test conditions would have
evaporated 7 x 1.0727 = 7.5 pounds from and at 212 deg. F. Hence, 7.5
pounds is called the "equivalent evaporation from and at 212 deg. F."
per pound of coal used.

As already stated, it takes 970.4 B. t. u. to make a pound of steam
from and at 212 deg. F. Then to make 7.5 pounds there would be required
7.5 x 970.4 = 7,278 B. t. u. This is the amount of heat required to
change 7.5 pounds of water at 212 deg. F. to steam at zero gage
pressure, but it is also the heat required to change 7 pounds of
water at 180 deg. F. to steam at 100 pounds gage pressure, because 7.5
pounds from and at 212 deg. F. is equivalent to 7 pounds from 180 deg. F.
to steam at 100 pounds. Therefore, the 7,278 B. t. u. is the amount
of heat usefully employed in making steam per pound of coal fired,
and so it is the _output_. Accordingly, the efficiency of the
boiler is--

Output 7,278
~ Efficiency = ------ = ------ = 0.54, nearly.
Input 13,500

In other words, the efficiency of the boiler is 0.54, or 54 per
cent, which means that only a little more than half of the heat in
the coal is usefully employed in making steam.

The chart shown in figure 3 is given to save the work of figuring
the efficiency. If the equivalent evaporation per pound of coal is
calculated and the heating value of the coal is known, the boiler
efficiency may be found directly from the chart. At the left-hand
side locate the point corresponding to the equivalent evaporation
and at the bottom locate the point corresponding to the heating
value of the coal. Follow the horizontal and vertical lines from
these two points until they cross, and note the diagonal line that
is nearest to the crossing point. The figures marked on the
diagonal line indicate the boiler efficiency.

Take the case just worked out, for example. The equivalent
evaporation is 7.5 pounds and the heating value of the fuel is
13,500 B. t. u. At the left of the chart locate the point 7.5
midway between 7 and 8 and at the bottom locate the point 13,500
midway between 13,000 and 14,000. Then follow the horizontal and
vertical lines from these two points until they cross, as indicated
by the dotted lines. The crossing point lies on the diagonal
corresponding to 54, and so the efficiency is 54 per cent.

BOILER HORSEPOWER OR CAPACITY.

The capacity of a boiler is usually stated in boiler horsepower. A
boiler