be an
average of the radiation for M intervals before firing and for M
intervals after the maximum is recorded, or
MR + MR' M M
C = ------- + (N - M)R' = - R + (N - -)R' (21)
2 2 2
In the case of Mr. Peabody's deductions M was found to be approximately
2 and formula (21) becomes directly, C = R + (N - 1)R' or formula (20).
The corrections to be made, as secured by the use of this formula, are
very close to those secured by Pfaundler's method, where the point of
maximum temperature is not more than five intervals later than the point
of firing. Where a longer period than this is indicated in the chart of
plotted temperatures, the approximate formula should not be used. As the
period between firing and the maximum temperature is increased, the
plotted results are further and further away from the theoretical
straight line curve. Where this period is not over five intervals, or
two and a half minutes, an approximation of the straight line curve may
be plotted by eye, and ordinarily the radiation correction to be applied
may be determined very closely from such an approximated curve.
Peabody's approximate formula has been found from a number of tests to
give results within .003 degrees Fahrenheit for the limits within which
its application holds good as described. The value of M, which is not
necessarily a whole number, should be determined for each test, though
in all probability such a value is a constant for any individual
calorimeter which is properly operated.
The correction for radiation as found on page 188 is in all instances to
be added to the range of temperature between the firing point and the
point chosen from which the final radiation is calculated. This
corrected range multiplied by the water equivalent of the calorimeter
gives the heat of combustion in calories of the coal burned in the
calorimeter together with that evolved by the burning of the fuse wire.
The heat evolved by the burning of the fuse wire is found from the
determination of the actual weight of wire burned and the heat of
combustion of one milligram of the wire (1.7 calories), _i. e._,
multiply the weight of wire used by 1.7, the result being in gram
calories or the heat required to raise one gram of water one degree
centigrade.
Other small corrections to be made are those for the formation of nitric
acid and for the combustion of sulphur to sulphuric acid instead of
sulphur dioxide, due to the more complete combustion in the presence of
oxygen than would be possible in the atmosphere.
To make these corrections the bomb of the calorimeter is carefully
washed out with water after each test and the amount of acid determined
from titrating this water with a standard solution of ammonia or of
caustic soda, all of the acid being assumed to be nitric acid. Each
cubic centimeter of the ammonia titrating solution used is equivalent to
a correction of 2.65 calories.
As part of acidity is due to the formation of sulphuric acid, a further
correction is necessary. In burning sulphuric acid the heat evolved per
gram of sulphur is 2230 calories in excess of the heat which would be
evolved if the sulphur burned to sulphur dioxide, or 22.3 calories for
each per cent of sulphur in the coal. One cubic centimeter of the
ammonia solution is equivalent to 0.00286 grams of sulphur as sulphuric
acid, or to 0.286 × 22.3 = 6.38 calories. It is evident therefore that
after multiplying the number of cubic centimeters used in titrating by
the heat factor for nitric acid (2.65) a further correction of
6.38 - 2.65 = 3.73 is necessary for each cubic centimeter used in
titrating sulphuric instead of nitric acid. This correction will be
3.73/0.297 = 13 units for each 0.01 gram of sulphur in the coal.
The total correction therefore for the aqueous nitric and sulphuric acid
is found by multiplying the ammonia by 2.65 and adding 13 calories for
each 0.01 gram of sulphur in the coal. This total correction is to be
deducted from the heat value as found from the corrected range and the
amount equivalent to the calorimeter.
After each test the pan in which the coal has been burned must be
carefully examined to make sure that all of the sample has undergone
complete combustion. The presence of black specks ordinarily indicates
unburned coal, and often will be found where the coal contains bone or
slate. Where such specks are found the tests should be repeated. In
testing any fuel where it is found difficult to completely consume a
sample, a weighed amount of naphthaline may be added, the total weight
of fuel and naphthaline being approximately one gram. The naphthaline
has a known heat of combustion, samples for this purpose being
obtainable from the United States Bureau of Standards, and from the
combined heat of combustion of the fuel and naphthaline that of the
former may be readily computed.
The heat evolved in burning of a definite weight of standard naphthaline
may also be used as a means of calibrating the calorimeter as a whole.
COMBUSTION OF COAL
The composition of coal varies over such a wide range, and the methods
of firing have to be altered so greatly to suit the various coals and
the innumerable types of furnaces in which they are burned, that any
instructions given for the handling of different fuels must of necessity
be of the most general character. For each kind of coal there is some
method of firing which will give the best results for each individual
set of conditions. General rules can be suggested, but the best results
can be obtained only by following such methods as experience and
practice show to be the best suited to the specific conditions.
The question of draft is an all important factor. If this be
insufficient, proper combustion is impossible, as the suction in the
furnace will not be great enough to draw the necessary amount of air
through the fuel bed, and the gases may pass off only partially
consumed. On the other hand, an excessive draft may cause losses due to
the excess quantities of air drawn through holes in the fire. Where coal
is burned however, there are rarely complaints from excessive draft, as
this can be and should be regulated by the boiler damper to give only
the draft necessary for the particular rate of combustion desired. The
draft required for various kinds of fuel is treated in detail in the
chapter on "Chimneys and Draft". In this chapter it will be assumed that
the draft is at all times ample and that it is regulated to give the
best results for each kind of coal.
TABLE 40
ANTHRACITE COAL SIZES
_________________________________________________________________
| | | |
| | | Testing Segments |
| | Round Mesh | Standard Square |
| | | Mesh |
| Trade Name |__________________|__________________|
| | | | | |
| | Through | Over | Through | Over |
| | Inches | Inches | Inches | Inches |
|___________________________|_________|________|_________|________|
| | | | | |
| Broken | 4-1/2 | 3-1/4 | 4 | 2-3/4 |
| Egg | 3-1/4 | 2-3/8 | 2-3/4 | 2 |
| Stove | 2-3/8 | 1-5/8 | 2 | 1-3/8 |
| Chestnut | 1-5/8 | 7/8 | 1-3/8 | 3/4 |
| Pea | 7/8 | 5/8 | 3/4 | 1/2 |
| No. 1 Buckwheat | 5/8 | 3/8 | 1/2 | 1/4 |
| No. 2 Buckwheat or Rice | 3/8 | 3/16 | 1/4 | 1/8 |
| No. 3 Buckwheat or Barley | 3/16 | 3/32 | 1/8 | 1/16 |
|___________________________|_________|________|_________|________|
Anthracite--Anthracite coal is ordinarily marketed under the names and
sizes given in Table 40.
The larger sizes of anthracite are rarely used for commercial steam
generating purposes as the demand for domestic use now limits the
supply. In commercial plants the sizes generally found are Nos. 1, 2 and
3 buckwheat. In some plants where the finer sizes are used, a small
percentage of bituminous coal, say, 10 per cent, is sometimes mixed with
the anthracite and beneficial results secured both in economy and
capacity.
Anthracite coal should be fired evenly, in small quantities and at
frequent intervals. If this method is not followed, dead spots will
appear in the fire, and if the fire gets too irregular through burning
in patches, nothing can be done to remedy it until the fire is cleaned
as a whole. After this grade of fuel has been fired it should be left
alone, and the fire tools used as little as possible. Owing to the
difficulty of igniting this fuel, care must be taken in cleaning fires.
The intervals of cleaning will, of course, depend upon the nature of the
coal and the rate of combustion. With the small sizes and moderately
high combustion rates, fires will have to be cleaned twice on each
eight-hour shift. As the fires become dirty the thickness of the fuel
bed will increase, until this depth may be 12 or 14 inches just before a
cleaning period. In cleaning, the following practice is usually
followed: The good coal on the forward half of the grate is pushed to
the rear half, and the refuse on the front portion either pulled out or
dumped. The good coal is then pulled forward onto the front part of the
grate and the refuse on the rear section dumped. The remaining good coal
is then spread evenly over the whole grate surface and the fire built up
with fresh coal.
A ratio of grate surface to heating surface of 1 to from 35 to 40 will
under ordinary conditions develop the rated capacity of a boiler when
burning anthracite buckwheat. Where the finer sizes are used, or where
overloads are desirable, however, this ratio should preferably be 1 to
25 and a forced blast should be used. Grates 10 feet deep with a slope
of 1½ inches to the foot can be handled comfortably with this class of
fuel, and grates 12 feet deep with the same slope can be successfully
handled. Where grates over 8 feet in depth are necessary, shaking grates
or overlapping dumping grates should be used. Dumping grates may be
applied either for the whole grate surface or to the rear section. Air
openings in the grate bars should be made from 3/16 inch in width for
No. 3 buckwheat to 5/16 inch for No. 1 buckwheat. It is important that
these air openings be uniformly distributed over the whole surface to
avoid blowing holes in the fire, and it is for this reason that
overlapping grates are recommended.
No air should be admitted over the fire. Steam is sometimes introduced
into the ashpit to soften any clinker that may form, but the quantity of
steam should be limited to that required for this purpose. The steam
that may be used in a steam jet blower for securing blast will in
certain instances assist in softening the clinker, but a much greater
quantity may be used by such an apparatus than is required for this
purpose. Combustion arches sprung above the grates have proved of
advantage in maintaining a high furnace temperature and in assisting in
the ignition of fresh coal.
Stacks used with forced blast should be of such size as to insure a
slight suction in the furnace under any conditions of operation. A blast
up to 3 inches of water should be available for the finer sizes supplied
by engine driven fans, automatically controlled by the boiler pressure.
The blast required will increase as the depth of the fuel bed increases,
and the slight suction should be maintained in the furnace by damper
regulation.
The use of blast with the finer sizes causes rapid fouling of the
heating surfaces of the boiler, the dust often amounting to over 10 per
cent of the total fuel fired. Economical disposal of dust and ashes is
of the utmost importance in burning fuel of this nature. Provision
should be made in the baffling of the boiler to accommodate and dispose
of this dust. Whenever conditions permit, the ashes can be economically
disposed of by flushing them out with water.
Bituminous Coals--There is no classification of bituminous coal as to
size that holds good in all localities. The American Society of
Mechanical Engineers suggests the following grading:
_Eastern Bituminous Coals_--
(A) Run of mine coal; the unscreened coal taken from the mine.
(B) Lump coal; that which passes over a bar-screen with openings 1¼
inches wide.
(C) Nut coal; that which passes through a bar-screen with 1¼-inch
openings and over one with ¾-inch openings.
(D) Slack coal; that which passes through a bar-screen with ¾-inch
openings.
_Western Bituminous Coals_--
(E) Run of mine coal; the unscreened coal taken from the mine.
(F) Lump coal; divided into 6-inch, 3-inch and 1¼-inch lump, according
to the diameter of the circular openings over which the respective
grades pass; also 6 × 3-inch lump and 3 × 1¼-inch lump, according as
the coal passes through a circular opening having the diameter of
the larger figure and over that of the smaller diameter.
(G) Nut coal; divided into 3-inch steam nut, which passes through an
opening 3 inches diameter and over 1¼ inches; 1¼ inch nut, which
passes through a 1¼-inch diameter opening and over a ¾-inch
diameter opening; ¾-inch nut, which passes through a ¾-inch
diameter opening and over a 5/8-inch diameter opening.
(H) Screenings; that which passes through a 1¼-inch diameter opening.
As the variation in character of bituminous coals is much greater than
in the anthracites, any rules set down for their handling must be the
more general. The difficulties in burning bituminous coals with economy
and with little or no smoke increases as the content of fixed carbon in
the coal decreases. It is their volatile content which causes the
difficulties and it is essential that the furnaces be designed to
properly handle this portion of the coal. The fixed carbon will take
care of itself, provided the volatile matter is properly burned.
Mr. Kent, in his "Steam Boiler Economy", described the action of
bituminous coal after it is fired as follows: "The first thing that the
fine fresh coal does is to choke the air spaces existing through the bed
of coke, thus shutting off the air supply which is needed to burn the
gases produced from the fresh coal. The next thing is a very rapid
evaporation of moisture from the coal, a chilling process, which robs
the furnace of heat. Next is the formation of water-gas by the chemical
reaction, C + H_{2}O = CO + 2H, the steam being decomposed, its oxygen
burning the carbon of the coal to carbonic oxide, and the hydrogen being
liberated. This reaction takes place when steam is brought in contact
with highly heated carbon. This also is a chilling process, absorbing
heat from the furnaces. The two valuable fuel gases thus generated would
give back all the heat absorbed in their formation if they could be
burned, but there is not enough air in the furnace to burn them.
Admitting extra air through the fire door at this time will be of no
service, for the gases being comparatively cool cannot be burned unless
the air is highly heated. After all the moisture has been driven off
from the coal, the distillation of hydrocarbons begins, and a
considerable portion of them escapes unburned, owing to the deficiency
of hot air, and to their being chilled by the relatively cool heating
surfaces of the boiler. During all this time great volumes of smoke are
escaping from the chimney, together with unburned hydrogen,
hydrocarbons, and carbonic oxide, all fuel gases, while at the same time
soot is being deposited on the heating surface, diminishing its
efficiency in transmitting heat to the water."
To burn these gases distilled from the coal, it is necessary that they
be brought into contact with air sufficiently heated to cause them to
ignite, that sufficient space be allowed for their mixture with the air,
and that sufficient time be allowed for their complete combustion before
they strike the boiler heating surfaces, since these surfaces are
comparatively cool and will lower the temperature of the gases below
their ignition point. The air drawn through the fire by the draft
suction is heated in its passage and heat is added by radiation from the
hot brick surfaces of the furnace, the air and volatile gases mixing as
this increase in temperature is taking place. Thus in most instances is
the first requirement fulfilled. The element of space for the proper
mixture of the gases with the air, and of time in which combustion is to
take place, should be taken care of by sufficiently large combustion
chambers.
Certain bituminous coals, owing to their high volatile content, require
that the air be heated to a higher temperature than it is possible for
it to attain simply in its passage through the fire and by absorption
from the side walls of the furnace. Such coals can be burned with the
best results under fire brick arches. Such arches increase the
temperature of the furnace and in this way maintain the heat that must
be present for ignition and complete combustion of the fuels in
question. These fuels too, sometimes require additional combustion
space, and an extension furnace will give this in addition to the
required arches.
As stated, the difficulty of burning bituminous coals successfully will
increase with the increase in volatile matter. This percentage of
volatile will affect directly the depth of coal bed to be carried and
the intervals of firing for the most satisfactory results. The variation
in the fuel over such wide ranges makes it impossible to definitely
state the thickness of fires for all classes, and experiment with the
class of fuel in use is the best method of determining how that
particular fuel should be handled. The following suggestions, which are
not to be considered in any sense hard and fast rules, may be of service
for general operating conditions for hand firing:
Semi-bituminous coals, such as Pocahontas, New River, Clearfield, etc.,
require fires from 10 to 14 inches thick; fresh coal should be fired at
intervals of 10 to 20 minutes and sufficient coal charged at each firing
to maintain a uniform thickness. Bituminous coals from Pittsburgh Region
require fires from 4 to 6 inches thick, and should be fired often in
comparatively small charges. Kentucky, Tennessee, Ohio and Illinois
coals require a thickness from 4 to 6 inches. Free burning coals from
Rock Springs, Wyoming, require from 6 to 8 inches, while the poorer
grades of Montana, Utah and Washington bituminous coals require a depth
of about 4 inches.
In general as thin fires are found necessary, the intervals of firing
should be made more frequent and the quantity of coal fired at each
interval smaller. As thin fires become necessary due to the character of
the coal, the tendency to clinker will increase if the thickness be
increased over that found to give the best results.
There are two general methods of hand firing: 1st, the spreading method;
and 2nd, the coking method.
[Illustration: Babcock & Wilcox Chain Grate Stoker]
In the spreading method but little fuel is fired at one time, and is
spread evenly over the fuel bed from front to rear. Where there is more
than one firing door the doors should be fired alternately. The
advantage of alternate firing is the whole surface of the fire is not
blanketed with green coal, and steam is generated more uniformly than if
all doors were fired at one time. Again, a better combustion results due
to the burning of more of the volatile matter directly after firing than
where all doors are fired at one time.
In the coking method, fresh coal is fired at considerable depth at the
front of the grate and after it is partially coked it is pushed back
into the furnace. The object of such a method is the preserving of a bed
of carbon at the rear of the grate, in passing over which the volatile
gases driven off from the green coal will be burned. This method is
particularly adaptable to a grate in which the gases are made to pass
horizontally over the fire. Modern practice for hand firing leans more
and more toward the spread firing method. Again the tendency is to work
bituminous coal fires less than formerly. A certain amount of slicing
and raking may be necessary with either method of firing, but in
general, the less the fire is worked the better the results.
Lignites--As the content of volatile matter and moisture in lignite is
higher than in bituminous coal, the difficulties encountered in burning
them are greater. A large combustion space is required and the best
results are obtained where a furnace of the reverberatory type is used,
giving the gases a long travel before meeting the tube surfaces. A fuel
bed from 4 to 6 inches in depth can be maintained, and the coal should
be fired in small quantities by the alternate method. Above certain
rates of combustion clinker forms rapidly, and a steam jet in the ashpit
for softening this clinker is often desirable. A considerable draft
should be available, but it should be carefully regulated by the boiler
damper to suit the condition of the fire. Smokelessness with hand firing
with this class of fuel is a practical impossibility. It has a strong
tendency to foul the heating surfaces rapidly and these surfaces should
be cleaned frequently. Shaking grates, intelligently handled, aid in
cleaning the fires, but their manipulation must be carefully watched to
prevent good coal being lost in the ashpit.
Stokers--The term "automatic stoker" oftentimes conveys the erroneous
impression that such an apparatus takes care of itself, and it must be
thoroughly understood that any stoker requires expert attention to as
high if not higher degree than do hand-fired furnaces.
Stoker-fired furnaces have many advantages over hand firing, but where a
stoker installation is contemplated there are many factors to be
considered. It is true that stokers feed coal to the fire automatically,
but if the coal has first to be fed to the stoker hopper by hand, its
automatic advantage is lost. This is as true of the removal of ash from
a stoker. In a general way, it may be stated that a stoker installation
is not advantageous except possibly for diminishing smoke, unless the
automatic feature is carried to the handling of the coal and ash, as
where coal and ash handling apparatus is not installed there is no
saving in labor. In large plants, however, stokers used in conjunction
with the modern methods of coal storage and coal and ash handling, make
possible a large labor saving. In small plants the labor saving for
stokers over hand-fired furnaces is negligible, and the expense of the
installation no less proportionately than in large plants. Stokers are,
therefore, advisable in small plants only where the saving in fuel will
be large, or where the smoke question is important.
Interest on investment, repairs, depreciation and steam required for
blast and stoker drive must all be considered. The upkeep cost will, in
general, be higher than for hand-fired furnaces. Stokers, however, make
possible the use of cheaper fuels with as high or higher economy than is
obtainable under operating conditions in hand-fired furnaces with a
better grade of fuel. The better efficiency obtainable with a good
stoker is due to more even and continuous firing as against the
intermittent firing of hand-fired furnaces; constant air supply as
against a variation in this supply to meet varying furnace conditions in
hand-fired furnaces; and the doing away to a great extent with the
necessity of working the fires.
Stokers under ordinary operating conditions will give more nearly
smokeless combustion than will hand-fired furnaces and for this reason
must often be installed regardless of other considerations. While a
constant air supply for a given power is theoretically secured by the
use of a stoker, and in many instances the draft is automatically
governed, the air supply should, nevertheless, be as carefully watched
and checked by flue gas analyses as in the case of hand-fired furnaces.
There is a tendency in all stokers to cause the loss of some good fuel
or siftings in the ashpit, but suitable arrangements may be made to
reclaim this.
In respect to efficiency of combustion, other conditions being equal,
there will be no appreciable difference with the different types of
stokers, provided that the proper type is used for the grade of fuel to
be burned and the conditions of operation to be fulfilled. No stoker
will satisfactorily handle all classes of fuel, and in making a
selection, care should be taken that the type is suited to the fuel and
the operating conditions. A cheap stoker is a poor investment. Only the
best stoker suited to the conditions which are to be met should be
adopted, for if there is to be a saving, it will more than cover the
cost of the best over the cheaper stoker.
Mechanical Stokers are of three general types: 1st, overfeed; 2nd,
underfeed; and 3rd, traveling grate. The traveling grate stokers are
sometimes classed as overfeed but properly should be classed by
themselves as under certain conditions they are of the underfeed rather
than the overfeed type.
Overfeed Stokers in general may be divided into two classes, the
distinction being in the direction in which the coal is fed relative to
the furnaces. In one class the coal is fed into hoppers at the front end
of the furnace onto grates with an inclination downward toward the rear
of about 45 degrees. These grates are reciprocated, being made to take
alternately level and inclined positions and this motion gradually
carries the fuel as it is burned toward the rear and bottom of the
furnace. At the bottom of the grates flat dumping sections are supplied
for completing the combustion and for cleaning. The fuel is partly
burned or coked on the upper portion of the grates, the volatile gases
driven off in this process for a perfect action being ignited and burned
in their passage over the bed of burning carbon lower on the grates, or
on becoming mixed with the hot gases in the furnace chamber. In the
second class the fuel is fed from the sides of the furnace for its full
depth from front to rear onto grates inclined toward the center of the
furnace. It is moved by rocking bars and is gradually carried to the
bottom and center of the furnace as combustion advances. Here some type
of a so-called clinker breaker removes the refuse.
Underfeed Stokers are either horizontal or inclined. The fuel is fed
from underneath, either continuously by a screw, or intermittently by
plungers. The principle upon which these stokers base their claims for
efficiency and smokelessness is that the green fuel is fed under the
coked and burning coal, the volatile gases from this fresh fuel being
heated and ignited in their passage through the hottest portion of the
fire on the top. In the horizontal classes of underfeed stokers, the
action of a screw carries the fuel back through a retort from which it
passes upward, as the fuel above is consumed, the ash being finally
deposited on dead plates on either side of the retort, from which it can
be removed. In the inclined class, the refuse is carried downward to the
rear of the furnace where there are dumping plates, as in some of the
overfeed types.
Underfeed stokers are ordinarily operated with a forced blast, this in
some cases being operated by the same mechanism as the stoker drive,
thus automatically meeting the requirements of various combustion rates.
Traveling Grates are of the class best illustrated by chain grate
stokers. As implied by the name these consist of endless grates composed
of short sections of bars, passing over sprockets at the front and rear
of the furnace. Coal is fed by gravity onto the forward end of the
grates through suitable hoppers, is ignited under ignition arches and is
carried with the grate toward the rear of the furnace as its combustion
progresses. When operated properly, the combustion is completed as the
fire reaches the end of the grate and the refuse is carried over this
rear end by the grate in making the turn over the rear sprocket. In some
cases auxiliary dumping grates at the rear of the chain grates are used
with success.
Chain grate stokers in general produce less smoke than either overfeed
or underfeed types, due to the fact that there are no cleaning periods
necessary. Such periods occur with the latter types of stokers at
intervals depending upon the character of the fuel used and the rate of
combustion. With chain grate stokers the cleaning is continuous and
automatic, and no periods occur when smoke will necessarily be produced.
In the earlier forms, chain grates had an objectionable feature in that
the admission of large amounts of excess air at the rear of the furnace
through the grates was possible. This objection has been largely
overcome in recent models by the use of some such device as the bridge
wall water box and suitable dampers. A distinct advantage of chain
grates over other types is that they can be withdrawn from the furnace
for inspection or repairs without interfering in any way with the boiler
setting.
This class of stoker is particularly successful in burning low grades of
coal running high in ash and volatile matter which can only be burned
with difficulty on the other types. The cost of up-keep in a chain
grate, properly constructed and operated, is low in comparison with the
same cost for other stokers.
The Babcock & Wilcox chain grate is representative of this design of
stoker.
Smoke--The question of smoke and smokelessness in burning fuels has
recently become a very important factor of the problem of combustion.
Cities and communities throughout the country have passed ordinances
relative to the quantities of smoke that may be emitted from a stack,
and the failure of operators to live up to the requirements of such
ordinances, resulting as it does in fines and annoyance, has brought
their attention forcibly to the matter.
The whole question of smoke and smokelessness is to a large extent a
comparative one. There are any number of plants burning a wide variety
of fuels in ordinary hand-fired furnaces, in extension furnaces and on
automatic stokers that are operating under service conditions,
practically without smoke. It is safe to say, however, that no plant
will operate smokelessly under any and all conditions of service, nor is
there a plant in which the degree of smokelessness does not depend
largely upon the intelligence of the operating force.
[Illustration: Fig. 26. Babcock & Wilcox Boiler and Superheater Equipped
with Babcock & Wilcox Chain Grate Stoker. This Setting has been
Particularly Successful in Minimizing Smoke]
When a condition arises in a boiler room requiring the fires to be
brought up quickly, the operatives in handling certain types of stokers
will use their slice bars freely to break up the green portion of the
fire over the bed of partially burned coal. In fact, when a load is
suddenly thrown on a station the steam pressure can often be maintained
only in this way, and such use of the slice bar will cause smoke with
the very best type of stoker. In a certain plant using a highly volatile
coal and operating boilers equipped with ordinary hand-fired furnaces,
extension hand-fired furnaces and stokers, in which the boilers with the
different types of furnaces were on separate stacks, a difference in
smoke from the different types of furnaces was apparent at light loads,
but when a heavy load was thrown on the plant, all three stacks would
smoke to the same extent, and it was impossible to judge which type of
furnace was on one or the other of the stacks.
In hand-fired furnaces much can be accomplished by proper firing. A
combination of the alternate and spreading methods should be used, the
coal being fired evenly, quickly, lightly and often, and the fires
worked as little as possible. Smoke can be diminished by giving the
gases a long travel under the action of heated brickwork before they
strike the boiler heating surfaces. Air introduced over the fires and
the use of heated arches, etc., for mingling the air with the gases
distilled from the coal will also diminish smoke. Extension furnaces
will undoubtedly lessen smoke where hand firing is used, due to the
increase in length of gas travel and the fact that this travel is
partially under heated brickwork. Where hand-fired grates are
immediately under the boiler tubes, and a high volatile coal is used, if
sufficient combustion space is not provided the volatile gases,
distilled as soon as the coal is thrown on the fire, strike the tube
surfaces and are cooled below the burning point before they are wholly
consumed and pass through as smoke. With an extension furnace, these
volatile gases are acted upon by the radiant heat from the extension
furnace arch and this heat, together with the added length of travel
causes their more complete combustion before striking the heating
surfaces than in the former case.
Smoke may be diminished by employing a baffle arrangement which gives
the gases a fairly long travel under heated brickwork and by introducing
air above the fire. In many cases, however, special furnaces for smoke
reduction are installed at the expense of capacity and economy.
From the standpoint of smokelessness, undoubtedly the best results are
obtained with a good stoker, properly operated. As stated above, the
best stoker will cause smoke under certain conditions. Intelligently
handled, however, under ordinary operating conditions, stoker-fired
furnaces are much more nearly smokeless than those which are hand fired,
and are, to all intents and purposes, smokeless. In practically all
stoker installations there enters the element of time for combustion,
the volatile gases as they are distilled being acted upon by ignition or
other arches before they strike the heating surfaces. In many instances
too, stokers are installed with an extension beyond the boiler front,
which gives an added length of travel during which, the gases are acted
upon by the radiant heat from the ignition or supplementary arches, and
here again we see the long travel giving time for the volatile gases to
be properly consumed.
To repeat, it must be emphatically borne in mind that the question of
smokelessness is largely one of degree, and dependent to an extent much
greater than is ordinarily appreciated upon the handling of the fuel and
the furnaces by the operators, be these furnaces hand fired or
automatically fired.
[Illustration: 3520 Horse-power Installation of Babcock & Wilcox Boilers
at the Portland Railway, Light and Power Co., Portland, Ore. These
Boilers are Equipped with Wood Refuse Extension Furnaces at the Front
and Oil Burning Furnaces at the Mud Drum End]
SOLID FUELS OTHER THAN COAL AND THEIR COMBUSTION
Wood--Wood is vegetable tissue which has undergone no geological change.
Usually the term is used to designate those compact substances
familiarly known as tree trunks and limbs. When newly cut, wood contains
moisture varying from 30 per cent to 50 per cent. When dried for a
period of about a year in the atmosphere, the moisture content will be
reduced to 18 or 20 per cent.
TABLE 41
ULTIMATE ANALYSES AND CALORIFIC VALUES OF DRY WOOD (GOTTLIEB)
_______________________________________________________
| | | | | | | |
| Kind | | | | | | B. t. u.|
| of | C | H | N | O | Ash | per |
| Wood | | | | | | Pound |
|________|_______|______|______|_______|______|_________|
| | | | | | | |
| Oak | 50.16 | 6.02 | 0.09 | 43.36 | 0.37 | 8316 |
| Ash | 49.18 | 6.27 | 0.07 | 43.91 | 0.57 | 8480 |
| Elm | 48.99 | 6.20 | 0.06 | 44.25 | 0.50 | 8510 |
| Beech | 49.06 | 6.11 | 0.09 | 44.17 | 0.57 | 8391 |
| Birch | 48.88 | 6.06 | 0.10 | 44.67 | 0.29 | 8586 |
| Fir | 50.36 | 5.92 | 0.05 | 43.39 | 0.28 | 9063 |
| Pine | 50.31 | 6.20 | 0.04 | 43.08 | 0.37 | 9153 |
| Poplar | 49.37 | 6.21 | 0.96 | 41.60 | 1.86 | 7834[40]|
| Willow | 49.96 | 5.96 | 0.96 | 39.56 | 3.37 | 7926[40]|
|________|_______|______|______|_______|______|_________|
Wood is usually classified as hard wood, including oak, maple, hickory,
birch, walnut and beech; and soft wood, including pine, fir, spruce,
elm, chestnut, poplar and willow. Contrary to general opinion, the heat
value per pound of soft wood is slightly greater than the same value per
pound of hard wood. Table 41 gives the chemical composition and the heat
values of the common woods. Ordinarily the heating value of wood is
considered equivalent to 0.4 that of bituminous coal. In considering the
calorific value of wood as given in this table, it is to be remembered
that while this value is based on air-dried wood, the moisture content
is still about 20 per cent of the whole, and the heat produced in
burning it will be diminished by this amount and by the heat required to
evaporate the moisture and superheat it to the temperature of the gases.
The heat so absorbed may be calculated by the formula giving the loss
due to moisture in the fuel, and the net calorific value determined.
In designing furnaces for burning wood, the question resolves itself
into: 1st, the essential elements to give maximum capacity and
efficiency with this class of fuel; and 2nd, the construction which will
entail the least labor in handling and feeding the fuel and removing the
refuse after combustion.
Wood, as used commercially for steam generating purposes, is usually a
waste product from some industrial process. At the present time refuse
from lumber and sawmills forms by far the greater part of this class of
fuel. In such refuse the moisture may run as high as 60 per cent and the
composition of the fuel may vary over wide ranges during different
portions of the mill operation. The fuel consists of sawdust, "hogged"
wood and slabs, and the percentage of each of these constituents may
vary greatly. Hogged wood is mill refuse and logs that have been passed
through a "hogging machine" or macerator. This machine, through the
action of revolving knives, cuts or shreds the wood into a state in
which it may readily be handled as fuel.
Table 42 gives the moisture content and heat value of typical sawmill
refuse from various woods.
TABLE 42
MOISTURE AND CALORIFIC VALUE OF SAWMILL REFUSE
_____________________________________________________________________
| | | | |
| | | Per Cent | B. t. u. |
| Kind of Wood | Nature of Refuse | Moisture | per Pound |
| | | | Dry Fuel |
|_____________________|_______________________|__________|____________|
| | | | |
| Mexican White Pine | Sawdust and Hog Chips | 51.90 | 9020 |
| Yosemite Sugar Pine | Sawdust and Hog Chips | 62.85 | 9010 |
| Redwood 75%, | Sawdust, Box Mill | | |
| Douglas Fir 25% | Refuse and Hog | 42.20 | 8977[41] |
| Redwood | Sawdust and Hog Chips | 52.98 | 9040[41] |
| Redwood | Sawdust and Hog Chips | 49.11 | 9204[41] |
| Fir, Hemlock, | | | |
| Spruce and Cedar | Sawdust | 42.06 | 8949[41] |
|_____________________|_______________________|__________|____________|
It is essential in the burning of this class of fuel that a large
combustion space be supplied, and on account of the usually high
moisture content there should be much heated brickwork to radiate heat
to the fuel bed and thus evaporate the moisture. Extension furnaces of
the proper size are usually essential for good results and when this
fuel is used alone, grates dropped to the floor line with an ashpit
below give additional volume for combustion and space for maintaining a
thick fuel bed. A thick fuel bed is necessary in order to avoid
excessive quantities of air passing through the boiler. Where the fuel
consists of hogged wood and sawdust alone, it is best to feed it
automatically into the furnace through chutes on the top of the
extension. The best results are secured when the fuel is allowed to pile
up in the furnace to a height of 3 or 4 feet in the form of a cone under
each chute. The fuel burns best when not disturbed in the furnace. Each
fuel chute, when a proper distance from the grates and with the piles
maintained at their proper height, will supply about 30 or 35 square
feet of grate surface. While large quantities of air are required for
burning this fuel, excess air is as harmful as with coal, and care must
be taken that such an excess is not admitted through fire doors or fuel
chutes. A strong natural draft usually is preferable to a blast with
this fuel. The action of blast is to make the regulation of the furnace
conditions more difficult and to blow over unconsumed fuel on the
heating surfaces and into the stack. This unconsumed fuel settling in
portions of the setting out of the direct path of the gases will have a
tendency to ignite provided any air reaches it, with results harmful to
the setting and breeching connection. This action is particularly
objectionable if these particles are carried over into the base of a
stack, where they will settle below the point at which the flue enters
and if ignited may cause the stack to become overheated and buckle.
Whether natural draft or blast is used, much of the fuel is carried onto
the heating surfaces and these should be cleaned regularly to maintain a
good efficiency. Collecting chambers in various portions of the setting
should be provided for this unconsumed fuel, and these should be kept
clean.
With proper draft conditions, 150 pounds of this fuel containing about
30 to 40 per cent of moisture can be burned per square foot of grate
surface per hour, and in a properly designed furnace one square foot of
grate surface can develop from 5 to 6 boiler horse power. Where the wood
contains 50 per cent of moisture or over, it is not usually safe to
figure on obtaining more than 3 to 4 horse power per square foot of
grate surface.
Dry sawdust, chips and blocks are also used as fuel in many wood-working
industries. Here, as with the wet wood, ample combustion space should be
supplied, but as this fuel is ordinarily kiln dried, large brickwork
surfaces in the furnace are not necessary for the evaporation of
moisture in the fuel. This fuel may be burned in extension furnaces
though these are not required unless they are necessary to secure an
added furnace volume, to get in sufficient grate surface, or where such
an arrangement must be used to allow for a fuel bed of sufficient
thickness. Depth of fuel bed with the dry fuel is as important as with
the moist fuel. If extension furnaces are used with this dry wood, care
must be taken in their design that there is no excessive throttling of
the gases in the furnace, or brickwork trouble will result. In Babcock &
Wilcox boilers this fuel may be burned without extension furnaces,
provided that the boilers are set at a sufficient height to provide
ample combustion space and to allow for proper depth of fuel bed.
Sometimes this is gained by lowering the grates to the floor line and
excavating for an ashpit. Where the fuel is largely sawdust, it may be
introduced over the fire doors through inclined chutes. The old methods
of handling and collecting sawdust by means of air suction and blast
were such that the amount of air admitted through such chutes was
excessive, but with improved methods the amount of air so admitted may
be reduced to a negligible quantity. The blocks and refuse which cannot
be handled through chutes may be fired through fire doors in the front
of the boiler, which should be made sufficiently large to accommodate
the larger sizes of fuel. As with wet fuel, there will be a quantity of
unconsumed wood carried over and the heating surfaces must be kept
clean.
In a few localities cord wood is burned. With this as with other classes
of wood fuel, a large combustion space is an essential feature. The
percentage of moisture in cord wood may make it necessary to use an
extension furnace, but ordinarily this is not required. Ample combustion
space is in most cases secured by dropping the grates to the floor line,
large double-deck fire doors being supplied at the usual fire door level
through which the wood is thrown by hand. Air is admitted under the
grates through an excavated ashpit. The side, front and rear walls of
the furnace should be corbelled out to cover about one-third of the
total grate surface. This prevents cold air from laneing up the sides of
the furnace and also reduces the grate surface. Cord wood and slabs form
an open fire through which the frictional loss of the air is much less
than in the case of sawdust or hogged material. The combustion rate with
cord wood is, therefore, higher and the grate surface may be
considerably reduced. Such wood is usually cut in lengths of 4 feet or 4
feet 6 inches, and the depth of the grates should be kept approximately
5 feet to get the best results.
Bagasse--Bagasse is the refuse of sugar cane from which the juice has
been extracted by pressure between the rolls of the mill. From the start
of the sugar industry bagasse has been considered the natural fuel for
sugar plantations, and in view of the importance of the industry a word
of history relative to the use of this fuel is not out of place.
When the manufacture of sugar was in its infancy the cane was passed
through but a single mill and the defecation and concentration of the
saccharine juice took place in a series of vessels mounted one after
another over a common fire at one end and connected to a stack at the
opposite end. This primitive method was known in the English colonies as
the "Open Wall" and in the Spanish-American countries as the "Jamaica
Train".
The evaporation and concentration of the juice in the open air and over
a direct fire required such quantities of fuel, and the bagasse, in
fact, played such an important part in the manufacture of sugar, that
oftentimes the degree of extraction, which was already low, would be
sacrificed to the necessity of obtaining a bagasse that might be readily
burned.
The furnaces in use with these methods were as primitive as the rest of
the apparatus, and the bagasse could be burned in them only by first
drying it. This naturally required an enormous quantity of handling of
the fuel in spreading and collecting and frequently entailed a shutting
down of the mill, because a shower would spoil the supply which had been
dried.
The difficulties arising from the necessity of drying this fuel caused a
widespread attempt on the part of inventors to the turning out of a
furnace which would successfully burn green bagasse. Some of the designs
were more or less clever, and about the year 1880 several such green
bagasse furnaces were installed. These did not come up to expectations,
however, and almost invariably they were abandoned and recourse had to
be taken to the old method of drying in the sun.
From 1880 the new era in the sugar industry may be dated. Slavery was
almost universally abolished and it became necessary to pay for labor.
The cost of production was thus increased, while growing competition of
European beet sugar lowered the prices. The only remedy for the new
state of affairs was the cheapening of the production by the increase of
extraction and improvement in manufacture. The double mill took the
place of the single, the open wall method of extraction was replaced by
vacuum evaporative apparatus and centrifugal machines were introduced to
do the work of the great curing houses. As opposed to these
improvements, however, the steam plants remained as they started,
consisting of double flue boilers externally fired with dry bagasse.
On several of the plantations horizontal multitubular boilers externally
fired were installed and at the time were considered the acme of
perfection. Numerous attempts were made to burn the bagasse green, among
others the step grates imported from Louisiana and known as the Leon
Marie furnaces, but satisfactory results were obtained in none of the
appliances tried.
The Babcock & Wilcox Co. at this time turned their attention to the
problem with the results which ultimately led to its solution. Their New
Orleans representative, Mr. Frederick Cook, invented a hot forced blast
bagasse furnace and conveyed the patent rights to this company. This
furnace while not as efficient as the standard of to-day, and expensive
in its construction, did, nevertheless, burn the bagasse green and
enabled the boilers to develop their normal rated capacity. The first
furnace of this type was installed at the Southwood and Mt. Houmas
plantations and on a small plantation in Florida. About the year 1888
two furnaces were erected in Cuba, one on the plantation Senado and the
other at the Central Hormiguero. The results obtained with these
furnaces were so remarkable in comparison with what had previously been
accomplished that the company was overwhelmed with orders. The expense
of auxiliary fuel, usually wood, which was costly and indispensable in
rainy weather, was done away with and as the mill could be operated on
bagasse alone, the steam production and the factory work could be
regulated with natural increase in daily output.
Progress and improvement in the manufacture itself was going on at a
remarkable rate, the single grinding had been replaced by a double
grinding, this in turn by a third grinding, and finally the maceration
and dilution of the bagasse was carried to the extraction of practically
the last trace of sugar contained in it. The quantity of juice to be
treated was increased in this way 20 or 30 per cent but was accompanied
by the reduction to a minimum of the bagasse available as a fuel, and
led to demands upon the furnace beyond its capacity.
With the improvements in the manufacture, planters had been compelled to
make enormous sacrifices to change radically their systems, and the
heavy disbursement necessary for mill apparatus left few in a financial
position to make costly installations of good furnaces. The necessity of
turning to something cheap in furnace construction but which was
nevertheless better than the early method of burning the fuel dry led to
the invention of numerous furnaces by all classes of engineers
regardless of their knowledge of the subject and based upon no
experience. None of the furnaces thus produced were in any sense
inventions but were more or less barefaced infringements of the patents
of The Babcock & Wilcox Co. As the company could not protect its rights
without hurting its clients, who in many cases against their own will
were infringing upon these patents, and as on the other hand they were
anxious to do something to meet the wants of the planters, a series of
experiments were started, at their own rather than at their customers'
expense, with a view to developing a furnace which, without being as
expensive, would still fulfill all the requirements of the manufacturer.
The result was the cold blast green bagasse furnace which is now
offered, and it has been adopted as standard for this class of work
after years of study and observation in our installations in the sugar
countries of the world. Such a furnace is described later in considering
the combustion of bagasse.
Composition and Calorific Value of Bagasse--The proportion of fiber
contained in the cane and density of the juice are important factors in
the relation the bagasse fuel will have to the total fuel necessary to
generate the steam required in a mill's operation. A cane rich in wood
fiber produces more bagasse than a poor one and a thicker juice is
subject to a higher degree of dilution than one not so rich.
Besides the percentage of bagasse in the cane, its physical condition
has a bearing on its calorific value. The factors here entering are the
age at which the cane must be cut, the locality in which it is grown,
etc. From the analysis of any sample of bagasse its approximate
calorific value may be calculated from the formula,
8550F + 7119S + 6750G - 972W
B. t. u. per pound bagasse = ---------------------------- (22)
100
Where F = per cent of fiber in cane, S = per cent sucrose, G = per cent
glucose, W = per cent water.
This formula gives the total available heat per pound of bagasse, that
is, the heat generated per pound less the heat required to evaporate its
moisture and superheat the steam thus formed to the temperature of the
stack gases.
Three samples of bagasse in which the ash is assumed to be 3 per cent
give from the formula:
F = 50 S and G = 4.5 W = 42.5 B. t. u. = 4183
F = 40 S and G = 6.0 W = 51.0 B. t. u. = 3351
F = 33.3 S and G = 7.0 W = 56.7 B. t. u. = 2797
A sample of Java bagasse having F = 46.5, S = 4.50, G = 0.5, W = 47.5
gives B. t. u. 3868.
These figures show that the dryer the bagasse is crushed, the higher the
calorific value, though this is accompanied by a decrease in sucrose.
The explanation lies in the fact that the presence of sucrose in an
analysis is accompanied by a definite amount of water, and that the
residual juice contains sufficient organic substance to evaporate the
water present when a fuel is burned in a furnace. For example, assume
the residual juice (100 per cent) to contain 12 per cent organic matter.
From the constant in formula,
12×7119 (100-12)×972
------- = 854.3 and ------------ = 855.4.
100 100
That is, the moisture in a juice containing 12 per cent of sugar will be
evaporated by the heat developed by the combustion of the contained
sugar. It would, therefore, appear that a bagasse containing such juice
has a calorific value due only to its fiber content. This is, of course,
true only where the highest products of oxidization are formed during
the combustion of the organic matter. This is not strictly the case,
especially with a bagasse of a high moisture content which will not burn
properly but which smoulders and produces a large quantity of products
of destructive distillation, chiefly heavy hydrocarbons, which escape
unburnt. The reasoning, however, is sufficient to explain the steam
making properties of bagasse of a low sucrose content, such as are
secured in Java, as when the sucrose content is lower, the heat value is
increased by extracting more juice, and hence more sugar from it. The
sugar operations in Java exemplify this and show that with a high
dilution by maceration and heavy pressure the bagasse meets all of the
steam requirements of the mills without auxiliary fuel.
A high percentage of silica or salts in bagasse has sometimes been
ascribed as the reason for the tendency to smoulder in certain cases of
soft fiber bagasse. This, however, is due to the large moisture content
of the sample resulting directly from the nature of the cane. Soluble
salts in the bagasse has also been given as the explanation of such
smouldering action of the fire, but here too the explanation lies solely
in the high moisture content, this resulting in the development of only
sufficient heat to evaporate the moisture.
TABLE 43
ANALYSES AND CALORIFIC VALUES OF BAGASSE
+---------------------------------------------------------------------+
|+----------+--------+-------+-------+-------+-------+-------+-------+|
|| | | | | | | |B.t.u. ||
|| | | | | | | | per ||
|| Source |Moisture| C | H | O | N | Ash | Pound ||
|| | | | | | | | Dry ||
|| | | | | | | |Bagasse||
|+----------+--------+-------+-------+-------+-------+-------+-------+|
||Cuba | 51.50 | 43.15 | 6.00 | 47.95 | | 2.90 | 7985 ||
||Cuba | 49.10 | 43.74 | 6.08 | 48.61 | | 1.57 | 8300 ||
||Cuba | 42.50 | 43.61 | 6.06 | 48.45 | | 1.88 | 8240 ||
||Cuba | 51.61 | 46.80 | 5.34 | 46.35 | | 1.51 | ||
||Cuba | 52.80 | 46.78 | 5.74 | 45.38 | | 2.10 | ||
||Porto Rico| 41.60 | 44.28 | 6.66 | 47.10 | 0.41 | 1.35 | 8359 ||
||Porto Rico| 43.50 | 44.21 | 6.31 | 47.72 | 0.41 | 1.35 | 8386 ||
||Porto Rico| 44.20 | 44.92 | 6.27 | 46.50 | 0.41 | 1.90 | 8380 ||
||Louisiana | 52.10 | | | | | 2.27 | 8230 ||
||Louisiana | 54.00 | | | | | | 8370 ||
||Louisiana | 51.80 | | | | | | 8371 ||
||Java | | 46.03 | 6.56 | 45.55 | 0.18 | 1.68 | 8681 ||
|+----------+--------+-------+-------+-------+-------+-------+-------+|
+---------------------------------------------------------------------+
Table 43 gives the analyses and heat values of bagasse from various
localities. Table 44 gives the value of mill bagasse at different
extractions, which data may be of service in making approximations as to
its fuel value as compared with that of other fuels.
TABLE 44
VALUE OF ONE POUND OF MILL BAGASSE AT DIFFERENT EXTRACTIONS
1: Per Cent Extraction of Weight of Cane
2: Per Cent Moisture in Bagasse
3: Per Cent in Bagasse
4: Fuel Value, B. t. u.
5: Per Cent in Bagasse
6: Fuel Value, B. t. u.
7: Per Cent in Bagasse
8: Fuel Value, B. t. u.
9: Total Heat Developed per Pound of Bagasse
10: Heat Required to Evaporate Moisture[42]
11: Heat Available for Steam Generation
12: Pounds of Bagasse Equivalent to one Pound of Coal of 14,000 B. t. u.
