Steam, Its Generation and Use
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Title: Steam, Its Generation and Use
Author: Babcock & Wilcox Company
Release date: September 18, 2007 [eBook #22657]
Most recently updated: June 29, 2021
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK STEAM, ITS GENERATION AND USE ***
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
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[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, howeve
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