Steam, Its Generation and Use | Page 9

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