+----------------------------------------------------------------+
|+---+-----+----------+---------+---------+----------------+----+|
|| | | | | |B.t.u. Value per| ||
|| | | Fiber | Sugar |Molasses |Pound of Bagasse| ||
|| | +-----+----+----+----+----+----+-----+----+-----+ ||
|| | | | | | | | | | | | ||
|| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 ||
|+---+-----+-----+----+----+----+----+----+-----+----+-----+----+|
|| BASED UPON CANE OF 12 PER CENT FIBER AND JUICE CONTAINING ||
||18 PER CENT OF SOLID MATTER. REPRESENTING TROPICAL CONDITIONS ||
|+---+-----+-----+----+----+----+----+----+-----+----+-----+----+|
||75 |42.64|48.00|3996|6.24|451 |3.12|217 |4664 |525 |4139 |3.38||
||77 |39.22|52.17|4343|5.74|414 |2.87|200 |4958 |483 |4475 |3.13||
||79 |35.15|57.14|4757|5.14|371 |2.57|179 |5307 |433 |4874 |2.87||
||81 |30.21|63.16|5258|4.42|319 |2.21|154 |5731 |372 |5359 |2.61||
||83 |24.12|70.59|5877|3.53|256 |1.76|122 |6255 |297 |5958 |2.35||
||85 |16.20|80.00|6660|2.40|173 |1.20| 83 |6916 |200 |6716 |2.08||
|+---+-----+-----+----+----+----+----+----+-----+----+-----+----+|
|| BASED UPON CANE OF 10 PER CENT FIBER AND JUICE CONTAINING ||
||15 PER CENT OF SOLID MATTER. REPRESENTING LOUISIANA CONDITIONS||
|+---+-----+-----+----+----+----+----+----+-----+----+-----+----+|
||75 |51.00|40.00|3330|6.00|433 |3.00|209 |3972 |678 |3294 |4.25||
||77 |48.07|43.45|3617|5.66|409 |2.82|196 |4222 |592 |3630 |3.86||
||79 |44.52|47.62|3964|5.24|378 |2.62|182 |4524 |548 |3976 |3.52||
||81 |40.18|52.63|4381|4.73|342 |2.36|164 |4887 |495 |4392 |3.19||
||83 |35.00|58.82|4897|4.12|298 |2.06|143 |5436 |431 |5005 |2.80||
||85 |28.33|66.67|5550|3.33|241 |1.67|116 |5907 |349 |5558 |2.52||
|+---+-----+-----+----+----+----+----+----+-----+----+-----+----+|
+----------------------------------------------------------------+
Furnace Design and the Combustion of Bagasse--With the advance in sugar
manufacture there came, as described, a decrease in the amount of
bagasse available for fuel. As the general efficiency of a plant of this
description is measured by the amount of auxiliary fuel required per ton
of cane, the relative importance of the furnace design for the burning
of this fuel is apparent.
In modern practice, under certain conditions of mill operation, and with
bagasse of certain physical properties, the bagasse available from the
cane ground will meet the total steam requirements of the plant as a
whole; such conditions prevail, as described, in Java. In the United
States, Cuba, Porto Rico and like countries, however, auxiliary fuel is
almost universally a necessity. The amount will vary, depending to a
great extent upon the proportion of fiber in the cane, which varies
widely with the locality and with the age at which it is cut, and to a
lesser extent upon the degree of purity of the manufactured sugar, the
use of the maceration water and the efficiency of the mill apparatus as
a whole.
[Illustration: Fig. 27. Babcock & Wilcox Boiler Set with Green Bagasse
Furnace]
Experience has shown that this fuel may be burned with the best results
in large quantities. A given amount of bagasse burned in one furnace
between two boilers will give better results than the same quantity
burned in a number of smaller furnaces. An objection has been raised
against such practice on the grounds that the necessity of shutting down
two boiler units when it is necessary for any reason to take off a
furnace, requires a larger combined boiler capacity to insure continuity
of service. As a matter of fact, several small furnaces will cost
considerably more than one large furnace, and the saving in original
furnace cost by such an installation, taken in conjunction with the
added efficiency of the larger furnace over the small, will probably
more than offset the cost of additional boiler units for spares.
The essential features in furnace design for this class of fuel are
ample combustion space and a length of gas travel sufficient to enable
the gases to be completely burned before the boiler heating surfaces are
encountered. Experience has shown that better results are secured where
the fuel is burned on a hearth rather than on grates, the objection to
the latter method being that the air for combustion enters largely
around the edges, where the fuel pile is thinnest. When burned on a
hearth the air for combustion is introduced into the furnace through
several rows of tuyeres placed above and symmetrically around the
hearth. An arrangement of such tuyeres over a grate, and a proper
manipulation of the ashpit doors, will overcome largely the objection to
grates and at the same time enable other fuel to be burned in the
furnace when necessary. This arrangement of grates and tuyeres is
probably the better from a commercially efficient standpoint. Where the
air is admitted through tuyeres over the grate or hearth line, it
impinges on the fuel pile as a whole and causes a uniform combustion.
Such tuyeres connect with an annular space in which, where a blast is
used, the air pressure is controlled by a blower.
All experience with this class of fuel indicates that the best results
are secured with high combustion rates. With a natural draft in the
furnace of, say, three-tenths inch of water, a combustion rate of from
250 to 300 pounds per square foot of grate surface per hour may be
obtained. With a blast of, say, five-tenths inch of water, this rate can
be increased to 450 pounds per square foot of grate surface per hour.
These rates apply to bagasse as fired containing approximately 50 per
cent of moisture. It would appear that the most economical results are
secured with a combustion rate of approximately 300 pounds per square
foot per hour which, as stated, may be obtained with natural draft.
Where a natural draft is available sufficient to give such a rate, it is
in general to be preferred to a blast.
Fig. 27 shows a typical bagasse furnace with which very satisfactory
results have been obtained. The design of this furnace may be altered to
suit the boilers to which it is connected. It may be changed slightly in
its proportions and in certain instances in its position relative to the
boiler. The furnace as shown is essentially a bagasse furnace and may be
modified somewhat to accommodate auxiliary fuel.
The fuel is ignited in a pit A on a hearth which is ordinarily
elliptical in shape. Air for combustion is admitted through the tuyeres
B connected to an annular space C through which the amount of air is
controlled. Above the pit the furnace widens out to form a combustion
space D which has a cylindrical or spherical roof with its top
ordinarily from 11 to 13 feet above the floor. The gases pass from this
space horizontally to a second combustion chamber E from which they are
led through arches F to the boiler. The arrangement of such arches is
modified to suit the boiler or boilers with which the furnace is
operated. A furnace of such design embodies the essential features of
ample combustion space and long gas travel.
The fuel should be fed to the furnace through an opening in the roof
above the pit by some mechanical means which will insure a constant fuel
feed and at the same time prevent the inrush of cold air into the
furnace.
This class of fuel deposits a considerable quantity of dust, which if
not removed promptly will fuse into a hard glass-like clinker. Ample
provision should be made for the removal of such dust from the furnace,
the gas ducts and the boiler setting, and these should be thoroughly
cleaned once in 24 hours.
Table 45 gives the results of several tests on Babcock & Wilcox boilers
using fuel of this character.
TABLE 45
TESTS OF BABCOCK & WILCOX BOILERS WITH GREEN BAGASSE
____________________________________________________________________
| Duration of Test | Hours | 12 | 10 | 10 | 10 |
| Rated Capacity of Boiler |Horse Power| 319 | 319 | 319 | 319 |
| Grate Surface |Square Feet| 33 | 33 | 16.5 | 16.5 |
| Draft in Furnace | Inches | .30 | .28 | .29 | .27 |
| Draft at Damper | Inches | .47 | .45 | .46 | .48 |
| Blast under Grates | Inches | ... | ... | ... | .34 |
| Temperature of Exit Gases | Degrees F.| 536 | 541 | 522 | 547 |
| /CO_{2} | Per Cent | 13.8 | 12.6 | 11.7 | 12.8 |
| Flue Gas Analysis { O | Per Cent | 5.9 | 7.6 | 8.2 | 6.9 |
| \CO | Per Cent | 0.0 | 0.0 | 0.0 | 0.0 |
| Bagasse per Hour as Fired | Pounds | 4980 | 4479 | 5040 | 5586 |
| Moisture in Bagasse | Per Cent |52.39 |52.93 |51.84 |51.71 |
| Dry Bagasse per Hour | Pounds | 2371 | 2108 | 2427 | 2697 |
| Dry Bagasse per Square Foot| | | | | |
| of Grate Surface per Hour| Pounds | 71.9 | 63.9 |147.1 |163.4 |
| Water per Hour from and at | | | | | |
| 212 Degrees | Pounds |10141 | 9850 |10430 |11229 |
| Per Cent of Rated Capacity | | | | | |
| Developed | Per Cent | 92.1 | 89.2 | 94.7 |102.0 |
|____________________________|___________|______|______|______|______|
Tan Bark--Tan bark, or spent tan, is the fibrous portion of bark
remaining after use in the tanning industry. It is usually very high in
its moisture content, a number of samples giving an average of 65 per
cent or about two-thirds of the total weight of the fuel. The weight of
the spent tan is about 2.13 times as great as the weight of the bark
ground. In calorific value an average of 10 samples gives 9500 B. t. u.
per pound dry.[43] The available heat per pound as fired, owing to the
great percentage of moisture usually found, will be approximately 2700
B. t. u. Since the weight of the spent tan as fired is 2.13 as great as
the weight of the bark as ground at the mill, one pound of ground bark
produces an available heat of approximately 5700 B. t. u. Relative to
bituminous coal, a ton of bark is equivalent to 0.4 ton of coal. An
average chemical analysis of the bark is, carbon 51.8 per cent, hydrogen
6.04, oxygen 40.74, ash 1.42.
Tan bark is burned in isolated cases and in general the remarks on
burning wet wood fuel apply to its combustion. The essential features
are a large combustion space, large areas of heated brickwork radiating
to the fuel bed, and draft sufficient for high combustion rates. The
ratings obtainable with this class of fuel will not be as high as with
wet wood fuel, because of the heat value and the excessive moisture
content. Mr. D. M. Meyers found in a series of experiments that an
average of from 1.5 to 2.08 horse power could be developed per square
foot of grate surface with horizontal return tubular boilers. This horse
power would vary considerably with the method in which the spent tan was
fired.
[Illustration: 686 Horse-power Babcock & Wilcox Boiler and Superheater
in Course of Erection at the Quincy, Mass., Station of the Bay State
Street Railway Co.]
LIQUID FUELS AND THEIR COMBUSTION
Petroleum is practically the only liquid fuel sufficiently abundant and
cheap to be used for the generation of steam. It possesses many
advantages over coal and is extensively used in many localities.
There are three kinds of petroleum in use, namely those yielding on
distillation: 1st, paraffin; 2nd, asphalt; 3rd, olefine. To the first
group belong the oils of the Appalachian Range and the Middle West of
the United States. These are a dark brown in color with a greenish
tinge. Upon their distillation such a variety of valuable light oils are
obtained that their use as fuel is prohibitive because of price.
To the second group belong the oils found in Texas and California. These
vary in color from a reddish brown to a jet black and are used very
largely as fuel.
The third group comprises the oils from Russia, which, like the second,
are used largely for fuel purposes.
The light and easily ignited constituents of petroleum, such as naphtha,
gasolene and kerosene, are oftentimes driven off by a partial
distillation, these products being of greater value for other purposes
than for use as fuel. This partial distillation does not decrease the
value of petroleum as a fuel; in fact, the residuum known in trade as
"fuel oil" has a slightly higher calorific value than petroleum and
because of its higher flash point, it may be more safely handled.
Statements made with reference to petroleum apply as well to fuel oil.
In general crude oil consists of carbon and hydrogen, though it also
contains varying quantities of moisture, sulphur, nitrogen, arsenic,
phosphorus and silt. The moisture contained may vary from less than 1 to
over 30 per cent, depending upon the care taken to separate the water
from the oil in pumping from the well. As in any fuel, this moisture
affects the available heat of the oil, and in contracting for the
purchase of fuel of this nature it is well to limit the per cent of
moisture it may contain. A large portion of any contained moisture can
be separated by settling and for this reason sufficient storage capacity
should be supplied to provide time for such action.
A method of obtaining approximately the percentage of moisture in crude
oil which may be used successfully, particularly with lighter oils, is
as follows. A burette graduated into 200 divisions is filled to the 100
mark with gasolene, and the remaining 100 divisions with the oil, which
should be slightly warmed before mixing. The two are then shaken
together and any shrinkage below the 200 mark filled up with oil. The
mixture should then be allowed to stand in a warm place for 24 hours,
during which the water and silt will settle to the bottom. Their
percentage by volume can then be correctly read on the burette
divisions, and the percentage by weight calculated from the specific
gravities. This method is exceedingly approximate and where accurate
results are required it should not be used. For such work, the
distillation method should be used as follows:
Gradually heat 100 cubic centimeters of the oil in a distillation flask
to a temperature of 150 degrees centigrade; collect the distillate in a
graduated tube and measure the resulting water. Such a method insures
complete removal of water and reduces the error arising from the slight
solubility of the water in gasolene. Two samples checked by the two
methods for the amount of moisture present gave,
_Distillation_ _Dilution_
_Per Cent_ _Per Cent_
8.71 6.25
8.82 6.26
TABLE 46
COMPOSITION AND CALORIFIC VALUE OF VARIOUS OILS
+-------------------------+-----+-----+----+--------+----+---+--------+-----+------------------------+
| Kind of Oil | %C | %H | %S | %O |S.G.|FP | %H2O |Btu |Authority |
+-------------------------+-----+-----+----+--------+----+---+--------+-----+------------------------+
|California, Coaling | | | | |.927|134| |17117|Babcock & Wilcox Co. |
|California, Bakersfield | | | | |.975| | |17600|Wade |
|California, Bakersfield | | |1.30| |.992| | |18257|Wade |
|California, Kern River | | | | |.950|140| |18845|Babcock & Wilcox Co. |
|California, Los Angeles | | |2.56| | | | |18328|Babcock & Wilcox Co. |
|California, Los Angeles | | | | |.957|196| |18855|Babcock & Wilcox Co. |
|California, Los Angeles | | | | |.977| | .40 |18280|Babcock & Wilcox Co. |
|California, Monte Christo| | | | |.966|205| |18878|Babcock & Wilcox Co. |
|California, Whittier | | | .98| |.944| |1.06 |18507|Wade |
|California, Whittier | | | .72| |.936| |1.06 |18240|Wade |
|California |85.04|11.52|2.45| .99[44]| | |1.40 |17871|Babcock & Wilcox Co. |
|California |81.52|11.51| .55|6.92[44]| |230| |18667|U.S.N. Liquid Fuel Board|
|California | | | .87| | | | .95 |18533|Blasdale |
|California | | | | |.891|257| |18655|Babcock & Wilcox Co. |
|California | | |2.45| |.973| |1.50[45]|17976|O'Neill |
|California | | |2.46| |.975| |1.32 |18104|Shepherd |
|Texas, Beaumont |84.6 |10.9 |1.63|2.87 |.924|180| |19060|U.S.N. Liquid Fuel Board|
|Texas, Beaumont |83.3 |12.4 | .50|3.83 |.926|216| |19481|U.S.N. Liquid Fuel Board|
|Texas, Beaumont |85.0 |12.3 |1.75| .92[44]| | | |19060|Denton |
|Texas, Beaumont |86.1 |12.3 |1.60| |.942| | |20152|Sparkes |
|Texas, Beaumont | | | | |.903|222| |19349|Babcock & Wilcox Co. |
|Texas, Sabine | | | | |.937|143| |18662|Babcock & Wilcox Co. |
|Texas |87.15|12.33|0.32| |.908|370| |19338|U. S. N. |
|Texas |87.29|12.32|0.43| |.910|375| |19659|U. S. N. |
|Ohio |83.4 |14.7 |0.6 |1.3 | | | |19580| |
|Pennsylvania |84.9 |13.7 | |1.4 |.886| | |19210|Booth |
|West Virginia |84.3 |14.1 | |1.6 |.841| | |21240| |
|Mexico | | | | |.921|162| |18840|Babcock & Wilcox Co. |
|Russia, Baku |86.7 |12.9 | | |.884| | |20691|Booth |
|Russia, Novorossick |84.9 |11.6 | |3.46 | | | |19452|Booth |
|Russia, Caucasus |86.6 |12.3 | |1.10 |.938| | |20138| |
|Java |87.1 |12.0 | | .9 |.923| | |21163| |
|Austria, Galicia |82.2 |12.1 |5.7 | |.870| | |18416| |
|Italy, Parma |84.0 |13.4 |1.8 | |.786| | | | |
|Borneo |85.7 |11.0 | |3.31 | | | |19240|Orde |
+-------------------------+-----+-----+----+--------+----+---+--------+-----+------------------------+
%C = Per Cent Carbon
%H = Per Cent Hydrogen
%S = Per Cent Sulphur
%O = Per Cent Oxygen
S.G. = Specific Gravity
FP = Degrees Flash Point
%H_{2}O = Per Cent Moisture
Btu = B. t. u. Per Pound
Calorific Value--A pound of petroleum usually has a calorific value of
from 18,000 to 22,000 B. t. u. If an ultimate analysis of an average
sample be, carbon 84 per cent, hydrogen 14 per cent, oxygen 2 per cent,
and assuming that the oxygen is combined with its equivalent of hydrogen
as water, the analysis would become, carbon 84 per cent, hydrogen 13.75
per cent, water 2.25 per cent, and the heat value per pound including
its contained water would be,
Carbon .8400 × 14,600 = 12,264 B. t. u.
Hydrogen .1375 × 62,100 = 8,625 B. t. u.
------[**Should be .1375 x 62,000 = 8,525]
Total 20,889 B. t. u.[**Would be Total = 20,789]
The nitrogen in petroleum varies from 0.008 to 1.0 per cent, while the
sulphur varies from 0.07 to 3.0 per cent.
Table 46, compiled from various sources, gives the composition,
calorific value and other data relative to oil from different
localities.
The flash point of crude oil is the temperature at which it gives off
inflammable gases. While information on the actual flash points of the
various oils is meager, it is, nevertheless, a question of importance in
determining their availability as fuels. In general it may be stated
that the light oils have a low, and the heavy oils a much higher flash
point. A division is sometimes made at oils having a specific gravity of
0.85, with a statement that where the specific gravity is below this
point the flash point is below 60 degrees Fahrenheit, and where it is
above, the flash point is above 60 degrees Fahrenheit. There are,
however, many exceptions to this rule. As the flash point is lower the
danger of ignition or explosion becomes greater, and the utmost care
should be taken in handling the oils with a low flash point to avoid
this danger. On the other hand, because the flash point is high is no
justification for carelessness in handling those fuels. With proper
precautions taken, in general, the use of oil as fuel is practically as
safe as the use of coal.
Gravity of Oils--Oils are frequently classified according to their
gravity as indicated by the Beaume hydrometer scale. Such a
classification is by no means an accurate measure of their relative
calorific values.
Petroleum as Compared with Coal--The advantages of the use of oil fuel
over coal may be summarized as follows:
1st. The cost of handling is much lower, the oil being fed by simple
mechanical means, resulting in,
2nd. A general labor saving throughout the plant in the elimination of
stokers, coal passers, ash handlers, etc.
3rd. For equal heat value, oil occupies very much less space than coal.
This storage space may be at a distance from the boiler without
detriment.
4th. Higher efficiencies and capacities are obtainable with oil than
with coal. The combustion is more perfect as the excess air is reduced
to a minimum; the furnace temperature may be kept practically constant
as the furnace doors need not be opened for cleaning or working fires;
smoke may be eliminated with the consequent increased cleanliness of the
heating surfaces.
5th. The intensity of the fire can be almost instantaneously regulated
to meet load fluctuations.
6th. Oil when stored does not lose in calorific value as does coal, nor
are there any difficulties arising from disintegration, such as may be
found when coal is stored.
7th. Cleanliness and freedom from dust and ashes in the boiler room with
a consequent saving in wear and tear on machinery; little or no damage
to surrounding property due to such dust.
The disadvantages of oil are:
1st. The necessity that the oil have a reasonably high flash point to
minimize the danger of explosions.
2nd. City or town ordinances may impose burdensome conditions relative
to location and isolation of storage tanks, which in the case of a plant
situated in a congested portion of the city, might make use of this fuel
prohibitive.
3rd. Unless the boilers and furnaces are especially adapted for the use
of this fuel, the boiler upkeep cost will be higher than if coal were
used. This objection can be entirely obviated, however, if the
installation is entrusted to those who have had experience in the work,
and the operation of a properly designed plant is placed in the hands of
intelligent labor.
TABLE 47
RELATIVE VALUE OF COAL AND OIL FUEL
+------+--------+-------+-----------------------------------------------+
|Gross | Net | Net | Water Evaporated from and at |
|Boiler| Boiler |Evap- | 212 Degrees Fahrenheit per Pound of Coal |
|Effic-|Effici- |oration+-----+-----+-----+-----+-----+-----+-----+-----+
| iency|ency[46]| from | | | | | | | | |
| with | with |and at | | | | | | | | |
| Oil | Oil | 212 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| Fuel | Fuel |Degrees| | | | | | | | |
| | |Fahren-| | | | | | | | |
| | | heit +-----+-----+-----+-----+-----+-----+-----+-----+
| | | per | |
| | | Pound | Pounds of Oil Equal to One Pound of Coal |
| | |of Oil | |
+------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+
| 73 | 71 | 13.54 |.3693|.4431|.5170|.5909|.6647|.7386|.8124|.8863|
| 74 | 72 | 13.73 |.3642|.4370|.5099|.5827|.6556|.7283|.8011|.8740|
| 75 | 73 | 13.92 |.3592|.4310|.5029|.5747|.6466|.7184|.7903|.8621|
| 76 | 74 | 14.11 |.3544|.4253|.4961|.5670|.6378|.7087|.7796|.8505|
| 77 | 75 | 14.30 |.3497|.4196|.4895|.5594|.6294|.6993|.7692|.8392|
| 78 | 76 | 14.49 |.3451|.4141|.4831|.5521|.6211|.6901|.7591|.8281|
| 79 | 77 | 14.68 |.3406|.4087|.4768|.5450|.6131|.6812|.7493|.8174|
| 80 | 78 | 14.87 |.3363|.4035|.4708|.5380|.6053|.6725|.7398|.8070|
| 81 | 79 | 15.06 |.3320|.3984|.4648|.5312|.5976|.6640|.7304|.7968|
| 82 | 80 | 15.25 |.3279|.3934|.4590|.5246|.5902|.6557|.7213|.7869|
| 83 | 81 | 15.44 |.3238|.3886|.4534|.5181|.5829|.6447|.7125|.7772|
+------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+
| | | Net | |
| | |Evap- | |
| | |oration| |
| | | from | |
| | |and at | |
| | | 212 | Barrels of Oil Equal to One Ton of Coal |
| | |Degrees| |
| | |Fahren-| |
| | | heit | |
| | | per | |
| | |Barrel | |
| | |of Oil | |
+------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+
| 73 | 71 | 4549 |2.198|2.638|3.077|3.516|3.955|4.395|4.835|5.275|
| 74 | 72 | 4613 |2.168|2.601|3.035|3.468|3.902|4.335|4.769|5.202|
| 75 | 73 | 4677 |2.138|2.565|2.993|3.420|3.848|4.275|4.703|5.131|
| 76 | 74 | 4741 |2.110|2.532|2.954|3.376|3.798|4.220|4.642|5.063|
| 77 | 75 | 4807 |2.082|2.498|2.914|3.330|3.746|4.162|4.578|4.994|
| 78 | 76 | 4869 |2.054|2.465|2.876|3.286|3.697|4.108|4.518|4.929|
| 79 | 77 | 4932 |2.027|2.433|2.838|3.243|3.649|4.054|4.460|4.865|
| 80 | 78 | 4996 |2.002|2.402|2.802|3.202|3.602|4.003|4.403|4.803|
| 81 | 79 | 5060 |1.976|2.371|2.767|3.162|3.557|3.952|4.348|4.743|
| 82 | 80 | 5124 |1.952|2.342|2.732|3.122|3.513|3.903|4.293|4.683|
| 83 | 81 | 5187 |1.927|2.313|2.699|3.085|3.470|3.856|4.241|4.627|
+------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+
[Illustration: City of San Francisco, Cal., Fire Fighting Station. No.
1. 2800 Horse Power of Babcock & Wilcox Boilers, Equipped for Burning
Oil Fuel]
Many tables have been published with a view to comparing the two fuels.
Such of these as are based solely on the relative calorific values of
oil and coal are of limited value, inasmuch as the efficiencies to be
obtained with oil are higher than that obtainable with coal. Table 47
takes into consideration the variation in efficiency with the two fuels,
but is based on a constant calorific value for oil and coal. This table,
like others of a similar nature, while useful as a rough guide, cannot
be considered as an accurate basis for comparison. This is due to the
fact that there are numerous factors entering into the problem which
affect the saving possible to a much greater extent than do the relative
calorific values of two fuels. Some of the features to be considered in
arriving at the true basis for comparison are the labor saving possible,
the space available for fuel storage, the facilities for conveying the
oil by pipe lines, the hours during which a plant is in operation, the
load factor, the quantity of coal required for banking fires, etc., etc.
The only exact method of estimating the relative advantages and costs of
the two fuels is by considering the operating expenses of the plant with
each in turn, including the costs of every item entering into the
problem.
Burning Oil Fuel--The requirements for burning petroleum are as follows:
1st. Its atomization must be thorough.
2nd. When atomized it must be brought into contact with the requisite
quantity of air for its combustion, and this quantity must be at the
same time a minimum to obviate loss in stack gases.
3rd. The mixture must be burned in a furnace where a refractory material
radiates heat to assist in the combustion, and the furnace must stand up
under the high temperatures developed.
4th. The combustion must be completed before the gases come into contact
with the heating surfaces or otherwise the flame will be extinguished,
possibly to ignite later in the flue connection or in the stack.
5th. There must be no localization of the heat on certain portions of
the heating surfaces or trouble will result from overheating and
blistering.
The first requirement is met by the selection of a proper burner.
The second requirement is fulfilled by properly introducing the air into
the furnace, either through checkerwork under the burners or through
openings around them, and by controlling the quantity of air to meet
variations in furnace conditions.
The third requirement is provided for by installing a furnace so
designed as to give a sufficient area of heated brickwork to radiate the
heat required to maintain a proper furnace temperature.
The fourth requirement is provided for by giving ample space for the
combustion of the mixture of atomized oil and air, and a gas travel of
sufficient length to insure that this combustion be completed before the
gases strike the heating surfaces.
The fifth requirement is fulfilled by the adoption of a suitable burner
in connection with the furnace meeting the other requirements. A burner
must be used from which the flame will not impinge directly on the
heating surface and must be located where such action cannot take place.
If suitable burners properly located are not used, not only is the heat
localized with disastrous results, but the efficiency is lowered by the
cooling of the gases before combustion is completed.
Oil Burners--The functions of an oil burner is to atomize or vaporize
the fuel so that it may be burned like a gas. All burners may be
classified under three general types: 1st, spray burners, in which the
oil is atomized by steam or compressed air; 2nd, vapor burners, in which
the oil is converted into vapor and then passed into the furnace; 3rd,
mechanical burners, in which the oil is atomized by submitting it to a
high pressure and passing it through a small orifice.
Vapor burners have never been in general use and will not be discussed.
Spray burners are almost universally used for land practice and the
simplicity of the steam atomizer and the excellent economy of the better
types, together with the low oil pressure and temperature required makes
this type a favorite for stationary plants, where the loss of fresh
water is not a vital consideration. In marine work, or in any case where
it is advisable to save feed water that otherwise would have to be added
in the form of "make-up", either compressed air or mechanical means are
used for atomization. Spray burners using compressed air as the
atomizing agent are in satisfactory operation in some plants, but their
use is not general. Where there is no necessity of saving raw feed
water, the greater simplicity and economy of the steam spray atomizer is
generally the most satisfactory. The air burners require blowers,
compressors or other apparatus which occupy space that might be
otherwise utilized and require attention that is not necessary where
steam is used.
Steam spray burners of the older types had disadvantages in that they
were so designed that there was a tendency for the nozzle to clog with
sludge or coke formed from the oil by the heat, without means of being
readily cleaned. This has been overcome in the more modern types.
Steam spray burners, as now used, may be divided into two classes: 1st,
inside mixers; and 2nd, outside mixers. In the former the steam and oil
come into contact within the burner and the mixture is atomized in
passing through the orifice of the burner nozzle.
[Illustration: Fig. 28. Peabody Oil Burner]
In the outside mixing class the steam flows through a narrow slot or
horizontal row of small holes in the burner nozzle; the oil flows
through a similar slot or hole above the steam orifice, and is picked up
by the steam outside of the burner and is atomized. Fig. 28 shows a type
of the Peabody burner of this class, which has given eminent
satisfaction. The construction is evident from the cut. It will be noted
that the portions of the burner forming the orifice may be readily
replaced in case of wear, or if it is desired to alter the form of the
flame.
Where burners of the spray type are used, heating the oil is of
advantage not only in causing it to be atomized more easily, but in
aiding economical combustion. The temperature is, of course, limited by
the flash point of the oil used, but within the limit of this
temperature there is no danger of decomposition or of carbon deposits on
the supply pipes. Such heating should be done close to the boiler to
minimize radiation loss. If the temperature is raised to a point where
an appreciable vaporization occurs, the oil will flow irregularly from
the burner and cause the flame to sputter.
On both steam and air atomizing types, a by-pass should be installed
between the steam or air and the oil pipes to provide for the blowing
out of the oil duct. Strainers should be provided for removing sludge
from the fuel and should be so located as to allow for rapid removal,
cleaning and replacing.
Mechanical burners have been in use for some time in European countries,
but their introduction and use has been of only recent occurrence in the
United States. Here as already stated, the means for atomization are
purely mechanical. The most successful of the mechanical atomizers up to
the present have been of the round flame type, and only these will be
considered. Experiments have been made with flat flame mechanical
burners, but their satisfactory action has been confined to instances
where it is only necessary to burn a small quantity of oil through each
individual burner.
This system of oil burning is especially adapted for marine work as the
quantity of steam for putting pressure on the oil is small and the
condensed steam may be returned to the system.
The only method by which successful mechanical atomization has been
accomplished is one by which the oil is given a whirling motion within
the burner tip. This is done either by forcing the oil through a passage
of helical form or by delivering it tangentially to a circular chamber
from which there is a central outlet. The oil is fed to these burners
under a pressure which varies with the make of the burner and the rates
at which individual burners are using oil. The oil particles fly off
from such a burner in straight lines in the form of a cone rather than
in the form of a spiral spray, as might be supposed.
With burners of the mechanical atomizing design, the method of
introducing air for combustion and the velocity of this air are of the
greatest importance in securing good combustion and in the effects on
the character and shape of the flame. Such burners are located at the
front of the furnace and various methods have been tried for introducing
the air for combustion. Where, in the spray burners, air is ordinarily
admitted through a checkerwork under the burner proper, with the
mechanical burner, it is almost universally admitted around the burner.
Early experiments with these air distributors were confined largely to
single or duplicate cones used with the idea of directing the air to the
axis of the burner. A highly successful method of such air introduction,
developed by Messrs. Peabody and Irish of The Babcock & Wilcox Co., is
by means of what they term an "impeller plate". This consists of a
circular metal disk with an opening at the center for the oil burner and
with radial metal strips from the center to the periphery turned at an
angle which in the later designs may be altered to give the air supply
demanded by the rate of combustion.
The air so admitted does not necessarily require a whirling motion, but
experiments show that where the air is brought into contact with the oil
spray with the right "twist", better combustion is secured and lower air
pressures and less refinement of adjustment of individual burners are
required.
Mechanical burners have a distinct advantage over those in which steam
is used as the atomizing agent in that they lend themselves more readily
to adjustment under wider variations of load. For a given horse power
there will ordinarily be installed a much greater number of mechanical
than steam atomizing burners. This in itself is a means to better
regulation, for with the steam atomizing burner, if one of a number is
shut off, there is a marked decrease in efficiency. This is due to the
fact that with the air admitted under the burner, it is ordinarily
passing through the checkerwork regardless of whether it is being
utilized for combustion or not. With a mechanical burner, on the other
hand, where individual burners are shut off, air that would be admitted
for such burner, were it in operation, may also be shut off and there
will be no undue loss from excess air.
Further adjustment to meet load conditions is possible by a change in
the oil pressure acting on all burners at once. A good burner will
atomize moderately heavy oil with an oil pressure as low as 30 pounds
per square inch and from that point up to 200 pounds or above. The
heating of the oil also has an effect on the capacity of individual
burners and in this way a third method of adjustment is given. Under
working conditions, the oil pressure remaining constant, the capacity of
each burner will decrease as the temperature of the oil is increased
though at low temperatures the reverse is the case. Some experiments
with a Texas crude oil having a flash point of 210 degrees showed that
the capacity of a mechanical atomizing burner of the Peabody type
increased from 80 degrees Fahrenheit to 110 degrees Fahrenheit, from
which point it fell off rapidly to 140 degrees and then more slowly to
the flash point.
The above methods, together with the regulation possible through
manipulation of the boiler dampers, indicate the wide range of load
conditions that may be handled with an installation of this class of
burners.
As has already been stated, results with mechanical atomizing burners
that may be considered very successful have been limited almost entirely
to cases where forced blast of some description has been used, the high
velocity of the air entering being of material assistance in securing
the proper mixture of air with the oil spray. Much has been done and is
being done in the way of experiment with this class of apparatus toward
developing a successful mechanical atomizing burner for use with natural
draft, and there appears to be no reason why such experiments should not
eventually produce satisfactory results.
Steam Consumption of Burners--The Bureau of Steam Engineering, U. S.
Navy, made in 1901 an exhaustive series of tests of various oil burners
that may be considered as representing, in so far as the performance of
the burners themselves is concerned, the practice of that time. These
tests showed that a burner utilizing air as an atomizing agent, required
for compressing the air from 1.06 to 7.45 per cent of the total steam
generated, the average being 3.18 per cent. Four tests of steam
atomizing burners showed a consumption of 3.98 to 5.77 per cent of the
total steam, the average being 4.8 per cent.
Improvement in burner design has largely reduced the steam consumption,
though to a greater degree in steam than in air atomizing burners.
Recent experiments show that a good steam atomizing burner will require
approximately 2 per cent of the total steam generated by the boiler
operated at or about its rated capacity. This figure will decrease as
the capacity is increased and is so low as to be practically negligible,
except in cases where the question of loss of feed water is all
important. There are no figures available as to the actual steam
consumption of mechanical atomizing burners but apparently this is small
if the requirement is understood to be entirely apart from the steam
consumption of the apparatus producing the forced blast.
Capacity of Burners--A good steam atomizing burner properly located in a
well-designed oil furnace has a capacity of somewhat over 400 horse
power. This question of capacity of individual burners is largely one of
the proper relation between the number of burners used and the furnace
volume. In some recent tests with a Babcock & Wilcox boiler of 640 rated
horse power, equipped with three burners, approximately 1350 horse power
was developed with an available draft of .55 inch at the damper or 450
horse power per burner. Four burners were also tried in the same furnace
but the total steam generated did not exceed 1350 horse power or in this
instance 338 horse power per burner.
From the nature of mechanical atomizing burners, individual burners have
not as large a capacity as the steam atomizing class. In some tests on a
Babcock & Wilcox marine boiler, equipped with mechanical atomizing
burners, the maximum horse power developed per burner was approximately
105. Here again the burner capacity is largely one of proper relation
between furnace volume and number of burners.
Furnace Design--Too much stress cannot be laid on the importance of
furnace design for the use of this class of fuel. Provided a good type
of burner is adopted the furnace arrangement and the method of
introducing air for combustion into the furnace are the all important
factors. No matter what the type of burner, satisfactory results cannot
be secured in a furnace not suited to the fuel.
The Babcock & Wilcox Co. has had much experience with the burning of oil
as fuel and an extended series of experiments by Mr. E. H. Peabody led
to the development and adoption of the Peabody furnace as being most
eminently suited for this class of work. Fig. 29 shows such a furnace
applied to a Babcock & Wilcox boiler, and with slight modification it
can be as readily applied to any boiler of The Babcock & Wilcox Co.
manufacture. In the description of this furnace, its points of advantage
cover the requirements of oil-burning furnaces in general.
The atomized oil is introduced into the furnace in the direction in
which it increases in height. This increase in furnace volume in the
direction of the flame insures free expansion and a thorough mixture of
the oil with the air, and the consequent complete combustion of the
gases before they come into contact with the tube heating surfaces. In
such a furnace flat flame burners should be used, preferably of the
Peabody type, in which the flame spreads outward toward the sides in the
form of a fan. There is no tendency of the flames to impinge directly on
the heating surfaces, and the furnace can handle any quantity of flame
without danger of tube difficulties. The burners should be so located
that the flames from individual burners do not interfere nor impinge to
any extent on the side walls of the furnace, an even distribution of
heat being secured in this manner. The burners are operated from the
boiler front and peepholes are supplied through which the operator may
watch the flame while regulating the burners. The burners can be
removed, inspected, or cleaned and replaced in a few minutes. Air is
admitted through a checkerwork of fire brick supported on the furnace
floor, the openings in the checkerwork being so arranged as to give the
best economic results in combustion.
[Illustration: Fig. 29. Babcock & Wilcox Boiler, Equipped with a Peabody
Oil Furnace]
With steam atomizing burners introduced through the front of the boiler
in stationary practice, it is usually in the direction in which the
furnace decreases in height and it is with such an arrangement that
difficulties through the loss of tubes may be expected. With such an
arrangement, the flame may impinge directly upon the tube surfaces and
tube troubles from this source may arise, particularly where the feed
water has a tendency toward rapid scale formation. Such difficulties may
be the result of a blowpipe action on the part of the burner, the over
heating of the tube due to oil or scale within, or the actual erosion of
the metal by particles of oil improperly atomized. Such action need not
be anticipated, provided the oil is burned with a short flame. The
flames from mechanical atomizing burners have a less velocity of
projection than those from steam atomizing burners and if introduced
into the higher end of the furnace, should not lead to tube difficulties
provided they are properly located and operated. This class of burner
also will give the most satisfactory results if introduced so that the
flames travel in the direction of increase in furnace volume. This is
perhaps best exemplified by the very good results secured with
mechanical atomizing burners and Babcock & Wilcox marine boilers in
which, due to the fact that the boilers are fired from the low end, the
flames from burners introduced through the front are in this direction.
Operation of Burners--When burners are not in use, or when they are
being started up, care must be taken to prevent oil from flowing and
collecting on the floor of the furnace before it is ignited. In starting
a burner, the atomized fuel may be ignited by a burning wad of oil-soaked
waste held before it on an iron rod. To insure quick ignition, the steam
supply should be cut down. But little practice is required to become an
adept at lighting an oil fire. When ignition has taken place and the
furnace brought to an even heat, the steam should be cut down to the
minimum amount required for atomization. This amount can be determined
from the appearance of the flame. If sufficient steam is not supplied,
particles of burning oil will drop to the furnace floor, giving a
scintillating appearance to the flame. The steam valves should be opened
just sufficiently to overcome this scintillating action.
Air Supply--From the nature of the fuel and the method of burning, the
quantity of air for combustion may be minimized. As with other fuels,
when the amount of air admitted is the minimum which will completely
consume the oil, the results are the best. The excess or deficiency of
air can be judged by the appearance of the stack or by observing the
gases passing through the boiler settings. A perfectly clear stack
indicates excess air, whereas smoke indicates a deficiency. With
properly designed furnaces the best results are secured by running near
the smoking point with a slight haze in the gases. A slight variation in
the air supply will affect the furnace conditions in an oil burning
boiler more than the same variation where coal is used, and for this
reason it is of the utmost importance that flue gas analysis be made
frequently on oil-burning boilers. With the air for combustion properly
regulated by adjustment of any checkerwork or any other device which may
be used, and the dampers carefully set, the flue gas analysis should
show, for good furnace conditions, a percentage of CO_{2} between 13 and
14 per cent, with either no CO or but a trace.
In boiler plant operation it is difficult to regulate the steam supply
to the burners and the damper position to meet sudden and repeated
variations in the load. A device has been patented which automatically
regulates by means of the boiler pressure the pressure of the steam to
the burners, the oil to the burners and the position of the boiler
damper. Such a device has been shown to give good results in plant
operation where hand regulation is difficult at best, and in many
instances is unfortunately not even attempted.
Efficiency with Oil--As pointed out in enumerating the advantages of oil
fuel over coal, higher efficiencies are obtainable with the former. With
boilers of approximately 500 horse power equipped with properly designed
furnaces and burners, an efficiency of 83 per cent is possible or making
an allowance of 2 per cent for steam used by burners, a net efficiency
of 81 per cent. The conditions under which such efficiencies are to be
secured are distinctly test conditions in which careful operation is a
prime requisite. With furnace conditions that are not conductive to the
best combustion, this figure may be decreased by from 5 to 10 per cent.
In large properly designed plants, however, the first named efficiency
may be approached for uniform running conditions, the nearness to which
it is reached depending on the intelligence of the operating crew. It
must be remembered that the use of oil fuel presents to the careless
operator possibilities for wastefulness much greater than in plants
where coal is fired, and it therefore pays to go carefully into this
feature.
Table 48 gives some representative tests with oil fuel.
TABLE 48
TESTS OF BABCOCK AND WILCOX BOILERS WITH OIL FUEL
_______________________________________________________________________
| | | | |
| |Pacific Light|Pacific Light|Miami Copper |
| | and Power | and Power | Company |
| Plant | Company | Company | |
| |Los Angeles, | | Miami, |
| | Cal. |Redondo, Cal.| Arizona |
|_____________________________|_____________|_____________|_____________|
| | | | | |
| Rated Capacity | Horse | | | |
| of Boiler | Power | 467 | 604 | 600 |
|__________________|__________|_____________|_____________|_____________|
| | | | | | | | |
| Duration of Test | Hours | 10 | 10 | 7 | 7 | 10 | 4 |
| | | | | | | | |
| Steam Pressure | | | | | | | |
| by Gauge | Pounds | 156.4| 156.9| 184.7| 184.9| 183.4| 189.5|
| | | | | | | | |
| Temperature of | Degrees | | | | | | |
| Feed Water | F. | 62.6| 61.1| 93.4| 101.2| 157.7| 156.6|
| | | | | | | | |
| Degrees of | Degrees | | | | | | |
| Superheat | F. | | | 83.7| 144.3| 103.4| 139.6|
| | | | | | | | |
| Factor of | | | | | | | |
| Evaporation | |1.2004|1.2020|1.2227|1.2475|1.1676|1.1886|
| | | | | | | | |
| Draft in Furnace | Inches | .02 | .05 | .014| .19 | .12 | .22 |
| | | | | | | | |
| Draft at Damper | Inches | .08 | .15 | .046| .47 | .19 | .67 |
| | | | | | | | |
| Temperature of | Degrees | | | | | | |
| Exit Gases | F. | 438 | 525 | 406 | 537 | 430 | 612 |
| _ | | | | | | | |
| Flue | CO_{2} | Per Cent | | | 14.3 | 12.1 | | |
| Gas | O | Per Cent | | | 3.8 | 6.8 | | |
| Analysis|_CO | Per Cent | | | 0.0 | 0.0 | | |
| | | | | | | | |
| Oil Burned | | | | | | | |
| per Hour | Pounds | 1147 | 1837 | 1439 | 2869 | 1404 | 3214 |
| | | | | | | | |
| Water Evaporated | | | | | | | |
| per Hour from | | | | | | | |
| from and at | Pounds | 18310| 27855| 22639| 40375| 21720| 42863|
| 212 Degrees | | | | | | | |
| | | | | | | | |
| Evaporation from | | | | | | | |
| and at 212 | | | | | | | |
| Degrees per | Pounds | 15.96| 15.16| 15.73| 14.07| 15.47| 13.34|
| Pound of Oil | | | | | | | |
| | | | | | | | |
| Per Cent of | | | | | | | |
| Rated Capacity | Pounds | 113.6| 172.9| 108.6| 193.8| 104.9| 207.1|
| Developed | | | | | | | |
| | | | | | | | |
| B. t. u. per | | | | | | | |
| Pound of Oil | B. t. u. | 18626| 18518| 18326| 18096| 18600| 18600|
| | | | | | | | |
| Efficiency | Per Cent | 83.15| 79.46| 83.29| 76.02| 80.70| 69.6 |
|__________________|__________|______|______|______|______|______|______|
Burning Oil in Connection with Other Fuels--Considerable attention has
been recently given to the burning of oil in connection with other
fuels, and a combination of this sort may be advisable either with the
view to increasing the boiler capacity to assist over peak loads, or to
keep the boiler in operation where there is the possibility of a
temporary failure of the primary fuel. It would appear from experiments
that such a combination gives satisfactory results from the standpoint
of both capacity and efficiency, if the two fuels are burned in separate
furnaces. Satisfactory results cannot ordinarily be obtained when it is
attempted to burn oil fuel in the same furnace as the primary fuel, as
it is practically impossible to admit the proper amount of air for
combustion for each of the two fuels simultaneously. The Babcock &
Wilcox boiler lends itself readily to a double furnace arrangement and
Fig. 30 shows an installation where oil fuel is burned as an auxiliary
to wood.
[Illustration: Fig. 30. Babcock & Wilcox Boiler Set with Combination Oil
and Wood-burning Furnace]
Water-gas Tar--Water-gas tar, or gas-house tar, is a by-product of the
coal used in the manufacture of water gas. It is slightly heavier than
crude oil and has a comparatively low flash point. In burning, it should
be heated only to a temperature which makes it sufficiently fluid, and
any furnace suitable for crude oil is in general suitable for water-gas
tar. Care should be taken where this fuel is used to install a suitable
apparatus for straining it before it is fed to the burner.
[Illustration: Babcock & Wilcox Boilers Fired with Blast Furnace Gas at
the Bethlehem Steel Co., Bethlehem, Pa. This Company Operates 12,900
Horse Power of Babcock & Wilcox Boilers]
GASEOUS FUELS AND THEIR COMBUSTION
Of the gaseous fuels available for steam generating purposes, the most
common are blast furnace gas, natural gas and by-product coke oven gas.
Blast furnace gas, as implied by its name, is a by-product from the
blast furnace of the iron industry. This gasification of the solid fuel
in a blast furnace results, 1st, through combustion by the oxygen of the
blast; 2nd, through contact with the incandescent ore (Fe_{2}O_{3} + C
= 2 FeO + CO and FeO + C = Fe + CO); and 3rd, through the agency of
CO_{2} either formed in the process of reduction or driven from the
carbonates charged either as ore or flux.
Approximately 90 per cent of the fuel consumed in all of the blast
furnaces of the United States is coke. The consumption of coke per ton
of iron made varies from 1600 to 3600 pounds per ton of 2240 pounds of
iron. This consumption depends upon the quality of the coal, the nature
of the ore, the quality of the pig iron produced and the equipment and
management of the plant. The average consumption, and one which is
approximately correct for ordinary conditions, is 2000 pounds of coke
per gross ton (2240 pounds) of pig iron. The gas produced in a gas
furnace per ton of pig iron is obtained from the weight of fixed carbon
gasified, the weight of the oxygen combined with the material of charge
reduced, the weight of the gaseous constituents of the flux and the
weight of air delivered by the blowing engine and the weight of volatile
combustible contained in the coke. Ordinarily, this weight of gas will
be found to be approximately five times the weight of the coke burned,
or 10,000 pounds per ton of pig iron produced.
