horsepower means the evaporation of 34.5 pounds of water per
hour from and at 212 deg. F. Therefore, to find the boiler horsepower
developed during a test, calculate the evaporation from and at 212 deg.
F. per hour and divide it by 34.5.

Take the test previously mentioned, for example. The evaporation
from and at 212 deg. F. or the equivalent evaporation, was 7.5 pounds
of water per pound of coal. The weight of coal burned per hour was
5,000 / 10 = 500 pounds. Then the equivalent evaporation was 7.5 x
500 = 3,750 pounds per hour. According to the foregoing definition
of a boiler horsepower, then--

3,750
Boiler horsepower = ----- = 109.
34.5

The "rated horsepower" of a boiler, or the "builders' rating," is
the number of square feet of heating surface in the boiler divided
by a number. In the case of stationary boilers this number is 10 or
12, but 10 is very commonly taken as the amount of heating surface
per horsepower. Assuming this value and assuming further that the
boiler tested had 1,500 square feet of heating surface, its rated
horsepower would be 1,500 / 10 = 150 boiler horsepower.

It is often desirable to know what per cent of the rated capacity
is developed in a test. This is found by dividing the horsepower
developed during the test by the builders' rating. In the case of
the boiler tested, 109 horsepower was developed. The percentage of
rated capacity developed, therefore, was 109 / 150 = 0.73, or 73
per cent.

HEATING SURFACE.

The heating surface of a boiler is the surface of metal exposed to
the fire or hot gases on one side and to water on the other side.
Thus, the internal surface of the tubes of a fire-tube boiler is
the heating surface of the tubes, but the outside surface of the
tubes of a water-tube boiler is the heating surface of those tubes.
In addition to the tubes, all other surfaces which have hot gases
on one side and water on the other must be taken into account. For
instance, in a fire-tube boiler from one-half to two-thirds of the
shell (depending on how the boiler is set) acts as heating surface.
In addition to this, the surface presented by both heads, below the
water level, has to be computed. The heating surface of each head
is equal to two-thirds its area minus the total area of the holes
cut away to receive the tubes.

COST OF EVAPORATION.

The cost of evaporation is usually stat