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.]