With the exception of the small amount of carbon in combination with
hydrogen as methane, and a very small percentage of free hydrogen,
ordinarily less than 0.1 per cent, the calorific value of blast furnace
gas is due to the CO content which when united with sufficient oxygen
when burned under a boiler, burns further to CO_{2}. The heat value of
such gas will vary in most cases from 85 to 100 B. t. u. per cubic foot
under standard conditions. In modern practice, where the blast is heated
by hot blast stoves, approximately 15 per cent of the total amount of
gas is used for this purpose, leaving 85 per cent of the total for use
under boilers or in gas engines, that is, approximately 8500 pounds of
gas per ton of pig iron produced. In a modern blast furnace plant, the
gas serves ordinarily as the only fuel required. Table 49 gives the
analyses of several samples of blast furnace gas.
TABLE 49
TYPICAL ANALYSES OF BLAST FURNACE GAS
+----------------------------------------------------------------+
|+-----------------------+------+----+-----+----+------+--------+|
|| |CO_{2}| O | CO | H |CH_{4}| N ||
|+-----------------------+------+----+-----+----+------+--------+|
||Bessemer Furnace | 9.85|0.36|32.73|3.14| .. |53.92 ||
||Bessemer Furnace | 11.4 | .. |27.7 |1.9 | 0.3 |58.7 ||
||Bessemer Furnace | 10.0 | .. |26.2 |3.1 | 0.2 |60.5 ||
||Bessemer Furnace | 9.1 | .. |28.7 |2.7 | 0.2 |59.3 ||
||Bessemer Furnace | 13.5 | .. |25.2 |1.43| .. |59.87 ||
||Bessemer Furnace[47] | 10.9 | .. |27.8 |2.8 | 0.2 |58.3 ||
||Ferro Manganese Furnace| 7.1 | .. |30.1 | .. | .. |62.8[48]||
||Basic Ore Furnace | 16.0 |0.2 |23.6 | .. | .. |60.2[48]||
|+-----------------------+------+----+-----+----+------+--------+|
+----------------------------------------------------------------+
Until recently, the important consideration in the burning of blast
furnace gas has been the capacity that can be developed with practically
no attention given to the aspect of efficiency. This phase of the
question is now drawing attention and furnaces especially designed for
good efficiency with this class of fuel are demanded. The essential
feature is ample combustion space, in which the combustion of gases may
be practically completed before striking the heating surfaces. The gases
have the power of burning out completely after striking the heating
surfaces, provided the initial temperature is sufficiently high, but
where the combustion is completed before such time, the results secured
are more satisfactory. A furnace volume of approximately 1 to 1.5 cubic
feet per rated boiler horse power will give a combustion space that is
ample.
Where there is the possibility of a failure of the gas supply, or where
steam is required when the blast furnace is shut down, coal fired grates
of sufficient size to get the required capacity should be installed.
Where grates of full size are not required, ignition grates should be
installed, which need be only large enough to carry a fire for igniting
the gas or for generating a small quantity of steam when the blast
furnace is shut down. The area of such grates has no direct bearing on
the size of the boiler. The grates may be placed directly under the gas
burners in a standard position or may be placed between two bridge walls
back of the gas furnace and fired from the side of the boiler. An
advantage is claimed for the standard grate position that it minimizes
the danger of explosion on the re-ignition of gas after a temporary
stoppage of the supply and also that a considerable amount of dirt, of
which there is a good deal with this class of fuel and which is
difficult to remove, deposits on the fire and is taken out when the
fires are cleaned. In any event, regardless of the location of the
grates, ample provision should be made for removing this dust, not only
from the furnace but from the setting as a whole.
Blast furnace gas burners are of two general types: Those in which the
air for combustion is admitted around the burner proper, and those in
which this air is admitted through the burner. Whatever the design of
burner, provision should be made for the regulation of both the air and
the gas supply independently. A gas opening of .8 square inch per rated
horse power will enable a boiler to develop its nominal rating with a
gas pressure in the main of about 2 inches. This pressure is ordinarily
from 6 to 8 inches and in this way openings of the above size will be
good for ordinary overloads. The air openings should be from .75 to .85
square inch per rated horse power. Good results are secured by inclining
the gas burners slightly downward toward the rear of the furnace. Where
the burners are introduced over coal fired grates, they should be set
high enough to give headroom for hand firing.
Ordinarily, individual stacks of 130 feet high with diameters as given
in Kent's table for corresponding horse power are large enough for this
class of work. Such a stack will give a draft sufficient to allow a
boiler to be operated at 175 per cent of its rated capacity, and beyond
this point the capacity will not increase proportionately with the
draft. When more than one boiler is connected with a stack, the draft
available at the damper should be equivalent to that which an individual
stack of 130 feet high would give. The draft from such a stack is
necessary to maintain a suction under all conditions throughout all
parts of the setting. If the draft is increased above that which such a
stack will give, difficulties arise from excess air for combustion with
consequent loss in efficiency.
A poor mixing or laneing action in the furnace may result in a pulsating
effect of the gases in the setting. This action may at times be remedied
by admitting more air to the furnace. On account of the possibility of a
pulsating action of the gases under certain conditions and the puffs or
explosions, settings for this class of work should be carefully
constructed and thoroughly buckstayed and tied.
Natural Gas--Natural gas from different localities varies considerably
in composition and heating value. In Table 50 there is given a number of
analyses and heat values for natural gas from various localities.
This fuel is used for steam generating purposes to a considerable extent
in some localities, though such use is apparently decreasing. It is best
burned by employing a large number of small burners, each being capable
of handling 30 nominal rated horse power. The use of a large number of
burners obviates the danger of any laneing or blowpipe action, which
might be present where large burners are used. Ordinarily, such a gas,
as it enters the burners, is under a pressure of about 8 ounces. For the
purpose of comparison, all observations should be based on gas reduced
to the standard conditions of temperature and pressure, namely 32
degrees Fahrenheit and 14.7 pounds per square inch. When the temperature
and pressure corresponding to meter readings are known, the volume of
gas under standard conditions may be obtained by multiplying the meter
readings in cubic feet by 33.54 P/T, in which P equals the absolute
pressure in pounds per square inch and T equals the absolute temperature
of the gas at the meter. In boiler testing work, the evaporation should
always be reduced to that per cubic foot of gas under standard
conditions.
TABLE 50
TYPICAL ANALYSES (BY VOLUME) AND CALORIFIC VALUES OF NATURAL
GAS FROM VARIOUS LOCALITIES
+----------------+-----+-----+-----+-----+-----+----+-------+------+--------+
|Locality of Well| H |CH_{4}| CO |CO_{2}| N | O | Heavy |H_{2}S|B. t. u.|
| | | | | | | |Hydro- | | per |
| | | | | | | |carbons| | Cubic |
| | | | | | | | | | Foot |
| | | | | | | | | |Calcul- |
| | | | | | | | | |ated[49]|
|----------------+-----+-----+-----+-----+-----+----+-------+------+--------+
|Anderson, Ind. | 1.86|93.07| 0.73| 0.26| 3.02|0.42| 0.47 | 0.15 | 1017 |
|Marion, Ind. | 1.20|93.16| 0.60| 0.30| 3.43|0.55| 0.15 | 0.20 | 1009 |
|Muncie, Ind. | 2.35|92.67| 0.45| 0.25| 3.53|0.35| 0.25 | 0.15 | 1004 |
|Olean, N. Y. | |96.50| 0.50| | |2.00| 1.00 | | 1018 |
|Findlay, O. | 1.64|93.35| 0.41| 0.25| 3.41|0.39| 0.35 | 0.20 | 1011 |
|St. Ive, Pa. | 6.10|75.54|Trace| 0.34| | | 18.12 | | 1117 |
|Cherry Tree, Pa.|22.50|60.27| | 2.28| 7.32|0.83| 6.80 | | 842 |
|Grapeville, Pa. |24.56|14.93|Trace|Trace|18.69|1.22| 40.60 | | 925 |
|Harvey Well, | | | | | | | | | |
| Butler Co., Pa.|13.50|80.00|Trace| 0.66| | | 5.72 | | 998 |
|Pittsburgh, Pa. | 9.64|57.85| 1.00| |23.41|2.10| 6.00 | | 748 |
|Pittsburgh, Pa. |20.02|72.18| 1.00| 0.80| |1.10| 4.30 | | 917 |
|Pittsburgh, Pa. |26.16|65.25| 0.80| 0.60| |0.80| 6.30 | | 899 |
+----------------+-----+-----+-----+-----+-----+----+-------+------+--------+
[Illustration: 1600 Horse-power Installation of Babcock & Wilcox Boilers
and Superheaters at the Carnegie Natural Gas Co., Underwood, W. Va.
Natural Gas is the Fuel Burned under these Boilers]
When natural gas is the only fuel, the burners should be evenly
distributed over the lower portion of the boiler front. If the fuel is
used as an auxiliary to coal, the burners may be placed through the fire
front. A large combustion space is essential and a volume of .75 cubic
feet per rated horse power will be found to give good results. The
burners should be of a design which give the gas and air a rotary motion
to insure a proper mixture. A checkerwork wall is sometimes placed in
the furnace about 3 feet from the burners to break up the flame, but
with a good design of burner this is unnecessary. Where the gas is
burned alone and no grates are furnished, good results are secured by
inclining the burner downward to the rear at a slight angle.
By-product Coke Oven Gas--By-product coke oven gas is a product of the
destructive distillation of coal in a distilling or by-product coke
oven. In this class of apparatus the gases, instead of being burned at
the point of their origin, as in a beehive or retort coke oven, are
taken from the oven through an uptake pipe, cooled and yield as
by-products tar, ammonia, illuminating and fuel gas. A certain portion
of the gas product is burned in the ovens and the remainder used or sold
for illuminating or fuel purposes, the methods of utilizing the gas
varying with plant operation and locality.
Table 51 gives the analyses and heat value of certain samples of
by-product coke oven gas utilized for fuel purposes.
This gas is nearer to natural gas in its heat value than is blast
furnace gas, and in general the remarks as to the proper methods of
burning natural gas and the features to be followed in furnace design
hold as well for by-product coke oven gas.
TABLE 51
TYPICAL ANALYSES OF BY-PRODUCT
COKE OVEN GAS
+----------------------------------------------+
|+------+-------------------------------------+|
||CO_{2}| O |CO |CH_{4}| H | N |B.t.u. per||
|| | | | | | |Cubic Foot||
|+------+-----+---+------+----+----+----------+|
|| 0.75 |Trace|6.0|28.15 |53.0|12.1| 505 ||
|| 2.00 |Trace|3.2|18.80 |57.2|18.0| 399 ||
|| 3.20 | 0.4 |6.3|29.60 |41.6|16.1| 551 ||
|| 0.80 | 1.6 |4.9|28.40 |54.2|10.1| 460 ||
|+------+-----+---+------+----+----+----------+|
+----------------------------------------------+
The essential difference in burning the two fuels is the pressure under
which it reaches the gas burner. Where this is ordinarily from 4 to 8
ounces in the case of natural gas, it is approximately 4 inches of water
in the case of by-product coke oven gas. This necessitates the use of
larger gas openings in the burners for the latter class of fuel than for
the former.
By-product coke oven gas comes to the burners saturated with moisture
and provision should be made for the blowing out of water of
condensation. This gas too, carries a large proportion of tar and
hydrocarbons which form a deposit in the burners and provision should be
made for cleaning this out. This is best accomplished by an attachment
which permits the blowing out of the burners by steam.
UTILIZATION OF WASTE HEAT
While it has been long recognized that the reclamation of heat from the
waste gases of various industrial processes would lead to a great saving
in fuel and labor, the problem has, until recently, never been given the
attention that its importance merits. It is true that installations have
been made for the utilization of such gases, but in general they have
consisted simply in the placing of a given amount of boiler heating
surface in the path of the gases and those making the installations have
been satisfied with whatever power has been generated, no attention
being given to the proportioning of either the heating surface or the
gas passages to meet the peculiar characteristics of the particular
class of waste gas available. The Babcock & Wilcox Co. has recently gone
into the question of the utilization of what has been known as waste
heat with great thoroughness, and the results secured by their
installations with practically all operations yielding such gases are
eminently successful.
TABLE 52
TEMPERATURE OF WASTE GASES FROM
VARIOUS INDUSTRIAL PROCESSES
+-----------------------------------------------------+
|+-----------------------------------+---------------+|
||Waste Heat From |Temperature[50]||
|| | Degrees ||
|+-----------------------------------+---------------+|
||Brick Kilns | 2000-2300 ||
||Zinc Furnaces | 2000-2300 ||
||Copper Matte Reverberatory Furnaces| 2000-2200 ||
||Beehive Coke Ovens | 1800-2000 ||
||Cement Kilns | 1200-1600[51]||
||Nickel Refining Furnaces | 1500-1750 ||
||Open Hearth Steel Furnaces | 1100-1400 ||
|+-----------------------------------+---------------+|
+-----------------------------------------------------+
The power that can be obtained from waste gases depends upon their
temperature and weight, and both of these factors vary widely in
different commercial operations. Table 52 gives a list of certain
processes yielding waste gases the heat of which is available for the
generation of steam and the approximate temperature of such gases. It
should be understood that the temperatures in the table are the average
of the range of a complete cycle of the operation and that the minimum
and maximum temperatures may vary largely from the figures given.
The maximum available horse power that may be secured from such gases is
represented by the formula:
W(T-t)s
H. P. = ------- (23)
33,479
Where W = the weight of gases passing per hour,
T = temperature of gases entering heating surface,
t = temperature leaving heating surface,
s = specific heat of gases.
The initial temperature and the weight or volume of gas will depend, as
stated, upon the process involved. The exit temperature will depend, to
a certain extent, upon the temperature of the entering gases, but will
be governed mainly by the efficiency of the heating surfaces installed
for the absorption of the heat.
Where the temperature of the gas available is high, approaching that
found in direct fired boiler practice, the problem is simple and the
question of design of boiler becomes one of adapting the proper amount
of heating surface to the volume of gas to be handled. With such
temperatures, and a volume of gas available approximately in accordance
with that found in direct fired boiler practice, a standard boiler or
one but slightly modified from the standard will serve the purpose
satisfactorily. As the temperatures become lower, however, the problem
is more difficult and the departure from standard practice more radical.
With low temperature gases, to obtain a heat transfer rate at all
comparable with that found in ordinary boiler practice, the lack of
temperature must be offset by an added velocity of the gases in their
passage over the heating surfaces. In securing the velocity necessary to
give a heat transfer rate with low temperature gases sufficient to make
the installation of waste heat boilers show a reasonable return on the
investment, the frictional resistance to the gases through the boiler
becomes greatly in excess of what would be considered good practice in
direct fired boilers. Practically all operations yielding waste gases
require that nothing be done in the way of impairing the draft at the
furnace outlet, as this might interfere with the operation of the
primary furnace. The installation of a waste heat boiler, therefore,
very frequently necessitates providing sufficient mechanical draft to
overcome the frictional resistance of the gases through the heating
surfaces and still leave ample draft available to meet the maximum
requirements of the primary furnace.
Where the temperature and volume of the gases are in line with what are
found in ordinary direct fired practice, the area of the gas passages
may be practically standard. With the volume of gas known, the draft
loss through the heating surfaces may be obtained from experimental data
and this additional draft requirement met by the installation of a stack
sufficient to take care of this draft loss and still leave draft enough
for operating the furnace at its maximum capacity.
Where the temperatures are low, the added frictional resistance will
ordinarily be too great to allow the draft required to be secured by
additional stack height and the installation of a fan is necessary. Such
a fan should be capable of handling the maximum volume of gas that the
furnace may produce, and of maintaining a suction equivalent to the
maximum frictional resistance of such volume through the boiler plus the
maximum draft requirement at the furnace outlet. Stacks and fans for
this class of work should be figured on the safe side. Where a fan
installation is necessary, the loss of draft in the fan connections
should be considered, and in figuring conservatively it should be
remembered that a fan of ample size may be run as economically as a
smaller fan, whereas the smaller fan, if overloaded, is operated with a
large loss in efficiency. In practically any installation where low
temperature gas requires a fan to give the proper heat transfer from the
gases, the cost of the fan and of the energy to drive it will be more
than offset by the added power from the boiler secured by its use.
Furthermore, the installation of such a fan will frequently increase the
capacity of the industrial furnace, in connection with which the waste
heat boilers are installed.
In proportioning heating surfaces and gas passages for waste heat work
there are so many factors bearing directly on what constitutes the
proper installation that it is impossible to set any fixed rules. Each
individual installation must be considered by itself as well as the
particular characteristics of the gases available, such as their
temperature and volume, and the presence of dust or tar-like substances,
and all must be given the proper weight in the determination of the
design of the heating surfaces and gas passages for the specific set of
conditions.
[Graph: Per Cent of Water Heating Surface passed over by Gases/Per Cent
of the Total Amount of Steam Generated in the Boiler
against Temperature in Degrees Fahrenheit of Hot Gases Sweeping Heating
Surface
Fig. 31. Curve Showing Relation Between Gas Temperature, Heating Surface
passed over, and Amount of Steam Generated. Ten Square Feet of Heating
Surface are Assumed as Equivalent to One Boiler Horse Power]
Fig. 31 shows the relation of gas temperatures, heating surface passed
over and work done by such surface for use in cases where the
temperatures approach those found in direct fired practice and where the
volume of gas available is approximately that with which one horse power
may be developed on 10 square feet of heating surface. The curve assumes
what may be considered standard gas passage areas, and further, that
there is no heat absorbed by direct radiation from the fire.
Experiments have shown that this curve is very nearly correct for the
conditions assumed. Such being the case, its application in waste heat
work is clear. Decreasing or increasing the velocity of the gases over
the heating surfaces from what might be considered normal direct fired
practice, that is, decreasing or increasing the frictional loss through
the boiler will increase or decrease the amount of heating surface
necessary to develop one boiler horse power. The application of Fig. 31
to such use may best be seen by an example:
Assume the entering gas temperatures to be 1470 degrees and that the
gases are cooled to 570 degrees. From the curve, under what are assumed
to be standard conditions, the gases have passed over 19 per cent of
the heating surface by the time they have been cooled 1470 degrees.
When cooled to 570 degrees, 78 per cent of the heating surface has been
passed over. The work done in relation to the standard of the curve is
represented by (1470 - 570) ÷ (2500 - 500) = 45 per cent. (These
figures may also be read from the curve in terms of the per cent of the
work done by different parts of the heating surfaces.) That is, 78 per
cent - 19 per cent = 59 per cent of the standard heating surface has
done 45 per cent of the standard amount of work. 59 ÷ 45 = 1.31, which
is the ratio of surface of the assumed case to the standard case of the
curve. Expressed differently, there will be required 13.1 square feet
of heating surface in the assumed case to develop a horse power as
against 10 square feet in the standard case.
The gases available for this class of work are almost invariably very
dirty. It is essential for the successful operation of waste-heat
boilers that ample provision be made for cleaning by the installation of
access doors through which all parts of the setting may be reached. In
many instances, such as waste-heat boilers set in connection with cement
kilns, settling chambers are provided for the dust before the gases
reach the boiler.
By-passes for the gases should in all cases be provided to enable the
boiler to be shut down for cleaning and repairs without interfering with
the operation of the primary furnace. All connections from furnace to
boilers should be kept tight to prevent the infiltration of air, with
the consequent lowering of gas temperatures.
Auxiliary gas or coal fired grates must be installed to insure
continuity in the operation of the boiler where the operation of the
furnace is intermittent or where it may be desired to run the boiler
with the primary furnace not in operation. Such grates are sometimes
used continuously where the gases available are not sufficient to
develop the required horse power from a given amount of heating surface.
Fear has at times been expressed that certain waste gases, such as those
containing sulphur fumes, will have a deleterious action on the heating
surface of the boiler. This feature has been carefully watched, however,
and from plants in operation it would appear that in the absence of
water or steam leaks within the setting, there is no such harmful
action.
[Illustration: Fig. 32. Babcock & Wilcox Boiler Arranged for Utilizing
Waste Heat from Open Hearth Furnace. This Setting may be Modified to
Take Care of Practically any Kind of Waste Gas]
CHIMNEYS AND DRAFT
The height and diameter of a properly designed chimney depend upon the
amount of fuel to be burned, its nature, the design of the flue, with
its arrangement relative to the boiler or boilers, and the altitude of
the plant above sea level. There are so many factors involved that as
yet there has been produced no formula which is satisfactory in taking
them all into consideration, and the methods used for determining stack
sizes are largely empirical. In this chapter a method sufficiently
comprehensive and accurate to cover all practical cases will be
developed and illustrated.
Draft is the difference in pressure available for producing a flow of
the gases. If the gases within a stack be heated, each cubic foot will
expand, and the weight of the expanded gas per cubic foot will be less
than that of a cubic foot of the cold air outside the chimney.
Therefore, the unit pressure at the stack base due to the weight of the
column of heated gas will be less than that due to a column of cold air.
This difference in pressure, like the difference in head of water, will
cause a flow of the gases into the base of the stack. In its passage to
the stack the cold air must pass through the furnace or furnaces of the
boilers connected to it, and it in turn becomes heated. This newly
heated gas will also rise in the stack and the action will be
continuous.
The intensity of the draft, or difference in pressure, is usually
measured in inches of water. Assuming an atmospheric temperature of 62
degrees Fahrenheit and the temperature of the gases in the chimney as
500 degrees Fahrenheit, and, neglecting for the moment the difference in
density between the chimney gases and the air, the difference between
the weights of the external air and the internal flue gases per cubic
foot is .0347 pound, obtained as follows:
Weight of a cubic foot of air at 62 degrees Fahrenheit = .0761 pound
Weight of a cubic foot of air at 500 degrees Fahrenheit = .0414 pound
------------------------
Difference = .0347 pound
Therefore, a chimney 100 feet high, assumed for the purpose of
illustration to be suspended in the air, would have a pressure exerted
on each square foot of its cross sectional area at its base of .0347 ×
100 = 3.47 pounds. As a cubic foot of water at 62 degrees Fahrenheit
weighs 62.32 pounds, an inch of water would exert a pressure of 62.32 ÷
12 = 5.193 pounds per square foot. The 100-foot stack would, therefore,
under the above temperature conditions, show a draft of 3.47 ÷ 5.193 or
approximately 0.67 inches of water.
The method best suited for determining the proper proportion of stacks
and flues is dependent upon the principle that if the cross sectional
area of the stack is sufficiently large for the volume of gases to be
handled, the intensity of the draft will depend directly upon the
height; therefore, the method of procedure is as follows:
1st. Select a stack of such height as will produce the draft required by
the particular character of the fuel and the amount to be burned per
square foot of grate surface.
2nd. Determine the cross sectional area necessary to handle the gases
without undue frictional losses.
The application of these rules follows:
Draft Formula--The force or intensity of the draft, not allowing for the
difference in the density of the air and of the flue gases, is given by
the formula:
/ 1 1 \
D = 0.52 H × P |--- - -----| (24)
\ T T_{1}/
in which
D = draft produced, measured in inches of water,
H = height of top of stack above grate bars in feet,
P = atmospheric pressure in pounds per square inch,
T = absolute atmospheric temperature,
T_{1} = absolute temperature of stack gases.
In this formula no account is taken of the density of the flue gases, it
being assumed that it is the same as that of air. Any error arising from
this assumption is negligible in practice as a factor of correction is
applied in using the formula to cover the difference between the
theoretical figures and those corresponding to actual operating
conditions.
The force of draft at sea level (which corresponds to an atmospheric
pressure of 14.7 pounds per square inch) produced by a chimney 100 feet
high with the temperature of the air at 60 degrees Fahrenheit and that
of the flue gases at 500 degrees Fahrenheit is,
/ 1 1 \
D = 0.52 × 100 × 14.7 | --- - --- | = 0.67
\ 521 961 /
Under the same temperature conditions this chimney at an atmospheric
pressure of 10 pounds per square inch (which corresponds to an altitude
of about 10,000 feet above sea level) would produce a draft of,
/ 1 1 \
D = 0.52 × 100 × 10 | --- - --- | = 0.45
\ 521 961 /
For use in applying this formula it is convenient to tabulate values of
the product
/ 1 1 \
0.52 × 14.7|--- - -----|
\ T T_{1}/
which we will call K, for various values of T_{1}. With these values
calculated for assumed atmospheric temperature and pressure (24) becomes
D = KH. (25)
For average conditions the atmospheric pressure may be considered 14.7
pounds per square inch, and the temperature 60 degrees Fahrenheit. For
these values and various stack temperatures K becomes:
_Temperature Stack Gases_ _Constant K_
750 .0084
700 .0081
650 .0078
600 .0075
550 .0071
500 .0067
450 .0063
400 .0058
350 .0053
Draft Losses--The intensity of the draft as determined by the above
formula is theoretical and can never be observed with a draft gauge or
any recording device. However, if the ashpit doors of the boiler are
closed and there is no perceptible leakage of air through the boiler
setting or flue, the draft measured at the stack base will be
approximately the same as the theoretical draft. The difference existing
at other times represents the pressure necessary to force the gases
through the stack against their own inertia and the friction against the
sides. This difference will increase with the velocity of the gases.
With the ashpit doors closed the volume of gases passing to the stack
are a minimum and the maximum force of draft will be shown by a gauge.
As draft measurements are taken along the path of the gases, the
readings grow less as the points at which they are taken are farther
from the stack, until in the boiler ashpit, with the ashpit doors open
for freely admitting the air, there is little or no perceptible rise in
the water of the gauge. The breeching, the boiler damper, the baffles
and the tubes, and the coal on the grates all retard the passage of the
gases, and the draft from the chimney is required to overcome the
resistance offered by the various factors. The draft at the rear of the
boiler setting where connection is made to the stack or flue may be 0.5
inch, while in the furnace directly over the fire it may not be over,
say, 0.15 inch, the difference being the draft required to overcome the
resistance offered in forcing the gases through the tubes and around the
baffling.
One of the most important factors to be considered in designing a stack
is the pressure required to force the air for combustion through the bed
of fuel on the grates. This pressure will vary with the nature of the
fuel used, and in many instances will be a large percentage of the total
draft. In the case of natural draft, its measure is found directly by
noting the draft in the furnace, for with properly designed ashpit doors
it is evident that the pressure under the grates will not differ
sensibly from atmospheric pressure.
Loss in Stack--The difference between the theoretical draft as
determined by formula (24) and the amount lost by friction in the stack
proper is the available draft, or that which the draft gauge indicates
when connected to the base of the stack. The sum of the losses of draft
in the flue, boiler and furnace must be equivalent to the available
draft, and as these quantities can be determined from record of
experiments, the problem of designing a stack becomes one of
proportioning it to produce a certain available draft.
The loss in the stack due to friction of the gases can be calculated
from the following formula:
f W² C H
[Delta]D = -------- (26)
A³
in which
[Delta]D = draft loss in inches of water,
W = weight of gas in pounds passing per second,
C = perimeter of stack in feet,
H = height of stack in feet,
f = a constant with the following values at sea level:
.0015 for steel stacks, temperature of gases 600 degrees
Fahrenheit.
.0011 for steel stacks, temperature of gases 350 degrees
Fahrenheit.
.0020 for brick or brick-lined stacks, temperature of gases
600 degrees Fahrenheit.
.0015 for brick or brick-lined stacks, temperature of gases
350 degrees Fahrenheit.
A = Area of stack in square feet.
[Illustration: 24,420 Horse-power Installation of Babcock & Wilcox
Boilers and Superheaters, Equipped with Babcock & Wilcox Chain Grate
Stokers in the Quarry Street Station of the Commonwealth Edison Co.,
Chicago, Ill.]
This formula can also be used for calculating the frictional losses for
flues, in which case, C = the perimeter of the flue in feet, H = the
length of the flue in feet, the other values being the same as for
stacks.
The available draft is equal to the difference between the theoretical
draft from formula (25) and the loss from formula (26), hence:
f W² C H
d^{1} = available draft = KH - -------- (27)
A³
Table 53 gives the available draft in inches that a stack 100 feet high
will produce when serving different horse powers of boilers with the
methods of calculation for other heights.
TABLE 53
AVAILABLE DRAFT
CALCULATED FOR 100-FOOT STACK OF DIFFERENT DIAMETERS ASSUMING STACK
TEMPERATURE OF 500 DEGREES FAHRENHEIT AND 100 POUNDS OF GAS PER HORSE POWER
FOR OTHER HEIGHTS OF STACK MULTIPLY DRAFT BY HEIGHT ÷ 100
+-----+-------------------------------------------------------------------+
|Horse| |
|Power| Diameter of Stack in Inches |
+-----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| |36 |42 |48 |54 |60 |66 |72 |78 |84 |90 |96 |102|108|114|120|132|144|
+-----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 100 |.64| | | | | | | | | | | | | | | | |
| 200 |.55|.62| | | | | | | | | | | | | | | |
| 300 |.41|.55|.61| | | | | | | | | | | | | | |
| 400 |.21|.46|.56|.61| | | | | | | | | | | | | |
| 500 | |.34|.50|.57|.61| | | | | | | | | | | | |
| 600 | |.19|.42|.53|.59| | | | | | | | | | | | |
| 700 | | |.34|.48|.56|.60|.63| | | | | | | | | | |
| 800 | | |.23|.43|.52|.58|.61|.63| | | | | | | | | |
| 900 | | | |.36|.49|.56|.60|.62|.64| | | | | | | | |
|1000 | | | |.29|.45|.53|.58|.61|.63|.64| | | | | | | |
|1100 | | | | |.40|.50|.56|.60|.62|.63|.64| | | | | | |
|1200 | | | | |.35|.47|.54|.58|.61|.63|.64|.65| | | | | |
|1300 | | | | |.29|.44|.52|.57|.60|.62|.63|.64|.65| | | | |
|1400 | | | | | |.40|.49|.55|.59|.61|.63|.64|.65|.65| | | |
|1500 | | | | | |.36|.47|.53|.58|.60|.62|.63|.64|.65|.65| | |
|1600 | | | | | |.31|.43|.52|.56|.59|.62|.63|.64|.65|.65| | |
|1700 | | | | | | |.41|.50|.55|.58|.61|.62|.64|.64|.65| | |
|1800 | | | | | | |.37|.47|.54|.57|.60|.62|.63|.64|.65| | |
|1900 | | | | | | |.34|.45|.52|.56|.59|.61|.63|.64|.64| | |
|2000 | | | | | | | |.43|.50|.55|.59|.61|.62|.63|.64| | |
|2100 | | | | | | | |.40|.49|.54|.58|.60|.62|.63|.64| | |
|2200 | | | | | | | |.38|.47|.53|.57|.59|.61|.62|.64| | |
|2300 | | | | | | | |.35|.45|.52|.56|.59|.61|.62|.63| | |
|2400 | | | | | | | |.32|.43|.50|.55|.58|.60|.62|.63| | |
|2500 | | | | | | | | |.41|.49|.54|.57|.60|.61|.63| | |
|2600 | | | | | | | | | |.47|.53|.56|.59|.61|.62|.64|.65|
|2700 | | | | | | | | | |.45|.52|.55|.58|.60|.62|.64|.65|
|2800 | | | | | | | | | |.44|.59|.55|.58|.60|.61|.64|.65|
|2900 | | | | | | | | | |.42|.49|.54|.57|.59|.61|.63|.65|
|3000 | | | | | | | | | |.40|.48|.53|.56|.59|.61|.63|.64|
|3100 | | | | | | | | | |.38|.47|.52|.56|.58|.60|.63|.64|
|3200 | | | | | | | | | | |.45|.51|.55|.58|.60|.63|.64|
|3300 | | | | | | | | | | |.44|.50|.54|.57|.59|.62|.64|
|3400 | | | | | | | | | | |.42|.49|.53|.56|.59|.62|.64|
|3500 | | | | | | | | | | |.40|.48|.52|.56|.58|.62|.64|
|3600 | | | | | | | | | | | |.47|.52|.55|.58|.61|.63|
|3700 | | | | | | | | | | | |.45|.51|.55|.57|.61|.63|
|3800 | | | | | | | | | | | |.44|.50|.54|.57|.61|.63|
|3900 | | | | | | | | | | | |.43|.49|.53|.56|.60|.63|
|4000 | | | | | | | | | | | |.42|.48|.52|.56|.60|.62|
|4100 | | | | | | | | | | | |.40|.47|.52|.55|.60|.62|
|4200 | | | | | | | | | | | |.39|.46|.51|.55|.59|.62|
|4300 | | | | | | | | | | | | |.45|.50|.54|.59|.62|
|4400 | | | | | | | | | | | | |.44|.49|.53|.59|.62|
|4500 | | | | | | | | | | | | |.43|.49|.53|.58|.61|
|4600 | | | | | | | | | | | | |.42|.48|.52|.58|.61|
|4700 | | | | | | | | | | | | |.41|.47|.51|.57|.61|
|4800 | | | | | | | | | | | | |.40|.46|.51|.57|.60|
|4900 | | | | | | | | | | | | | |.45|.50|.57|.60|
|5000 | | | | | | | | | | | | | |.44|.49|.56|.60|
+-----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
FOR OTHER STACK TEMPERATURES ADD OR DEDUCT BEFORE MULTIPLYING BY
HEIGHT ÷ 100 AS FOLLOWS[52]
For 750 Degrees F. Add .17 inch.
For 700 Degrees F. Add .14 inch.
For 650 Degrees F. Add .11 inch.
For 600 Degrees F. Add .08 inch.
For 550 Degrees F. Add .04 inch.
For 450 Degrees F. Deduct .04 inch.
For 400 Degrees F. Deduct .09 inch.
For 350 Degrees F. Deduct .14 inch.
[Graph: Horse Power of Boilers against Diameter of Stack in Inches
Fig. 33. Diameter of Stacks and Horse Power they will Serve
Computed from Formula (28). For brick or brick-lined stacks, increase
the diameter 6 per cent]
Height and Diameter of Stacks--From this formula (27) it becomes evident
that a stack of certain diameter, if it be increased in height, will
produce the same available draft as one of larger diameter, the
additional height being required to overcome the added frictional loss.
It follows that among the various stacks that would meet the
requirements of a particular case there must be one which can be
constructed more cheaply than the others. It has been determined from
the relation of the cost of stacks to their diameters and heights, in
connection with the formula for available draft, that the minimum cost
stack has a diameter dependent solely upon the horse power of the
boilers it serves, and a height proportional to the available draft
required.
Assuming 120 pounds of flue gas per hour for each boiler horse power,
which provides for ordinary overloads and the use of poor coal, the
method above stated gives:
For an unlined steel stack--
diameter in inches = 4.68 (H. P.)^{2/5} (28)
For a stack lined with masonry--
diameter in inches = 4.92 (H. P.)^{2/5} (29)
In both of these formulae H. P. = the rated horse power of the boiler.
From this formula the curve, Fig. 33, has been calculated and from it
the stack diameter for any boiler horse power can be selected.
For stoker practice where a large stack serves a number of boilers, the
area is usually made about one-third more than the above rules call for,
which allows for leakage of air through the setting of any idle boilers,
irregularities in operating conditions, etc.
Stacks with diameters determined as above will give an available draft
which bears a constant ratio of the theoretical draft, and allowing for
the cooling of the gases in their passage upward through the stack, this
ratio is 8. Using this factor in formula (25), and transposing, the
height of the chimney becomes,
d^{1}
H = ----- (30)
.8 K
Where H = height of stack in feet above the level of the grates,
d^{1} = available draft required,
K = constant as in formula.
Losses in Flues--The loss of draft in straight flues due to friction and
inertia can be calculated approximately from formula (26), which was
given for loss in stacks. It is to be borne in mind that C in this
formula is the actual perimeter of the flue and is least, relative to
the cross sectional area, when the section is a circle, is greater for a
square section, and greatest for a rectangular section. The retarding
effect of a square flue is 12 per cent greater than that of a circular
flue of the same area and that of a rectangular with sides as 1 and 1½,
15 per cent greater. The greater resistance of the more or less uneven
brick or concrete flue is provided for in the value of the constants
given for formula (26). Both steel and brick flues should be short and
should have as near a circular or square cross section as possible.
Abrupt turns are to be avoided, but as long easy sweeps require valuable
space, it is often desirable to increase the height of the stack rather
than to take up added space in the boiler room. Short right-angle turns
reduce the draft by an amount which can be roughly approximated as equal
to 0.05 inch for each turn. The turns which the gases make in leaving
the damper box of a boiler, in entering a horizontal flue and in turning
up into a stack should always be considered. The cross sectional areas
of the passages leading from the boilers to the stack should be of ample
size to provide against undue frictional loss. It is poor economy to
restrict the size of the flue and thus make additional stack height
necessary to overcome the added friction. The general practice is to
make flue areas the same or slightly larger than that of the stack;
these should be, preferably, at least 20 per cent greater, and a safe
rule to follow in figuring flue areas is to allow 35 square feet per
1000 horse power. It is unnecessary to maintain the same size of flue
the entire distance behind a row of boilers, and the areas at any point
may be made proportional to the volume of gases that will pass that
point. That is, the areas may be reduced as connections to various
boilers are passed.
[Illustration: 6000 Horse-power Installation of Babcock & Wilcox Boilers
at the United States Navy Yard, Washington, D. C.]
With circular steel flues of approximately the same size as the stacks,
or reduced proportionally to the volume of gases they will handle, a
convenient rule is to allow 0.1 inch draft loss per 100 feet of flue
length and 0.05 inch for each right-angle turn. These figures are also
good for square or rectangular steel flues with areas sufficiently large
to provide against excessive frictional loss. For losses in brick or
concrete flues, these figures should be doubled.
Underground flues are less desirable than overhead or rear flues for the
reason that in most instances the gases will have to make more turns
where underground flues are used and because the cross sectional area of
such flues will oftentimes be decreased on account of an accumulation of
dirt or water which it may be impossible to remove.
In tall buildings, such as office buildings, it is frequently necessary
in order to carry spent gases above the roofs, to install a stack the
height of which is out of all proportion to the requirements of the
boilers. In such cases it is permissible to decrease the diameter of a
stack, but care must be taken that this decrease is not sufficient to
cause a frictional loss in the stack as great as the added draft
intensity due to the increase in height, which local conditions make
necessary.
In such cases also the fact that the stack diameter is permissibly
decreased is no reason why flue sizes connecting to the stack should be
decreased. These should still be figured in proportion to the area of
the stack that would be furnished under ordinary conditions or with an
allowance of 35 square feet per 1000 horse power, even though the cross
sectional area appears out of proportion to the stack area.
Loss in Boiler--In calculating the available draft of a chimney 120
pounds per hour has been used as the weight of the gases per boiler
horse power. This covers an overload of the boiler to an extent of 50
per cent and provides for the use of poor coal. The loss in draft
through a boiler proper will depend upon its type and baffling and will
increase with the per cent of rating at which it is run. No figures can
be given which will cover all conditions, but for approximate use in
figuring the available draft necessary it may be assumed that the loss
through a boiler will be 0.25 inch where the boiler is run at rating,
0.40 inch where it is run at 150 per cent of its rated capacity, and
0.70 inch where it is run at 200 per cent of its rated capacity.
Loss in Furnace--The draft loss in the furnace or through the fuel bed
varies between wide limits. The air necessary for combustion must pass
through the interstices of the coal on the grate. Where these are large,
as is the case with broken coal, but little pressure is required to
force the air through the bed; but if they are small, as with bituminous
slack or small sizes of anthracite, a much greater pressure is needed.
If the draft is insufficient the coal will accumulate on the grates and
a dead smoky fire will result with the accompanying poor combustion; if
the draft is too great, the coal may be rapidly consumed on certain
portions of the grate, leaving the fire thin in spots and a portion of
the grates uncovered with the resulting losses due to an excessive
amount of air.
[Graph: Force of Draft between Furnace and Ash Pit--Inches of Water
against Pounds of Coal burned per Square Foot of Grate Surface per Hour
Fig. 34. Draft Required at Different Combustion Rates for Various Kinds
of Coal]
Draft Required for Different Fuels--For every kind of fuel and rate of
combustion there is a certain draft with which the best general results
are obtained. A comparatively light draft is best with the free burning
bituminous coals and the amount to use increases as the percentage of
volatile matter diminishes and the fixed carbon increases, being highest
for the small sizes of anthracites. Numerous other factors such as the
thickness of fires, the percentage of ash and the air spaces in the
grates bear directly on this question of the draft best suited to a
given combustion rate. The effect of these factors can only be found by
experiment. It is almost impossible to show by one set of curves the
furnace draft required at various rates of combustion for all of the
different conditions of fuel, etc., that may be met. The curves in Fig.
34, however, give the furnace draft necessary to burn various kinds of
coal at the combustion rates indicated by the abscissae, for a general
set of conditions. These curves have been plotted from the records of
numerous tests and allow a safe margin for economically burning coals of
the kinds noted.
Rate of Combustion--The amount of coal which can be burned per hour per
square foot of grate surface is governed by the character of the coal
and the draft available. When the boiler and grate are properly
proportioned, the efficiency will be practically the same, within
reasonable limits, for different rates of combustion. The area of the
grate, and the ratio of this area to the boiler heating surface will
depend upon the nature of the fuel to be burned, and the stack should be
so designed as to give a draft sufficient to burn the maximum amount of
fuel per square foot of grate surface corresponding to the maximum
evaporative requirements of the boiler.
Solution of a Problem--The stack diameter can be determined from the
curve, Fig. 33. The height can be determined by adding the draft losses
in the furnace, through the boiler and flues, and computing from formula
(30) the height necessary to give this draft.
Example: Proportion a stack for boilers rated at 2000 horse power,
equipped with stokers, and burning bituminous coal that will evaporate 8
pounds of water from and at 212 degrees Fahrenheit per pound of fuel;
the ratio of boiler heating surface to grate surface being 50:1; the
flues being 100 feet long and containing two right-angle turns; the
stack to be able to handle overloads of 50 per cent; and the rated horse
power of the boilers based on 10 square feet of heating surface per
horse power.
The atmospheric temperature may be assumed as 60 degrees Fahrenheit and
the flue temperatures at the maximum overload as 550 degrees Fahrenheit.
The grate surface equals 400 square feet.
2000 × 34½
The total coal burned at rating = ---------- = 8624 pounds.
8
The coal per square foot of grate surface per hour at rating =
8624
---- = 22 pounds.
400
For 50 per cent overload the combustion rate will be approximately 60
per cent greater than this or 1.60 × 22 = 35 pounds per square foot of
grate surface per hour. The furnace draft required for the combustion
rate, from the curve, Fig. 34, is 0.6 inch. The loss in the boiler will
be 0.4 inch, in the flue 0.1 inch, and in the turns 2 × 0.05 = 0.1 inch.
The available draft required at the base of the stack is, therefore,
_Inches_
Boiler 0.4
Furnace 0.6
Flues 0.1
Turns 0.1
---
Total 1.2
Since the available draft is 80 per cent of the theoretical draft, this
draft due to the height required is 1.2 ÷ .8 = 1.5 inch.
The chimney constant for temperatures of 60 degrees Fahrenheit and 550
degrees Fahrenheit is .0071 and from formula (30),
1.5
H = ----- = 211 feet.
.0071
Its diameter from curve in Fig. 33 is 96 inches if unlined, and 102
inches inside if lined with masonry. The cross sectional area of the
flue should be approximately 70 square feet at the point where the total
amount of gas is to be handled, tapering to the boiler farthest from the
stack to a size which will depend upon the size of the boiler units
used.
Correction in Stack Sizes for Altitudes--It has ordinarily been assumed
that a stack height for altitude will be increased inversely as the
ratio of the barometric pressure at the altitude to that at sea level,
and that the stack diameter will increase inversely as the two-fifths
power of this ratio. Such a relation has been based on the assumption of
constant draft measured in inches of water at the base of the stack for
a given rate of operation of the boilers, regardless of altitude.
If the assumption be made that boilers, flues and furnace remain the
same, and further that the increased velocity of a given weight of air
passing through the furnace at a higher altitude would have no effect on
the combustion, the theory has been advanced[53] that a different law
applies.
Under the above assumptions, whenever a stack is working at its maximum
capacity at any altitude, the entire draft is utilized in overcoming the
various resistances, each of which is proportional to the square of the
velocity of the gases. Since boiler areas are fixed, all velocities may
be related to a common velocity, say, that within the stack, and all
resistances may, therefore, be expressed as proportional to the square
of the chimney velocity. The total resistance to flow, in terms of
velocity head, may be expressed in terms of weight of a column of
external air, the numerical value of such head being independent of the
barometric pressure. Likewise the draft of a stack, expressed in height
of column of external air, will be numerically independent of the
barometric pressure. It is evident, therefore, that if a given boiler
plant, with its stack operated with a fixed fuel, be transplanted from
sea level to an altitude, assuming the temperatures remain constant, the
total draft head measured in height of column of external air will be
numerically constant. The velocity of chimney gases will, therefore,
remain the same at altitude as at sea level and the weight of gases
flowing per second with a fixed velocity will be proportional to the
atmospheric density or inversely proportional to the normal barometric
pressure.
To develop a given horse power requires a constant weight of chimney gas
and air for combustion. Hence, as the altitude is increased, the density
is decreased and, for the assumptions given above, the velocity through
the furnace, the boiler passes, breeching and flues must be
correspondingly greater at altitude than at sea level. The mean
velocity, therefore, for a given boiler horse power and constant weight
of gases will be inversely proportional to the barometric pressure and
the velocity head measured in column of external air will be inversely
proportional to the square of the barometric pressure.
For stacks operating at altitude it is necessary not only to increase
the height but also the diameter, as there is an added resistance within
the stack due to the added friction from the additional height. This
frictional loss can be compensated by a suitable increase in the
diameter and when so compensated, it is evident that on the assumptions
as given, the chimney height would have to be increased at a ratio
inversely proportional to the square of the normal barometric pressure.
In designing a boiler for high altitudes, as already stated, the
assumption is usually made that a given grade of fuel will require the
same draft measured in inches of water at the boiler damper as at sea
level, and this leads to making the stack height inversely as the
barometric pressures, instead of inversely as the square of the
barometric pressures. The correct height, no doubt, falls somewhere
between the two values as larger flues are usually used at the higher
altitudes, whereas to obtain the ratio of the squares, the flues must be
the same size in each case, and again the effect of an increased
velocity of a given weight of air through the fire at a high altitude,
on the combustion, must be neglected. In making capacity tests with coal
fuel, no difference has been noted in the rates of combustion for a
given draft suction measured by a water column at high and low
altitudes, and this would make it appear that the correct height to use
is more nearly that obtained by the inverse ratio of the barometric
readings than by the inverse ratio of the squares of the barometric
readings. If the assumption is made that the value falls midway between
the two formulae, the error in using a stack figured in the ordinary way
by making the height inversely proportional to the barometric readings
would differ about 10 per cent in capacity at an altitude of 10,000
feet, which difference is well within the probable variation of the size
determined by different methods. It would, therefore, appear that ample
accuracy is obtained in all cases by simply making the height inversely
proportional to the barometric readings and increasing the diameter so
that the stacks used at high altitudes have the same frictional
resistance as those used at low altitudes, although, if desired, the
stack may be made somewhat higher at high altitudes than this rule calls
for in order to be on the safe side.
