y 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 asce