The increase of stack diameter necessary to maintain the same friction
loss is inversely as the two-fifths power of the barometric pressure.
Table 54 gives the ratio of barometric readings of various altitudes to
sea level, values for the square of this ratio and values of the
two-fifths power of this ratio.
TABLE 54
STACK CAPACITIES, CORRECTION FACTORS FOR
ALTITUDES
_______________________________________________________________________
| | | | | |
| Altitude | | R | | R^{2/5} |
| Height in Feet | Normal | Ratio Barometer | | Ratio Increase |
| Above | Barometer | Reading | R² | in Stack |
| Sea Level | | Sea Level to | | Diameter |
| | | Altitude | | |
|________________|___________|_________________|_______|________________|
| | | | | |
| 0 | 30.00 | 1.000 | 1.000 | 1.000 |
| 1000 | 28.88 | 1.039 | 1.079 | 1.015 |
| 2000 | 27.80 | 1.079 | 1.064 | 1.030 |
| 3000 | 26.76 | 1.121 | 1.257 | 1.047 |
| 4000 | 25.76 | 1.165 | 1.356 | 1.063 |
| 5000 | 24.79 | 1.210 | 1.464 | 1.079 |
| 6000 | 23.87 | 1.257 | 1.580 | 1.096 |
| 7000 | 22.97 | 1.306 | 1.706 | 1.113 |
| 8000 | 22.11 | 1.357 | 1.841 | 1.130 |
| 9000 | 21.28 | 1.410 | 1.988 | 1.147 |
| 10000 | 20.49 | 1.464 | 2.144 | 1.165 |
|________________|___________|_________________|_______|________________|
These figures show that the altitude affects the height to a much
greater extent than the diameter and that practically no increase in
diameter is necessary for altitudes up to 3000 feet.
For high altitudes the increase in stack height necessary is, in some
cases, such as to make the proportion of height to diameter
impracticable. The method to be recommended in overcoming, at least
partially, the great increase in height necessary at high altitudes is
an increase in the grate surface of the boilers which the stack serves,
in this way reducing the combustion rate necessary to develop a given
power and hence the draft required for such combustion rate.
TABLE 55
STACK SIZES BY KENT'S FORMULA
ASSUMING 5 POUNDS OF COAL PER HORSE POWER
____________________________________________________________________
| | | | |
| | | Height of Stack in Feet |Side of|
| | |______________________________________________|Equiva-|
| Dia- | Area | | | | | | | | | | | lent |
| meter|Square| 50| 60| 70| 80 | 90 | 100| 110| 125| 150| 175|Square |
|Inches| Feet |___|___|___|____|____|____|____|____|____|____| Stack |
| | | |Inches |
| | | Commercial Horse Power | |
|______|______|______________________________________________|_______|
| | | | | | | | | | | | | |
| 33 | 5.94|106|115|125| 133| 141| 149| | | | | 30 |
| 36 | 7.07|129|141|152| 163| 173| 182| | | | | 32 |
| 39 | 8.30|155|169|183| 196| 208| 219| 229| 245| | | 35 |
| 42 | 9.62|183|200|216| 231| 245| 258| 271| 289| 316| | 38 |
| 48 | 12.57|246|269|290| 311| 330| 348| 365| 389| 426| 460| 43 |
| 54 | 15.90|318|348|376| 402| 427| 449| 472| 503| 551| 595| 48 |
| 60 | 19.64|400|437|473| 505| 536| 565| 593| 632| 692| 748| 54 |
| 66 | 23.76|490|537|580| 620| 658| 694| 728| 776| 849| 918| 59 |
| 72 | 28.27|591|646|698| 747| 792| 835| 876| 934|1023|1105| 64 |
| 78 | 33.18|700|766|828| 885| 939| 990|1038|1107|1212|1310| 70 |
| 84 | 38.48|818|896|968|1035|1098|1157|1214|1294|1418|1531| 75 |
|______|______|___|___|___|____|____|____|____|____|____|____|_______|
| | | | |
| | | Height of Stack in Feet |Side of|
| | |______________________________________________|Equiva-|
| Dia- | Area | | | | | | | | | lent |
| meter|Square| 100| 110 | 125 | 150 | 175 | 200 | 225 | 250 |Square |
|Inches| Feet |____|_____|_____|_____|_____|_____|_____|_____| Stack |
| | | |Inches |
| | | Commercial Horse Power | |
|______|______|______________________________________________|_______|
| | | | | | | | | | | |
| 90 | 44.18|1338| 1403| 1496| 1639| 1770| 1893| 2008| 2116| 80 |
| 96 | 50.27|1532| 1606| 1713| 1876| 2027| 2167| 2298| 2423| 86 |
| 102 | 56.75|1739| 1824| 1944| 2130| 2300| 2459| 2609| 2750| 91 |
| 108 | 63.62|1959| 2054| 2190| 2392| 2592| 2770| 2939| 3098| 98 |
| 114 | 70.88|2192| 2299| 2451| 2685| 2900| 3100| 3288| 3466| 101 |
| 120 | 78.54|2438| 2557| 2726| 2986| 3226| 3448| 3657| 3855| 107 |
| 126 | 86.59|2697| 2829| 3016| 3303| 3568| 3814| 4046| 4265| 112 |
| 132 | 95.03|2970| 3114| 3321| 3637| 3929| 4200| 4455| 4696| 117 |
| 144 |113.10|3554| 3726| 3973| 4352| 4701| 5026| 5331| 5618| 128 |
| 156 |132.73|4190| 4393| 4684| 5131| 5542| 5925| 6285| 6624| 138 |
| 168 |153.94|4878| 5115| 5454| 5974| 6454| 6899| 7318| 7713| 150 |
|______|______|____|_____|_____|_____|_____|_____|_____|_____|_______|
Kent's Stack Tables--Table 55 gives, in convenient form for approximate
work, the sizes of stacks and the horse power of boilers which they will
serve. This table is a modification of Mr. William Kent's stack table
and is calculated from his formula. Provided no unusual conditions are
encountered, it is reliable for the ordinary rates of combustion with
bituminous coals. It is figured on a consumption of 5 pounds of coal
burned per hour per boiler horse power developed, this figure giving a
fairly liberal allowance for the use of poor coal and for a reasonable
overload. When the coal used is a low grade bituminous of the Middle or
Western States, it is strongly recommended that these sizes be increased
materially, such an increase being from 25 to 60 per cent, depending
upon the nature of the coal and the capacity desired. For the coal
burned per hour for any size stack given in the table, the values should
be multiplied by 5.
A convenient rule for large stacks, 200 feet high and over, is to
provide 30 square feet of cross sectional area per 1000 rated horse
power.
Stacks for Oil Fuel--The requirements of stacks connected to boilers
under which oil fuel is burned are entirely different from those where
coal is used. While more attention has been paid to the matter of stack
sizes for oil fuel in recent years, there has not as yet been gathered
the large amount of experimental data available for use in designing
coal stacks.
In the case of oil-fired boilers the loss of draft through the fuel bed
is partially eliminated. While there may be practically no loss through
any checkerwork admitting air to the furnace when a boiler is new, the
areas for the air passage in this checkerwork will in a short time be
decreased, due to the silt which is present in practically all fuel oil.
The loss in draft through the boiler proper at a given rating will be
less than in the case of coal-fired boilers, this being due to a
decrease in the volume of the gases. Further, the action of the oil
burner itself is to a certain extent that of a forced draft. To offset
this decrease in draft requirement, the temperature of the gases
entering the stack will be somewhat lower where oil is used than where
coal is used, and the draft that a stack of a given height would give,
therefore, decreases. The factors as given above, affecting as they do
the intensity of the draft, affect directly the height of the stack to
be used.
As already stated, the volume of gases from oil-fired boilers being less
than in the case of coal, makes it evident that the area of stacks for
oil fuel will be less than for coal. It is assumed that these areas will
vary directly as the volume of the gases to be handled, and this volume
for oil may be taken as approximately 60 per cent of that for coal.
In designing stacks for oil fuel there are two features which must not
be overlooked. In coal-firing practice there is rarely danger of too
much draft. In the burning of oil, however, this may play an important
part in the reduction of plant economy, the influence of excessive draft
being more apparent where the load on the plant may be reduced at
intervals. The reason for this is that, aside from a slight decrease in
temperature at reduced loads, the tendency, due to careless firing, is
toward a constant gas flow through the boiler regardless of the rate of
operation, with the corresponding increase of excess air at light loads.
With excessive stack height, economical operation at varying loads is
almost impossible with hand control. With automatic control, however,
where stacks are necessarily high to take care of known peaks, under
lighter loads this economical operation becomes less difficult. For this
reason the question of designing a stack for a plant where the load is
known to be nearly a constant is easier than for a plant where the load
will vary over a wide range. While great care must be taken to avoid
excessive draft, still more care must be taken to assure a draft suction
within all parts of the setting under any and all conditions of
operation. It is very easily possible to more than offset the economy
gained through low draft, by the losses due to setting deterioration,
resulting from such lack of suction. Under conditions where the suction
is not sufficient to carry off the products of combustion, the action of
the heat on the setting brickwork will cause its rapid failure.
[Illustration: 7800 Horse-power Installation of Babcock & Wilcox
Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at the
Metropolitan West Side Elevated Ry. Co., Chicago, Ill.]
It becomes evident, therefore, that the question of stack height for
oil-fired boilers is one which must be considered with the greatest of
care. The designer, on the one hand, must guard against the evils of
excessive draft with the view to plant economy, and, on the other,
against the evils of lack of draft from the viewpoint of upkeep cost.
Stacks for this work should be proportioned to give ample draft for the
maximum overload that a plant will be called upon to carry, all
conditions of overload carefully considered. At the same time, where
this maximum overload is figured liberally enough to insure a draft
suction within the setting under all conditions, care must be taken
against the installation of a stack which would give more than this
maximum draft.
TABLE 56
STACK SIZES FOR OIL FUEL
ADAPTED FROM C. R. WEYMOUTH'S TABLE (TRANS.
A. S. M. E. VOL. 34)
+----------------------------------------------------+
|+--------+-----------------------------------------+|
|| | Height in Feet Above Boiler Room Floor ||
||Diameter+------+------+------+-----+--------------+|
|| Inches | 80 | 90 | 100 | 120 | 140 | 160 ||
|+--------+------+------+------+------+------+------+|
|| 33 | 161 | 206 | 233 | 270 | 306 | 315 ||
|| 36 | 208 | 253 | 295 | 331 | 363 | 387 ||
|| 39 | 251 | 303 | 343 | 399 | 488 | 467 ||
|| 42 | 295 | 359 | 403 | 474 | 521 | 557 ||
|| 48 | 399 | 486 | 551 | 645 | 713 | 760 ||
|| 54 | 519 | 634 | 720 | 847 | 933 | 1000 ||
|| 60 | 657 | 800 | 913 | 1073 | 1193 | 1280 ||
|| 66 | 813 | 993 | 1133 | 1333 | 1480 | 1593 ||
|| 72 | 980 | 1206 | 1373 | 1620 | 1807 | 1940 ||
|| 84 | 1373 | 1587 | 1933 | 2293 | 2560 | 2767 ||
|| 96 | 1833 | 2260 | 2587 | 3087 | 3453 | 3740 ||
|| 108 | 2367 | 2920 | 3347 | 4000 | 4483 | 4867 ||
|| 120 | 3060 | 3660 | 4207 | 5040 | 5660 | 6160 ||
|+--------+------+------+------+------+------+------+|
+----------------------------------------------------+
Figures represent nominal rated horse power. Sizes as given good for 50
per cent overloads.
Based on centrally located stacks, short direct flues and ordinary
operating efficiencies.
Table 56 gives the sizes of stacks, and horse power which they will
serve for oil fuel. This table is, in modified form, one calculated by
Mr. C. R. Weymouth after an exhaustive study of data pertaining to the
subject, and will ordinarily give satisfactory results.
Stacks for Blast Furnace Gas Work--For boilers burning blast furnace
gas, as in the case of oil-fired boilers, stack sizes as suited for coal
firing will have to be modified. The diameter of stacks for this work
should be approximately the same as for coal-fired boilers. The volume
of gases would be slightly greater than from a coal fire and would
decrease the draft with a given stack, but such a decrease due to volume
is about offset by an increase due to somewhat higher temperatures in
the case of the blast furnace gases.
Records show that with this class of fuel 175 per cent of the rated
capacity of a boiler can be developed with a draft at the boiler damper
of from 0.75 inch to 1.0 inch, and it is well to limit the height of
stacks to one which will give this draft as a maximum. A stack of proper
diameter, 130 feet high above the ground, will produce such a draft and
this height should ordinarily not be exceeded. Until recently the
question of economy in boilers fired with blast furnace gas has not been
considered, but, aside from the economical standpoint, excessive draft
should be guarded against in order to lower the upkeep cost.
Stacks should be made of sufficient height to produce a draft that will
develop the maximum capacity required, and this draft decreased
proportionately for loads under the maximum by damper regulation. The
amount of gas fed to a boiler for any given rating is a fixed quantity
and if a draft in excess of that required for that particular rate of
operation is supplied, economy is decreased and the wear and tear on the
setting is materially increased. Excess air which is drawn in, either
through or around the gas burners by an excessive draft, will decrease
economy, as in any other class of work. Again, as in oil-fired practice,
it is essential on the other hand that a suction be maintained within
all parts of the setting, in this case not only to provide against
setting deterioration but to protect the operators from leakage of gas
which is disagreeable and may be dangerous. Aside from the intensity of
the draft, a poor mixture of the gas and air or a "laneing" action may
lead to secondary combustion with the possibility of dangerous
explosions within the setting, may cause a pulsating action within the
setting, may increase the exit temperatures to a point where there is
danger of burning out damper boxes, and, in general, is hard on the
setting. It is highly essential, therefore, that the furnace be properly
constructed to meet the draft which will be available.
Stacks for Wood-fired Boilers--For boilers using wood as fuel, there is
but little data upon which to base stack sizes. The loss of draft
through the bed of fuel will vary over limits even wider than in the
case of coal, for in this class of fuel the moisture may run from
practically 0.0 per cent to over 60 per cent, and the methods of
handling and firing are radically different for the different classes of
wood (see chapter on Wood-burning Furnaces). As economy is ordinarily of
little importance, high stack temperatures may be expected, and often
unavoidably large quantities of excess air are supplied due to the
method of firing. In general, it may be stated that for this class of
fuel the diameter of stacks should be at least as great as for coal-fired
boilers, while the height may be slightly decreased. It is far the best
plan in designing a stack for boilers using wood fuel to consider each
individual set of conditions that exist, rather than try to follow any
general rule.
One factor not to be overlooked in stacks for wood burning is their
location. The fine particles of this fuel are often carried unconsumed
through the boiler, and where the stack is not on top of the boiler,
these particles may accumulate in the base of the stack below the point
at which the flue enters. Where there is any air leakage through the
base of such a stack, this fuel may become ignited and the stack burned.
Where there is a possibility of such action taking place, it is well to
line the stack with fire brick for a portion of its height.
Draft Gauges--The ordinary form of draft gauge, Fig. 35, which consists
of a U-tube, containing water, lacks sensitiveness in measuring such
slight pressure differences as usually exist, and for that reason gauges
which multiply the draft indications are more convenient and are much
used.
[Illustration: Fig. 35. U-tube Draft Gauge]
[Illustration: Fig. 36. Barrus Draft Gauge]
An instrument which has given excellent results is one introduced by Mr.
G. H. Barrus, which multiplies the ordinary indications as many times as
desired. This is illustrated in Fig. 36, and consists of a U-tube made
of one-half inch glass, surmounted by two larger tubes, or chambers,
each having a diameter of 2½ inches. Two different liquids which will
not mix, and which are of different color, are used, usually alcohol
colored red and a certain grade of lubricating oil. The movement of the
line of demarcation is proportional to the difference in the areas of
the chambers and the U-tube connecting them. The instrument is
calibrated by comparison with the ordinary U-tube gauge.
In the Ellison form of gauge the lower portion of the ordinary U-tube
has been replaced by a tube slightly inclined to the horizontal, as
shown in Fig. 37. By this arrangement any vertical motion in the
right-hand upright tube causes a very much greater travel of the liquid
in the inclined tube, thus permitting extremely small variation in the
intensity of the draft to be read with facility.
[Illustration: Fig. 37. Ellison Draft Gauge]
The gauge is first leveled by means of the small level attached to it,
both legs being open to the atmosphere. The liquid is then adjusted
until its meniscus rests at the zero point on the left. The right-hand
leg is then connected to the source of draft by means of a piece of
rubber tubing. Under these circumstances, a rise of level of one inch in
the right-hand vertical tube causes the meniscus in the inclined tube to
pass from the point 0 to 1.0. The scale is divided into tenths of an
inch, and the sub-divisions are hundredths of an inch.
The makers furnish a non-drying oil for the liquid, usually a 300
degrees test refined petroleum.
A very convenient form of the ordinary U-tube gauge is known as the
Peabody gauge, and it is shown in Fig. 38. This is a small modified
U-tube with a sliding scale between the two legs of the U and with
connections such that either a draft suction or a draft pressure may be
taken. The tops of the sliding pieces extending across the tubes are
placed at the bottom of the meniscus and accurate readings in hundredths
of an inch are obtained by a vernier.
[Illustration: Fig. 38. Peabody Draft Gauge]
EFFICIENCY AND CAPACITY OF BOILERS
Two of the most important operating factors entering into the
consideration of what constitutes a satisfactory boiler are its
efficiency and capacity. The relation of these factors to one another
will be considered later under the selection of boilers with reference
to the work they are to accomplish. The present chapter deals with the
efficiency and capacity only with a view to making clear exactly what is
meant by these terms as applied to steam generating apparatus, together
with the methods of determining these factors by tests.
Efficiency--The term "efficiency", specifically applied to a steam
boiler, is the ratio of heat absorbed by the boiler in the generation of
steam to the total amount of heat available in the medium utilized in
securing such generation. When this medium is a solid fuel, such as
coal, it is impossible to secure the complete combustion of the total
amount fed to the boiler. A portion is bound to drop through the grates
where it becomes mixed with the ash and, remaining unburned, produces no
heat. Obviously, it is unfair to charge the boiler with the failure to
absorb the portion of available heat in the fuel that is wasted in this
way. On the other hand, the boiler user must pay for such waste and is
justified in charging it against the combined boiler and furnace. Due to
this fact, the efficiency of a boiler, as ordinarily stated, is in
reality the combined efficiency of the boiler, furnace and grate, and
Efficiency of boiler,} Heat absorbed per pound of fuel
furnace and grate } = ------------------------------- (31)
Heat value per pound of fuel
The efficiency will be the same whether based on dry fuel or on fuel as
fired, including its content of moisture. For example: If the coal
contained 3 per cent of moisture, the efficiency would be
Heat absorbed per pound of dry coal × 0.97
------------------------------------------
Heat value per pound of dry coal × 0.97
where 0.97 cancels and the formula becomes (31).
The heat supplied to the boiler is due to the combustible portion of
fuel which is actually burned, irrespective of what proportion of the
total combustible fired may be.[54] This fact has led to the use of a
second efficiency basis on combustible and which is called the
efficiency of boiler and furnace[55], namely,
Efficiency of boiler and furnace[55]
Heat absorbed per pound of combustible[56]
= -------------------------------------- (32)
Heat value per pound of combustible
The efficiency so determined is used in comparing the relative
performance of boilers, irrespective of the type of grates used under
them. If the loss of fuel through the grates could be entirely overcome,
the efficiencies obtained by (31) and (32) would obviously be the same.
Hence, in the case of liquid and gaseous fuels, where there is
practically no waste, these efficiencies are almost identical.
As a matter of fact, it is extremely difficult, if not impossible, to
determine the actual efficiency of a boiler alone, as distinguished from
the combined efficiency of boiler, grate and furnace. This is due to the
fact that the losses due to excess air cannot be correctly attributed to
either the boiler or the furnace, but only to a combination of the
complete apparatus. Attempts have been made to devise methods for
dividing the losses proportionately between the furnace and the boiler,
but such attempts are unsatisfactory and it is impossible to determine
the efficiency of a boiler apart from that of a furnace in such a way as
to make such determination of any practical value or in a way that might
not lead to endless dispute, were the question to arise in the case of a
guaranteed efficiency. From the boiler manufacturer's standpoint, the
only way of establishing an efficiency that has any value when
guarantees are to be met, is to require the grate or stoker manufacturer
to make certain guarantees as to minimum CO_{2}, maximum CO, and that
the amount of combustible in the ash and blown away with the flue gases
does not exceed a certain percentage. With such a guarantee, the
efficiency should be based on the combined furnace and boiler.
General practice, however, has established the use of the efficiency
based upon combustible as representing the efficiency of the boiler
alone. When such an efficiency is used, its exact meaning, as pointed
out on opposite page, should be realized.
The computation of the efficiencies described on opposite page is best
illustrated by example.
Assume the following data to be determined from an actual boiler trial.
Steam pressure by gauge, 200 pounds.
Feed temperature, 180 degrees.
Total weight of coal fired, 17,500 pounds.
Percentage of moisture in coal, 3 per cent.
Total ash and refuse, 2396 pounds.
Total water evaporated, 153,543 pounds.
Per cent of moisture in steam, 0.5 per cent.
Heat value per pound of dry coal, 13,516.
Heat value per pound of combustible, 15,359.
The factor of evaporation for such a set of conditions is 1.0834. The
actual evaporation corrected for moisture in the steam is 152,775 and
the equivalent evaporation from and at 212 degrees is, therefore,
165,516 pounds.
The total dry fuel will be 17,500 × .97 = 16,975, and the evaporation
per pound of dry fuel from and at 212 degrees will be 165,516 ÷ 16,975 =
9.75 pounds. The heat absorbed per pound of dry fuel will, therefore, be
9.75 × 970.4 = 9461 B. t. u. Hence, the efficiency by (31) will be 9461
÷ 13,516 = 70.0 per cent. The total combustible burned will be 16,975
- 2396 = 14,579, and the evaporation from and at 212 degrees per pound
of combustible will be 165,516 ÷ 14,579 = 11.35 pounds. Hence, the
efficiency based on combustible from (32) will be (11.35 × 97.04) ÷
15,359 = 71.79.[**should be 71.71]
For approximate results, a chart may be used to take the place of a
computation of efficiency. Fig. 39 shows such a chart based on the
evaporation per pound of dry fuel and the heat value per pound of dry
fuel, from which efficiencies may be read directly to within one-half of
one per cent. It is used as follows: From the intersection of the
horizontal line, representing the evaporation per pound of fuel, with
the vertical line, representing the heat value per pound, the efficiency
is read directly from the diagonal scale of efficiencies. This chart may
also be used for efficiency based upon combustible when the evaporation
from and at 212 degrees and the heat values are both given in terms of
combustible.
[Graph: Evaporation from and at 212° per Pound of Dry Fuel
against B.T.U. per Pound of Dry Fuel
Fig. 39. Efficiency Chart. Calculated from Marks and Davis Tables
Diagonal Lines Represent Per Cent Efficiency]
Boiler efficiencies will vary over a wide range, depending on a great
variety of factors and conditions. The highest efficiencies that have
been secured with coal are in the neighborhood of 82 per cent and from
that point efficiencies are found all the way down to below 50 per cent.
Table 59[57] of tests of Babcock & Wilcox boilers under varying
conditions of fuel and operation will give an idea of what may be
obtained with proper operating conditions.
The difference between the efficiency secured in any boiler trial and
the perfect efficiency, 100 per cent, includes the losses, some of which
are unavoidable in the present state of the art, arising in the
conversion of the heat energy of the coal to the heat energy in the
steam. These losses may be classified as follows:
1st. Loss due to fuel dropped through the grate.
2nd. Loss due to unburned fuel which is carried by the draft, as small
particles, beyond the bridge wall into the setting or up the stack.
3rd. Loss due to the utilization of a portion of the heat in heating the
moisture contained in the fuel from the temperature of the atmosphere to
212 degrees; to evaporate it at that temperature and to superheat the
steam thus formed to the temperature of the flue gases. This steam, of
course, is first heated to the temperature of the furnace but as it
gives up a portion of this heat in passing through the boiler, the
superheating to the temperature of the exit gases is the correct degree
to be considered.
4th. Loss due to the water formed and by the burning of the hydrogen in
the fuel which must be evaporated and superheated as in item 3.
5th. Loss due to the superheating of the moisture in the air supplied
from the atmospheric temperature to the temperature of the flue gases.
6th. Loss due to the heating of the dry products of combustion to the
temperature of the flue gases.
7th. Loss due to the incomplete combustion of the fuel when the carbon
is not completely consumed but burns to CO instead of CO_{2}. The CO
passes out of the stack unburned as a volatile gas capable of further
combustion.
8th. Loss due to radiation of heat from the boiler and furnace settings.
Obviously a very elaborate test would have to be made were all of the
above items to be determined accurately. In ordinary practice it has
become customary to summarize these losses as follows, the methods of
computing the losses being given in each instance by a typical example:
(A) Loss due to the heating of moisture in the fuel from the atmospheric
temperature to 212 degrees, evaporate it at that temperature and
superheat it to the temperature of the flue gases. This in reality is
the total heat above the temperature of the air in the boiler room, in
one pound of superheated steam at atmospheric pressure at the
temperature of the flue gases, multiplied by the percentage of moisture
in the fuel. As the total heat above the temperature of the air would
have to be computed in each instance, this loss is best expressed by:
Loss in B. t. u. per pound = W(212-t+970.4+.47(T-212)) (33)
Where W = per cent of moisture in coal,
t = the temperature of air in the boiler room,
T = temperature of the flue gases,
.47 = the specific heat of superheated steam at the atmospheric
pressure and at the flue gas temperature,
(212-t) = B. t. u. necessary to heat one pound of water from the
temperature of the boiler room to 212 degrees,
970.4 = B. t. u. necessary to evaporate one pound of water at 212
degrees to steam at atmospheric pressure,
.47(T-212) = B. t. u. necessary to superheat one pound of steam at
atmospheric pressure from 212 degrees to temperature T.
[Illustration: Portion of 15,000 Horse-power Installation of Babcock &
Wilcox Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at
the Northumberland, Pa., Plant of the Atlas Portland Cement Co. This
Company Operates a Total of 24,000 Horse Power of Babcock & Wilcox
Boilers in its Various Plants]
(B) Loss due to heat carried away in the steam produced by the burning
of the hydrogen component of the fuel. In burning, one pound of hydrogen
unites with 8 pounds of oxygen to form 9 pounds of steam. Following the
reasoning of item (A), therefore, this loss will be:
Loss in B. t. u. per pound = 9H((212-t)+970.4+.47(T-212)) (34)
where H = the percentage by weight of hydrogen.
This item is frequently considered as a part of the unaccounted for
loss, where an ultimate analysis of the fuel is not given.
(C) Loss due to heat carried away by dry chimney gases. This is
dependent upon the weight of gas per pound of coal which may be
determined by formula (16), page 158.
Loss in B. t. u. per pound = (T-t)×.24×W.
Where T and t have values as in (33),
.24 = specific heat of chimney gases,
W = weight of dry chimney gas per pound of coal.
(D) Loss due to incomplete combustion of the carbon content of the fuel,
that is, the burning of the carbon to CO instead of CO_{2}.
10,150 CO
Loss in B. t. u. per pound = C×--------- (35)
CO_{2}+CO
C = per cent of carbon in coal by ultimate analysis,
CO and CO_{2} = per cent of CO and CO_{2} by volume from flue gas
analysis.
10,150 = the number of heat units generated by burning to CO_{2} one
pound of carbon contained in carbon monoxide.
(E) Loss due to unconsumed carbon in the ash (it being usually assumed
that all the combustible in the ash is carbon).
Loss in B. t. u. per pound =
per cent C × per cent ash × B. t. u. per pound of combustible in the ash
(usually taken as 14,600 B. t. u.) (36)
The loss incurred in this way is, directly, the carbon in the ash in
percentage terms of the total dry coal fired, multiplied by the heat
value of carbon.
To compute this item, which is of great importance in comparing the
relative performances of different designs of grates, an analysis of the
ash must be available.
The other losses, namely, items 2, 5 and 8 of the first classification,
are ordinarily grouped under one item, as unaccounted for losses, and
are obviously the difference between 100 per cent and the sum of the
heat utilized and the losses accounted for as given above. Item 5, or
the loss due to the moisture in the air, may be readily computed, the
moisture being determined from wet and dry bulb thermometer readings,
but it is usually disregarded as it is relatively small, averaging, say,
one-fifth to one-half of one per cent. Lack of data may, of course, make
it necessary to include certain items of the second and ordinary
classification in this unaccounted for group.
TABLE 57
DATA FROM WHICH HEAT BALANCE
(TABLE 58) IS COMPUTED
+------------------------------------------------------+
|+----------------------------------------------------+|
||Steam Pressure by Gauge, Pounds | 192 ||
||Temperature of Feed, Degrees Fahrenheit | 180 ||
||Degrees of Superheat, Degrees Fahrenheit |115.2||
||Temperature of Boiler Room, Degrees Fahrenheit| 81 ||
||Temperature of Exit Gases, Degrees Fahrenheit | 480 ||
||Weight of Coal Used per Hour, Pounds | 5714||
||Moisture, Per Cent | 1.83||
||Dry Coal Per Hour, Pounds | 5609||
||Ash and Refuse per Hour, Pounds | 561||
||Ash and Refuse (of Dry Coal), Per Cent |10.00||
||Actual Evaporation per Hour, Pounds |57036||
|| .- C, Per Cent |78.57||
|| | H, Per Cent | 5.60||
||Ultimate | O, Per Cent | 7.02||
||Analysis -+ N, Per Cent | 1.11||
||Dry Coal | Ash, Per Cent | 6.52||
|| '- Sulphur, Per Cent | 1.18||
||Heat Value per Pound Dry Coal, B. t. u. |14225||
||Heat Value per Pound Combustible, B. t. u. |15217||
||Combustible in Ash by Analysis, Per Cent | 17.9||
|| .- CO_{2}, Per Cent |14.33||
||Flue Gas -+ O, Per Cent | 4.54||
||Analysis | CO, Per Cent | 0.11||
|| '- N, Per Cent |81.02||
|+----------------------------------------------+-----+|
+------------------------------------------------------+
A schedule of the losses as outlined, requires an evaporative test of
the boiler, an analysis of the flue gases, an ultimate analysis of the
fuel, and either an ultimate or proximate analysis of the ash. As the
amount of unaccounted for losses forms a basis on which to judge the
accuracy of a test, such a schedule is called a "heat balance".
A heat balance is best illustrated by an example: Assume the data as
given in Table 57 to be secured in an actual boiler test.
From this data the factor of evaporation is 1.1514 and the evaporation
per hour from and at 212 degrees is 65,671 pounds. Hence the evaporation
from and at 212 degrees per pound of dry coal is 65,671÷5609 = 11.71
pounds. The efficiency of boiler, furnace and grate is:
(11.71×970.4)÷14,225 = 79.88 per cent.
The heat losses are:
(A) Loss due to moisture in coal,
= .01831 ((212-81)+970.4+.47(480-212))
= 22. B. t. u.,
= 0.15 per cent.
(B) The loss due to the burning of hydrogen:
= 9×.0560((212-81)+970.4+.47(480-212))
= 618 B. t. u.,
= 4.34 per cent.
(C) To compute the loss in the heat carried away by dry chimney gases
per pound of coal the weight of such gases must be first determined.
This weight per pound of coal is:
(11CO_{2}+8O+7(CO+N))
(-------------------)C
( 3(CO_{2}+CO) )
where CO_{2}, O, CO and H are the percentage by volume as determined by
the flue gas analysis and C is the percentage by weight of carbon in the
dry fuel. Hence the weight of gas per pound of coal will be,
(11×14.33+8×4.54+7(0.11+81.02))
(-----------------------------)×78.57 = 13.7 pounds.
( 3(14.33+0.11) )
Therefore the loss of heat in the dry gases carried up the chimney =
13.7×0.24(480-81) = 1311 B. t. u.,
= 9.22 per cent.
(D) The loss due to incomplete combustion as evidenced by the presence
of CO in the flue gas analysis is:
0.11
----------×.7857×10,150 = 61. B. t. u.,
14.33+0.11 = .43 per cent.
(E) The loss due to unconsumed carbon in the ash:
The analysis of the ash showed 17.9 per cent to be combustible matter,
all of which is assumed to be carbon. The test showed 10.00 of the total
dry fuel fired to be ash. Hence 10.00×.179 = 1.79 per cent of the total
fuel represents the proportion of this total unconsumed in the ash and
the loss due to this cause is
1.79 per cent × 14,600 = 261 B. t. u.,
= 1.83 per cent.
The heat absorbed by the boilers per pound of dry fuel is 11.71×970.4 =
11,363 B. t. u. This quantity plus losses (A), (B), (C), (D) and (E), or
11,363+22+618+1311+61+261 = 13,636 B. t. u. accounted for. The heat
value of the coal, 14,225 B. t. u., less 13,636 B. t. u., leaves 589
B. t. u., unaccounted for losses, or 4.15 per cent.
The heat balance should be arranged in the form indicated by Table 58.
TABLE 58
HEAT BALANCE
B. T. U. PER POUND DRY COAL 14,225
+----------------------------------------------------------------------+
|+--------------------------------------------------------------------+|
|| |B. t. u.|Per Cent||
|+--------------------------------------------------+--------+--------+|
||Heat absorbed by Boiler | 11,363 | 79.88 ||
||Loss due to Evaporation of Moisture in Fuel | 22 | 0.15 ||
||Loss due to Moisture formed by Burning of Hydrogen| 618 | 4.34 ||
||Loss due to Heat carried away in Dry Chimney Gases| 1311 | 9.22 ||
||Loss due to Incomplete Combustion of Carbon | 61 | 0.43 ||
||Loss due to Unconsumed Carbon in the Ash | 261 | 1.83 ||
||Loss due to Radiation and Unaccounted Losses | 589 | 4.15 ||
|+--------------------------------------------------+--------+--------+|
||Total | 14,225 | 100.00 ||
|+--------------------------------------------------+--------+--------+|
+----------------------------------------------------------------------+
Application of Heat Balance--A heat balance should be made in connection
with any boiler trial on which sufficient data for its computation has
been obtained. This is particularly true where the boiler performance
has been considered unsatisfactory. The distribution of the heat is thus
determined and any extraordinary loss may be detected. Where accurate
data for computing such a heat balance is not available, such a
calculation based on certain assumptions is sometimes sufficient to
indicate unusual losses.
The largest loss is ordinarily due to the chimney gases, which depends
directly upon the weight of the gas and its temperature leaving the
boiler. As pointed out in the chapter on flue gas analysis, the lower
limit of the weight of gas is fixed by the minimum air supplied with
which complete combustion may be obtained. As shown, where this supply
is unduly small, the loss caused by burning the carbon to CO instead of
to CO_{2} more than offsets the gain in decreasing the weight of gas.
The lower limit of the stack temperature, as has been shown in the
chapter on draft, is more or less fixed by the temperature necessary to
create sufficient draft suction for good combustion. With natural draft,
this lower limit is probably between 400 and 450 degrees.
Capacity--Before the capacity of a boiler is considered, it is necessary
to define the basis to which such a term may be referred. Such a basis
is the so-called boiler horse power.
The unit of motive power in general use among steam engineers is the
"horse power" which is equivalent to 33,000 foot pounds per minute.
Stationary boilers are at the present time rated in horse power, though
such a basis of rating may lead and has often led to a misunderstanding.
_Work_, as the term is used in mechanics, is the overcoming of
resistance through space, while _power_ is the _rate_ of work or the
amount done per unit of time. As the operation of a boiler in service
implies no motion, it can produce no power in the sense of the term as
understood in mechanics. Its operation is the generation of steam, which
acts as a medium to convey the energy of the fuel which is in the form
of heat to a prime mover in which that heat energy is converted into
energy of motion or work, and power is developed.
If all engines developed the same amount of power from an equal amount
of heat, a boiler might be designated as one having a definite horse
power, dependent upon the amount of engine horse power its steam would
develop. Such a statement of the rating of boilers, though it would
still be inaccurate, if the term is considered in its mechanical sense,
could, through custom, be interpreted to indicate that a boiler was of
the exact capacity required to generate the steam necessary to develop a
definite amount of horse power in an engine. Such a basis of rating,
however, is obviously impossible when the fact is considered that the
amount of steam necessary to produce the same power in prime movers of
different types and sizes varies over very wide limits.
To do away with the confusion resulting from an indefinite meaning of
the term boiler horse power, the Committee of Judges in charge of the
boiler trials at the Centennial Exposition, 1876, at Philadelphia,
ascertained that a good engine of the type prevailing at the time
required approximately 30 pounds of steam per hour per horse power
developed. In order to establish a relation between the engine power and
the size of a boiler required to develop that power, they recommended
that an evaporation of 30 pounds of water from an initial temperature of
100 degrees Fahrenheit to steam at 70 pounds gauge pressure be
considered as _one boiler horse power_. This recommendation has been
generally accepted by American engineers as a standard, and when the
term boiler horse power is used in connection with stationary
boilers[58] throughout this country,[59] without special definition, it
is understood to have this meaning.
Inasmuch as an equivalent evaporation from and at 212 degrees Fahrenheit
is the generally accepted basis of comparison[60], it is now customary
to consider the standard boiler horse power as recommended by the
Centennial Exposition Committee, in terms of equivalent evaporation from
and at 212 degrees. This will be 30 pounds multiplied by the factor of
evaporation for 70 pounds gauge pressure and 100 degrees feed
temperature, or 1.1494. 30 × 1.1494 = 34.482, or approximately 34.5
pounds. Hence, _one boiler horse power is equal to an evaporation of
34.5 pounds of water per hour from and at 212 degrees Fahrenheit_. The
term boiler horse power, therefore, is clearly a measure of evaporation
and not of power.
A method of basing the horse power rating of a boiler adopted by boiler
manufacturers is that of heating surfaces. Such a method is absolutely
arbitrary and changes in no way the definition of a boiler horse power
just given. It is simply a statement by the manufacturer that his
product, under ordinary operating conditions or conditions which may be
specified, will evaporate 34.5 pounds of water from and at 212 degrees
per definite amount of heating surface provided. The amount of heating
surface that has been considered by manufacturers capable of evaporating
34.5 pounds from and at 212 degrees per hour has changed from time to
time as the art has progressed. At the present time 10 square feet of
heating surface is ordinarily considered the equivalent of one boiler
horse power among manufacturers of stationary boilers. In view of the
arbitrary nature of such rating and of the widely varying rates of
evaporation possible per square foot of heating surface with different
boilers and different operating conditions, such a basis of rating has
in reality no particular bearing on the question of horse power and
should be considered merely as a convenience.
The whole question of a unit of boiler capacity has been widely
discussed with a view to the adoption of a standard to which there would
appear to be a more rational and definite basis. Many suggestions have
been offered as to such a basis but up to the present time there has
been none which has met with universal approval or which would appear
likely to be generally adopted.
With the meaning of boiler horse power as given above, that is, a
measure of evaporation, it is evident that the capacity of a boiler is a
measure of the power it can develop expressed in boiler horse power.
Since it is necessary, as stated, for boiler manufacturers to adopt a
standard for reasons of convenience in selling, the horse power for
which a boiler is sold is known as its normal rated capacity.
The efficiency of a boiler and the maximum capacity it will develop can
be determined accurately only by a boiler test. The standard methods of
conducting such tests are given on the following pages, these methods
being the recommendations of the Power Test Committee of the American
Society of Mechanical Engineers brought out in 1913.[61] Certain changes
have been made to incorporate in the boiler code such portions of the
"Instructions Regarding Tests in General" as apply to boiler testing.
Methods of calculation and such matter as are treated in other portions
of the book have been omitted from the code as noted.
[Illustration: Portion of 2600 Horse-power Installation of Babcock &
Wilcox Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at
the Peter Schoenhofen Brewing Co., Chicago, Ill.]
1. OBJECT
Ascertain the specific object of the test, and keep this in view not
only in the work of preparation, but also during the progress of the
test, and do not let it be obscured by devoting too close attention to
matters of minor importance. Whatever the object of the test may be,
accuracy and reliability must underlie the work from beginning to end.
If questions of fulfillment of contract are involved, there should be a
clear understanding between all the parties, preferably in writing, as
to the operating conditions which should obtain during the trial, and as
to the methods of testing to be followed, unless these are already
expressed in the contract itself.
Among the many objects of performance tests, the following may be noted:
Determination of capacity and efficiency, and how these compare
with standard or guaranteed results.
Comparison of different conditions or methods of operation.
Determination of the cause of either inferior or superior
results.
Comparison of different kinds of fuel.
Determination of the effect of changes of design or proportion
upon capacity or efficiency, etc.
2. PREPARATIONS
_(A) Dimensions:_
Measure the dimensions of the principal parts of the apparatus to be
tested, so far as they bear on the objects in view, or determine these
from correct working drawings. Notice the general features of the same,
both exterior and interior, and make sketches, if needed, to show
unusual points of design.
The dimensions of the heating surfaces of boilers and
superheaters to be found are those of surfaces in contact with
the fire or hot gases. The submerged surfaces in boilers at the
mean water level should be considered as water-heating surfaces,
and other surfaces which are exposed to the gases as
superheating surfaces.
_(B) Examination of Plant:_
Make a thorough examination of the physical condition of all parts of
the plant or apparatus which concern the object in view, and record the
conditions found, together with any points in the matter of operation
which bear thereon.
In boilers, examine for leakage of tubes and riveted or other
metal joints. Note the condition of brick furnaces, grates and
baffles. Examine brick walls and cleaning doors for air leaks,
either by shutting the damper and observing the escaping smoke
or by candle-flame test. Determine the condition of heating
surfaces with reference to exterior deposits of soot and
interior deposits of mud or scale.
See that the steam main is so arranged that condensed and
entrained water cannot flow back into the boiler.
If the object of the test is to determine the highest efficiency or
capacity obtainable, any physical defects, or defects of operation,
tending to make the result unfavorable should first be remedied; all
foul parts being cleaned, and the whole put in first-class condition.
If, on the other hand, the object is to ascertain the performance under
existing conditions, no such preparation is either required or desired.
_(C) General Precautions against Leakage:_
In steam tests make sure that there is no leakage through blow-offs,
drips, etc., or any steam or water connections of the plant or apparatus
undergoing test, which would in any way affect the results. All such
connections should be blanked off, or satisfactory assurance should be
obtained that there is leakage neither out nor in. This is a most
important matter, and no assurance should be considered satisfactory
unless it is susceptible of absolute demonstration.
3. FUEL
Determine the character of fuel to be used.[62] For tests of maximum
efficiency or capacity of the boiler to compare with other boilers, the
coal should be of some kind which is commercially regarded as a standard
for the locality where the test is made.
In the Eastern States the standards thus regarded for
semi-bituminous coals are Pocahontas (Va. and W. Va.) and New
River (W. Va.); for anthracite coals those of the No. 1
buckwheat size, fresh-mined, containing not over 13 per cent ash
by analysis; and for bituminous coals, Youghiogheny and
Pittsburgh coals. In some sections east of the Allegheny
Mountains the semi-bituminous Clearfield (Pa.) and Cumberland
(Md.) are also considered as standards. These coals when of good
quality possess the essentials of excellence, adaptability to
various kinds of furnaces, grates, boilers, and methods of
firing required, besides being widely distributed and generally
accessible in the Eastern market. There are no special grades of
coal mined in the Western States which are widely and generally
considered as standards for testing purposes; the best coal
obtainable in any particular locality being regarded as the
standard of comparison.
A coal selected for maximum efficiency and capacity tests, should be the
best of its class, and especially free from slagging and unusual
clinker-forming impurities.
For guarantee and other tests with a specified coal containing not more
than a certain amount of ash and moisture, the coal selected should not
be higher in ash and in moisture than the stated amounts, because any
increase is liable to reduce the efficiency and capacity more than the
equivalent proportion of such increase.
The size of the coal, especially where it is of the anthracite class,
should be determined by screening a suitable sample.
4. APPARATUS AND INSTRUMENTS[63]
The apparatus and instruments required for boiler tests are:
(A) Platform scales for weighing coal and ashes.
(B) Graduated scales attached to the water glasses.
(C) Tanks and platform scales for weighing water (or water
meters calibrated in place). Wherever practicable the feed water
should be weighed, especially for guarantee tests. The most
satisfactory and reliable apparatus for this purpose consists of
one or more tanks each placed on platform scales, these being
elevated a sufficient distance above the floor to empty into a
receiving tank placed below, the latter being connected to the
feed pump. Where only one weighing tank is used the receiving
tank should be of larger size than the weighing tank, to afford
sufficient reserve supply to the pump while the upper tank is
filling. If a single weighing tank is used it should preferably
be of such capacity as to require emptying not oftener than
every 5 minutes. If two or more are used the intervals between
successive emptyings should not be less than 3 minutes.
(D) Pressure gauges, thermometers, and draft gauges.
(E) Calorimeters for determining the calorific value of fuel and
the quality of steam.
(F) Furnaces pyrometers.
(G) Gas analyzing apparatus.
5. OPERATING CONDITIONS
Determine what the operating conditions and method of firing should be
to conform to the object in view, and see that they prevail throughout
the trial, as nearly as possible.
Where uniformity in the rate of evaporation is required,
arrangement can be usually made to dispose of the steam so that
this result can be attained. In a single boiler it may be
accomplished by discharging steam through a waste pipe and
regulating the amount by means of a valve. In a battery of
boilers, in which only one is tested, the draft may be regulated
on the remaining boilers to meet the varying demands for steam,
leaving the test boiler to work under a steady rate of
evaporation.
6. DURATION
The duration of tests to determine the efficiency of a hand-fired
boiler, should be 10 hours of continuous running, or such time as may be
required to burn a total of 250 pounds of coal per square foot of grate.
In the case of a boiler using a mechanical stoker, the duration, where
practicable, should be at least 24 hours. If the stoker is of a type
that permits the quantity and condition of the fuel bed at beginning and
end of the test to be accurately estimated, the duration may be reduced
to 10 hours, or such time as may be required to burn the above noted
total of 250 pounds per square foot.
In commercial tests where the service requires continuous
operation night and day, with frequent shifts of firemen, the
duration of the test, whether the boilers are hand fired or
stoker fired, should be at least 24 hours. Likewise in
commercial tests, either of a single boiler or of a plant of
several boilers, which operate regularly a certain number of
hours and during the balance of the day the fires are banked,
the duration should not be less than 24 hours.
The duration of tests to determine the maximum evaporative
capacity of a boiler, without determining the efficiency, should
not be less than 3 hours.
7. STARTING AND STOPPING
The conditions regarding the temperature of the furnace and boiler, the
quantity and quality of the live coal and ash on the grates, the water
level, and the steam pressure, should be as nearly as possible the same
at the end as at the beginning of the test.
To secure the desired equality of conditions with hand-fired boilers,
the following method should be employed:
The furnace being well heated by a preliminary run, burn the
fire low, and thoroughly clean it, leaving enough live coal
spread evenly over the grate (say 2 to 4 inches),[64] to serve
as a foundation for the new fire. Note quickly the thickness of
the coal bed as nearly as it can be estimated or measured; also
the water level,[65] the steam pressure, and the time, and
record the latter as the starting time. Fresh coal should then
be fired from that weighed for the test, the ashpit throughly
cleaned, and the regular work of the test proceeded with. Before
the end of the test the fire should again be burned low and
cleaned in such a manner as to leave the same amount of live
coal on the grate as at the start. When this condition is
reached, observe quickly the water level,[65] the steam
pressure, and the time, and record the latter as the stopping
time. If the water level is not the same as at the beginning a
correction should be made by computation, rather than by feeding
additional water after the final readings are taken. Finally
remove the ashes and refuse from the ashpit. In a plant
containing several boilers where it is not practicable to clean
them simultaneously, the fires should be cleaned one after the
other as rapidly as may be, and each one after cleaning charged
with enough coal to maintain a thin fire in good working
condition. After the last fire is cleaned and in working
condition, burn all the fires low (say 4 to 6 inches), note
quickly the thickness of each, also the water levels, steam
pressure, and time, which last is taken as the starting time.
Likewise when the time arrives for closing the test, the fires
should be quickly cleaned one by one, and when this work is
completed they should all be burned low the same as the start,
and the various observations made as noted. In the case of a
large boiler having several furnace doors requiring the fire to
be cleaned in sections one after the other, the above directions
pertaining to starting and stopping in a plant of several
boilers may be followed.
To obtain the desired equality of conditions of the fire when a
mechanical stoker other than a chain grate is used, the procedure should
be modified where practicable as follows:
Regulate the coal feed so as to burn the fire to the low
condition required for cleaning. Shut off the coal-feeding
mechanism and fill the hoppers level full. Clean the ash or dump
plate, note quickly the depth and condition of the coal on the
grate, the water level,[66] the steam pressure, and the time,
and record the latter as the starting time. Then start the
coal-feeding mechanism, clean the ashpit, and proceed with the
regular work of the test.
When the time arrives for the close of the test, shut off the
coal-feeding mechanism, fill the hoppers and burn the fire to
the same low point as at the beginning. When this condition is
reached, note the water level, the steam pressure, and the time,
and record the latter as the stopping time. Finally clean the
ashplate and haul the ashes.
In the case of chain grate stokers, the desired operating
conditions should be maintained for half an hour before starting
a test and for a like period before its close, the height of the
throat plate and the speed of the grate being the same during
both of these periods.
8. RECORDS
A log of the data should be entered in notebooks or on blank sheets
suitably prepared in advance. This should be done in such manner that
the test may be divided into hourly periods, or if necessary, periods of
less duration, and the leading data obtained for any one or more periods
as desired, thereby showing the degree of uniformity obtained.
Half-hourly readings of the instruments are usually sufficient. If there
are sudden and wide fluctuations, the readings in such cases should be
taken every 15 minutes, and in some instances oftener.
The coal should be weighed and delivered to the firemen in
portions sufficient for one hour's run, thereby ascertaining the
degree of uniformity of firing. An ample supply of coal should
be maintained at all times, but the quantity on the floor at the
end of each hour should be as small as practicable, so that the
same may be readily estimated and deducted from the total
weight.
The records should be such as to ascertain also the consumption
of feed water each hour and thereby determine the degree of
uniformity of evaporation.
9. QUALITY OF STEAM[67]
If the boiler does not produce superheated steam the percentage of
moisture in the steam should be determined by the use of a throttling or
separating calorimeter. If the boiler has superheating surface, the
temperature of the steam should be determined by the use of a
thermometer inserted in a thermometer well.
For saturated steam construct a sampling pipe or nozzle made of one-half
inch iron pipe and insert it in the steam main at a point where the
entrained moisture is likely to be most thoroughly mixed. The inner end
of the pipe, which should extend nearly across to the opposite side of
the main, should be closed and interior portion perforated with not less
than twenty one-eighth inch holes equally distributed from end to end
and preferably drilled in irregular or spiral rows, with the first hole
not less than half an inch from the wall of the pipe.
The sampling pipe should not be placed near a point where water
may pocket or where such water may effect the amount of moisture
contained in the sample. Where non-return valves are used, or
there are horizontal connections leading from the boiler to a
vertical outlet, water may collect at the lower end of the
uptake pipe and be blown upward in a spray which will not be
carried away by the steam owing to a lack of velocity. A sample
taken from the lower part of this pipe will show a greater
amount of moisture than a true sample. With goose-neck
connections a small amount of water may collect on the bottom of
the pipe near the upper end where the inclination is such that
the tendency to flow backward is ordinarily counterbalanced by
the flow of steam forward over its surface; but when the
velocity momentarily decreases the water flows back to the lower
end of the goose-neck and increases the moisture at that point,
making it an undesirable location for sampling. In any case it
must be borne in mind that with low velocities the tendency is
for drops of entrained water to settle to the bottom of the
pipe, and to be temporarily broken up into spray whenever an
abrupt bend or other disturbance is met.
If it is necessary to attach the sampling nozzle at a point near the end
of a long horizontal run, a drip pipe should be provided a short
distance in front of the nozzle, preferably at a pocket formed by some
fitting and the water running along the bottom of the main drawn off,
weighed, and added to the moisture shown by the calorimeter; or, better,
a steam separator should be installed at the point noted.
In testing a stationary boiler the sampling pipe should be located as
near as practicable to the boiler, and the same is true as regards the
thermometer well when the steam is superheated. In an engine or turbine
test these locations should be as near as practicable to throttle valve.
In the test of a plant where it is desired to get complete information,
especially where the steam main is unusually long, sampling nozzles or
thermometer wells should be provided at both points, so as to obtain
data at either point as may be required.
10. SAMPLING AND DRYING COAL
During the progress of test the coal should be regularly sampled for the
purpose of analysis and determination of moisture.
Select a representative shovelful from each barrow-load as it is drawn
from the coal pile or other source of supply, and store the samples in a
cool place in a covered metal receptacle. When all the coal has thus
been sampled, break up the lumps, thoroughly mix the whole quantity, and
finally reduce it by the process of repeated quartering and crushing to
a sample weighing about 5 pounds, the largest pieces being about the
size of a pea. From this sample two one-quart air-tight glass fruit
jars, or other air-tight vessels, are to be promptly filled and
preserved for subsequent determinations of moisture, calorific value,
and chemical composition. These operations should be conducted where the
air is cool and free from drafts.
[Illustration: 3460 Horse-power Installation of Babcock & Wilcox Boilers
at the Chicago, Ill., Shops of the Chicago and Northwestern Ry. Co.]
When the sample lot of coal has been reduced by quartering to, say, 100
pounds, a portion weighing, say, 15 to 20 pounds should be withdrawn for
the purpose of immediate moisture determination. This is placed in a
shallow iron pan and dried on the hot iron boiler flue for at least 12
hours, being weighed before and after drying on scales reading to
quarter ounces.
The moisture thus determined is approximately reliable for anthracite
and semi-bituminous coals, but not for coals containing much inherent
moisture. For such coals, and for all absolutely reliable determinations
the method to be pursued is as follows:
Take one of the samples contained in the glass jars, and subject
it to a thorough air drying, by spreading it in a thin layer and
exposing it for several hours to the atmosphere of a warm room,
weighing it before and after, thereby determining the quantity
of surface moisture it contains.[68] Then crush the whole of it
by running it through an ordinary coffee mill or other suitable
crusher adjusted so as to produce somewhat coarse grains (less
than 1/16 inch), thoroughly mix the crushed sample, select from
it a portion of from 10 to 50 grams,[69] weigh it in a balance
which will easily show a variation as small as 1 part in 1000,
and dry it for one hour in an air or sand bath at a temperature
between 240 and 280 degrees Fahrenheit. Weigh it and record the
loss, then heat and weigh again until the minimum weight has
been reached. The difference between the original and the
minimum weight is the moisture in the air-dried coal. The sum of
the moisture thus found and that of the surface moisture is the
total moisture.
11. ASHES AND REFUSE
The ashes and refuse withdrawn from the furnace and ashpit during the
progress of the test and at its close should be weighed so far as
possible in a dry state. If wet the amount of moisture should be
ascertained and allowed for, a sample being taken and dried for this
purpose. This sample may serve also for analysis and the determination
of unburned carbon and fusing temperature.
The method above described for sampling coal may also be followed for
obtaining a sample of the ashes and refuse.
12. CALORIFIC TESTS AND ANALYSES OF COAL
The quality of the fuel should be determined by calorific tests and
analysis of the coal sample above referred to.[70]
13. ANALYSES OF FLUE GASES
For approximate determinations of the composition of the flue gases, the
Orsat apparatus, or some modification thereof, should be employed. If
momentary samples are obtained the analyses should be made as frequently
as possible, say, every 15 to 30 minutes, depending on the skill of the
operator, noting at the time the sample is drawn the furnace and firing
conditions. If the sample drawn is a continuous one, the intervals may
be made longer.
14. SMOKE OBSERVATIONS[71]
In tests of bituminous coals requiring a determination of the amount of
smoke produced, observations should be made regularly throughout the
trial at intervals of 5 minutes (or if necessary every minute), noting
at the same time the furnace and firing conditions.
15. CALCULATION OF RESULTS
The methods to be followed in expressing and calculating those results
which are not self-evident are explained as follows:
(A) _Efficiency._ The "efficiency of boiler, furnace and
grate" is the relation between the heat absorbed per pound of
coal fired, and the calorific value of one pound of coal.
The "efficiency of boiler and furnace" is the relation between
the heat absorbed per pound of combustible burned, and the
calorific value of one pound of combustible. This expression of
efficiency furnishes a means for comparing one boiler and
furnace with another, when the losses of unburned coal due to
grates, cleanings, etc., are eliminated.
The "combustible burned" is determined by subtracting from the
weight of coal supplied to the boiler, the moisture in the coal,
the weight of ash and unburned coal withdrawn from the furnace
and ashpit, and the weight of dust, soot, and refuse, if any,
withdrawn from the tubes, flues, and combustion chambers,
including ash carried away in the gases, if any, determined from
the analysis of coal and ash. The "combustible" used for
determining the calorific value is the weight of coal less the
moisture and ash found by analysis.
The "heat absorbed" per pound of coal, or combustible, is
calculated by multiplying the equivalent evaporation from and at
212 degrees per pound of coal or combustible by 970.4.
Other items in this section which have been treated elsewhere are:
(B) Corrections for moisture in steam.
(C) Correction for live steam used.
(D) Equivalent evaporation.
(E) Heat balance.
(F) Total heat of combustion of coal.
(G) Air for combustion and the methods recommended for
calculating these results are in accordance with those described
in different portions of this book.
16. DATA AND RESULTS
The data and results should be reported in accordance with either the
short form or the complete form, adding lines for data not provided for,
or omitting those not required, as may conform to the object in view.
17. CHART
In trials having for an object the determination and exposition of the
complete boiler performance, the entire log of readings and data should
be plotted on a chart and represented graphically.
18. TESTS WITH OIL AND GAS FUELS
Tests of boilers using oil or gas for fuel should accord with the rules
here given, excepting as they are varied to conform to the particular
characteristics of the fuel. The duration in such cases may be reduced,
and the "flying" method of starting and stopping employed.
The table of data and results should contain items stating
character of furnace and burner, quality and composition of oil
or gas, temperature of oil, pressure of steam used for
vaporizing and quantity of steam used for both vaporizing and
for heating.
TABLE DATA AND RESULTS OF EVAPORATIVE TEST
SHORT FORM, CODE OF 1912
1 Test of.................boiler located at................................
to determine...............conducted by..............................
2 Kind of furnace..........................................................
3 Grate surface.................................................square feet
4 Water-heating surface.........................................square feet
5 Superheating surface..........................................square feet
6 Date.....................................................................
7 Duration............................................................hours
8 Kind and size of coal....................................................
AVERAGE PRESSURES, TEMPERATURES, ETC.
9 Steam pressure by gauge............................................pounds
10 Temperature of feed water entering boiler.........................degrees
11 Temperature of escaping gases leaving boiler......................degrees
12 Force of draft between damper and boiler...........................inches
13 Percentage of moisture in steam,
or number degrees of superheating..................per cent or degrees
TOTAL QUANTITIES
14 Weight of coal as fired[72]........................................pounds
15 Percentage of moisture in coal...................................per cent
16 Total weight of dry coal consumed..................................pounds
17 Total ash and refuse...............................................pounds
18 Percentage of ash and refuse in dry coal.........................per cent
19 Total weight of water fed to the boiler[73]........................pounds
20 Total water evaporated, corrected for moisture in steam............pounds
21 Total equivalent evaporation from and at 212 degrees...............pounds
HOURLY QUANTITIES AND RATES
22 Dry coal consumed per hour.........................................pounds
23 Dry coal per square feet of grate surface per hour.................pounds
24 Water evaporated per hour corrected for quality of steam...........pounds
25 Equivalent evaporation per hour from and at 212 degrees............pounds
26 Equivalent evaporation per hour from and at 212 degrees
per square foot of water-heating surface........................pounds
CAPACITY
27 Evaporation per hour from and at 212 degrees (same as Line 25).....pounds
28 Boiler horse power developed (Item 27÷34½).............boiler horse power
29 Rated capacity, in evaporation from and at 212 degrees per hour....pounds
30 Rated boiler horse power...............................boiler horse power
31 Percentage of rated capacity developed...........................per cent
ECONOMY RESULTS
32 Water fed per pound of coal fired (Item 19÷Item 14)................pounds
33 Water evaporated per pound of dry coal (Item 20÷Item 16)...........pounds
34 Equivalent evaporation from and at 212 degrees per pound
of dry coal (Item 21÷Item 16)...................................pounds
35 Equivalent evaporation from and at 212 degrees per pound
of combustible [Item 21÷(Item 16-Item 17)]......................pounds
EFFICIENCY
36 Calorific value of one pound of dry coal.........................B. t. u.
37 Calorific value of one pound of combustible......................B. t. u.
( Item 34×970.4)
38 Efficiency of boiler, furnace and grate (100 × -------------)....per cent
( Item 36 )
( Item 35×970.4)
39 Efficiency of boiler and furnace (100 × -------------)...........per cent
( Item 37 )
COST OF EVAPORATION
40 Cost of coal per ton of......pounds delivered in boiler room......dollars
41 Cost of coal required for evaporating 1000 pounds of water
from and at 212 degrees........................................dollars
[Illustration: Portion of 3600 Horse-power Installation of Babcock &
Wilcox Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at
the Loomis Street Plant of the Peoples Gas Light & Coke Co., Chicago,
Ill. This Company has Installed 7780 Horse Power of Babcock & Wilcox
Boilers]
THE SELECTION OF BOILERS WITH A CONSIDERATION OF THE FACTORS DETERMINING
SUCH SELECTION
The selection of steam boilers is a matter to which the most careful
thought and attention may be well given. Within the last twenty years,
radical changes have taken place in the methods and appliances for the
generation and distribution of power. These changes have been made
largely in the prime movers, both as to type and size, and are best
illustrated by the changes in central station power-plant practice. It
is hardly within the scope of this work to treat of power-plant design
and the discussion will be limited to a consideration of the boiler end
of the power plant.
As stated, the changes have been largely in prime movers, the steam
generating equipment having been considered more or less of a standard
piece of apparatus whose sole function is the transfer of the heat
liberated from the fuel by combustion to the steam stored or circulated
in such apparatus. When the fact is considered that the cost of steam
generation is roughly from 65 to 80 per cent of the total cost of power
production, it may be readily understood that the most fruitful field
for improvement exists in the boiler end of the power plant. The
efficiency of the plant as a whole will vary with the load it carries
and it is in the boiler room where such variation is largest and most
subject to control.
The improvements to be secured in the boiler room results are not simply
a matter of dictation of operating methods. The securing of perfect
combustion, with the accompanying efficiency of heat transfer, while
comparatively simple in theory, is difficult to obtain in practical
operation. This fact is perhaps best exemplified by the difference
between test results and those obtained in daily operation even under
the most careful supervision. This difference makes it necessary to
establish a standard by which operating results may be judged, a
standard not necessarily that which might be possible under test
conditions but one which experiment shows can be secured under the very
best operating conditions.
The study of the theory of combustion, draft, etc., as already given,
will indicate that the question of efficiency is largely a matter of
proper relation between fuel, furnace and generator. While the
possibility of a substantial saving through added efficiency cannot be
overlooked, the boiler design of the future must, even more than in the
past, be considered particularly from the aspect of reliability and
simplicity. A flexibility of operation is necessary as a guarantee of
continuity of service.
In view of the above, before the question of the selection of boilers
can be taken up intelligently, it is necessary to consider the subjects
of boiler efficiency and boiler capacity, together with their relation
to each other.
The criterion by which the efficiency of a boiler plant is to be judged
is the cost of the production of a definite amount of steam. Considered
in this sense, there must be included in the efficiency of a boiler
plant the simplicity of operation, flexibility and reliability of the
boiler used. The items of repair and upkeep cost are often high because
of the nature of the service. The governing factor in these items is
unquestionably the type of boiler selected.
The features entering into the plant efficiency are so numerous that it
is impossible to make a statement as to a means of securing the highest
efficiency which will apply to all cases. Such efficiency is to be
secured by the proper relation of fuel, furnace and boiler heating
surface, actual operating conditions, which allow the approaching of the
potential efficiencies made possible by the refinement of design, and a
systematic supervision of the operation assisted by a detailed record of
performances and conditions. The question of supervision will be taken
up later in the chapter on "Operation and Care of Boilers".
The efficiencies that may be expected from the combination of
well-designed boilers and furnaces are indicated in Table 59 in which
are given a number of tests with various fuels and under widely
different operating conditions.
It is to be appreciated that the results obtained as given in this table
are practically all under test conditions. The nearness with which
practical operating conditions can approach these figures will depend
upon the character of the supervision of the boiler room and the
intelligence of the operating crew. The size of the plant will
ordinarily govern the expense warranted in securing the right sort of
supervision.
The bearing that the type of boiler has on the efficiency to be expected
can only be realized from a study of the foregoing chapters.
Capacity--Capacity, as already defined, is the ability of a definite
amount of boiler-heating surface to generate steam. Boilers are
ordinarily purchased under a manufacturer's specification, which rates a
boiler at a nominal rated horse power, usually based on 10 square feet
of heating surface per horse power. Such a builders' rating is
absolutely arbitrary and implies nothing as to the limiting amount of
water that this amount of heating surface will evaporate. It does not
imply that the evaporation of 34.5 pounds of water from and at 212
degrees with 10 square feet of heating surface is the limit of the
capacity of the boiler. Further, from a statement that a boiler is of a
certain horse power on the manufacturer's basis, it is not to be
understood that the boiler is in any state of strain when developing
more than its rated capacity.
Broadly stated, the evaporative capacity of a certain amount of heating
surface in a well-designed boiler, that is, the boiler horse power it is
capable of producing, is limited only by the amount of fuel that can be
burned under the boiler. While such a statement would imply that the
question of capacity to be secured was simply one of making an
arrangement by which sufficient fuel could be burned under a definite
amount of heating surface to generate the required amount of steam,
there are limiting features that must be weighed against the advantages
of high capacity developed from small heating surfaces. Briefly stated,
these factors are as follows:
1st. Efficiency. As the capacity increases, there will in general be a
decrease in efficiency, this loss above a certain point making it
inadvisable to try to secure more than a definite horse power from a
given boiler. This loss of efficiency with increased capacity is treated
below in detail, in considering the relation of efficiency to capacity.
2nd. Grate Ratio Possible or Practicable. All fuels have a maximum rate
of combustion, beyond which satisfactory results cannot be obtained,
regardless of draft available or which may be secured by mechanical
means. Such being the case, it is evident that with this maximum
combustion rate secured, the only method of obtaining added capacity
will be through the addition of grate surface. There is obviously a
point beyond which the grate surface for a given boiler cannot be
increased. This is due to the impracticability of handling grates above
a certain maximum size, to the enormous loss in draft pressure through a
boiler resulting from an attempt to force an abnormal quantity of gas
through the heating surface and to innumerable details of design and
maintenance that would make such an arrangement wholly unfeasible.
3rd. Feed Water. The difficulties that may arise through the use of poor
feed water or that are liable to happen through the use of practically
any feed water have already been pointed out. This question of feed is
frequently the limiting factor in the capacity obtainable, for with an
increase in such capacity comes an added concentration of such
ingredients in the feed water as will cause priming, foaming or rapid
scale formation. Certain waters which will give no trouble that cannot
be readily overcome with the boiler run at ordinary ratings will cause
difficulties at higher ratings entirely out of proportion to any
advantage secured by an increase in the power that a definite amount of
heating surface may be made to produce.
Where capacity in the sense of overload is desired, the type of boiler
selected will play a large part in the successful operation through such
periods. A boiler must be selected with which there is possible a
furnace arrangement that will give flexibility without undue loss in
efficiency over the range of capacity desired. The heating surface must
be so arranged that it will be possible to install in a practical
manner, sufficient grate surface at or below the maximum combustion rate
to develop the amount of power required. The design of boiler must be
such that there will be no priming or foaming at high overloads and that
any added scale formation due to such overloads may be easily removed.
Certain boilers which deliver commercially dry steam when operated at
about their normal rated capacity will prime badly when run at overloads
and this action may take place with a water that should be easily
handled by a properly designed boiler at any reasonable load. Such
action is ordinarily produced by the lack of a well defined, positive
circulation.
Relation of Efficiency and Capacity--The statement has been made that in
general the efficiency of a boiler will decrease as the capacity is
increased. Considering the boiler alone, apart from the furnace, this
statement may be readily explained.
Presupposing a constant furnace temperature, regardless of the capacity
at which a given boiler is run; to assure equal efficiencies at low and
high ratings, the exit temperature in the two instances would
necessarily be the same. For this temperature at the high rating, to be
identical with that at the low rating, the rate of heat transfer from
the gases to the heating surfaces would have to vary directly as the
weight or volume of such gases. Experiment has shown, however, that this
is not true but that this rate of transfer varies as some power of the
volume of gas less than one. As the heat transfer does not, therefore,
increase proportionately with the volume of gases, the exit temperature
for a given furnace temperature will be increased as the volume of gases
increases. As this is the measure of the efficiency of the heating
surface, the boiler efficiency will, therefore, decrease as the volume
of gases increases or the capacity at which the boiler is operated
increases.
Further, a certain portion of the heat absorbed by the heating surface
is through direct radiation from the fire. Again, presupposing a
constant furnace temperature; the heat absorbed through radiation is
solely a function of the amount of surface exposed to such radiation.
Hence, for the conditions assumed, the amount of heat absorbed by
radiation at the higher ratings will be the same as at the lower ratings
but in proportion to the total absorption will be less. As the added
volume of gas does not increase the rate of heat transfer, there are
therefore two factors acting toward the decrease in the efficiency of a
boiler with an increase in the capacity.
TABLE 59
TESTS OF BABCOCK & WILCOX BOILERS WITH VARIOUS FUELS
______________________________________________________________________
|Number| | | | Rated |
| of | Name and Location | Kind of Coal | Kind of | Horse |
| Test | of Plant | | Furnace |Power of|
| | | | | Boiler |
| | | | | |
|______|___________________________|________________|_________|________|
| |Susquehanna Coal Co., |No. 1 Anthracite|Hand | |
| 1 |Shenandoah, Pa. |Buckwheat |Fired | 300 |
|______|___________________________|________________|_________|________|
| |Balbach Smelting & |No. 2 Buckwheat |Wilkenson| |
| 2 |Refining Co., Newark, N. J.|and Bird's-eye | Stoker | 218 |
|______|___________________________|________________|_________|________|
| |H. R. Worthington, |No. 2 Anthracite|Hand | |
| 3 |Harrison N. J. |Buckwheat |Fired | 300 |
|______|___________________________|________________|_________|________|
| |Raymond Street Jail, |Anthracite Pea |Hand | |
| 4 |Brooklyn, N. Y. | |Fired | 155 |
|______|___________________________|________________|_________|________|
| |R. H. Macy & Co., |No. 3 Anthracite|Hand | |
| 5 |New York, N. Y. |Buckwheat |Fired | 293 |
|______|___________________________|________________|_________|________|
| |National Bureau of |Anthracite Egg |Hand | |
| 6 |Standards, Washington, D.C.| |Fired | 119 |
|______|___________________________|________________|_________|________|
| |Fred. Loeser & Co., |No. 1 Anthracite|Hand | |
| 7 |Brooklyn, N. Y. |Buckwheat |Fired | 300 |
|______|___________________________|________________|_________|________|
| |New York Edison Co., |No. 2 Anthracite|Hand | |
| 8 |New York City |Buckwheat |Fired | 374 |
|______|___________________________|________________|_________|________|
| |Sewage Pumping Station, |Hocking Valley |Hand | |
| 9 |Cleveland, O. |Lump, O. |Fired | 150 |
|______|___________________________|________________|_________|________|
| |Scioto River Pumping Sta., |Hocking Valley, |Hand | |
| 10 |Cleveland, O. |O. |Fired | 300 |
|______|___________________________|________________|_________|________|
| |Consolidated Gas & Electric|Somerset, Pa. |Hand | |
| 11 |Co., Baltimore, Md. | |Fired | 640 |
|______|___________________________|________________|_________|________|
| |Consolidated Gas & Electric|Somerset, Pa. |Hand | |
| 12 |Co., Baltimore, Md. | |Fired | 640 |
|______|___________________________|________________|_________|________|
| |Merrimac Mfg. Co., |Georges Creek, |Hand | |
| 13 |Lowell, Mass. |Md. |Fired | 321 |
|______|___________________________|________________|_________|________|
| |Great West'n Sugar Co., |Lafayette, Col.,|HandFired| |
| 14 |Ft. Collins, Col. |Mine Run |Extension| 351 |
|______|___________________________|________________|_________|________|
| |Baltimore Sewage Pumping |New River |Hand | |
| 15 | Station | |Fired | 266 |
|______|___________________________|________________|_________|________|
| |Tennessee State Prison, |Brushy Mountain,|Hand | |
| 16 |Nashville, Tenn. |Tenn. |Fired | 300 |
|______|___________________________|________________|_________|________|
| |Pine Bluff Corporation, |Arkansas Slack |Hand | |
| 17 |Pine Bluff, Ark. | |Fired | 298 |
|______|___________________________|________________|_________|________|
| |Pub. Serv. Corporation |Valley, Pa., |Roney | |
| 18 |of N. J., Hoboken |Mine Run |Stoker | 520 |
|______|___________________________|________________|_________|________|
| |Pub. Serv. Corporation |Valley, Pa., |Roney | |
| 19 |of N. J., Hoboken |Mine Run |Stoker | 520 |
|______|___________________________|________________|_________|________|
| |Frick Building, |Pittsburgh Nut |American | |
| 20 |Pittsburgh, Pa. |and Slack |Stoker | 300 |
|______|___________________________|________________|_________|________|
| |New York Edison Co., |Loyal Hanna, Pa.|Taylor | |
| 21 |New York City | |Stoker | 604 |
|______|___________________________|________________|_________|________|
| |City of Columbus, O., |Hocking Valley, |Detroit | |
| 22 |Dept. Lighting |O. |Stoker | 300 |
|______|___________________________|________________|_________|________|
| |Edison Elec. Illum. Co., |New River |Murphy | |
| 23 |Boston, Mass. | |Stoker | 508 |
|______|___________________________|________________|_________|________|
| |Colorado Springs & |Pike View, Col.,|Green Chn| |
| 24 |Interurban Ry., Col. |Mine Run |Grate | 400 |
|______|___________________________|________________|_________|________|
| |Pub. Serv. Corporation |Lancashire, Pa. |B&W.Chain| |
| 25 |of N. J., Marion | |Grate | 600 |
|______|___________________________|________________|_________|________|
| |Pub. Serv. Corporation |Lancashire, Pa. |B&W.Chain| |
| 26 |of N. J., Marion | |Grate | 600 |
|______|___________________________|________________|_________|________|
| |Erie County Electric Co., |Mercer County, |B&W.Chain| |
| 27 |Erie, Pa. |Pa. |Grate | 508 |
|______|___________________________|________________|_________|________|
| |Union Elec. Lt. & Pr. Co., |Mascouth, Ill. |B&W.Chain| |
| 28 |St. Louis, Mo. | |Grate | 508 |
|______|___________________________|________________|_________|________|
| |Union Elec. Lt. & Pr. Co., |St. Clair |B&W.Chain| |
| 29 |St. Louis, Mo. |County, Ill. |Grate | 508 |
|______|___________________________|________________|_________|________|
| |Commonwealth Edison Co., |Carterville, |B&W.Chain| |
| 30 |Chicago, Ill. |Ill., Screenings|Grate | 508 |
|______|___________________________|________________|_________|________|
________________________________________________________________
|Number|Grate |Dura-|Steam |Temper-|Degrees|Factor| Draft |
| of |Surf. | tion|Pres. | ature | Super | of | In | At |
| Test |Square|Test | By | Water | -heat |Evapo-|Furnace|Boiler|
| | Feet |Hours|Gauge |Degrees|Degrees|ration|Inches |Damper|
| | | |Pounds| Fahr. | Fahr. | |Upr/Lwr|Inches|
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 1 | 84 | 8 | 68 | 53.9 | |1.1965| +.41 | .21 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | +.65 | |
| 2 | 51.6 | 7 | 136.3| 203 | 150 |1.1480| .47 | .56 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 3 | 67.6 | 8 | 139 | 139.6 | 139 |1.1984| .70 | .96 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 4 | 40 | 8 | 110.2| 137 | |1.1185| .33 | .43 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 5 | 59.5 | 10 | 133.2| 75.2 | |1.1849| .19 | .40 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 6 | 26.5 | 18 | 132.1| 70.5 | |1.1897| .33 | |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | +.51 | |
| 7 | 48.9 | 7 | 101. | 121.3 | |1.1333| -.20 | .30 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 8 | 59.5 | 6 | 191.8| 88.3 | |1.1771| .50 | |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 9 | 27 | 24 | 156.3| 58 | |1.2051| .10 | .24 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 10 | | 24 | 145 | 75 | |1.1866| .26 | .46 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 11 | 118 | 8 | 170 | 186.1 | 66.7 |1.1162| .34 | .42 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 12 | 118 | 7.92| 173 | 180.2 | 75.2 |1.1276| .44 | .58 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 13 | 52 | 24 | 75 | 53.3 | |1.1987| .25 | .35 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 14 | 59.5 | 8 | 105 | 35.8 | |1.2219| .17 | .38 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 15 | 59.5 | 24 | 170.1| 133 | |1.1293| .12 | .43 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 16 | 51.3 | 10 | 105 | 75.1 | |1.1814| .21 | .42 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 17 | 59.5 | 8 | 149.2| 71 | |1.1910| .35 | .59 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 18 | 103.2| 10 | 133.2| 65.3 | 65.9 |1.2346| .05 | .49 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 19 | 103.2| 9 | 139 | 64 | 80.2 |1.2358| .18 | .57 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 20 | 53 | 9 | 125 | 76.6 | |1.1826| +1.64 | .64 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 21 | 75 | 8 | 198.5| 165.1 | 104 |1.1662| +3.05 | .60 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 22 | | 9 | 140 | 67 | 180 |1.2942| .22 | .35 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 23 | 90 |16.25| 199 | 48.4 | 136.5 |1.2996| .23 | 1.27 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 24 | 103 | 8 | 129 | 56 | |1.2002| .23 | .30 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | +.52 | |
| 25 | 132 | 8 | 200 | 57.2 | 280.4 |1.3909| +.19 | .52 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | +.15 | |
| 26 | 132 | 8 | 199 | 60.7 | 171.0 |1.3191| .04 | .52 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 27 | 90 | 8 | 120 | 69.9 | |1.1888| .31 | .58 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 28 | 103.5| 8 | 180 | 46 | 113 |1.2871| .62 | 1.24 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 29 | 103.5| 8 | 183 | 53.1 | 104 |1.2725| .60 | 1.26 |
|______|______|_____|______|_______|_______|______|_______|______|
| | | | | | | | | |
| 30 | 90 | 7 | 184 | 127.1 | 180 |1.2393| .68 | 1.15 |
|______|______|_____|______|_______|_______|______|_______|______|
______________________________________________________________
|Number|Temper-| Coal |
| of | ature | Total | Moist-| Total |Ash and| Total |DryCoal|
| Test |FlueGas|Weight:| ure | dry | Refuse|Combus-|/sq.ft.|
| |Degrees| Fired | Per | Coal | Per | tible | Grate |
| | Fahr. |Pounds | Cent | Pounds| Cent | Pounds|/Hr.Lb.|
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 1 | | 11670 | 4.45 | 11151 | 26.05 | 8248 | 16.6 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 2 | 487 | 8800 | 7.62 | 8129 | 29.82 | 5705 | 19.71 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 3 | 559 | 10799 | 6.42 | 10106 | 20.02 | 8081 | 21.77 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 4 | 427 | 5088 | 4.00 | 4884 | 19.35 | 3939 | 15.26 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 5 | 414 | 9440 | 2.14 | 9238 | 11.19 | 8204 | 15.52 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 6 | 410 | 8555 | 3.62 | 8245 | 15.73 | 6948 | 17.28 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 7 | 480 | 7130 | 7.38 | 6604 | 18.35 | 5392 | 19.29 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 8 | 449 | 7500 | 2.70 | 7298 | 27.94 | 5259 | 14.73 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 9 | 410 | 15087 | 7.50 | 13956 | 11.30 | 12379 | 21.5 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 10 | 503 | 29528 | 7.72 | 27248 | | | 24.7 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 11 | 487 | 20400 | 2.84 | 19821 | 7.83 | 18269 | 21.00 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 12 | 494 | 21332 | 2.29 | 20843 | 8.23 | 19127 | 22.31 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 13 | 516 | 24584 | 4.29 | 23529 | 7.63 | 21883 | 18.85 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 14 | 523 | 15540 | 18.64 | 12643 | | | 28.59 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 15 | 474 | 18330 | 2.03 | 17958 | 16.36 | 16096 | 12.57 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 16 | 536 | 12243 | 2.14 | 11981 | | | 23.40 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 17 | 534 | 10500 | 3.04 | 10181 | | | 21.40 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 18 | 458 | 18600 | 3.40 | 17968 | 18.38 | 14665 | 17.41 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 19 | 609 | 23400 | 2.56 | 22801 | 16.89 | 18951 | 24.55 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 20 | 518 | 10500 | 1.83 | 10308 | 12.22 | 9048 | 21.56 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 21 | 536 | 25296 | 2.20 | 24736 | | | 41.0 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 22 | 511 | 14263 | 8.63 | 13032 | | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 23 | 560 | 39670 | 4.22 | 37996 | 4.32 | 36355 | 25.98 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 24 | 538 | 23000 | 23.73 | 17542 | | | 21.36 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 25 | 590 | 32205 | 4.03 | 30907 | 15.65 | 26070 | 29.26 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 26 | 529 | 24243 | 4.09 | 23251 | 12.33 | 20385 | 22.01 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 27 | 533 | 22328 | 4.42 | 21341 | 16.88 | 17739 | 29.64 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 28 | 523 | 32163 | 13.74 | 27744 | | | 33.50 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 29 | 567 | 36150 | 14.62 | 30865 | | | 37.28 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 30 | | 30610 | 11.12 | 27206 | 14.70 | 23198 | 43.20 |
|______|_______|_______|_______|_______|_______|_______|_______|
______________________________________________________________
|Number| Water | | Flue Gas Analysis |
| of |Actual | Equiv.|ditto /|% Rated|CO_{2} | O | CO |
| Test |Evapor-|Evap. @|sq.ft. |Cap'ty.| Per | Per | Per |
| | ation |>=212° |Heating|Develpd| Cent | Cent | Cent |
| |/Hr.Lb.|/Hr.Lb.|Surface|PerCent| | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 1 | 10268 | 12286 | 4.10 | 118.7 | | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 2 | 8246 | 9466 | 4.34 | 125.7 | | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 3 | 9145 | 10959 | 3.65 | 105.9 | | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 4 | 5006 | 5599 | 3.61 | 104.7 | 12.26 | 7.88 | 0.0 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 5 | 7434 | 8809 | 3.06 | 87.2 | | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 6 | 2903 | 3454 | 2.91 | 84.4 | | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 7 | 7464 | 8459 | 2.82 | 81.7 | | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 8 | 9164 | 10787 | 2.88 | 83.5 | | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 9 | 4374 | 5271 | 3.51 | 101.8 | 11.7 | 7.3 | 0.07 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 10 | 8688 | 10309 | 3.44 | 99.6 | 12.9 | 5.0 | 0.2 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 11 | 24036 | 26829 | 4.19 | 121.5 | 12.5 | 6.4 | 0.5 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 12 | 25313 | 28544 | 4.46 | 129.3 | 13.3 | 5.1 | 0.5 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 13 | 9168 | 10990 | 3.42 | 99.3 | 9.6 | 8.8 | 0.4 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 14 | 11202 | 13689 | 3.91 | 113.5 | 9.1 | 9.9 | 0.0 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 15 | 7565 | 8543 | 3.21 | 93.1 | 10.71 | 9.10 | 0.0 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 16 | 9512 | 11237 | 3.74 | 108.6 | | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 17 | 9257 | 11025 | 3.70 | 107.2 | | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 18 | 15887 | 19614 | 3.77 | 108.7 | 11.7 | 7.7 | 0.0 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 19 | 21320 | 26347 | 5.06 | 146.7 | 11.9 | 7.8 | 0.0 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 20 | 9976 | 11978 | 3.93 | 112.0 | 11.3 | 7.5 | 0.0 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 21 | 28451 | 33066 | 5.47 | 158.6 | 12.3 | 6.4 | 0.7 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 22 | 10467 | 13526 | 4.51 | 130.7 | 11.9 | 7.2 | 0.04 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 23 | 20700 | 26902 | 5.30 | 153.5 | 11.1 | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 24 | 14650 | 17583 | 4.40 | 127.4 | | | |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 25 | 28906 | 40205 | 6.70 | 194.2 | 10.5 | 8.3 | 0.0 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 26 | 23074 | 30437 | 5.07 | 147.0 | 10.1 | 9.0 | 0.0 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 27 | 20759 | 24678 | 4.85 | 140.8 | 10.1 | 9.1 | 0.0 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 28 | 21998 | 28314 | 5.67 | 161.5 | 8.7 | 10.6 | 0.0 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 29 | 24386 | 31031 | 6.11 | 177.1 | 8.9 | 10.7 | 0.2 |
|______|_______|_______|_______|_______|_______|_______|_______|
| | | | | | | | |
| 30 | 30505 | 37805 | 7.43 | 215.7 | 10.4 | 9.4 | 0.2 |
|______|_______|_______|_______|_______|_______|_______|_______|
_______________________________________________________
|Number| Proximate Analysis Dry Coal | Equiv.|Combnd.|
| of |Volatl.| Fixed | Ash |B.t.u./|Evap. @|Efficy.|
| Test |Matter |Carbon | Per | Pound |>=212°/|Boiler |
| | Per | Per | Cent | Dry | Pound |& Grate|
| | Cent | Cent | | Coal |DryCoal|PerCent|
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 1 | | | 26.05 | 11913 | 8.81 | 71.8 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 2 | | | | 11104 | 8.15 | 72.1 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 3 | 5.55 | 80.60 | 13.87 | 12300 | 8.67 | 68.4 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 4 | 7.74 | 77.48 | 14.78 | 12851 | 9.17 | 69.2 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 5 | | | | 13138 | 9.53 | 69.6 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 6 | 6.13 | 84.86 | 9.01 | 13454 | 9.57 | 69.0 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 7 | | | | 12224 | 8.97 | 71.2 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 8 | 0.55 | 86.73 | 12.72 | 12642 | 8.87 | 68.1 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 9 | 39.01 | 48.08 | 12.91 | 12292 | 9.06 | 71.5 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 10 | 38.33 | 46.71 | 14.96 | 12284 | 9.08 | 71.7 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 11 | 19.86 | 73.02 | 7.12 | 14602 | 10.83 | 72.0 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 12 | 20.24 | 72.26 | 7.50 | 14381 | 10.84 | 73.2 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 13 | | | | 14955 | 11.21 | 72.7 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 14 | 39.60 | 54.46 | 5.94 | 11585 | 8.66 | 72.5 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 15 | 17.44 | 76.42 | 5.84 | 15379 | 11.42 | 72.1 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 16 | 33.40 | 54.73 | 11.87 | 12751 | 9.38 | 71.4 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 17 | 15.42 | 62.48 | 22.10 | 12060 | 8.66 | 69.6 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 18 | 14.99 | 75.13 | 9.88 | 14152 | 10.92 | 74.88 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 19 | 14.40 | 74.33 | 11.27 | 14022 | 10.40 | 71.97 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 20 | 32.44 | 56.71 | 10.85 | 13510 | 10.30 | 74.6 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 21 | 19.02 | 72.09 | 8.89 | 14105 | 10.69 | 73.5 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 22 | 32.11 | 53.93 | 13.96 | 12435 | 9.41 | 73.4 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 23 | 19.66 | 75.41 | 4.93 | 14910 | 11.51 | 74.9 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 24 | 43.57 | 46.22 | 10.21 | 11160 | 8.02 | 69.7 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 25 | 22.84 | 69.91 | 7.25 | 13840 | 10.41 | 72.6 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 26 | 32.36 | 60.67 | 6.97 | 14027 | 10.47 | 72.1 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 27 | 33.26 | 54.03 | 12.71 | 12742 | 9.25 | 70.4 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 28 | 28.96 | 46.88 | 24.16 | 10576 | 8.16 | 74.9 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 29 | 36.50 | 41.20 | 22.30 | 10849 | 8.04 | 71.9 |
|______|_______|_______|_______|_______|_______|_______|
| | | | | | | |
| 30 | | | 10.24 | 13126 | 9.73 | 71.9 |
|______|_______|_______|_______|_______|_______|_______|
[Illustration: 15400 Horse-power Installation of Babcock & Wilcox
Boilers and Superheaters, Equipped with Babcock & Wilcox Chain Grate
Stokers at the Plant of the Twin City Rapid Transit Co., Minneapolis,
Minn.]
This increase in the efficiency of the boiler alone with the decrease in
the rate at which it is operated, will hold to a point where the
radiation of heat from the boiler setting is proportionately large
enough to be a governing factor in the total amount of heat absorbed.
The second reason given above for a decrease of boiler efficiency with
increase of capacity, viz., the effect of radiant heat, is to a greater
extent than the first reason dependent upon a constant furnace
temperature. Any increase in this temperature will affect enormously the
amount of heat absorbed by radiation, as this absorption will vary as
the fourth power of the temperature of the radiating body. In this way
it is seen that but a slight increase in furnace temperature will be
necessary to bring the proportional part, due to absorption by
radiation, of the total heat absorbed, up to its proper proportion at
the higher ratings. This factor of furnace temperature more properly
belongs to the consideration of furnace efficiency than of boiler
efficiency. There is a point, however, in any furnace above which the
combustion will be so poor as to actually reduce the furnace temperature
and, therefore, the proportion of heat absorbed through radiation by a
given amount of exposed heating surface.
Since it is thus true that the efficiency of the boiler considered alone
will increase with a decreased capacity, it is evident that if the
furnace conditions are constant regardless of the load, that the
combined efficiency of boiler and furnace will also decrease with
increasing loads. This fact was clearly proven in the tests of the
boilers at the Detroit Edison Company.[74] The furnace arrangement of
these boilers and the great care with which the tests were run made it
possible to secure uniformly good furnace conditions irrespective of
load, and here the maximum efficiency was obtained at a point somewhat
less than the rated capacity of the boilers.
In some cases, however, and especially in the ordinary operation of the
plant, the furnace efficiency will, up to a certain point, increase with
an increase in power. This increase in furnace efficiency is ordinarily
at a greater rate as the capacity increases than is the decrease in
boiler efficiency, with the result that the combined efficiency of
boiler and furnace will to a certain point increase with an increase in
capacity. This makes the ordinary point of maximum combined efficiency
somewhat above the rated capacity of the boiler and in many cases the
combined efficiency will be practically a constant over a considerable
range of ratings. The features limiting the establishing of the point of
maximum efficiency at a high rating are the same as those limiting the
amount of grate surface that can be installed under a boiler. The
relative efficiency of different combinations of boilers and furnaces at
different ratings depends so largely upon the furnace conditions that
what might hold for one combination would not for another.
In view of the above, it is impossible to make a statement of the
efficiency at different capacities of a boiler and furnace which will
hold for any and all conditions. Fig. 40 shows in a general form the
relation of efficiency to capacity. This curve has been plotted from a
great number of tests, all of which were corrected to bring them to
approximately the same conditions. The curve represents test conditions.
The efficiencies represented are those which may be secured only under
such conditions. The general direction of the curve, however, will be
found to hold approximately correct for operating conditions when used
only as a guide to what may be expected.
[Graph: Combined Efficiency of Boiler and Furnace Per Cent
against Per Cent of Boiler's Rated Capacity Developed
Fig. 40. Approximate Variation of Efficiency with Capacity under Test
Conditions]
Economical Loads--With the effect of capacity on economy in mind, the
question arises as to what constitutes the economical load to be
carried. In figuring on the economical load for an individual plant, the
broader economy is to be considered, that in which, against the boiler
efficiency, there is to be weighed the plant first cost, returns on such
investment, fuel cost, labor, capacity, etc., etc. This matter has been
widely discussed, but unfortunately such discussion has been largely
limited to central power station practice. The power generated in such
stations, while representing an enormous total, is by no means the
larger proportion of the total power generated throughout the country.
The factors determining the economic load for the small plant, however,
are the same as in a large, and in general the statements made relative
to the question are equally applicable.
The economical rating at which a boiler plant should be run is dependent
solely upon the load to be carried by that individual plant and the
nature of such load. The economical load for each individual plant can
be determined only from the careful study of each individual set of
conditions or by actual trial.
The controlling factor in the cost of the plant, regardless of the
nature of the load, is the capacity to carry the maximum peak load that
may be thrown on the plant under any conditions.
While load conditions, do, as stated, vary in every individual plant, in
a broad sense all loads may be grouped in three classes: 1st, the
approximately constant 24-hour load; 2nd, the steady 10 or 12-hour load
usually with a noonday period of no load; 3rd, the 24-hour variable
load, found in central station practice. The economical load at which
the boiler may be run will vary with these groups:
1st. For a constant load, 24 hours in the day, it will be found in most
cases that, when all features are considered, the most economical load
or that at which a given amount of steam can be produced the most
cheaply will be considerably over the rated horse power of the boiler.
How much above the rated capacity this most economic load will be, is
dependent largely upon the cost of coal at the plant, but under ordinary
conditions, the point of maximum economy will probably be found to be
somewhere between 25 and 50 per cent above the rated capacity of the
boilers. The capital investment must be weighed against the coal saving
through increased thermal efficiency and the labor account, which
increases with the number of units, must be given proper consideration.
When the question is considered in connection with a plant already
installed, the conditions are different from where a new plant is
contemplated. In an old plant, where there are enough boilers to operate
at low rates of capacity, the capital investment leads to a fixed
charge, and it will be found that the most economical load at which
boilers may be operated will be lower than where a new plant is under
consideration.
2nd. For a load of 10 or 12 hours a day, either an approximately steady
load or one in which there is a peak, where the boilers have been banked
over night, the capacity at which they may be run with the best economy
will be found to be higher than for uniform 24-hour load conditions.
This is obviously due to original investment, that is, a given amount of
invested capital can be made to earn a larger return through the higher
overload, and this will hold true to a point where the added return more
than offsets the decrease in actual boiler efficiency. Here again the
determining factors of what is the economical load are the fuel and
labor cost balanced against the thermal efficiency. With a load of this
character, there is another factor which may affect the economical plant
operating load. This is from the viewpoint of spare boilers. That such
added capacity in the way of spares is necessary is unquestionable.
Since they must be installed, therefore, their presence leads to a fixed
charge and it is probable that for the plant, as a whole, the economical
load will be somewhat lower than if the boilers were considered only as
spares. That is, it may be found best to operate these spares as a part
of the regular equipment at all times except when other boilers are off
for cleaning and repairs, thus reducing the load on the individual
boilers and increasing the efficiency. Under such conditions, the added
boiler units can be considered as spares only during such time as some
of the boilers are not in operation.
Due to the operating difficulties that may be encountered at the higher
overloads, it will ordinarily be found that the most economical ratings
at which to run boilers for such load conditions will be between 150 and
175 per cent of rating. Here again the maximum capacity at which the
boilers may be run for the best plant economy is limited by the point at
which the efficiency drops below what is warranted in view of the first
cost of the apparatus.
3rd. The 24-hour variable load. This is a class of load carried by the
central power station, a load constant only in the sense that there are
no periods of no load and which varies widely with different portions of
the 24 hours. With such a load it is particularly difficult to make any
assertion as to the point of maximum economy that will hold for any
station, as this point is more than with any other class of load
dependent upon the factors entering into the operation of each
individual plant.
The methods of handling a load of this description vary probably more
than with any other kind of load, dependent upon fuel, labor, type of
stoker, flexibility of combined furnace and boiler etc., etc.
In general, under ordinary conditions such as appear in city central
power station work where the maximum peaks occur but a few times a year,
the plant should be made of such size as to enable it to carry these
peaks at the maximum possible overload on the boilers, sufficient margin
of course being allowed for insurance against interruption of service.
With the boilers operating at this maximum overload through the peaks a
large sacrifice in boiler efficiency is allowable, provided that by such
sacrifice the overload expected is secured.
[Illustration: Portion of 4890 Horse-power Installation of Babcock &
Wilcox Boilers at the Billings Sugar Co., Billings, Mont. 694 Horse
Power of these Boilers are Equipped with Babcock and Wilcox Chain Grate
Stokers]
Some methods of handling a load of this nature are given below:
Certain plant operating conditions make it advisable, from the
standpoint of plant economy, to carry whatever load is on the plant at
any time on only such boilers as will furnish the power required when
operating at ratings of, say, 150 to 200 per cent. That is, all boilers
which are in service are operated at such ratings at all times, the
variation in load being taken care of by the number of boilers on the
line. Banked boilers are cut in to take care of increasing loads and
peaks and placed again on bank when the peak periods have passed. It is
probable that this method of handling central station load is to-day the
most generally used.
Other conditions of operation make it advisable to carry the load on a
definite number of boiler units, operating these at slightly below their
rated capacity during periods of light or low loads and securing the
overload capacity during peaks by operating the same boilers at high
ratings. In this method there are no boilers kept on banked fires, the
spares being spares in every sense of the word.
A third method of handling widely varying loads which is coming somewhat
into vogue is that of considering the plant as divided, one part to take
care of what may be considered the constant plant load, the other to
take care of the floating or variable load. With such a method that
portion of the plant carrying the steady load is so proportioned that
the boilers may be operated at the point of maximum efficiency, this
point being raised to a maximum through the use of economizers and the
general installation of any apparatus leading to such results. The
variable load will be carried on the remaining boilers of the plant
under either of the methods just given, that is, at the high ratings of
all boilers in service and banking others, or a variable capacity from
all boilers in service.
The opportunity is again taken to indicate the very general character of
any statements made relative to the economical load for any plant and to
emphasize the fact that each individual case must be considered
independently, with the conditions of operations applicable thereto.
With a thorough understanding of the meaning of boiler efficiency and
capacity and their relation to each other, it is possible to consider
more specifically the selection of boilers.
The foremost consideration is, without question, the adaptability of the
design selected to the nature of the work to be done. An installation
which is only temporary in its nature would obviously not warrant the
first cost that a permanent plant would. If boilers are to carry an
intermittent and suddenly fluctuating load, such as a hoisting load or a
reversing mill load, a design would have to be selected that would not
tend to prime with the fluctuations and sudden demand for steam. A
boiler that would give the highest possible efficiency with fuel of one
description, would not of necessity give such efficiency with a
different fuel. A boiler of a certain design which might be good for
small plant practice would not, because of the limitations in
practicable size of units, be suitable for large installations. A
discussion of the relative value of designs can be carried on almost
indefinitely but enough has been said to indicate that a given design
will not serve satisfactorily under all conditions and that the
adaptability to the service required will be dependent upon the fuel
available, the class of labor procurable, the feed water that must be
used, the nature of the plant's load, the size of the plant and the
first cost warranted by the service the boiler is to fulfill.
TABLE 60
ACTUAL EVAPORATION FOR DIFFERENT PRESSURES AND TEMPERATURES OF FEED
WATER CORRESPONDING TO ONE HORSE POWER (34½ POUNDS PER HOUR FROM AND AT 212 DEGREES FAHRENHEIT)
-----------------------------------------------------------------------------------------------------------------------------------------
Temperature| |
of | Pressure by Gauge--Pounds per Square Inch |
Feed | |
Degrees | |
Fahrenheit | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 210 | 220 | 230 | 240 | 250 |
-----------+-----------------------------------------------------------------------------------------------------------------------------|
32 |28.41|28.36|28.29|28.24|28.20|28.16|28.13|28.09|28.07|28.04|28.02|27.99|27.97|27.95|27.94|27.92|27.90|27.89|27.87|27.86|27.83|
40 |28.61|28.54|28.49|28.44|28.40|28.35|28.32|28.29|28.26|28.23|28.21|28.18|28.16|28.14|28.12|28.11|28.09|28.07|28.06|28.05|28.03|
50 |28.85|28.79|28.73|28.68|28.64|28.60|28.56|28.53|28.50|28.47|28.45|28.43|28.40|28.38|28.36|28.35|28.33|28.31|28.30|28.28|28.27|
60 |29.10|29.04|28.98|28.93|28.88|28.84|28.81|28.77|28.74|28.72|28.69|28.67|28.65|28.62|28.60|28.59|28.57|28.55|28.54|28.52|28.51|
70 |29.36|29.29|29.23|29.18|29.14|29.09|29.06|29.02|28.99|28.96|28.94|28.92|28.89|28.87|28.85|28.83|28.82|28.80|28.78|28.77|28.76|
80 |29.62|29.55|29.49|29.44|29.39|29.35|29.31|29.27|29.24|29.22|29.19|29.17|29.14|29.12|29.10|29.08|29.07|29.05|29.03|29.02|29.00|
90 |29.88|29.81|29.75|29.70|29.65|29.61|29.57|29.53|29.50|29.47|29.45|29.42|29.40|29.38|29.36|29.34|29.32|29.30|29.29|29.27|29.25|
100 |30.15|30.08|30.02|29.96|29.91|29.87|29.83|29.80|29.76|29.73|29.71|29.68|29.66|29.63|29.61|29.60|29.58|29.56|29.54|29.53|29.51|
110 |30.42|30.35|30.29|30.23|30.18|30.14|30.10|30.06|30.03|30.00|29.97|29.95|29.92|29.90|29.88|29.86|29.84|29.82|29.81|29.79|29.77|
120 |30.70|30.63|30.56|30.51|30.46|30.41|30.37|30.33|30.30|30.27|30.24|30.22|30.19|30.17|30.15|30.13|30.11|30.09|30.07|30.06|30.04|
130 |30.99|30.91|30.84|30.79|30.73|30.69|30.65|30.61|30.57|30.54|30.52|30.49|30.47|30.44|30.42|30.40|30.38|30.36|30.35|30.33|30.31|
140 |31.28|31.20|31.13|31.07|31.02|30.97|30.93|30.89|30.86|30.83|30.80|30.77|30.75|30.72|30.70|30.68|30.66|30.64|30.62|30.61|30.59|
150 |31.58|31.49|31.42|31.36|31.31|31.26|31.22|31.18|31.14|31.11|31.08|31.06|31.03|31.01|30.98|30.96|30.94|30.92|30.91|30.89|30.87|
160 |31.87|31.79|31.72|31.66|31.61|31.56|31.51|31.47|31.44|31.40|31.37|31.35|31.32|31.29|31.27|31.25|31.23|31.21|31.19|31.18|31.16|
170 |32.18|32.10|32.02|31.96|31.91|31.86|31.81|31.77|31.73|31.70|31.67|31.64|31.62|31.59|31.57|31.54|31.52|31.50|31.49|31.47|31.46|
180 |32.49|32.41|32.33|32.27|32.22|32.16|32.12|32.08|32.04|32.00|31.97|31.95|31.92|31.89|31.87|31.84|31.82|31.80|31.79|31.77|31.75|
190 |32.81|32.72|32.65|32.59|32.53|32.47|32.43|32.38|32.35|32.32|32.29|32.26|32.23|32.20|32.17|32.15|32.13|32.11|32.09|32.07|32.05|
200 |33.13|33.05|32.97|32.91|32.85|32.79|32.75|32.70|32.66|32.63|32.60|32.57|32.54|32.51|32.49|32.46|32.44|32.42|32.40|32.38|32.36|
210 |33.47|33.38|33.30|33.24|33.18|33.13|33.08|33.03|32.99|32.95|32.92|32.89|32.86|32.83|32.81|32.79|32.76|32.74|32.72|32.70|32.68|
-----------------------------------------------------------------------------------------------------------------------------------------
The proper consideration can be given to the adaptability of any boiler
for the service in view only after a thorough understanding of the
requirements of a good steam boiler, with the application of what has
been said on the proper operation to the special requirements of each
case. Of almost equal importance to the factors mentioned are the
experience, the skill and responsibility of the manufacturer.
With the design of boiler selected that is best adapted to the service
required, the next step is the determination of the boiler power
requirements.
The amount of steam that must be generated is determined from the steam
consumption of the prime movers. It has already been indicated that such
consumption can vary over wide limits with the size and type of the
apparatus used, but fortunately all types have been so tested that
manufacturers are enabled to state within very close limits the actual
consumption under any given set of conditions. It is obvious that
conditions of operation will have a bearing on the steam consumption
that is as important as the type and size of the apparatus itself. This
being the case, any tabular information that can be given on such steam
consumption, unless it be extended to an impracticable size, is only of
use for the most approximate work and more definite figures on this
consumption should in all cases be obtained from the manufacturer of the
apparatus to be used for the conditions under which it will operate.
To the steam consumption of the main prime movers, there is to be added
that of the auxiliaries. Again it is impossible to make a definite
statement of what this allowance should be, the figure depending wholly
upon the type and the number of such auxiliaries. For approximate work,
it is perhaps best to allow 15 or 20 per cent of the steam requirements
of the main engines, for that of auxiliaries. Whatever figure is used
should be taken high enough to be on the conservative side.
When any such figures are based on the actual weight of steam required,
Table 60, which gives the actual evaporation for various pressures and
temperatures of feed corresponding to one boiler horse power (34.5
pounds of water per hour from and at 212 degrees), may be of service.
With the steam requirements known, the next step is the determination of
the number and size of boiler units to be installed. This is directly
affected by the capacity at which a consideration of the economical load
indicates is the best for the operating conditions which will exist. The
other factors entering into such determination are the size of the plant
and the character of the feed water.
The size of the plant has its bearing on the question from the fact that
higher efficiencies are in general obtained from large units, that labor
cost decreases with the number of units, the first cost of brickwork is
lower for large than for small size units, a general decrease in the
complication of piping, etc., and in general the cost per horse power of
any design of boiler decreases with the size of units. To illustrate
this, it is only necessary to consider a plant of, say, 10,000 boiler
horse power, consisting of 40-250 horse-power units or 17-600
horse-power units.
The feed water available has its bearing on the subject from the other
side, for it has already been shown that very large units are not
advisable where the feed water is not of the best.
The character of an installment is also a factor. Where, say, 1000 horse
power is installed in a plant where it is known what the ultimate
capacity is to be, the size of units should be selected with the idea of
this ultimate capacity in mind rather than the amount of the first
installation.
Boiler service, from its nature, is severe. All boilers have to be
cleaned from time to time and certain repairs to settings, etc., are a
necessity. This makes it necessary, in determining the number of boilers
to be installed, to allow a certain number of units or spares to be
operated when any of the regular boilers must be taken off the line.
With the steam requirements determined for a plant of moderate size and
a reasonably constant load, it is highly advisable to install at least
two spare boilers where a continuity of service is essential. This
permits the taking off of one boiler for cleaning or repairs and still
allows a spare boiler in the event of some unforeseen occurrence, such
as the blowing out of a tube or the like. Investment in such spare
apparatus is nothing more nor less than insurance on the necessary
continuity of service. In small plants of, say, 500 or 600 horse power,
two spares are not usually warranted in view of the cost of such
insurance. A large plant is ordinarily laid out in a number of sections
or panels and each section should have its spare boiler or boilers even
though the sections are cross connected. In central station work, where
the peaks are carried on the boilers brought up from the bank, such
spares are, of course, in addition to these banked boilers. From the
aspect of cleaning boilers alone, the number of spare boilers is
determined by the nature of any scale that may be formed. If scale is
formed so rapidly that the boilers cannot be kept clean enough for good
operating results, by cleaning in rotation, one at a time, the number of
spares to take care of such proper cleaning will naturally increase.
In view of the above, it is evident that only a suggestion can be made
as to the number and size of units, as no recommendation will hold for
all cases. In general, it will be found best to install units of the
largest possible size compatible with the size of the plant and
operating conditions, with the total power requirements divided among
such a number of units as will give proper flexibility of load, with
such additional units for spares as conditions of cleaning and insurance
against interruption of service warrant.
In closing the subject of the selection of boilers, it may not be out of
place to refer to the effect of the builder's guarantee upon the
determination of design to be used. Here in one of its most important
aspects appears the responsibility of the manufacturer. Emphasis has
been laid on the difference between test results and those secured in
ordinary operating practice. That such a difference exists is well known
and it is now pretty generally realized that it is the responsible
manufacturer who, where guarantees are necessary, submits the
conservative figures, figures which may readily be exceeded under test
conditions and which may be closely approached under the ordinary plant
conditions that will be met in daily operation.
OPERATION AND CARE OF BOILERS
The general subject of boiler room practice may be considered from two
aspects. The first is that of the broad plant economy, with a suggestion
as to the methods to be followed in securing the best economical results
with the apparatus at hand and procurable. The second deals rather with
specific recommendations which should be followed in plant practice,
recommendations leading not only to economy but also to safety and
continuity of service. Such recommendations are dictated from an
understanding of the nature of steam generating apparatus and its
operation, as covered previously in this book.
It has already been pointed out that the attention given in recent years
to steam generating practice has come with a realization of the wide
difference existing between the results being obtained in every-day
operation and those theoretically possible. The amount of such attention
and regulation given to the steam generating end of a power plant,
however, is comparatively small in relation to that given to the balance
of the plant, but it may be safely stated that it is here that there is
the greatest assurance of a return for the attention given.
In the endeavor to increase boiler room efficiency, it is of the utmost
importance that a standard basis be set by which average results are to
be judged. With the theoretical efficiency obtainable varying so widely,
this standard cannot be placed at the highest efficiency that has been
obtained regardless of operating conditions. It is better set at the
best obtainable results for each individual plant under its conditions
of installation and daily operation.
With an individual standard so set, present practice can only be
improved by a systematic effort to approach this standard. The degree
with which operating results will approximate such a standard will be
found to be directly proportional to the amount of intelligent
supervision given the operation. For such supervision to be given, it is
necessary to have not only a full realization of what the plant can do
under the best operating conditions but also a full and complete
knowledge of what it is doing under all of the different conditions that
may arise. What the plant is doing should be made a matter of continuous
record so arranged that the results may be directly compared for any
period or set of conditions, and where such results vary from the
standard set, steps must be taken immediately to remedy the causes of
such failings. Such a record is an important check in the losses in the
plant.
As the size of the plant and the fuel consumption increase, such a check
of losses and recording of results becomes a necessity. In the larger
plants, the saving of but a fraction of one per cent in the fuel bill
represents an amount running into thousands of dollars annually, while
the expense of the proper supervision to secure such saving is small.
The methods of supervision followed in the large plants are necessarily
elaborate and complete. In the smaller plants the same methods may be
followed on a more moderate scale with a corresponding saving in fuel
and an inappreciable increase in either plant organization or expense.
There has been within the last few years a great increase in the
practicability and reliability of the various types of apparatus by
which the records of plant operation may be secured. Much of this
apparatus is ingenious and, considering the work to be done, is
remarkably accurate. From the delicate nature of some of the apparatus,
the liability to error necessitates frequent calibration but even where
the accuracy is known to be only within limits of, say, 5 per cent
either way, the records obtained are of the greatest service in
considering relative results. Some of the records desirable and the
apparatus for securing them are given below.
[Illustration: 2400 Horse-power Installation of Cross Drum Babcock &
Wilcox Boilers and Superheaters at the Westinghouse Electric and
Manufacturing Co., East Pittsburgh, Pa.]
Inasmuch as the ultimate measure of the efficiency of the boiler plant
is the cost of steam generation, the important records are those of
steam generated and fuel consumed Records of temperature, analyses,
draft and the like, serve as a check on this consumption, indicating the
distribution of the losses and affording a means of remedying conditions
where improvement is possible.
Coal Records--There are many devices on the market for conveniently
weighing the coal used. These are ordinarily accurate within close
limits, and where the size or nature of the plant warrants the
investment in such a device, its use is to be recommended. The coal
consumption should be recorded by some other method than from the
weights of coal purchased. The total weight gives no way of dividing the
consumption into periods and it will unquestionably be found to be
profitable to put into operation some scheme by which the coal is
weighed as it is used. In this way, the coal consumption, during any
specific period of the plant's operation, can be readily seen. The
simplest of such methods which may be used in small plants is the actual
weighing on scales of the fuel as it is brought into the fire room and
the recording of such weights.
Aside from the actual weight of the fuel used, it is often advisable to
keep other coal records, coal and ash analyses and the like, for the
evaporation to be expected will be dependent upon the grade of fuel used
and its calorific value, fusibility of its ash, and like factors.
The highest calorific value for unit cost is not necessarily the
indication of the best commercial results. The cost of fuel is governed
by this calorific value only when such value is modified by local
conditions of capacity, labor and commercial efficiency. One of the
important factors entering into fuel cost is the consideration of the
cost of ash handling and the maintenance of ash handling apparatus if
such be installed. The value of a fuel, regardless of its calorific
value, is to be based only on the results obtained in every-day plant
operation.
Coal and ash analyses used in connection with the amount of fuel
consumed, are a direct indication of the relation between the results
being secured and the standard of results which has been set for the
plant. The methods of such analyses have already been described. The
apparatus is simple and the degree of scientific knowledge necessary is
only such as may be readily mastered by plant operatives.
The ash content of a fuel, as indicated from a coal analysis checked
against ash weights as actually found in plant operation, acts as a
check on grate efficiency. The effect of any saving in the ashes, that
is, the permissible ash to be allowed in the fuel purchased, is
determined by the point at which the cost of handling, combined with the
falling off in the evaporation, exceeds the saving of fuel cost through
the use of poorer coal.
Water Records--Water records with the coal consumption, form the basis
for judging the economic production of steam. The methods of securing
such records are of later introduction than for coal, but great advances
have been made in the apparatus to be used. Here possibly, to a greater
extent than in any recording device, are the records of value in
determining relative evaporation, that is, an error is rather allowable
provided such an error be reasonably constant.
The apparatus for recording such evaporation is of two general classes:
Those measuring water before it is fed to the boiler and those measuring
the steam as it leaves. Of the first, the venturi meter is perhaps the
best known, though recently there has come into considerable vogue an
apparatus utilizing a weir notch for the measuring of such water. Both
methods are reasonably accurate and apparatus of this description has an
advantage over one measuring steam in that it may be calibrated much
more readily. Of the steam measuring devices, the one in most common use
is the steam flow meter. Provided the instruments are selected for a
proper flow, etc., they are of inestimable value in indicating the steam
consumption. Where such instruments are placed on the various engine
room lines, they will immediately indicate an excessive consumption for
any one of the units. With a steam flow meter placed on each boiler, it
is possible to fix relatively the amount produced by each boiler and,
considered in connection with some of the "check" records described
below, clearly indicate whether its portion of the total steam produced
is up to the standard set for the over-all boiler room efficiency.
Flue Gas Analysis--The value of a flue gas analysis as a measure of
furnace efficiency has already been indicated. There are on the market a
number of instruments by which a continuous record of the carbon dioxide
in the flue gases may be secured and in general the results so recorded
are accurate. The limitations of an analysis showing only CO_{2} and the
necessity of completing such an analysis with an Orsat, or like
apparatus, and in this way checking the automatic device, have already
been pointed out, but where such records are properly checked from time
to time and are used in conjunction with a record of flue temperatures,
the losses due to excess air or incomplete combustion and the like may
be directly compared for any period. Such records act as a means for
controlling excess air and also as a check on individual firemen.
Where the size of a plant will not warrant the purchase of an expensive
continuous CO_{2} recorder, it is advisable to make analyses of samples
for various conditions of firing and to install an apparatus whereby a
sample of flue gas covering a period of, say, eight hours, may be
obtained and such a sample afterwards analyzed.
Temperature Records--Flue gas temperatures, feed water temperatures and
steam temperatures are all taken with recording thermometers, any number
of which will, when properly calibrated, give accurate results.
A record of flue temperatures is serviceable in checking stack losses
and, in general, the cleanliness of the boiler. A record of steam
temperatures, where superheaters are used, will indicate excessive
fluctuations and lead to an investigation of their cause. Feed
temperatures are valuable in showing that the full benefit of the
exhaust steam is being derived.
Draft Regulation--As the capacity of a boiler varies with the combustion
rate and this rate with the draft, an automatic apparatus satisfactorily
varying this draft with the capacity demands on the boiler will
obviously be advantageous.
As has been pointed out, any fuel has some rate of combustion at which
the best results will be obtained. In a properly designed plant where
the load is reasonably steady, the draft necessary to secure such a rate
may be regulated automatically.
Automatic apparatus for the regulation of draft has recently reached a
stage of perfection which in the larger plants at any rate makes its
installation advisable. The installation of a draft gauge or gauges is
strongly to be recommended and a record of such drafts should be kept as
being a check on the combustion rates.
An important feature to be considered in the installing of all recording
apparatus is its location. Thermometers, draft gauges and flue gas
sampling pipes should be so located as to give as nearly as possible an
average of the conditions, the gases flowing freely over the ends of the
thermometers, couples and sampling pipes. With the location permanent,
there is no security that the samples may be considered an average but
in any event comparative results will be secured which will be useful in
plant operation. The best permanent location of apparatus will vary
considerably with the design of the boiler.
It may not be out of place to refer briefly to some of the shortcomings
found in boiler room practice, with a suggestion as to a means of
overcoming them.
1st. It is sometimes found that the operating force is not fully
acquainted with the boilers and apparatus. Probably the most general of
such shortcomings is the fixed idea in the heads of the operatives that
boilers run above their rated capacity are operating under a state of
strain and that by operating at less than their rated capacity the most
economical service is assured, whereas, by determining what a boiler
will do, it may be found that the most economical rating under the
conditions of the plant will be considerably in excess of the builder's
rating. Such ideas can be dislodged only by demonstrating to the
operatives what maximum load the boilers can carry, showing how the
economy will vary with the load and the determining of the economical
load for the individual plant in question.
2nd. Stokers. With stoker-fired boilers, it is essential that the
operators know the limitations of their stokers as determined by their
individual installation. A thorough understanding of the requirements of
efficient handling must be insisted upon. The operatives must realize
that smokeless stacks are not necessarily the indication of good
combustion for, as has been pointed out, absolute smokelessness is
oftentimes secured at an enormous loss in efficiency through excess air.
Another feature in stoker-fired plants is in the cleaning of fires. It
must be impressed upon the operatives that before the fires are cleaned
they should be put into condition for such cleaning. If this cleaning is
done at a definite time, regardless of whether the fires are in the best
condition for cleaning, there will be a great loss of good fuel with the
ashes.
3rd. It is necessary that in each individual plant there be a basis on
which to judge the cleanliness of a boiler. From the operative's
standpoint, it is probably more necessary that there be a thorough
understanding of the relation between scale and tube difficulties than
between scale and efficiency. It is, of course, impossible to keep
boilers absolutely free from scale at all times, but experience in each
individual plant determines the limit to which scale can be allowed to
form before tube difficulties will begin or a perceptible falling off in
efficiency will take place. With such a limit of scale formation fixed,
the operatives should be impressed with the danger of allowing it to be
exceeded.
4th. The operatives should be instructed as to the losses resulting from
excess air due to leaks in the setting and as to losses in efficiency
and capacity due to the by-passing of gases through the setting, that
is, not following the path of the baffles as originally installed. In
replacing tubes and in cleaning the heating surfaces, care must be taken
not to dislodge baffle brick or tile.
[Illustration: 2000 Horse-power Installation of Babcock & Wilcox
Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at the
Sunnyside Plant of the Pennsylvania Tunnel and Terminal Railroad Co.,
Long Island City, N. Y.]
5th. That an increase in the temperature of the feed reduces the amount
of work demanded from the boiler has been shown. The necessity of
keeping the feed temperature as high as the quantity of exhaust steam
will allow should be thoroughly understood. As an example of this, there
was a case brought to our attention where a large amount of exhaust
steam was wasted simply because the feed pump showed a tendency to leak
if the temperature of feed water was increased above 140 degrees. The
amount wasted was sufficient to increase the temperature to 180 degrees
but was not utilized simply because of the slight expense necessary to
overhaul the feed pump.
The highest return will be obtained when the speed of the feed pumps is
maintained reasonably constant for should the pumps run very slowly at
times, there may be a loss of the steam from other auxiliaries by
blowing off from the heaters.
6th. With a view to checking steam losses through the useless blowing of
safety valves, the operative should be made to realize the great amount
of steam that it is possible to get through a pipe of a given size.
Oftentimes the fireman feels a sense of security from objections to a
drop in steam simply because of the blowing of safety valves, not
considering the losses due to such a cause and makes no effort to check
this flow either by manipulation of dampers or regulation of fires.
The few of the numerous shortcomings outlined above, which may be found
in many plants, are almost entirely due to lack of knowledge on the part
of the operating crew as to the conditions existing in their own plants
and the better performances being secured in others. Such shortcomings
can be overcome only by the education of the operatives, the showing of
the defects of present methods, and instruction in better methods. Where
such instruction is necessary, the value of records is obvious. There is
fortunately a tendency toward the employment of a better class of labor
in the boiler room, a tendency which is becoming more and more marked as
the realization of the possible saving in this end of the plant
increases.
The second aspect of boiler room management, dealing with specific
recommendations as to the care and operation of the boilers, is dictated
largely by the nature of the apparatus. Some of the features to be
watched in considering this aspect follow.
Before placing a new boiler in service, a careful and thorough
examination should be made of the pressure parts and the setting. The
boiler as erected should correspond in its baffle openings, where
baffles are adjustable, with the prints furnished for its erection, and
such baffles should be tight. The setting should be so constructed that
the boiler is free to expand without interfering with the brickwork.
This ability to expand applies also to blow-off and other piping. After
erection all mortar and chips of brick should be cleaned from the
pressure parts. The tie rods should be set up snug and then slacked
slightly until the setting has become thoroughly warm after the first
firing. The boiler should be examined internally before starting to
insure the absence of dirt, any foreign material such as waste, and
tools. Oil and paint are sometimes found in the interior of a new boiler
and where such is the case, a quantity of soda ash should be placed
within it, the boiler filled with water to its normal level and a slow
fire started. After twelve hours of slow simmering, the fire should be
allowed to die out, the boiler cooled slowly and then opened and washed
out thoroughly. Such a proceeding will remove all oil and grease from
the interior and prevent the possibility of foaming and tube
difficulties when the boiler is placed in service.
The water column piping should be examined and known to be free and
clear. The water level, as indicated by the gauge glass, should be
checked by opening the gauge cocks.
The method of drying out a brick setting before placing a boiler in
operation is described later in the discussion of boiler settings.
A boiler should not be cut into the line with other boilers until the
pressure within it is approximately that in the steam main. The boiler
stop valve should be opened very slowly until it is fully opened. The
arrangement of piping should be such that there can be no possibility of
water collecting in a pocket between the boiler and the main, from which
it can be carried over into the steam line when a boiler is cut in.
In regular operation the safety valve and steam gauge should be checked
daily. In small plants the steam pressure should be raised sufficiently
to cause the safety valves to blow, at which time the steam gauge should
indicate the pressure at which the valve is known to be set. If it does
not, one is in error and the gauge should be compared with one of known
accuracy and any error at once rectified.
In large plants such a method of checking would result in losses too
great to be allowed. Here the gauges and valves are ordinarily checked
at the time a boiler is cut out, the valves being assured of not
sticking by daily instantaneous opening through manipulation by hand of
the valve lever. The daily blowing of the safety valve acts not only as
a check on the gauge but insures the valve against sticking.
The water column should be blown down thoroughly at least once on every
shift and the height of water indicated by the glass checked by the
gauge cocks. The bottom blow-offs should be kept tight. These should be
opened at least once daily to blow from the mud drum any sediment that
may have collected and to reduce the concentration. The amount of
blowing down and the frequency is, of course, determined by the nature
of the feed water used.
In case of low water, resulting either from carelessness or from some
unforeseen condition of operation, the essential object to be obtained
is the extinguishing of the fire in the quickest possible manner. Where
practicable, this is best accomplished by the playing of a heavy stream
of water from a hose on the fire. Another method, perhaps not so
efficient, but more generally recommended, is the covering of the fire
with wet ashes or fresh fuel. A boiler so treated should be cut out of
line after such an occurrence and a thorough inspection made to
ascertain what damage, if any, has been done before it is again placed
in service.
The efficiency and capacity depend to an extent very much greater than
is ordinarily realized upon the cleanliness of the heating surfaces,
both externally and internally, and too much stress cannot be put upon
the necessity for systematic cleaning as a regular feature in the plant
operation.
The outer surfaces of the tubes should be blown free from soot at
regular intervals, the frequency of such cleaning periods being
dependent upon the class of fuel used. The most efficient way of blowing
soot from the tubes is by means of a steam lance with which all parts of
the surfaces are reached and swept clean. There are numerous soot
blowing devices on the market which are designed to be permanently fixed
within the boiler setting. Where such devices are installed, there are
certain features that must be watched to avoid trouble. If there is any
leakage of water of condensation within the setting coming into contact
with the boiler tubes, it will tend toward corrosion, or if in contact
with the heated brickwork will cause rapid disintegration of the
setting. If the steam jets are so placed that they impinge directly
against the tubes, erosion may take place. Where such permanent soot
blowers are installed, too much care cannot be taken to guard against
these possibilities.
Internally, the tubes must be kept free from scale, the ingredients of
which a study of the chapter on the impurities of water indicates are
present in varying quantities in all feed waters. Not only has the
presence of scale a direct bearing on the efficiency and capacity to be
obtained from a boiler but its absence is an assurance against the
burning out of tubes.
In the absence of a blow-pipe action of the flames, it is impossible to
burn a metal surface where water is in intimate contact with that
surface.
In stoker-fired plants where a blast is used, and the furnace is not
properly designed, there is a danger of a blow-pipe action if the fires
are allowed to get too thin. The rapid formation of steam at such points
of localized heat may lead to the burning of the metal of the tubes.
Any formation of scale on the interior surface of a boiler keeps the
water from such a surface and increases its tendency to burn. Particles
of loose scale that may become detached will lodge at certain points in
the tubes and localize this tendency at such points. It is because of
the danger of detaching scale and causing loose flakes to be present
that the use of a boiler compound is not recommended for the removal of
scale that has already formed in a boiler. This question is covered in
the treatment of feed waters. If oil is allowed to enter a boiler, its
action is the same as that of scale in keeping the water away from the
metal surfaces.
[Illustration: Fig. 41]
It has been proven beyond a doubt that a very large percentage of tube
losses is due directly to the presence of scale which, in many
instances, has been so thin as to be considered of no moment, and the
importance of maintaining the boiler heating surfaces in a clean
condition cannot be emphasized too strongly.
The internal cleaning can best be accomplished by means of an air or
water-driven turbine, the cutter heads of which may be changed to handle
various thicknesses of scale. Fig. 41 shows a turbine cleaner with
various cutting heads, which has been found to give satisfactory
service.
Where a water-driven turbine is used, it should be connected to a pump
which will deliver at least 120 gallons per minute per cleaner at 150
pounds pressure. This pressure should never be less than 90 pounds if
satisfactory results are desired. Where an air-driven turbine is used,
the pressure should be at least 100 pounds, though 150 pounds is
preferable, and sufficient water should be introduced into the tube to
keep the cutting head cool and assist in washing down the scale as it is
chipped off.
Where scale has been allowed to accumulate to an excessive thickness,
the work of removal is difficult and tedious. Where such a heavy scale
is of sulphate formation, its removal may be assisted by filling the
boiler with water to which there has been added a quantity of soda ash,
a bucketful to each drum, starting a low fire and allowing the water to
boil for twenty-four hours with no pressure on the boiler. It should be
cooled slowly, drained, and the turbine cleaner used immediately, as the
scale will tend to harden rapidly under the action of the air.
Where oil has been allowed to get into a boiler, it should be removed
before placing the boiler in service, as described previously where
reference is made to its removal by boiling out with soda ash.
Where pitting or corrosion is noted, the parts affected should be
carefully cleaned and the interior of the drums should be painted with
white zinc if the boiler is to remain idle. The cause of such action
should be immediately ascertained and steps taken to apply the proper
remedy.
When making an internal inspection of a boiler or when cleaning the
interior heating surfaces, great care must be taken to guard against the
possibility of steam entering the boiler in question from other boilers
on the same line either through the careless opening of the boiler stop
valve or some auxiliary valve or from an open blow-off. Bad accidents
through scalding have resulted from the neglect of this precaution.
Boiler brickwork should be kept pointed up and all cracks filled. The
boiler baffles should be kept tight to prevent by-passing of any gases
through the heating surfaces.
Boilers should be taken out of service at regular intervals for cleaning
and repairs. When this is done, the boiler should be cooled slowly, and
when possible, be allowed to stand for twenty-four hours after the fire
is drawn before opening. The cooling process should not be hurried by
allowing cold air to rush through the setting as this will invariably
cause trouble with the brickwork. When a boiler is off for cleaning, a
careful examination should be made of its condition, both external and
internal, and all leaks of steam, water and air through the setting
stopped. If water is allowed to come into contact with brickwork that is
heated, rapid disintegration will take place. If water is allowed to
come into contact with the metal of the boiler when out of service,
there is a likelihood of corrosion.
If a boiler is to remain idle for some time, its deterioration may be
much more rapid than when in service. If the period for which it is to
be laid off is not to exceed three months, it may be filled with water
while out of service. The boiler should first be cleaned thoroughly,
internally and externally, all soot and ashes being removed from the
exterior of the pressure parts and any accumulation of scale removed
from the interior surfaces. It should then be filled with water, to
which five or six pails of soda ash have been added, a slow fire started
to drive the air from the boiler, the fire drawn and the boiler pumped
full. In this condition it may be kept for some time without bad
effects.
If the boiler is to be out of service for more than three months, it
should be emptied, drained and thoroughly dried after being cleaned. A
tray of quick lime should be placed in each drum, the boiler closed, the
grates covered and a quantity of quick lime placed on top of the
covering. Special care should be taken to prevent air, steam or water
leaks into the boiler or onto the pressure parts to obviate danger of
corrosion.
[Illustration: 3000 Horse-power Installation of Babcock & Wilcox Boilers
in the Main Power Plant, Chicago & Northwestern Ry. Depot, Chicago,
Ill.]
BRICKWORK BOILER SETTINGS
A consideration of the losses in boiler efficiency, due to the effects
of excess air, clearly indicates the necessity of maintaining the brick
setting of a boiler tight and free from air leaks. In view of the
temperatures to which certain portions of such a setting are subjected,
the material to be used in its construction must be of the best
procurable.
Boiler settings to-day consist almost universally of brickwork--two
kinds being used, namely, red brick and fire brick.
The red brick should only be used in such portions of the setting as are
well protected from the heat. In such location, their service is not so
severe as that of fire brick and ordinarily, if such red brick are
sound, hard, well burned and uniform, they will serve their purpose.
The fire brick should be selected with the greatest care, as it is this
portion of the setting that has to endure the high temperatures now
developed in boiler practice. To a great extent, the life of a boiler
setting is dependent upon the quality of the fire brick used and the
care exercised in its laying.
The best fire brick are manufactured from the fire clays of
Pennsylvania. South and west from this locality the quality of fire clay
becomes poorer as the distance increases, some of the southern fire
clays containing a considerable percentage of iron oxide.
Until very recently, the important characteristic on which to base a
judgment of the suitability of fire brick for use in connection with
boiler settings has been considered the melting point, or the
temperature at which the brick will liquify and run. Experience has
shown, however, that this point is only important within certain limits
and that the real basis on which to judge material of this description
is, from the boiler man's standpoint, the quality of plasticity under a
given load. This tendency of a brick to become plastic occurs at a
temperature much below the melting point and to a degree that may cause
the brick to become deformed under the stress to which it is subjected.
The allowable plastic or softening temperature will naturally be
relative and dependent upon the stress to be endured.
With the plasticity the determining factor, the perfect fire brick is
one whose critical point of plasticity lies well above the working
temperature of the fire. It is probable that there are but few brick on
the market which would not show, if tested, this critical temperature at
the stress met with in arch construction at a point less than 2400
degrees. The fact that an arch will stand for a long period under
furnace temperatures considerably above this point is due entirely to
the fact that its temperature as a whole is far below the furnace
temperature and only about 10 per cent of its cross section nearest the
fire approaches the furnace temperature. This is borne out by the fact
that arches which are heated on both sides to the full temperature of an
ordinary furnace will first bow down in the middle and eventually fall.
A method of testing brick for this characteristic is given in the
Technologic Paper No. 7 of the Bureau of Standards dealing with "The
testing of clay refractories with special reference to their load
carrying capacity at furnace temperatures." Referring to the test for
this specific characteristic, this publication recommends the following:
"When subjected to the load test in a manner substantially as described
in this bulletin, at 1350 degrees centigrade (2462 degrees Fahrenheit),
and under a load of 50 pounds per square inch, a standard fire brick
tested on end should show no serious deformation and should not be
compressed more than one inch, referred to the standard length of nine
inches."
In the Bureau of Standards test for softening temperature, or critical
temperature of plasticity under the specified load, the brick are tested
on end. In testing fire brick for boiler purposes such a method might be
criticised, because such a test is a compression test and subject to
errors from unequal bearing surfaces causing shear. Furthermore, a
series of samples, presumably duplicates, will not fail in the same way,
due to the mechanical variation in the manufacture of the brick. Arches
that fail through plasticity show that the tensile strength of the brick
is important, this being evidenced by the fact that the bottom of a
wedge brick in an arch that has failed is usually found to be wider than
the top and the adjacent bricks are firmly cemented together.
A better method of testing is that of testing the brick as a beam
subjected to its own weight and not on end. This method has been used
for years in Germany and is recommended by the highest authorities in
ceramics. It takes into account the failure by tension in the brick as
well as by compression and thus covers the tension element which is
important in arch construction.
The plastic point under a unit stress of 100 pounds per square inch,
which may be taken as the average maximum arch stress, should be above
2800 degrees to give perfect results and should be above 2400 degrees to
enable the brick to be used with any degree of satisfaction.
The other characteristics by which the quality of a fire brick is to be
judged are:
Fusion point. In view of the fact that the critical temperature of
plasticity is below the fusion point, this is only important as an
indication from high fusion point of a high temperature of plasticity.
Hardness. This is a relative quality based on an arbitrary scale of 10
and is an indication of probable cracking and spalling.
Expansion. The lineal expansion per brick in inches. This characteristic
in conjunction with hardness is a measure of the physical movement of
the brick as affecting a mass of brickwork, such movement resulting in
cracked walls, etc. The expansion will vary between wide limits in
different brick and provided such expansion is not in excess of, say,
.05 inch in a 9-inch brick, when measured at 2600 degrees, it is not
particularly important in a properly designed furnace, though in general
the smaller the expansion the better.
Compression. The strength necessary to cause crushing of the brick at
the center of the 4½ inch face by a steel block one inch square. The
compression should ordinarily be low, a suggested standard being that a
brick show signs of crushing at 7500 pounds.
Size of Nodules. The average size of flint grains when the brick is
carefully crushed. The scale of these sizes may be considered: Small,
size of anthracite rice; large, size of anthracite pea.
Ratio of Nodules. The percentage of a given volume occupied by the flint
grains. This scale may be considered: High, 90 to 100 per cent; medium,
50 to 90 per cent; low, 10 to 50 per cent.
The statement of characteristics suggested as desirable, are for arch
purposes where the hardest service is met. For side wall purposes the
compression and hardness limit may be raised considerably and the
plastic point lowered.
Aside from the physical properties by which a fire brick is judged, it
is sometimes customary to require a chemical analysis of the brick. Such
an analysis is only necessary as determining the amount of total basic
fluxes (K_{2}O, Na_{2}O, CaO, MgO and FeO). These fluxes are ordinarily
combined into one expression, indicated by the symbol RO. This total
becomes important only above 0.2 molecular equivalent as expressed in
ceramic empirical formulae, and this limit should not be exceeded.[75]
From the nature of fire brick, their value can only be considered from a
relative standpoint. Generally speaking, what are known as first-grade
fire brick may be divided into three classes, suitable for various
conditions of operation, as follows:
Class A. For stoker-fired furnaces where high overloads are to be
expected or where other extreme conditions of service are apt to occur.
Class B. For ordinary stoker settings where there will be no excessive
overloads required from the boiler or any hand-fired furnaces where the
rates of driving will be high for such practice.
Class C. For ordinary hand-fired settings where the presumption is that
the boilers will not be overloaded except at rare intervals and for
short periods only.
Table 61 gives the characteristics of these three classes according to
the features determining the quality. This table indicates that the
hardness of the brick in general increases with the poorer qualities.
Provided the hardness is sufficient to enable the brick to withstand its
load, additional hardness is a detriment rather than an advantage.
TABLE 61
APPROXIMATE CLASSIFICATION OF FIRE BRICK
________________________________________________________________________
| | | | |
| Characteristics | Class A | Class B | Class C |
|_____________________|________________|________________|________________|
| | | | |
| Fuse Point, Degrees | Safe at Degrees| Safe at Degrees| Safe at Degrees|
| Fahrenheit | 3200-3300 | 2900-3200 | 2900-3000 |
| | | | |
| Compression Pounds | 6500-7500 | 7500-11,000 | 8500-15,000 |
| | | | |
| Hardness Relative | 1-2 | 2-4 | 4-6 |
| | | | |
| Size of Nodules | Medium | Medium to |Medium to Large |
| | | Medium Large | |
| | | | |
| Ratio of Nodules | High | Medium to High | Medium Low |
| | | | to Medium |
|_____________________|________________|________________|________________|
An approximate determination of the quality of a fire brick may be made
from the appearance of a fracture. Where such a fracture is open, clean,
white and flinty, the brick in all probability is of a good quality. If
this fracture has the fine uniform texture of bread, the brick is
probably poor.
In considering the heavy duty of brick in boiler furnaces, experience
shows that arches are the only part that ordinarily give trouble. These
fail from the following causes:
Bad workmanship in laying up of brick. This feature is treated below.
The tendency of a brick to become plastic at a temperature below the
fusing point. The limits of allowable plastic temperature have already
been pointed out.
Spalling. This action occurs on the inner ends of combustion arches
where they are swept by gases at a high velocity at the full furnace
temperature. The most troublesome spalling arises through cold air
striking the heated brickwork. Failure from this cause is becoming rare,
due to the large increase in number of stoker installations in which
rapid temperature changes are to a great degree eliminated. Furthermore,
there are a number of brick on the market practically free from such
defects and where a new brick is considered, it can be tried out and if
the defect exists, can be readily detected and the brick discarded.
Failures of arches from the expansive power of brick are also rare, due
to the fact that there are a number of brick in which the expansion is
well within the allowable limits and the ease with which such defects
may be determined before a brick is used.
Failures through chemical disintegration. Failure through this cause is
found only occasionally in brick containing a high percentage of iron
oxide.
With the grade of brick selected best suited to the service of the
boiler to be set, the other factor affecting the life of the setting is
the laying. It is probable that more setting difficulties arise from the
improper workmanship in the laying up of brick than from poor material,
and to insure a setting which will remain tight it is necessary that the
masonry work be done most carefully. This is particularly true where the
boiler is of such a type as to require combustion arches in the furnace.
Red brick should be laid in a thoroughly mixed mortar composed of one
volume of Portland cement, 3 volumes of unslacked lime and 16 volumes of
clear sharp sand. Not less than 2½ bushels of lime should be used in the
laying up of 1000 brick. Each brick should be thoroughly embedded and
all joints filled. Where red brick and fire brick are both used in the
same wall, they should be carried up at the same time and thoroughly
bonded to each other.
All fire brick should be dry when used and protected from moisture until
used. Each brick should be dipped in a thin fire clay wash, "rubbed and
shoved" into place, and tapped with a wooden mallet until it touches the
brick next below it. It must be recognized that fire clay is not a
cement and that it has little or no holding power. Its action is that of
a filler rather than a binder and no fire-clay wash should be used which
has a consistency sufficient to permit the use of a trowel.
All fire-brick linings should be laid up four courses of headers and one
stretcher. Furnace center walls should be entirely of fire brick. If the
center of such walls are built of red brick, they will melt down and
cause the failure of the wall as a whole.
Fire-brick arches should be constructed of selected brick which are
smooth, straight and uniform. The frames on which such arches are built,
called arch centers, should be constructed of batten strips not over 2
inches wide. The brick should be laid on these centers in courses, not
in rings, each joint being broken with a bond equal to the length of
half a brick. Each course should be first tried in place dry, and
checked with a straight edge to insure a uniform thickness of joint
between courses. Each brick should be dipped on one side and two edges
only and tapped into place with a mallet. Wedge brick courses should be
used only where necessary to keep the bottom faces of the straight brick
course in even contact with the centers. When such contact cannot be
exactly secured by the use of wedge brick, the straight brick should
lean away from the center of the arch rather than toward it. When the
arch is approximately two-thirds completed, a trial ring should be laid
to determine whether the key course will fit. When some cutting is
necessary to secure such a fit, it should be done on the two adjacent
courses on the side of the brick away from the key. It is necessary that
the keying course be a true fit from top to bottom, and after it has
been dipped and driven it should not extend below the surface of the
arch, but preferably should have its lower ledge one-quarter inch above
this surface. After fitting, the keys should be dipped, replaced
loosely, and the whole course driven uniformly into place by means of a
heavy hammer and a piece of wood extending the full length of the keying
course. Such a driving in of this course should raise the arch as a
whole from the center. The center should be so constructed that it may
be dropped free of the arch when the key course is in place and removed
from the furnace without being burned out.
[Illustration: A Typical Steel Casing for a Babcock & Wilcox Boiler
Built by The Babcock & Wilcox Co.]
Care of Brickwork--Before a boiler is placed in service, it is essential
that the brickwork setting be thoroughly and properly dried, or
otherwise the setting will invariably crack. The best method of starting
such a process is to block open the boiler damper and the ashpit doors
as soon as the brickwork is completed and in this way maintain a free
circulation of air through the setting. If possible, such preliminary
drying should be continued for several days before any fire is placed in
the furnace. When ready for the drying out fire, wood should be used at
the start in a light fire which may be gradually built up as the walls
become warm. After the walls have become thoroughly heated, coal may be
fired and the boiler placed in service.
As already stated, the life of a boiler setting is dependent to a large
extent upon the material entering into its construction and the care
with which such material is laid. A third and equally important factor
in the determining of such life is the care given to the maintaining of
the setting in good condition after the boiler is placed in operation.
This feature is discussed more fully in the chapter dealing with general
boiler room management.
Steel Casings--In the chapter dealing with the losses operating against
high efficiencies as indicated by the heat balance, it has been shown
that a considerable portion of such losses is due to radiation and to
air infiltration into the boiler setting. These losses have been
variously estimated from 2 to 10 per cent, depending upon the condition
of the setting and the amount of radiation surface, the latter in turn
being dependent upon the size of the boiler used. In the modern efforts
after the highest obtainable plant efficiencies much has been done to
reduce such losses by the use of an insulated steel casing covering the
brickwork. In an average size boiler unit the use of such casing, when
properly installed, will reduce radiation losses from one to two per
cent., over what can be accomplished with the best brick setting without
such casing and, in addition, prevent the loss due to the infiltration
of air, which may amount to an additional five per cent., as compared
with brick settings that are not maintained in good order. Steel plate,
or steel plate backed by asbestos mill-board, while acting as a
preventative against the infiltration of air through the boiler setting,
is not as effective from the standpoint of decreasing radiation losses
as a casing properly insulated from the brick portion of the setting by
magnesia block and asbestos mill-board. A casing which has been found to
give excellent results in eliminating air leakage and in the reduction
of radiation losses is clearly illustrated on page 306.
Many attempts have been made to use some material other than brick for
boiler settings but up to the present nothing has been found that may be
considered successful or which will give as satisfactory service under
severe conditions as properly laid brickwork.
BOILER ROOM PIPING
In the design of a steam plant, the piping system should receive the
most careful consideration. Aside from the constructive details, good
practice in which is fairly well established, the important factors are
the size of the piping to be employed and the methods utilized in
avoiding difficulties from the presence in the system of water of
condensation and the means employed toward reducing radiation losses.
Engineering opinion varies considerably on the question of material of
pipes and fittings for different classes of work, and the following is
offered simply as a suggestion of what constitutes good representative
practice.
All pipe should be of wrought iron or soft steel. Pipe at present is
made in "standard", "extra strong"[76] and "double extra strong"
weights. Until recently, a fourth weight approximately 10 per cent
lighter than standard and known as "Merchants" was built but the use of
this pipe has largely gone out of practice. Pipe sizes, unless otherwise
stated, are given in terms of nominal internal diameter. Table 62 gives
the dimensions and some general data on standard and extra strong
wrought-iron pipe.
TABLE 62
DIMENSIONS OF STANDARD AND EXTRA STRONG[76]
WROUGHT-IRON AND STEEL PIPE
_______________________________________________________________
| | | |
| | Diameter | Circumference |
| |__________________________|__________________________|
| | | | | |
| |External| Internal |External| Internal |
| |Standard|_________________|Standard|_________________|
| | and | | | and | | |
| Nominal | Extra |Standard| Extra | Extra |Standard| Extra |
| Size | Strong | | Strong | Strong | | Strong |
|_________|________|________|________|________|________|________|
| | | | | | | |
| 1/8 | .405 | .269 | .215 | 1.272 | .848 | .675 |
| 1/4 | .540 | .364 | .302 | 1.696 | 1.144 | .949 |
| 3/8 | .675 | .493 | .423 | 2.121 | 1.552 | 1.329 |
| 1/2 | .840 | .622 | .546 | 2.639 | 1.957 | 1.715 |
| 3/4 | 1.050 | .824 | .742 | 3.299 | 2.589 | 2.331 |
| 1 | 1.315 | 1.049 | .957 | 4.131 | 3.292 | 3.007 |
| 1-1/4 | 1.660 | 1.380 | 1.278 | 5.215 | 4.335 | 4.015 |
| 1-1/2 | 1.900 | 1.610 | 1.500 | 5.969 | 5.061 | 4.712 |
| 2 | 2.375 | 2.067 | 1.939 | 7.461 | 6.494 | 6.092 |
| 2-1/2 | 2.875 | 2.469 | 2.323 | 9.032 | 7.753 | 7.298 |
| 3 | 3.500 | 3.068 | 2.900 | 10.996 | 9.636 | 9.111 |
| 3-1/2 | 4.000 | 3.548 | 3.364 | 12.566 | 11.146 | 10.568 |
| 4 | 4.500 | 4.026 | 3.826 | 14.137 | 12.648 | 12.020 |
| 4-1/2 | 5.000 | 4.506 | 4.290 | 15.708 | 14.162 | 13.477 |
| 5 | 5.563 | 5.047 | 4.813 | 17.477 | 15.849 | 15.121 |
| 6 | 6.625 | 6.065 | 5.761 | 20.813 | 19.054 | 18.099 |
| 7 | 7.625 | 7.023 | 6.625 | 23.955 | 22.063 | 20.813 |
| 8 | 8.625 | 7.981 | 7.625 | 27.096 | 25.076 | 23.955 |
| 9 | 9.625 | 8.941 | 8.625 | 30.238 | 28.089 | 27.096 |
| 10 | 10.750 | 10.020 | 9.750 | 33.772 | 31.477 | 30.631 |
| 11 | 11.750 | 11.000 | 10.750 | 36.914 | 34.558 | 33.772 |
| 12 | 12.750 | 12.000 | 11.750 | 40.055 | 37.700 | 36.914 |
|_________|________|________|________|________|________|________|
__________________________________________________________
| | | | |
| | | Length | |
| | Internal | of | Nominal Weight |
| | Transverse |Pipe in | Pounds per |
| | Area |Feet per| Foot |
| |_____________________| Square |_________________|
| | | |Foot of | | |
| Nominal | Standard | Extra |External|Standard| Extra |
| Size | | Strong |Surface | | Strong |
|_________|__________|__________|________|________|________|
| | | | | | |
| 1/8 | .0573 | .0363 | 9.440 | .244 | .314 |
| 1/4 | .1041 | .0716 | 7.075 | .424 | .535 |
| 3/8 | .1917 | .1405 | 5.657 | .567 | .738 |
| 1/2 | .3048 | .2341 | 4.547 | .850 | 1.087 |
| 3/4 | .5333 | .4324 | 3.637 | 1.130 | 1.473 |
| 1 | .8626 | .7193 | 2.904 | 1.678 | 2.171 |
| 1-1/4 | 1.496 | 1.287 | 2.301 | 2.272 | 2.996 |
| 1-1/2 | 2.038 | 1.767 | 2.010 | 2.717 | 3.631 |
| 2 | 3.356 | 2.953 | 1.608 | 3.652 | 5.022 |
| 2-1/2 | 4.784 | 4.238 | 1.328 | 5.793 | 7.661 |
| 3 | 7.388 | 6.605 | 1.091 | 7.575 | 10.252 |
| 3-1/2 | 9.887 | 8.888 | .955 | 9.109 | 12.505 |
| 4 | 12.730 | 11.497 | .849 | 10.790 | 14.983 |
| 4-1/2 | 15.961 | 14.454 | .764 | 12.538 | 17.611 |
| 5 | 19.990 | 18.194 | .687 | 14.617 | 20.778 |
| 6 | 28.888 | 26.067 | .577 | 18.974 | 28.573 |
| 7 | 38.738 | 34.472 | .501 | 23.544 | 38.048 |
| 8 | 50.040 | 45.664 | .443 | 28.544 | 43.388 |
| 9 | 62.776 | 58.426 | .397 | 33.907 | 48.728 |
| 10 | 78.839 | 74.662 | .355 | 40.483 | 54.735 |
| 11 | 95.033 | 90.763 | .325 | 45.557 | 60.075 |
| 12 | 113.098 | 108.43 | .299 | 49.562 | 65.415 |
|_________|__________|__________|________|________|________|
Dimensions are nominal and except where noted are in inches.
In connection with pipe sizes, Table 63, giving certain tube data may be
found to be of service.
TABLE 63
TUBE DATA, STANDARD OPEN HEARTH OR LAP WELDED STEEL TUBES
+-----+--+----+-----+------+------+------+------+-------+-------+-------+
|S E D|B | T | I D |Circumference| Transverse |Square |Length |Nominal|
|i x i|. | h | n i | | Area | Feet |in Feet|Weight |
|z t a|W | i | t a | |Square Inches| of | per |Pounds |
|e e m|. | c | e m +------+------+------+------+ Exter |Square | per |
| r e| | k | r e |Exter-|Inter-|Exter-|Inter-| -nal |Foot of| Foot |
| n t|G | n | n t | nal | nal | nal | nal |Surface| Exter | |
| a e|a | e | a e | | | | | per | -nal | |
| l r|u | s | l r | | | | |Foot of|Surface| |
| |g | s | | | | | |Length | | |
| |e | | | | | | | | | |
+-----+--+----+-----+------+------+------+------+-------+-------+-------+
|1-1/2|10|.134|1.232| 4.712| 3.870|1.7671|1.1921| .392 | 2.546 | 1.955 |
|1-1/2| 9|.148|1.204| 4.712| 3.782|1.7671|1.1385| .392 | 2.546 | 2.137 |
|1-1/2| 8|.165|1.170| 4.712| 3.676|1.7671|1.0751| .392 | 2.546 | 2.353 |
| 2 |10|.134|1.732| 6.283| 5.441|3.1416|2.3560| .523 | 1.909 | 2.670 |
| 2 | 9|.148|1.704| 6.283| 5.353|3.1416|2.2778| .523 | 1.909 | 2.927 |
| 2 | 8|.165|1.670| 6.283| 5.246|3.1416|2.1904| .523 | 1.909 | 3.234 |
|3-1/4|11|.120|3.010|10.210| 9.456|8.2958|7.1157| .850 | 1.175 | 4.011 |
|3-1/4|10|.134|2.982|10.210| 9.368|8.2958|6.9840| .850 | 1.175 | 4.459 |
|3-1/4| 9|.148|2.954|10.210| 9.280|8.2958|6.8535| .850 | 1.175 | 4.903 |
| 4 |10|.134|3.732|12.566|11.724|12.566|10.939| 1.047 | .954 | 5.532 |
| 4 | 9|.148|3.704|12.566|11.636|12.566|10.775| 1.047 | .954 | 6.000 |
| 4 | 8|.165|3.670|12.566|11.530|12.566|10.578| 1.047 | .954 | 6.758 |
+-----+--+----+-----+------+------+------+------+-------+-------+-------+
Dimensions are nominal and except where noted are in inches.
Pipe Material and Thickness--For saturated steam pressures not exceeding
160 pounds, all pipe over 14 inches should be 3/8 inch thick O. D. pipe.
All other pipe should be standard full weight, except high pressure
feed[77] and blow-off lines, which should be extra strong.
For pressures above 150 pounds up to 200 pounds with superheated steam,
all high pressure feed and blow-off lines, high pressure steam lines
having threaded flanges, and straight runs and bends of high pressure
steam lines 6 inches and under having Van Stone joints should be extra
strong. All piping 7 inches and over having Van Stone joints should be
full weight soft flanging pipe of special quality. Pipe 14 inches and
over should be 3/8 inch thick O. D. pipe. All pipes for these pressures
not specified above should be full weight pipe.
Flanges--For saturated steam, 160 pounds working pressure, all flanges
for wrought-iron pipe should be cast-iron threaded. All high pressure
threaded flanges should have the diameter thickness and drilling in
accordance with the "manufacturer's standard" for "extra heavy" flanges.
All low pressure flanges should have diameter, thickness and drilling in
accordance with "manufacturer's standard" for "standard flanges."
The flanges on high pressure lines should be counterbored to receive
pipe and prevent the threads from shouldering. The pipe should be
screwed through the flange at least 1/16 inch, placed in machine and
after facing off the end one smooth cut should be taken over the face of
the flange to make it square with the axis of the pipe.
[Illustration: 2000 Horse-power Installation of Babcock & Wilcox Boilers
and Superheaters, Equipped with Babcock & Wilcox Chain Grate Stokers at
the Kentucky Electric Co., Louisville, Ky.]
For pressures above 160 pounds, where superheated steam is used, all
high pressure steam lines 4 inches and over should have solid rolled
steel flanges and special upset lapped joints. In the manufacture of
such joints, the ends of the pipe are heated and upset against the face
of a holding mandrel conforming to the shape of the flange, the lapped
portion of the pipe being flattened out against the face of the mandrel,
the upsetting action maintaining the desired thickness of the lap. When
cool, both sides of the lap are faced to form a uniform thickness and an
even bearing against flange and gasket. The joint, therefore, is a
strictly metal to metal joint, the flanges merely holding the lapped
ends of the pipe against the gasket.
A special grade of soft flanging pipe is selected to prevent breaking.
The bending action is a severe test of the pipe and if it withstands the
bending process and the pressure tests, the reliability of the joint is
assured. Such a joint is called a Van Stone joint, though many
modifications and improvements have been made since the joint was
originally introduced.
The diameter and thickness of such flanges should be special extra
heavy. Such flanges should be turned to diameter, their fronts faced and
the backs machined in lieu of spot facing.
In lines other than given for pressures over 150 pounds, all flanges for
wrought-iron pipe should be threaded. All threaded flanges for high
pressure superheated lines 3½ inches and under should be "semi-steel"
extra heavy. Flanges for other than steam lines should be manufacturer's
standard extra heavy.
Welded flanges are frequently used in place of those described with
satisfactory results.
Fittings--For saturated steam under pressures up to 160 pounds, all
fittings 3½ inches and under should be screwed. Fittings 4 inches and
over should have flanged ends. Fittings for this pressure should be of
cast iron and should have heavy leads and full taper threads. Flanged
fittings in high pressure lines should be extra heavy, and in low
pressure lines standard weight. Where possible in high pressure flanges
and fittings, bolt surfaces should be spot faced to provide suitable
bearing for bolt heads and nuts.
Fittings for superheated steam up to 70 degrees at pressures above 160
pounds are sometimes of cast iron.[78] For superheat above 70 degrees
such fittings should be "steel castings" and in general these fittings
are recommended for any degree of superheat. Fittings for other than
high pressure work may be of cast iron, except where superheated steam
is carried, where they should be of "wrought steel" or "hard metal".
Fittings 3½ inches and under should be screwed, 4 inches and over
flanged.
Flanges for pressures up to 160 pounds in pipes and fittings for low
pressure lines, and any fittings for high pressure lines should have
plain faces, smooth tool finish, scored with V-shaped grooves for rubber
gaskets. High pressure line flanges should have raised faces, projecting
the full available diameter inside the bolt holes. These faces should be
similarly scored.
All pipe ½ inch and under should have ground joint unions suitable for
the pressure required. Pipe ¾ inch and over should have cast-iron
flanged unions. Unions are to be preferred to wrought-iron couplings
wherever possible to facilitate dismantling.
Valves--For 150 pounds working pressure, saturated steam, all valves 2
inches and under may have screwed ends; 2½ inches and over should be
flanged. All high pressure steam valves 6 inches and over should have
suitable by-passes. All valves for use with superheated steam should be
of special construction. For pressures above 160 pounds, where the
superheat does not exceed 70 degrees, valve bodies, caps and yokes are
sometimes made of cast iron, though ordinarily semi-steel will give
better satisfaction. The spindles of such valves should be of bronze and
there should be special necks with condensing chambers to prevent the
superheated steam from blowing through the packing. For pressures over
160 pounds and degrees of superheat above 70, all valves 3 inches and
over should have valve bodies, caps and yokes of steel castings.
Spindles should be of some non-corrosive metal, such as "monel metal".
Seat rings should be removable of the same non-corrosive metal as should
the spindle seats and plug faces.
All salt water valves should have bronze spindles, sleeves and packing
seats.
The suggestions as to flanges for different classes of service made on
page 311 hold as well for valve flanges, except that such flanges are
not scored.
Automatic stop and check valves are coming into general use with boilers
and such use is compulsory under the boiler regulations of certain
communities. Where used, they should be preferably placed directly on
the boiler nozzle. Where two or more boilers are on one line, in
addition to the valve at the boiler, whether this be an automatic valve
or a gate valve, there should be an additional gate valve on each boiler
branch at the main steam header.
Relief valves should be furnished at the discharge side of each feed
pump and on the discharge side of each feed heater of the closed type.
Feed Lines--Feed lines should in all instances be made of extra strong
pipe due to the corrosive action of hot feed water. While it has been
suggested above that cast-iron threaded flanges should be used in such
lines, due to the sudden expansion of such pipe in certain instances
cast-iron threaded flanges crack before they become thoroughly heated
and expand, and for this reason cast-steel threaded flanges will give
more satisfactory results. In some instances, wrought-steel and Van
Stone joints have been used in feed lines and this undoubtedly is better
practice than the use of cast-steel threaded work, though the additional
cost is not warranted in all stations.
Feed valves should always be of the globe pattern. A gate valve cannot
be closely regulated and often clatters owing to the pulsations of the
feed pump.
Gaskets--For steam and water lines where the pressure does not exceed
160 pounds, wire insertion rubber gaskets 1/16 inch thick will be found
to give good service. For low pressure lines, canvas insertion black
rubber gaskets are ordinarily used. For oil lines special gaskets are
necessary.
For pressure above 160 pounds carrying superheated steam, corrugated
steel gaskets extending the full available diameter inside of the bolt
holes give good satisfaction. For high pressure water lines wire
inserted rubber gaskets are used, and for low pressure flanged joints
canvas inserted rubber gaskets.
Size of Steam Lines--The factors affecting the proper size of steam
lines are the radiation from such lines and the velocity of steam within
them. As the size of the steam line increases, there will be an increase
in the radiation.[79] As the size decreases, the steam velocity and the
pressure drop for a given quantity of steam naturally increases.
There is a marked tendency in modern practice toward higher steam
velocities, particularly in the case of superheated steam. It was
formerly considered good practice to limit this velocity to 6000 feet
per minute but this figure is to-day considered low.
In practice the limiting factor in the velocity advisable is the
allowable pressure drop. In the description of the action of the
throttling calorimeter, it has been demonstrated that there is no loss
accompanying a drop in pressure, the difference in energy between the
higher and lower pressures appearing as heat, which, in the case of
steam flowing through a pipe, may evaporate any condensation present or
may be radiated from the pipe. A decrease in pipe area decreases the
radiating surface of the pipe and thus the possible condensation. As the
heat liberated by the pressure drop is utilized in overcoming or
diminishing the tendency toward condensation and the heat loss through
radiation, the steam as it enters the prime mover will be drier or more
highly superheated where high steam velocities are used than where they
are lower, and if enough excess pressure is carried at the boilers to
maintain the desired pressure at the prime mover, the pressure drop
results in an actual saving rather than a loss. The whole is analogous
to standard practice in electrical distributing systems where generator
voltage is adjusted to suit the loss in the feeder lines.
In modern practice, with superheated steam, velocities of 15,000 feet
per minute are not unusual and this figure is very frequently exceeded.
Piping System Design--With the proper size of pipe to be used
determined, the most important factor is the provision for the removal
of water of condensation that will occur in any system. Such
condensation cannot be wholly overcome and if the water of condensation
is carried to the prime mover, difficulties will invariably result.
Water is practically incompressible and its effect when traveling at
high velocities differs little from that of a solid body of equal
weight, hence impact against elbows, valves or other obstructions, is
the equivalent of a heavy hammer blow that may result in the fracture of
the pipe. If there is not sufficient water in the system to produce this
result, it will certainly cause knocking and vibration in the pipe,
resulting eventually in leaky joints. Where the water reaches the prime
mover, its effect will vary from disagreeable knocking to disruption.
Too frequently when there are disastrous results from such a cause the
boilers are blamed for delivering wet steam when, as a matter of fact,
the evil is purely a result of poor piping design, the most common cause
of such an action being the pocketing of the water in certain parts of
the piping from whence it is carried along in slugs by the steam. The
action is particularly severe if steam is admitted to a cold pipe
containing water, as the water may then form a partial vacuum by
condensing the steam and be projected at a very high velocity through
the pipes producing a characteristic sharp metallic knock which often
causes bursting of the pipe or fittings. The amount of water present
through condensation may be appreciated when it is considered that
uncovered 6-inch pipe 150 feet long carrying 3600 pounds of high
pressure steam per hour will condense approximately 6 per cent of the
total steam carried through radiation. It follows that efficient means
of removing condensation water are absolutely imperative and the
following suggestions as to such means may be of service:
The pitch of all pipe should be in the direction of the flow of steam.
Wherever a rise is necessary, a drain should be installed. All main
headers and important branches should end in a drop leg and each such
drop leg and any low points in the system should be connected to the
drainage pump. A similar connection should be made to every fitting
where there is danger of a water pocket.
Branch lines should never be taken from the bottom of a main header but
where possible should be taken from the top. Each engine supply pipe
should have its own separator placed as near the throttle as possible.
Such separators should be drained to the drainage system.
Check valves are frequently placed in drain pipes to prevent steam from
entering any portion of the system that may be shut off.
Valves should be so located that they cannot form water pockets when
either open or closed. Globe valves will form a water pocket in the
piping to which they are connected unless set with the stem horizontal,
while gate valves may be set with the spindle vertical or at an angle.
Where valves are placed directly on the boiler nozzle, a drain should be
provided above them.
High pressure drains should be trapped to both feed heaters and waste
headers. Traps and meters should be provided with by-passes. Cylinder
drains, heater blow-offs and drains, boiler blow-offs and similar lines
should be led to waste. The ends of cylinder drains should not extend
below the surface of water, for on starting up or on closing the
throttle valve with the drains open, water may be drawn back into the
cylinders.
TABLE 64
RADIATION FROM COVERED AND UNCOVERED STEAM PIPES
CALCULATED FOR 160 POUNDS PRESSURE AND 60 DEGREES TEMPERATURE
+---------------------------------------------------------------------+
|+------+---------------------------+----+----+----+-----+-----+-----+|
|| | | | | | | | ||
|| Pipe | |1/2 |3/4 | 1 |1-1/4|1-1/2| ||
||Inches| Thickness of Covering |inch|inch|inch|inch |inch |Bare ||
|+------+---------------------------+----+----+----+-----+-----+-----+|
|| |B. t. u. per lineal foot | | | | | | ||
|| | per hour |149 |118 | 99 | 86 | 79 | 597 ||
|| |B. t. u. per square foot | | | | | | ||
|| | per hour |240 |190 |161 | 138 | 127 | 959 ||
|| 2 |B. t. u. per square foot | | | | | | ||
|| | per hour per one degree | | | | | | ||
|| | difference in temperature|.770|.613|.519|.445 |.410 |3.198||
|+------+---------------------------+----+----+----+-----+-----+-----+|
|| |B. t. u. per lineal foot | | | | | | ||
|| | per hour |247 |193 |160 | 139 | 123 |1085 ||
|| |B. t. u. per square foot | | | | | | ||
|| | per hour |210 |164 |136 | 118 | 104 | 921 ||
|| 4 |B. t. u. per square foot | | | | | | ||
|| | per hour per one degree | | | | | | ||
|| | difference in temperature|.677|.592|.439|.381 |.335 |2.970||
|+------+---------------------------+----+----+----+-----+-----+-----+|
|| |B. t. u. per lineal foot | | | | | | ||
|| | per hour |352 |269 |221 | 190 | 167 |1555 ||
|| |B. t. u. per square foot | | | | | | ||
|| | per hour |203 |155 |127 | 110 | 96 | 897 ||
|| 6 |B. t. u. per square foot | | | | | | ||
|| | per hour per one degree | | | | | | ||
|| | difference in temperature|.655|.500|.410|.355 |.310 |2.89 ||
|+------+---------------------------+----+----+----+-----+-----+-----+|
|| |B. t. u. per lineal foot | | | | | | ||
|| | per hour |443 |337 |276 | 235 | 207 |1994 ||
|| |B. t. u. per square foot | | | | | | ||
|| | per hour |196 |149 |122 | 104 | 92 | 883 ||
|| 8 |B. t. u. per square foot | | | | | | ||
|| | per hour per one degree | | | | | | ||
|| | difference in temperature|.632|.481|.394|.335 |.297 |2.85 ||
|+------+---------------------------+----+----+----+-----+-----+-----+|
|| |B. t. u. per lineal foot | | | | | | ||
|| | per hour |549 |416 |337 | 287 | 250 |2468 ||
|| |B. t. u. per square foot | | | | | | ||
|| | per hour |195 |148 |120 | 102 | 89 | 877 ||
|| 10 |B. t. u. per square foot | | | | | | ||
|| | per hour per one degree | | | | | | ||
|| | difference in temperature|.629|.477|.387|.329 |.287 |2.83 ||
|+------+---------------------------+----+----+----+-----+-----+-----+|
+---------------------------------------------------------------------+
Covering--Magnesia, canvas covered.
For calculating radiation for pressure and temperature other than 160
pounds, and 60 degrees, use B. t. u. figures for one degree difference.
Radiation from Pipes--The evils of the presence of condensed steam in
piping systems have been thoroughly discussed above and in some of the
previous articles. Condensation resulting from radiation, while it
cannot be wholly obviated, can, by proper installation, be greatly
reduced.
Bare pipe will radiate approximately 3 B. t. u. per hour per square foot
of exposed surface per one degree of difference in temperature between
the steam contained and the external air. This figure may be reduced to
from 0.3 to 0.4 B. t. u. for the same conditions by a 1½ inch insulating
covering. Table 64 gives the radiation losses for bare and covered pipes
with different thicknesses of magnesia covering.
Many experiments have been made as to the relative efficiencies of
different kinds of covering. Table 65 gives some approximately relative
figures based on one inch covering from experiments by Paulding,
Jacobus, Brill and others.
TABLE 65
APPROXIMATE
EFFICIENCIES OF VARIOUS
COVERINGS REFERRED TO
BARE PIPES
+--------------------------------+
|+-------------------+----------+|
|| Covering |Efficiency||
|+-------------------+----------+|
||Asbestocel | 76.8 ||
||Gast's Air Cell | 74.4 ||
||Asbesto Sponge Felt| 85.0 ||
||Magnesia | 83.5 ||
||Asbestos Navy Brand| 82.0 ||
||Asbesto Sponge Hair| 86.0 ||
||Asbestos Fire Felt | 73.5 ||
|+-------------------+----------+|
+--------------------------------+
Based on one-inch covering.
The following suggestions may be of service:
Exposed radiating surfaces of all pipes, all high pressure steam
flanges, valve bodies and fittings, heaters and separators, should be
covered with non-conducting material wherever such covering will improve
plant economy. All main steam lines, engine and boiler branches, should
be covered with 2 inches of 85 per cent carbonate of magnesia or the
equivalent. Other lines may be covered with one inch of the same
material. All covering should be sectional in form and large surfaces
should be covered with blocks, except where such material would be
difficult to install, in which case plastic material should be used. In
the case of flanges the covering should be tapered back from the flange
in order that the bolts may be removed.
All surfaces should be painted before the covering is applied. Canvas is
ordinarily placed over the covering, held in place by wrought-iron or
brass bands.
Expansion and Support of Pipe--It is highly important that the piping be
so run that there will be no undue strains through the action of
expansion. Certain points are usually securely anchored and the
expansion of the piping at other points taken care of by providing
supports along which the piping will slide or by means of flexible
hangers. Where pipe is supported or anchored, it should be from the
building structure and not from boilers or prime movers. Where supports
are furnished, they should in general be of any of the numerous sliding
supports that are available. Expansion is taken care of by such a method
of support and by the providing of large radius bends where necessary.
It was formerly believed that piping would actually expand under steam
temperatures about one-half the theoretical amount due to the fact that
the exterior of the pipe would not reach the full temperature of the
steam contained. It would appear, however from recent experiments that
such actual expansion will in the case of well-covered pipe be very
nearly the theoretical amount. In one case noted, a steam header 293
feet long when heated under a working pressure of 190 pounds, the steam
superheated approximately 125 degrees, expanded 8¾ inches; the
theoretical amount of expansion under the conditions would be
approximately 9-35/64 inches.
[Illustration: Bankers Trust Building, New York City, Operation 900
Horse Power of Babcock & Wilcox Boilers]
FLOW OF STEAM THROUGH PIPES AND ORIFICES
Various formulae for the flow of steam through pipes have been advanced,
all having their basis upon Bernoulli's theorem of the flow of water
through circular pipes with the proper modifications made for the
variation in constants between steam and water. The loss of energy due
to friction in a pipe is given by Unwin (based upon Weisbach) as
f 2 v² W L
E_{f} = ---------- (37)
gd
where E is the energy loss in foot pounds due to the friction of W units
of weight of steam passing with a velocity of v feet per second through
a pipe d feet in diameter and L feet long; g represents the acceleration
due to gravity (32.2) and f the coefficient of friction.
Numerous values have been given for this coefficient of friction, f,
which, from experiment, apparently varies with both the diameter of pipe
and the velocity of the passing steam. There is no authentic data on the
rate of this variation with velocity and, as in all experiments, the
effect of change of velocity has seemed less than the unavoidable errors
of observation, the coefficient is assumed to vary only with the size of
the pipe.
Unwin established a relation for this coefficient for steam at a
velocity of 100 feet per second,
/ 3 \
f = K| 1 + --- | (38)
\ 10d /
where K is a constant experimentally determined, and d the internal
diameter of the pipe in feet.
If h represents the loss of head in feet, then
f 2 v² W L
E_{f} = Wh = ---------- (39)
gd
f 2 v² L
and h = -------- (40)
gd
If D represents the density of the steam or weight per cubic foot, and p
the loss of pressure due to friction in pounds per square inch, then
hD
p = --- (41)
144
and from equations (38), (40) and (41),
D v² L / 3 \
p = -------- × K | 1 + --- | (42)
72 g d \ 10d /
To convert the velocity term and to reduce to units ordinarily used, let
d_{1} the diameter of pipe in inches = 12d, and w = the flow in pounds
per minute; then
[pi] / d_{1}\
w = 60v × --- | ---- |^{2} D
4 \ 12 /
9.6 w
and v = --------------
[pi] d_{1}^2 D
Substituting this value and that of d in formula (42)
/ 3.6 \ w^{2} L
p = 0.04839 K | 1 + ----- | ----------- (43)
\ d_{1} / D d_{1}^{5}
Some of the experimental determinations for the value of K are:
K = .005 for water (Unwin).
K = .005 for air (Arson).
K = .0028 for air (St. Gothard tunnel experiments).
K = .0026 for steam (Carpenter at Oriskany).
K = .0027 for steam (G. H. Babcock).
The value .0027 is apparently the most nearly correct, and substituting
in formula (43) gives,
/ 3.6 \ w^{2} L
p = 0.000131 | 1 + ---- | ----------- (44)
\ d_{1}/ D d_{1}^{5}
/ pDd_{1}^{5} \
w = 87 | -------------- |^{½} (45)
| / 3.6 \ |
| | 1 + ---- | L |
\ \ d_{1}/ /
Where w = the weight of steam passing in pounds per minute,
p = the difference in pressure between the two ends of the pipe in
pounds per square inch,
D = density of steam or weight per cubic foot,[80]
d_{1} = internal diameter of pipe in inches,
L = length of pipe in feet.
TABLE 66
FLOW OF STEAM THROUGH PIPES
+---------------------------------------------------------------------------------------+
|Initl|Diameter[81] of Pipe in Inches, Length of Pipe = 240 Diameters |
|Gauge|---------------------------------------------------------------------------------+
|Press| ¾ | 1 | 1½ | 2 | 2½ | 3 | 4 | 5 | 6 | 8 | 10 | 12 | 15 | 18 |
|Pound|---------------------------------------------------------------------------------+
|/SqIn| Weight of Steam per Minute, in Pounds, With One Pound Loss of Pressure |
+-----+---------------------------------------------------------------------------------+
| 1 |1.16|2.07| 5.7|10.27|15.45|25.38| 46.85| 77.3|115.9|211.4| 341.1| 502.4| 804|1177|
| 10 |1.44|2.57| 7.1|12.72|19.15|31.45| 58.05| 95.8|143.6|262.0| 422.7| 622.5| 996|1458|
| 20 |1.70|3.02| 8.3|14.94|22.49|36.94| 68.20|112.6|168.7|307.8| 496.5| 731.3|1170|1713|
| 30 |1.91|3.40| 9.4|16.84|25.35|41.63| 76.84|126.9|190.1|346.8| 559.5| 824.1|1318|1930|
| 40 |2.10|3.74|10.3|18.51|27.87|45.77| 84.49|139.5|209.0|381.3| 615.3| 906.0|1450|2122|
| 50 |2.27|4.04|11.2|20.01|30.13|49.48| 91.34|150.8|226.0|412.2| 665.0| 979.5|1567|2294|
| 60 |2.43|4.32|11.9|21.38|32.19|52.87| 97.60|161.1|241.5|440.5| 710.6|1046.7|1675|2451|
| 70 |2.57|4.58|12.6|22.65|34.10|56.00|103.37|170.7|255.8|466.5| 752.7|1108.5|1774|2596|
| 80 |2.71|4.82|13.3|23.82|35.87|58.91|108.74|179.5|269.0|490.7| 791.7|1166.1|1866|2731|
| 90 |2.83|5.04|13.9|24.92|37.52|61.62|113.74|187.8|281.4|513.3| 828.1|1219.8|1951|2856|
| 100 |2.95|5.25|14.5|25.96|39.07|64.18|118.47|195.6|293.1|534.6| 862.6|1270.1|2032|2975|
| 120 |3.16|5.63|15.5|27.85|41.93|68.87|127.12|209.9|314.5|573.7| 925.6|1363.3|2181|3193|
| 150 |3.45|6.14|17.0|30.37|45.72|75.09|138.61|228.8|343.0|625.5|1009.2|1486.5|2378|3481|
+---------------------------------------------------------------------------------------+
This formula is the most generally accepted for the flow of steam in
pipes. Table 66 is calculated from this formula and gives the amount of
steam passing per minute that will flow through straight smooth pipes
having a length of 240 diameters from various initial pressures with one
pound difference between the initial and final pressures.
To apply this table for other lengths of pipe and pressure losses other
than those assumed, let L = the length and d the diameter of the pipe,
both in inches; l, the loss in pounds; Q, the weight under the
conditions assumed in the table, and Q_{1}, the weight for the changed
conditions.
For any length of pipe, if the weight of steam passing is the same as
given in the table, the loss will be,
L
l = ---- (46)
240d
If the pipe length is the same as assumed in the table but the loss is
different, the quantity of steam passing per minute will be,
Q_{1} = Ql^{½} (47)
For any assumed pipe length and loss of pressure, the weight will be,
/240dl\
Q_{1} = Q|-----|^{½} (48)
\ L /
TABLE 67
FLOW OF STEAM THROUGH PIPES
LENGTH OF PIPE 1000 FEET
+--------------------------------------------------++----------------------------------------+
| Discharge in Pounds per Minute corresponding to || Drop in Pressure in |
| Drop in Pressure on Right for Pipe Diameters || Pounds per Square Inch corresponding |
| in Inches in Top Line || to Discharge on Left: Densities |
| || and corresponding Absolute Pressures |
| || per Square Inch in First Two Lines |
+--------------------------------------------------++----------------------------------------+
| Diameter[82]--Discharge || Density--Pressure--Drop |
+--------------------------------------------------++----------------------------------------+
| 12 | 10 | 8 | 6 | 4 | 3 | 2½| 2 | 1½| 1 ||.208 |.230|.284|.328|.401|.443|.506|.548|
| In | In | In | In | In | In | In | In | In | In || 90 | 100| 125| 150| 180| 200| 230| 250|
+--------------------------------------------------++-------+--------------------------------+
|2328|1443| 799| 371|123. |55.9|28.8|18.1|6.81|2.52||18.10|16.4|13.3|11.1|9.39|8.50|7.44|6.87|
|2165|1341| 742| 344|114.6|51.9|27.6|16.8|6.52|2.34||15.60|14.1|11.4|9.60|8.09|7.33|6.41|5.92|
|1996|1237| 685| 318|106.0|47.9|26.4|15.5|6.24|2.16||13.3 |12.0|9.74|8.18|6.90|6.24|5.47|5.05|
|1830|1134| 628| 292| 97.0|43.9|25.2|14.2|5.95|1.98||11.1 |10.0|8.13|6.83|5.76|5.21|4.56|4.21|
|1663|1031| 571| 265| 88.2|39.9|24.0|12.9|5.67|1.80|| 9.25|8.36|6.78|5.69|4.80|4.34|3.80|3.51|
|1580| 979| 542| 252| 83.8|37.9|22.8|12.3|5.29|1.71|| 8.33|7.53|6.10|5.13|4.32|3.91|3.42|3.16|
|1497| 928| 514| 239| 79.4|35.9|21.6|11.6|5.00|1.62|| 7.48|6.76|5.48|4.60|3.88|3.51|3.07|2.84|
|1414| 876| 485| 226| 75.0|33.9|20.4|10.9|4.72|1.53|| 6.67|6.03|4.88|4.10|3.46|3.13|2.74|2.53|
|1331| 825| 457| 212| 70.6|31.9|19.2|10.3|4.43|1.44|| 5.91|5.35|4.33|3.64|3.07|2.78|2.43|2.24|
|1248| 873| 428| 199| 66.2|23.9|18.0|9.68|4.15|1.35|| 5.19|4.69|3.80|3.19|2.69|2.44|2.13|1.97|
|1164| 722| 400| 186| 61.7|27.9|16.8|9.03|3.86|1.26|| 4.52|4.09|3.31|2.78|2.34|2.12|1.86|1.72|
|1081| 670| 371| 172| 57.3|25.9|15.6|8.38|3.68|1.17|| 3.90|3.53|2.86|2.40|2.02|1.83|1.60|1.48|
| 998| 619| 343| 159| 52.9|23.9|14.4|7.74|3.40|1.08|| 3.32|3.00|2.43|2.04|1.72|1.56|1.36|1.26|
| 915| 567| 314| 146| 48.5|21.9|13.2|7.10|3.11|0.99|| 2.79|2.52|2.04|1.72|1.45|1.31|1.15|1.06|
| 832| 516| 286| 132| 44.1|20.0|12.0|6.45|2.83|0.90|| 2.31|2.09|1.69|1.42|1.20|1.08|.949|.877|
| 748| 464| 257| 119| 39.7|18.0|10.8|5.81|2.55|0.81|| 1.87|1.69|1.37|1.15| .97|.878|.769|.710|
| 665| 412| 228| 106| 35.3|16.0| 9.6|5.16|2.26|0.72|| 1.47|1.33|1.08|.905|.762|.690|.604|.558|
| 582| 361| 200|92.8| 30.9|14.0| 8.4|4.52|1.98|0.63|| 1.13|1.02|.828|.695|.586|.531|.456|.429|
+--------------------------------------------------++----------------------------------------+
To get the pressure drop for lengths other than 1000 feet, multiply by
lengths in feet ÷ 1000.
Example: Find the weight of steam at 100 pounds initial gauge pressure,
which will pass through a 6-inch pipe 720 feet long with a pressure drop
of 4 pounds. Under the conditions assumed in the table, 293.1 pounds
would flow per minute; hence, Q = 293.1, and
_ _
| 240×6×4 |
Q_{1} = 293.1 | ------- |^{½} = 239.9 pounds
|_ 720×12_|
Table 67 may be frequently found to be of service in problems involving
the flow of steam. This table was calculated by Mr. E. C. Sickles for a
pipe 1000 feet long from formula (45), except that from the use of a
value of the constant K = .0026 instead of .0027, the constant in the
formula becomes 87.45 instead of 87.
In using this table, the pressures and densities to be considered, as
given at the top of the right-hand portion, are the mean of the initial
and final pressures and densities. Its use is as follows: Assume an
allowable drop of pressure through a given length of pipe. From the
value as found in the right-hand column under the column of mean
pressure, as determined by the initial and final pressures, pass to the
left-hand portion of the table along the same line until the quantity is
found corresponding to the flow required. The size of the pipe at the
head of this column is that which will carry the required amount of
steam with the assumed pressure drop.
The table may be used conversely to determine the pressure drop through
a pipe of a given diameter delivering a specified amount of steam by
passing from the known figure in the left to the column on the right
headed by the pressure which is the mean of the initial and final
pressures corresponding to the drop found and the actual initial
pressure present.
For a given flow of steam and diameter of pipe, the drop in pressure is
proportional to the length and if discharge quantities for other lengths
of pipe than 1000 feet are required, they may be found by proportion.
TABLE 68
FLOW OF STEAM INTO THE ATMOSPHERE
__________________________________________________________________
| | | | | |
| Absolute | Velocity | Actual | Discharge | Horse Power |
| Initial | of Outflow | Velocity | per Square | per Square |
| Pressure | at Constant | of Outflow | Inch of | Inch of |
| per Square | Density | Expanded | Orifice | Orifice if |
| Inch | Feet per | Feet per | per Minute | Horse Power |
| Pounds | Second | Second | Pounds | = 30 Pounds |
| | | | | per Hour |
|____________|_____________|____________|____________|_____________|
| | | | | |
| 25.37 | 863 | 1401 | 22.81 | 45.6 |
| 30. | 867 | 1408 | 26.84 | 53.7 |
| 40. | 874 | 1419 | 35.18 | 70.4 |
| 50. | 880 | 1429 | 44.06 | 88.1 |
| 60. | 885 | 1437 | 52.59 | 105.2 |
| 70. | 889 | 1444 | 61.07 | 122.1 |
| 75. | 891 | 1447 | 65.30 | 130.6 |
| 90. | 895 | 1454 | 77.94 | 155.9 |
| 100. | 898 | 1459 | 86.34 | 172.7 |
| 115. | 902 | 1466 | 98.76 | 197.5 |
| 135. | 906 | 1472 | 115.61 | 231.2 |
| 155. | 910 | 1478 | 132.21 | 264.4 |
| 165. | 912 | 1481 | 140.46 | 280.9 |
| 215. | 919 | 1493 | 181.58 | 363.2 |
|____________|_____________|____________|____________|_____________|
Elbows, globe valves and a square-ended entrance to pipes all offer
resistance to the passage of steam. It is customary to measure the
resistance offered by such construction in terms of the diameter of the
pipe. Many formulae have been advanced for computing the length of pipe
in diameters equivalent to such fittings or valves which offer
resistance. These formulae, however vary widely and for ordinary
purposes it will be sufficiently accurate to allow for resistance at the
entrance of a pipe a length equal to 60 times the diameter; for a right
angle elbow, a length equal to 40 diameters, and for a globe valve a
length equal to 60 diameters.
The flow of steam of a higher toward a lower pressure increases as the
difference in pressure increases to a point where the external pressure
becomes 58 per cent of the absolute initial pressure. Below this point
the flow is neither increased nor decreased by a reduction of the
external pressure, even to the extent of a perfect vacuum. The lowest
pressure for which this statement holds when steam is discharged into
the atmosphere is 25.37 pounds. For any pressure below this figure, the
atmospheric pressure, 14.7 pounds, is greater than 58 per cent of the
initial pressure. Table 68, by D. K. Clark, gives the velocity of
outflow at constant density, the actual velocity of outflow expanded
(the atmospheric pressure being taken as 14.7 pounds absolute, and the
ratio of expansion in the nozzle being 1.624), and the corresponding
discharge per square inch of orifice per minute.
Napier deduced an approximate formula for the outflow of steam into the
atmosphere which checks closely with the figures just given. This
formula is:
pa
W = ---- (49)
70
Where W = the pounds of steam flowing per second,
p = the absolute pressure in pounds per square inch,
and a = the area of the orifice in square inches.
In some experiments made by Professor C. H. Peabody, in the flow of
steam through pipes from ¼ inch to 1½ inches long and ¼ inch in
diameter, with rounded entrances, the greatest difference from Napier's
formula was 3.2 per cent excess of the experimental over the calculated
results.
For steam flowing through an orifice from a higher to a lower pressure
where the lower pressure is greater than 58 per cent of the higher, the
flow per minute may be calculated from the formula:
W = 1.9AK ((P - d)d)^{½} (50)
Where W = the weight of steam discharged in pounds per minute,
A = area of orifice in square inches,
P = the absolute initial pressure in pounds per square inch,
d = the difference in pressure between the two sides in pounds
per square inch,
K = a constant = .93 for a short pipe, and .63 for a hole in a
thin plate or a safety valve.
[Illustration: Vesta Coal Co., California, Pa., Operating at this Plant
3160 Horse Power of Babcock & Wilcox Boilers]
HEAT TRANSFER
The rate at which heat is transmitted from a hot gas to a cooler metal
surface over which the gas is flowing has been the subject of a great
deal of investigation both from the experimental and theoretical side. A
more or less complete explanation of this process is necessary for a
detailed analysis of the performance of steam boilers. Such information
at the present is almost entirely lacking and for this reason a boiler,
as a physical piece of apparatus, is not as well understood as it might
be. This, however, has had little effect in its practical development
and it is hardly possible that a more complete understanding of the
phenomena discussed will have any radical effect on the present design.
The amount of heat that is transferred across any surface is usually
expressed as a product, of which one factor is the slope or linear rate
of change in temperature and the other is the amount of heat transferred
per unit's difference in temperature in unit's length. In Fourier's
analytical theory of the conduction of heat, this second factor is taken
as a constant and is called the "conductivity" of the substance.
Following this practice, the amount of heat absorbed by any surface from
a hot gas is usually expressed as a product of the difference in
temperature between the gas and the absorbing surface into a factor
which is commonly designated the "transfer rate". There has been
considerable looseness in the writings of even the best authors as to
the way in which the gas temperature difference is to be measured. If
the gas varies in temperature across the section of the channel through
which it is assumed to flow, and most of them seem to consider that this
would be the case, there are two mean gas temperatures, one the mean of
the actual temperatures at any time across the section, and the other
the mean temperature of the entire volume of the gas passing such a
section in any given time. Since the velocity of flow will of a
certainty vary across the section, this second mean temperature, which
is one tacitly assumed in most instances, may vary materially from the
first. The two mean temperatures are only approximately equal when the
actual temperature measured across the section is very nearly a
constant. In what follows it will be assumed that the mean temperature
measured in the second way is referred to. In English units the
temperature difference is expressed in Fahrenheit degrees and the
transfer rate in B. t. u.'s per hour per square foot of surface. Pecla,
who seems to have been one of the first to consider this subject
analytically, assumed that the transfer rate was constant and
independent both of the temperature differences and the velocity of the
gas over the surface. Rankine, on the other hand, assumed that the
transfer rate, while independent of the velocity of the gas, was
proportional to the temperature difference, and expressed the total
amount of heat absorbed as proportional to the square of the difference
in temperature. Neither of these assumptions has any warrant in either
theory or experiment and they are only valuable in so far as their use
determine formulae that fit experimental results. Of the two, Rankine's
assumption seems to lead to formulae that more nearly represent actual
conditions. It has been quite fully developed by William Kent in his
"Steam Boiler Economy". Professor Osborne Reynolds, in a short paper
reprinted in Volume I of his "Scientific Papers", suggests that the
transfer rate is proportional to the product of the density and velocity
of the gas and it is to be assumed that he had in mind the mean
velocity, density and temperature over the section of the channel
through which the gas was assumed to flow. Contrary to prevalent
opinion, Professor Reynolds gave neither a valid experimental nor a
theoretical explanation of his formula and the attempts that have been
made since its first publication to establish it on any theoretical
basis can hardly be considered of scientific value. Nevertheless,
Reynolds' suggestion was really the starting point of the scientific
investigation of this subject and while his formula cannot in any sense
be held as completely expressing the facts, it is undoubtedly correct to
a first approximation for small temperature differences if the additive
constant, which in his paper he assumed as negligible, is given a
value.[83]
Experimental determinations have been made during the last few years of
the heat transfer rate in cylindrical tubes at comparatively low
temperatures and small temperature differences. The results at different
velocities have been plotted and an empirical formula determined
expressing the transfer rate with the velocity as a factor. The exponent
of the power of the velocity appearing in the formula, according to
Reynolds, would be unity. The most probable value, however, deduced from
most of the experiments makes it less than unity. After considering
experiments of his own, as well as experiments of others, Dr. Wilhelm
Nusselt[84] concludes that the evidence supports the following formulae:
_ _
[lambda]_{w} | w c_{p} [delta] |
a = b ------------ | --------------- |^{u}
d^{1-u} |_ [lambda] _|
Where a is the transfer rate in calories per hour per square meter
of surface per degree centigrade difference in temperature,
u is a physical constant equal to .786 from Dr. Nusselt's
experiments,
b is a constant which, for the units given below, is 15.90,
w is the mean velocity of the gas in meters per second,
c_{p} is the specific heat of the gas at its mean temperature
and pressure in calories per kilogram,
[delta] is the density in kilograms per cubic meter,
[lambda] is the conductivity at the mean temperature and pressure in
calories per hour per square meter per degree centigrade
temperature drop per meter,
[lambda]_{w} is the conductivity of the steam at the temperature of the
tube wall,
d is the diameter of the tube in meters.
If the unit of time for the velocity is made the hour, and in the place
of the product of the velocity and density is written its equivalent,
the weight of gas flowing per hour divided by the area of the tube, this
equation becomes:
_ _
[lambda]_{w} | Wc_{p} |
a = .0255 ------------ | --------- |^{.786}
d^{.214} |_ A[lambda] _|
where the quantities are in the units mentioned, or, since the constants
are absolute constants, in English units,
a is the transfer rate in B. t. u. per hour per square foot
of surface per degree difference in temperature,
W is the weight in pounds of the gas flowing through the tube
per hour,
A is the area of the tube in square feet,
d is the diameter of the tube in feet,
c_{p} is the specific heat of the gas at constant pressure,
[lambda] is the conductivity of the gas at the mean temperature and
pressure in B. t. u. per hour per square foot of surface
per degree Fahrenheit drop in temperature per foot,
[lambda]_{w} is the conductivity of the steam at the temperature of the
wall of the tube.
The conductivities of air, carbonic acid gas and superheated steam, as
affected by the temperature, in English units, are:
Conductivity of air .0122 (1 + .00132 T)
Conductivity of carbonic acid gas .0076 (1 + .00229 T)
Conductivity of superheated steam .0119 (1 + .00261 T)
where T is the temperature in degrees Fahrenheit.
Nusselt's formulae can be taken as typical of the number of other
formulae proposed by German, French and English writers.[85] Physical
properties, in addition to the density, are introduced in the form of
coefficients from a consideration of the physical dimensions of the
various units and of the theoretical formulae that are supposed to
govern the flow of the gas and the transfer of heat. All assume that the
correct method of representing the heat transfer rate is by the use of
one term, which seems to be unwarranted and probably has been adopted on
account of the convenience in working up the results by plotting them
logarithmically. This was the method Professor Reynolds used in
determining his equation for the loss in head in fluids flowing through
cylindrical pipes and it is now known that the derived equation cannot
be considered as anything more than an empirical formula. It, therefore,
is well for anyone considering this subject to understand at the outset
that the formulae discussed are only of an empirical nature and
applicable to limited ranges of temperature under the conditions
approximately the same as those surrounding the experiments from which
the constants of the formula were determined.
It is not probable that the subject of heat transfer in boilers will
ever be on any other than an experimental basis until the mathematical
expression connecting the quantity of fluid which will flow through a
channel of any section under a given head has been found and some
explanation of its derivation obtained. Taking the simplest possible
section, namely, a circle, it is found that at low velocities the loss
of head is directly proportional to the velocity and the fluid flows in
straight stream lines or the motion is direct. This motion is in exact
accordance with the theoretical equations of the motion of a viscous
fluid and constitutes almost a direct proof that the fundamental
assumptions on which these equations are based are correct. When,
however, the velocity exceeds a value which is determinable for any size
of tube, the direct or stream line motion breaks down and is replaced by
an eddy or mixing flow. In this flow the head loss by friction is
approximately, although not exactly, proportional to the square of the
velocity. No explanation of this has ever been found in spite of the
fact that the subject has been treated by the best mathematicians and
physicists for years back. It is to be assumed that the heat transferred
during the mixing flow would be at a much higher rate than with the
direct or stream line flow, and Professors Croker and Clement[86] have
demonstrated that this is true, the increase in the transfer being so
marked as to enable them to determine the point of critical velocity
from observing the rise in temperature of water flowing through a tube
surrounded by a steam jacket.
The formulae given apply only to a mixing flow and inasmuch as, from
what has just been stated, this form of motion does not exist from zero
velocity upward, it follows that any expression for the heat transfer
rate that would make its value zero when the velocity is zero, can
hardly be correct. Below the critical velocity, the transfer rate seems
to be little affected by change in velocity and Nusselt,[87] in another
paper which mathematically treats the direct or stream line flow,
concludes that, while it is approximately constant as far as the
velocity is concerned in a straight cylindrical tube, it would vary from
point to point of the tube, growing less as the surface passed over
increased.
It should further be noted that no account in any of this experimental
work has been taken of radiation of heat from the gas. Since the common
gases absorb very little radiant heat at ordinary temperatures, it has
been assumed that they radiate very little at any temperature. This may
or may not be true, but certainly a visible flame must radiate as well
as absorb heat. However this radiation may occur, since it would be a
volume phenomenon rather than a surface phenomenon it would be
considered somewhat differently from ordinary radiation. It might apply
as increasing the conductivity of the gas which, however independent of
radiation, is known to increase with the temperature. It is, therefore,
to be expected that at high temperatures the rate of transfer will be
greater than at low temperatures. The experimental determinations of
transfer rates at high temperatures are lacking.
Although comparatively nothing is known concerning the heat radiation
from gases at high temperatures, there is no question but what a large
proportion of the heat absorbed by a boiler is received direct as
radiation from the furnace. Experiments show that the lower row of tubes
of a Babcock & Wilcox boiler absorb heat at an average rate per square
foot of surface between the first baffle and the front headers
equivalent to the evaporation of from 50 to 75 pounds of water from and
at 212 degrees Fahrenheit per hour. Inasmuch as in these experiments no
separation could be made between the heat absorbed by the bottom of the
tube and that absorbed by the top, the average includes both maximum and
minimum rates for those particular tubes and it is fair to assume that
the portion of the tubes actually exposed to the furnace radiations
absorb heat at a higher rate. Part of this heat was, of course absorbed
by actual contact between the hot gases and the boiler heating surface.
A large portion of it, however, must have been due to radiation. Whether
this radiant heat came from the fire surface and the brickwork and
passed through the gases in the furnace with little or no absorption, or
whether, on the other hand, the radiation were absorbed by the furnace
gases and the heat received by the boiler was a secondary radiation from
the gases themselves and at a rate corresponding to the actual gas
temperature, is a question. If the radiations are direct, then the term
"furnace temperature", as usually used has no scientific meaning, for
obviously the temperature of the gas in the furnace would be entirely
different from the radiation temperature, even were it possible to
attach any significance to the term "radiation temperature", and it is
not possible to do this unless the radiations are what are known as
"full radiations" from a so-called "black body". If furnace radiation
takes place in this manner, the indications of a pyrometer placed in a
furnace are hard to interpret and such temperature measurements can be
of little value. If the furnace gases absorb the radiations from the
fire and from the brickwork of the side walls and in their turn radiate
heat to the boiler surface, it is scientifically correct to assume that
the actual or sensible temperature of the gas would be measured by a
pyrometer and the amount of radiation could be calculated from this
temperature by Stefan's law, which is to the effect that the rate of
radiation is proportional to the fourth power of the absolute
temperature, using the constant with the resulting formula that has been
determined from direct experiment and other phenomena. With this
understanding of the matter, the radiations absorbed by a boiler can be
taken as equal to that absorbed by a flat surface, covering the portion
of the boiler tubes exposed to the furnace and at the temperature of the
tube surface, when completely exposed on one side to the radiations from
an atmosphere at the temperature in the furnace. With this assumption,
if S^{1} is the area of the surface, T the absolute temperature of the
furnace gases, t the absolute temperature of the tube surface of the
boiler, the heat absorbed per hour measured in B. t. u.'s is equal to
_ _
| / T \ / t \ |
1600 | |----|^{4} - |----|^{4}| S^{1}
|_\1000/ \1000/ _|
In using this formula, or in any work connected with heat transfer, the
external temperature of the boiler heating surface can be taken as that
of saturated steam at the pressure under which the boiler is working,
with an almost negligible error, since experiments have shown that with
a surface clean internally, the external surface is only a few degrees
hotter than the water in contact with the inner surface, even at the
highest rates of evaporation. Further than this, it is not conceivable
that in a modern boiler there can be much difference in the temperature
of the boiler in the different parts, or much difference between the
temperature of the water and the temperature of the steam in the drums
which is in contact with it.
If the total evaporation of a boiler measured in B. t. u.'s per hour is
represented by E, the furnace temperature by T_{1}, the temperature of
the gas leaving the boiler by T_{2}, the weight of gas leaving the
furnace and passing through the setting per hour by W, the specific heat
of the gas by C, it follows from the fact that the total amount of heat
absorbed is equal to the heat received from radiation plus the heat
removed from the gases by cooling from the temperature T_{1} to the
temperature T_{2}, that
_ _
| / T \ / t \ |
E = 1600 | |----|^{4} - |----|^{4}| S^{1} + WC(T_{1} - T_{2})
|_\1000/ \1000/ _|
This formula can be used for calculating the furnace temperature when E,
t and T_{2} are known but it must be remembered that an assumption
which is probably, in part at least, incorrect is implied in using it or
in using any similar formula. Expressed in this way, however, it seems
more rational than the one proposed a few years ago by Dr. Nicholson[88]
where, in place of the surface exposed to radiation, he uses the grate
surface and assumes the furnace gas temperature as equal to the fire
temperature.
If the heat transfer rate is taken as independent of the gas temperature
and the heat absorbed by an element of the surface in a given time is
equated to the heat given out from the gas passing over this surface in
the same time, a single integration gives
Rs
(T - t) = (T_{1} - t) e^{- --}
WC
where s is the area of surface passed over by the gases from the furnace
to any point where the gas temperature T is measured, and the rate of
heat transfer is R. As written, this formula could be used for
calculating the temperature of the gas at any point in the boiler
setting. Gas temperatures, however, calculated in this way are not to be
depended upon as it is known that the transfer rate is not independent
of the temperature. Again, if the transfer rate is assumed as varying
directly with the weight of the gases passing, which is Reynolds'
suggestion, it is seen that the weight of the gases entirely disappears
from the formula and as a consequence if the formula was correct, as
long as the temperature of the gas entering the surface from the furnace
was the same, the temperatures throughout the setting would be the same.
This is known also to be incorrect. If, however, in place of T is
written T_{2} and in place of s is written S, the entire surface of the
boiler, and the formula is re-arranged, it becomes:
_ _
WC | T_{1} - t |
R = --- Log[89]| --------- |
S |_ T_{2} - t _|
This formula can be considered as giving a way of calculating an average
transfer rate. It has been used in this way for calculating the average
transfer rate from boiler tests in which the capacity has varied from an
evaporation of a little over 3 pounds per square foot of surface up to
15 pounds. When plotted against the gas weights, it was found that the
points were almost exactly on a line. This line, however, did not pass
through the zero point but started at a point corresponding to
approximately a transfer rate of 2. Checked out against many other
tests, the straight line law seems to hold generally and this is true
even though material changes are made in the method of calculating the
furnace temperature. The inclination of the line, however, varied
inversely as the average area for the passage of the gas through the
boiler. If A is the average area between all the passes of the boiler,
the heat transfer rate in Babcock & Wilcox type boilers with ordinary
clean surfaces can be determined to a rather close approximation from
the formula:
W
R = 2.00 + .0014 -
A
The manner in which A appears in this formula is the same as it would
appear in any formula in which the heat transfer rate was taken as
depending upon the product of the velocity and the density of the gas
jointly, since this product, as pointed out above, is equivalent to W/A.
Nusselt's experiments, as well as those of others, indicate that the
ratio appears in the proper way.
While the underlying principles from which the formula for this average
transfer rate was determined are questionable and at best only
approximately correct, it nevertheless follows that assuming the
transfer rate as determined experimentally, the formula can be used in
an inverse way for calculating the amount of surface required in a
boiler for cooling the gases through a range of temperature covered by
the experiments and it has been found that the results bear out this
assumption. The practical application of the theory of heat transfer, as
developed at present, seems consequently to rest on these last two
formulae, which from their nature are more or less empirical.
Through the range in the production of steam met with in boilers now in
service which in the marine type extends to the average evaporation of
12 to 15 pounds of water from and at 212 degrees Fahrenheit per square
foot of surface, the constant 2 in the approximate formula for the
average heat transfer rate constitutes quite a large proportion of the
total. The comparative increase in the transfer rate due to a change in
weight of the gases is not as great consequently as it would be if this
constant were zero. For this reason, with the same temperature of the
gases entering the boiler surface, there will be a gradual increase in
the temperature of the gases leaving the surface as the velocity or
weight of flow increases and the proportion of the heat contained in the
gases entering the boiler which is absorbed by it is gradually reduced.
It is, of course, possible that the weight of the gases could be
increased to such an amount or the area for their passage through the
boiler reduced by additional baffles until the constant term in the heat
transfer formula would be relatively unimportant. Under such conditions,
as pointed out previously, the final gas temperature would be unaffected
by a further increase in the velocity of the flow and the fraction of
the heat carried by the gases removed by the boiler would be constant.
Actual tests of waste heat boilers in which the weight of gas per square
foot of sectional area for its passage is many times more than in
ordinary installations show, however, that this condition has not been
attained and it will probably never be attained in any practical
installation. It is for this reason that the conclusions of Dr.
Nicholson in the paper referred to and of Messrs. Kreisinger and Ray in
the pamphlet "The Transmission of Heat into Steam Boilers", published by
the Department of the Interior in 1912, are not applicable without
modification to boiler design.
In superheaters the heat transfer is effected in two different stages;
the first transfer is from the hot gas to the metal of the superheater
tube and the second transfer is from the metal of the tube to the steam
on the inside. There is, theoretically, an intermediate stage in the
transfer of the heat from the outside to the inside surface of the tube.
The conductivity of steel is sufficient, however, to keep the
temperatures of the two sides of the tube very nearly equal to each
other so that the effect of the transfer in the tube itself can be
neglected. The transfer from the hot gas to the metal of the tube takes
place in the same way as with the boiler tubes proper, regard being paid
to the temperature of the tube which increases as the steam is heated.
The transfer from the inside surface of the tube to the steam is the
inverse of the process of the transfer of the heat on the outside and
seems to follow the same laws. The transfer rate, therefore, will
increase with the velocity of the steam through the tube. For this
reason, internal cores are quite often used in superheaters and actually
result in an increase in the amount of superheat obtained from a given
surface. The average transfer rate in superheaters based on a difference
in mean temperature between the gas on the outside of the tubes and the
steam on the inside of the tubes is if R is the transfer rate from the
gas to the tube and r the rate from the tube to the steam:
Rr
-----
R + r
and is always less than either R or r. This rate is usually greater than
the average transfer rate for the boiler as computed in the way outlined
in the preceding paragraphs. Since, however, steam cannot, under any
imagined set of conditions, take up more heat from a tube than would
water at the same average temperature, this fact supports the contention
made that the actual transfer rate in a boiler must increase quite
rapidly with the temperatures. The actual transfer rates in superheaters
are affected by so many conditions that it has not so far been possible
to evolve any formula of practical value.
[Illustration: Iron City Brewery of the Pittsburgh Brewing Co.,
Pittsburgh, Pa, Operating in this Plant 2000 Horse Power of Babcock &
Wilcox Boilers]
INDEX
PAGE
Absolute pressure 117
Absolute zero 80
Accessibility of Babcock & Wilcox boiler 59
Acidity in boiler feed water 106
Actual evap. corresponding to boiler horse power 288
Advantages of Babcock & Wilcox boilers 61
Stoker firing 195
Water tube over fire tube boilers 61
Air, composition of 147
In boiler feed water 106
Properties of 147
Required for combustion 152, 156
Specific heat of 148
Supplied for combustion 157
Vapor in 149
Volume of 147
Weight of 147
Alkalinity in boiler feed water 103
Testing feed for 103
Altitude, boiling point of water at 97
Chimney sizes corrected for 248
Alum in feed water treatment 106
A. S. M. E. code for boiler testing 267
Analyses, comparison of proximate and ultimate 183
Proximate coal, and heating values 177
Analysis, coal, proximate, methods of 176
Coal, ultimate 173
Determination of heating value from 173
Analysis, Flue gas 155
Flue gas, methods of 160
Flue gas, object of 155
Anthracite coal 166
Combustion rates with 246
Distribution of 167
Draft required for 246
Firing 190
Grate ratio for 191
Semi 166
Sizes of 190
Steam as aid to burning 191
Thickness of fires with 191
Arches, fire brick, as aid to combustion 190
Fire brick, for 304
Fire brick, laying 305
Automatic stokers, advantages of 195
Overfeed 196
Traveling grate 197
Traveling grate, Babcock & Wilcox 194
Underfeed 196
Auxiliaries, exhaust from, in heating feed water 113
Superheated steam with 142
Auxiliary grates, with blast furnace gas 228
With oil fuel 225
With waste heat 235
Babcock, G. H., lecture on circulation of water in Boilers 28
Lecture on theory of steam making 92
Babcock & Wilcox Co., Works at Barberton, Ohio 7
Works at Bayonne, N. J. 6
Babcock & Wilcox boiler, accessibility of 59
Advantages of 61
Circulation of water in 57, 66
Construction of 49
Cross boxes 50
Cross drum 53
Cross drum, dry steam with 71
Drumheads 49
Drums 49
Durability 75
Evolution of 39
Fittings 55
Fixtures 55
Fronts 53
Handhole fittings 50, 51
Headers 50, 51
Inclined header, wrought steel 54
Inspection 75
Life of 76
Materials entering into the construction of 59
Mud drums 51
Path of gases in 57
Path of water in 57
Rear tube doors of 53, 74
Repairs 75
Safety of 66
Sections 50
Set for utilizing waste heat 236
Set with Babcock & Wilcox chain grate stoker 12
Set with bagasse furnace 208
Set with Peabody oil furnace 222
Supports, cross drum 53
Supports, longitudinal drum 52
Tube doors 53
Vertical header, cast iron 58
Vertical header, wrought steel 48
Babcock & Wilcox chain grate stoker 194
Babcock & Wilcox superheater 136
Bagasse, composition of 206
Furnace 209
Heat, value of 206
Tests of Babcock & Wilcox boilers with 210
Value of diffusion 207
Barium carbonate in feed water treatment 106
Barium hydrate in feed water treatment 106
Barrus draft gauge 254
Bituminous coal, classification of 167
Combustion rates with 246
Composition of 177
Distribution of 168
Firing methods 193
Semi 166
Sizes of 191
Thickness of fire with 193
Blast furnace gas, burners for 228
Combustion of 228
Composition of 227
Stacks for 228
Boiler, Blakey's 23
Brickwork, care of 307
Circulation of water in steam 28
Compounds 109
Development of water tube 23
Eve's 24
Evolution of Babcock & Wilcox 39
Fire tube, compared with water tube 61
Guerney's 24
Horse power 263
Loads, economical 283
Perkins' 24
Room piping 108
Room practice 297
Rumsey's 23
Stevens', John 23
Stevens', John Cox 23
Units, number of 289
Units, size of 289
Wilcox's 25
Woolf's 23
Boilers, capacity of 278
Care of 291
Efficiency of 256
Horse power of 265
Operation of 291
Requirements of steam 27
Testing 267
Boiling point 86
Of various substances 86
Of water as affected by altitude 97
Brick, fire 304
Arches 305
Classification of 304
Compression of 303
Expansion of 303
Hardness of 303
Laying up 305
Nodules, ratio of 303
Nodules, size of 303
Plasticity of 302
Brick, red 302
Brickwork, care of 307
British thermal unit 83
Burners, blast furnace gas 228
By-product coke oven gas 231
Natural gas 231
Oil 217
Oil, capacity of 221
Oil, mechanical atomizing 219
Oil, operation of 223
Oil, steam atomizing 218
Oil, steam consumption of 220
Burning hydrogen, loss due to moisture formed in 261
By-product coke oven gas burners 231
By-product coke oven gas, combustion of 231
By-product coke oven gas, composition and heat value of 231
Calorie 83
Calorific value (see Heat value).
Calorimeter, coal, Mahler bomb 184
Mahler bomb, method of correction 187
Mahler bomb, method of operation of 185
Calorimeter, steam, compact type of throttling 132
Correction for 131
Location of nozzles for 134
Normal reading 131
Nozzles 134
Separating 133
Throttling 129
Capacity of boilers 264, 278
As affecting economy 276
Economical loads 283
With bagasse 210
With blast furnace gas 228
With coal 280
With oil fuel 224
Capacity of natural gas burners 229
Capacity of oil burners 221
Carbon dioxide in flue gases 154
Unreliability of readings taken alone 162
Carbon, fixed 165
Incomplete combustion of, loss due to 158
Monoxide, heat value of 151
Monoxide, in flue gases 155
Unconsumed in ash, loss due to 261
Care of boilers when out of service 300
Casings, boilers 307
Causticity of feed water 103
Testing for 105
Celsius thermometer scale 79
Centigrade thermometer scale 79
Chain grate stoker, Babcock & Wilcox 194
Chemicals required in feed water treatment 105
Chimney gases, losses in 158, 159
Chimneys (see Draft).
Correction in dimensions for altitude 248
Diameter of 243
Draft available from 241
Draft loss in 239
For blast furnace gas 253
For oil fuel 251
For wood fuel 254
Height of 243
Horse power they will serve 250
Circulation of water in Babcock & Wilcox boilers 57, 66
Of water in steam boilers 28
Results of defective 62, 66, 67
Classification of coals 166
Fire brick 304
Feed water difficulties 100
Fuels 165
Cleaners, turbine tube 299
Cleaning, ease of, Babcock & Wilcox boilers 73
Closed feed water heaters 111
Coal, Alaska 169
Analyses and heat value 177
Analysis, proximate 176
Analysis, ultimate 173
Anthracite 166
Bituminous 167
Cannel 167
Classification of 165, 166
Combustion of 190
Comparison with oil 214
Consumption, increase due to superheat 139
Distribution of 167
Formation of 165
Lignite 167
Records 293
Semi-anthracite 166
Semi-bituminous 166
Sizes of anthracite 190
Sizes of bituminous 191
Code of A. S. M. E. for boiler testing 267
Coefficient of expansion of various substances 87
Coke 171
Oven gas, by-product, burners 231
Oven gas, by-product, combustion of 231
Oven gas, by-product, composition and heat value of 231
Coking method of firing 195
Color as indication of temperature 91
Combination furnaces 224
Combustible in fuels 150
Combustion 150
Air required for 152, 156
Air supplied for 157
Combustion of coal 190
Of gaseous fuels 227
Of liquid fuels 212
Of solid fuels other than coal 201
Composition of bagasse 205
Blast furnace gas 227
By-product coke oven gas 231
Coals 177
Natural gas 229
Oil 213
Wood 201
Compounds, boiler 109
Compressibility of water 97
Compression of fire brick 303
Condensation, effect of superheated steam on 140
In steam pipes 313
Consumption, heat, of engines 141
Correction, stem, for thermometers 80
For normal reading in steam calorimeter 131
For radiation, bomb calorimeter 187
Corrosion 101, 106
Coverings, pipe 315
Cross drum, Babcock & Wilcox boiler 52, 53, 60
Dry steam with 71
Draft area as affecting economy in Babcock & Wilcox boilers 70
Available from chimneys 241
Draft loss in chimneys 239
Loss in boilers 245
Loss in flues 243
Loss in furnaces 245
Draft required for anthracite 246
Required for various fuels 246
Drums, Babcock & Wilcox, cross 53
Cross, boxes 50
Heads 49
Longitudinal 49
Manholes 49
Nozzles on 50
Dry steam in Babcock & Wilcox boilers 71
Density of gases 147
Steam 115
Dulong's formula for heating value 173
Ebullition, point of 86
Economizers 111
Efficiency of boilers, chart of 258
Combustible basis 256
Dry coal basis 256
Increase in, due to superheaters 139
Losses in (see Heat balance) 259
Testing 267
Test _vs._ operating 278
Variation in, with capacity 284
With coal 288
With oil 224
Ellison draft gauge 254
Engine, Hero's 13
Engines, superheated steam with 141
Equivalent evaporation from and at 212 degrees 116
Eve's boiler 24
Evolution of Babcock & Wilcox boiler 39
Exhaust steam from auxiliaries 113
Expansion, coefficient of 87
Of fire brick 303
Of pipe 315
Pyrometer 89
Factor of evaporation 117
Fahrenheit thermometer scale 79
Fans, use of, in waste heat work 233
Feed water, air in 106
As affecting capacity 279
Boiler 100
Feed water heaters, closed 111
Economizers 111
Open 111
Feed water heating, methods of 111
Saving by 110
Feed water, impurities in 100
Lines 312
Method of feeding 110
Feed water treatment 102
Chemical 102
Chemical, lime and soda process 102
Chemical, lime process 102
Chemical, soda process 102
Chemicals used in lime and soda process 105
Combined heat and chemical 105
Heat 102
Less usual reagents 106
Firing, advantages of stoker 195
Methods for anthracite 190
Bituminous 193
Lignite 195
Fittings, handhole in Babcock & Wilcox boilers 50, 51
Pipe 311
Superheated steam 145
With Babcock & Wilcox boilers 55
Fixtures with Babcock & Wilcox boilers 55
Flanges, pipe 309
Flow of steam into pressure above atmosphere 317
Into the atmosphere 328
Through orifices 317
Through pipes 317
Flue gas analysis 155
Conversion of volumetric to weight 161
Methods of making 160
Object of 155
Orsat apparatus 159
Flue gas, composition of 155
Losses in 158, 159
Weight per pound of carbon in fuel 158
Weight per pound of fuel 158
Weight resulting from combustion 157
Foaming 102, 107
Fuel analysis, proximate 176
Ultimate 173
Fuel calorimeter, Mabler bomb 184
Tests, method of making 186
Fuels, classification of 165
Gaseous, and their combustion 227
Fuels, liquid, and their combustion 212
Solid, coal 190
Solid, other than coal 201
Furnace, bagasse 209
Blast furnace gas 228
By-product coke oven gas 231
Combination wood and oil 225
Efficiency of 283
Natural gas 229
Peabody oil 222
Webster 55
Wood burning 201, 202
Galvanic action 107
Gas, blast furnace, burners 228
Combustion of 228
Composition of 227
Gas, by-product coke oven, burners 231
Combustion of 231
Composition of and heat value 231
Gas, natural, burners 229
Combustion of 229
Composition and heat value of 229
Gases, chimney, losses in 158, 159
Density of 163
Flue (see Flue gases).
Path of in Babcock & Wilcox boilers 57
Waste (see Waste heat) 232
Gaskets 312
Gauges, draft, Barrus 254
Ellison 255
Peabody 255
U-tube 254
Gauges, vacuum 117
Grate ratio for anthracite 191
Gravity of oils 214
Grooving 102
Guerney's boiler 24
Handhole fittings for Babcock & Wilcox boilers 50, 51
Handholes in Babcock & Wilcox boilers 50, 51
Hardness of boiler feed water 102
Permanent 102
Temporary 102
Testing for 105
Hardness of fire brick 303
Heat and chemical methods of treating feed water 105
And its measurement 79
Balance 262
Consumption of engines 141
Latent 84
Of liquid 120
Sensible 84
Specific (see Specific heat) 83
Total 86
Transfer 323
Heat value of bagasse 205
By-product coke oven gas 231
Coal 177
Heat value of fuels, determination of 173
Determination of Kent's approximate method 183
High and low 174
Heat value of natural gas 229
Oil 215
Wood 201
Heat waste (see Waste heat) 232
Heaters, feed water, closed 111
Economizers 111
Open 111
Heating feed water, saving by 110
Hero's engine 13
High and low heat value of fuels 174
High pressure steam, advantages of use of 119
High temperature measurements, accuracy of 89
Horse power, boiler 265
Evaporation (actual) corresponding to 288
Rated boiler 265
Stacks for various, of boilers 250
Hydrogen in flue gases 156
Ice, specific heat of 99
"Idalia", tests with superheated steam on yacht 143
Impurities in boiler feed water 100
Incomplete combustion of carbon, loss due to 158
Injectors, efficiency of 112
Relative efficiency of, and pumps 112
Iron alum in feed water treatment 106
Kent, Wm., determination of heat value from analysis 183
Stack table 250
Kindling point 150
Latent heat 84, 115
Laying of fire brick 305
Red brick 305
Lignite, analyses of 181
Combustion of 195
Lime and soda treatment of boiler feed 102
Used in chemical treatment of feed 105
Lime treatment of boiler feed water 102
Liquid fuels and their combustion 212
Loads, economical boiler 283
Losses due to excess air 158
Due to unburned carbon 158
Due to unconsumed carbon in the ash 261
Losses in efficiency (see Heat balance).
In flue gases 158, 159
Low water in boilers 298
Melting points of metals 91
Mercurial pyrometers 89
Moisture in coal, determination of 176
In fuels, losses due to 259
In steam, determination of 129
Mud drum of Babcock & Wilcox boiler 51
Napier's formula for flow of steam 321
Natural gas, burners for 229
Combustion of 229
Composition and heat value of 229
Nitrate of silver in testing feed water 105
Nitrogen, as indication of excess air 157
In air 147
In flue gases 157
Nodules, fire brick, ratio of 303
Size of 303
Normal reading, throttling calorimeter 131
Nozzles, steam sampling for calorimeter 134
Location of 134
Oil fuel, burners (see Burners).
Capacity with 224
Combustion of 217
Comparison with coal 214
Composition and heat value of 213
Efficiency with 224
Furnaces for 221
Gravity of 214
In combination with other fuels 224
Stacks for 251
Tests with 224
Open hearth furnace, Babcock & Wilcox boiler set
for utilizing waste heat from 236
Open heaters, feed water 111
Operation of boilers 291
Optical pyrometers 91
Orsat apparatus 160
Oxalate of soda in feed water treatment 106
Oxygen in air 147
Flue gases 155
Peabody draft gauge 255
Formulae for coal calorimeter correction 188
Furnace for oil fuel 221, 222
Oil burner 218
Peat 167
Perkins' boiler 24
Pfaundler's method of coal calorimeter radiation correction 187
Pipe coverings 315
Data 308
Expansion of 315
Pipe fittings 311
Flanges 309
Flow of steam through 317
Radiation from bare and covered 314
Sizes 312
Supports for 315
Piping, boiler room 308
Pitting 102
Plant records, coal 293
Draft 294
Temperature 294
Water 293
Plasticity of fire brick 302
Pressed fuels 171
Priming in boilers 102
Methods of treating for 107
Properties of water 96
Proximate analyses of coal 177
Proximate analysis 173
Method of making 176
Pulverized fuels 170
Pump, efficiency of feed 112
Pyrometers, expansion 89
Mercurial 89
Optical 91
Radiation 90
Thermo-electric 90
Quality of steam 129
Radiation correction for coal calorimeter 187, 188
Correction for steam calorimeter 131
Effect of superheated steam on 140
From pipes 314
Losses in efficiency due to 307
Pyrometers 90
Ratio of air supplied to that required for combustion 157
Reagents, less usual in feed treatment 106
Records, plant, coal 293
Draft 294
Temperature 294
Water 293
Requirements of steam boilers 27
As indicated by evolution of Babcock & Wilcox 45
Rumsey's boiler 23
Safety of Babcock & Wilcox boilers 66
Salts responsible for scale 101
Solubility of 101
Sampling coal 271
Nozzles for steam 134
Nozzles for steam, location of 134
Steam 134
Steam, errors in 135
Saturated air 149
Saving by heating feed 110
With superheat in "Idalia" tests 143
With superheat in prime movers 140, 142
Scale (see Thermometers) 101
Sea water, composition of 97
Sections, Babcock & Wilcox boiler 50
Selection of boilers 277
Sensible heat 84
Separating steam calorimeter 132
Sizes of anthracite coal 190
Bituminous coal 191
Smoke, methods of eliminating 197
Smokelessness, relative nature of 197
With hand-fired furnaces 199
With stoker-fired furnaces 199
Soda, lime and, treatment of feed 103
Oxalate of, in treatment of feed 106
Removal of scale aided by 300
Silicate of, in treatment of feed 106
Treatment of boiler feed 103
Space occupied by Babcock & Wilcox boilers 66
Specific heat 83
Specific heat of air 148
Ice 99
Saturated steam 99
Specific heat of superheated steam 137
Various solids, liquids and gases 85
Water 99
Spreading method of firing 193
Stacks and draft (see Chimneys) 237
Stacks for blast furnace gas 228
Oil fuel 251
Wood 202, 254
Stayed surfaces, absence of, in Babcock & Wilcox boilers 69
Difficulties arising from use of 67
Steam 115
As aid to combustion of anthracite 191
As aid to combustion of lignite 195
Consumption of prime movers 289
Density of 115
Flow of, into atmosphere 320
Flow of, into pressure above atmosphere 318
Flow of, through pipes 317
High pressure, advantage of 119
History of generation and use of 13
Making, theory of 92
Moisture in 129
Properties of, for vacuum 119
Properties of saturated 122
Properties of superheated 125
Quality of 129
Saturated 115
Specific heat of saturated 99
Specific heat of superheated 137
Specific volume of 115
Superheated 137
Superheaters (see Superheated steam).
Steaming, quick, with Babcock & Wilcox boilers 73
Stem Correction, thermometer 80
Stevens, John, boiler 23
Stevens, John Cox, boiler 23
Stokers, automatic, advantages of 195
Babcock & Wilcox chain grate 194
Overfeed 196
Smokelessness with 199
Traveling grate 197
Underfeed 196
Superheated steam 137
Additional fuel for 139
Effect on condensation 140
Effect on radiation 140
Fittings for use with 145
"Idalia" tests with 143
Specific heat of 137
Variation in temperature of 145
With turbines 142
Superheater, Babcock & Wilcox 136
Effect of on boiler efficiency 139
Supports, Babcock & Wilcox boiler 52, 53
Tan bark 210
Tar, water gas 225
Temperature, accuracy of high, measurements 89
As indicated by color 91
Of waste gases 232
Records 294
Test conditions _vs._ operating conditions 278
Testing, boiler, A. S. M. E. code for 267
Tests of Babcock & Wilcox boilers with bagasse 210
Coal 280
Oil 224
Theory of steam making 92
Thermo-electric pyrometers 90
Thermometer scale, celsius 79
Thermometer scale, centigrade 76
Fahrenheit 79
Réaumur 79
Thermometer scales, comparison of 80
Conversion of 80
Thermometer stem correction for 80
Thermometers, glass for 79
Throttling calorimeter 129
Total heat 86, 115
Treatment of boiler feed water (see Feed water) 102
Chemicals used in 105
Less usual reagents in 106
Tube data 309
Doors in Babcock & Wilcox boilers 53
Tubes in Babcock & Wilcox boilers 50
Ultimate analyses of coal 183
Analysis of fuels 173
Unaccounted losses in efficiency 261
Unconsumed carbon in ash 261
Units, boiler, number of 289
Size of 289
Units, British thermal 83
Unreliability of CO_{2} readings alone 162
Vacuum gauges 117
Properties of steam for 119
Valves used with superheated steam 312
Variation in properties of saturated steam 119
Superheat from boilers 145
Volume of air 147
Water 96
Volume, specific, of steam 115
Waste heat, auxiliary grates with boilers for 235
Babcock & Wilcox boilers set for use with 236
Boiler design for 233
Curve of temperature, heat absorption, and heating surface 235
Draft for 233
Fans for use with 233
Power obtainable from 232
Temperature of, from various processes 232
Utilization of 232
Water, air in boiler feed 106
Boiling points of 97
Compressibility of 97
Water feed, impurities in 100
Methods of feeding to boiler 132
Saving by heating 110
Treatment (see Feed water).
Water-gas tar 225
Heat of the liquid 120
Path of, in Babcock & Wilcox boilers 57
Properties of 96
Records 293
Specific heat of 99
Volume of 96
Weight of 96, 120
Watt, James 17
Weathering of coal 169
Webster furnace 55
Weight of air 147
Wilcox boiler 25
Wood, combustion of dry 202
Wet 203
Composition and heat value of 201
Furnace design for 201
Moisture in 201
Sawmill refuse 202
Woolf s boiler 24
Zero, absolute 81
FOOTNOTES
[Footnote 1: See discussion by George H. Babcock, of Stirling's paper on
"Water-tube and Shell Boilers", in Transactions, American Society of
Mechanical Engineers, Volume VI., Page 601.]
[Footnote 2: When one temperature alone is given the "true" specific
heat is given; otherwise the value is the "mean" specific heat for the
range of temperature given.]
[Footnote 3: For variation, see Table 13.]
[Footnote 4: Where range of temperature is given, coefficient is mean
over range.]
[Footnote 5: Coefficient of cubical expansion.]
[Footnote 6: Le Chatelier's Investigations.]
[Footnote 7: Burgess-Le Chatelier.]
[Footnote 8: For accuracy of high temperature measurements, see Table
7.]
[Footnote 9: Messrs. White & Taylor Trans. A. S. M. E., Vol. XXI, 1900.]
[Footnote 10: See Scientific American Supplement, 624, 625, December,
1887.]
[Footnote 11: 460 degrees below the zero of Fahrenheit. This is the
nearest approximation in whole degrees to the latest determinations of
the absolute zero of temperature]
[Footnote 12: Marks and Davis]
[Footnote 13: See page 120.]
[Footnote 14: See Trans., A. S. M. E., Vol. XIV., Page 79.]
[Footnote 15: Some waters, not naturally acid, become so at high
temperatures, as when chloride of magnesia decomposes with the formation
of free hydrochloride acid; such phenomena become more serious with an
increase in pressure and temperature.]
[Footnote 16: L. M. Booth Company.]
[Footnote 17: Based on lime containing 90 per cent calcium oxide.]
[Footnote 18: Based on soda containing 58 per cent sodium oxide.]
[Footnote 19: See Stem Correction, page 80.]
[Footnote 20: See pages 125 to 127.]
[Footnote 21: The actual specific heat at a particular temperature and
pressure is that corresponding to a change of one degree one way or the
other and differs considerably from the average value for the particular
temperature and pressure given in the table. The mean values given in
the table give correct results when employed to determine the factor of
evaporation whereas the actual values at the particular temperatures and
pressures would not.]
[Footnote 22: See page 117.]
[Footnote 23: Ratio by weight of O to N in air.]
[Footnote 24: 4.32 pounds of air contains one pound of O.]
[Footnote 25: Per pound of C in the CO.]
[Footnote 26: Ratio by volume of O to N in air.]
[Footnote 27: Available hydrogen.]
[Footnote 28: See Table 31, page 151.]
[Footnote 29: This formula is equivalent to (10) given in chapter on
combustion. 34.56 = theoretical air required for combustion of one pound
of H (see Table 31).]
[Footnote 30: For degree of accuracy of this formula, see Transactions,
A. S. M. E., Volume XXI, 1900, page 94.]
[Footnote 31: For loss per pound of coal multiply by per cent of carbon
in coal by ultimate analysis.]
[Footnote 32: For loss per pound of coal multiply by per cent of carbon
in coal by ultimate analysis.]
[Footnote 33: The Panther Creek District forms a part of what is known
as the Southern Field; in the matter of hardness, however, these coals
are more nearly akin to Lehigh coals.]
[Footnote 34: Sometimes called Western Middle or Northern Schuylkill
Field.]
[Footnote 35: Geographically, the Shamokin District is part of the
Western Middle Mahanoy Field, but the coals found in this section
resemble more closely those of the Wyoming Field.]
[Footnote 36: See page 161.]
[Footnote 37: U. S. Geological Survey.]
[Footnote 38: See "Steam Boiler Economy", page 47, First Edition.]
[Footnote 39: To agree with Pfaundler's formula the end ordinates should
be given half values in determining T", _i. e._, T" = ((Temp. at B +
Temp. at C) ÷ 2 + Temp. all other ordinates) ÷ N]
[Footnote 40: B. t. u. calculated.]
[Footnote 41: Average of two samples.]
[Footnote 42: Assuming bagasse temperature = 80 degrees Fahrenheit and
exit gas temperature = 500 degrees Fahrenheit.]
[Footnote 43: Dr. Henry C. Sherman. Columbia University.]
[Footnote 44: Includes N.]
[Footnote 45: Includes silt.]
[Footnote 46: Net efficiency = gross efficiency less 2 per cent for
steam used in atomizing oil.
Heat value of oil = 18500 B. t. u.
One ton of coal weighs 2000 pounds. One barrel of oil weighs 336 pounds.
One gallon of oil weighs 8 pounds.]
[Footnote 47: Average of 20 samples.]
[Footnote 48: Includes H and CH_{4}.]
[Footnote 49: B. t. u. approximate. For method of calculation, see page
175.]
[Footnote 50: Temperatures are average over one cycle of operation and
may vary widely as to maximum and minimum.]
[Footnote 51: Dependant upon length of kiln.]
[Footnote 52: Results secured by this method will be approximately
correct.]
[Footnote 53: See "Chimneys for Crude Oil", C. R. Weymouth, Trans.
A. S. M. E., Dec. 1912.]
[Footnote 54: To determine the portion of the fuel which is actually
burned, the weight of ashes should be computed from the total weight of
coal burned and the coal and ash analyses in order to allow for any ash
that may be blown away with the flue gases. In many cases the ash so
computed is considerably higher than that found in the test.]
[Footnote 55: As distinguished from the efficiency of boiler, furnace
and grate.]
[Footnote 56: To obtain the efficiency of the boiler as an absorber of
the heat contained in the hot gases, this should be the heat generated
per pound of combustible corrected so that any heat lost through
incomplete combustion will not be charged to the boiler. This, however,
does not eliminate the furnace as the presence of excess air in the
gases lowers the efficiency and the ability to run without excess air
depends on the design and operation of the furnace. The efficiency based
on the total heat value per pound of combustible is, however, ordinarily
taken as the efficiency of the boiler notwithstanding the fact that it
necessarily involves the furnace.]
[Footnote 57: See pages 280 and 281.]
[Footnote 58: Where the horse power of marine boilers is stated, it
generally refers to and is synonymous with the horse power developed by
the engines which they serve.]
[Footnote 59: In other countries, boilers are ordinarily rated not in
horse power but by specifying the quantity of water they are capable of
evaporating from and at 212 degrees or under other conditions.]
[Footnote 60: See equivalent evaporation from and at 212 degrees, page
116.]
[Footnote 61: The recommendations are those made in the preliminary
report of the Committee on Power Tests and at the time of going to press
have not been finally accepted by the Society as a whole.]
[Footnote 62: This code relates primarily to tests made with coal.]
[Footnote 63: The necessary apparatus and instruments are described
elsewhere. No definite rules can be given for location of instruments.
For suggestions on location, see A. S. M. E. Code of 1912, Appendix 24.
For calibration of instruments, see Code, Vol. XXXIV, Trans.,
A. S. M. E., pages 1691-1702 and 1713-14.]
[Footnote 64: One to two inches for small anthracite coals.]
[Footnote 65: Do not blow down the water-glass column for at least one
hour before these readings are taken. An erroneous indication may
otherwise be caused by a change of temperature and density of the water
within the column and connecting pipe.]
[Footnote 66: Do not blow down the water-glass column for at least one
hour before these readings are taken. An erroneous indication may
otherwise be caused by a change of temperature and density of the water
within the column and connecting pipe.]
[Footnote 67: For calculations relating to quality of steam, see page
129.]
[Footnote 68: Where the coal is very moist, a portion of the moisture
will cling to the walls of the jar, and in such case the jar and fuel
together should be dried out in determining the total moisture.]
[Footnote 69: Say ½ ounce to 2 ounces.]
[Footnote 70: For methods of analysis, see page 176.]
[Footnote 71: For suggestions relative to Smoke Observations, see
A. S. M. E. Code of 1912, Appendix 16 and 17.]
[Footnote 72: The term "as fired" means actual condition including
moisture, corrected for estimated difference in weight of coal on the
grate at beginning and end.]
[Footnote 73: Corrected for inequality of water level and steam pressure
at beginning and end.]
[Footnote 74: See Transactions, A. S. M. E., Volume XXXIII, 1912.]
[Footnote 75: For methods of determining, see Technologic Paper No. 7,
Bureau of Standards, page 44.]
[Footnote 76: Often called extra heavy pipe.]
[Footnote 77: See Feed Piping, page 312.]
[Footnote 78: See Superheat Chapter, page 145.]
[Footnote 79: See Radiation from Steam Lines, page 314.]
[Footnote 80: D, the density, is taken as the mean of the density at the
initial and final pressures.]
[Footnote 81: Diameters up to 5 inches, inclusive, are _actual_
diameters of standard pipe, see Table 62, page 308.]
[Footnote 82: Diameters up to 4 inches, inclusive, are _actual_ internal
diameters, see Table 62, page 308.]
[Footnote 83: H. P. Jordan, "Proceedings of the Institute of Mechanical
Engineers", 1909.]
[Footnote 84: "Zeitschrift des Vereines Deutscher Ingenieur", 1909, page
1750.]
[Footnote 85: Heinrich Gröber--Zeit. d. Ver. Ing., March 1912, December
1912. Leprince-Ringuet--Revue de Mecanique. July 1911. John Perry--"The
Steam Engine". T. E. Stanton--Philosophical Transactions, 1897. Dr.
J. T. Nicholson--Proceedings Institute of Engineers & Shipbuilders in
Scotland, 1910. W. E. Dally--Proceedings Institute of Mechanical
Engineers, 1909.]
[Footnote 86: Proceedings Royal Society, Vol. LXXI.]
[Footnote 87: Zeitschrift des Vereines Deutscher Ingenieur, 1910, page
1154.]
[Footnote 88: Proceedings Institute of Engineers and Shipbuilders,
1910.]
[Footnote 89: Natural or Hyperbolic Logarithm.]
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