By general agreement a horse power as applied to steam boilers is
thirty (30) pounds of feed water at a temperature of 100 degrees Fahr.
converted into steam in 1 hour at 70 pounds gauge pressure.
The standard is all that can be asked because the same test will
determine two things; first the steam making capacity of the boiler and
second its evaporative efficiency, which is all that is necessary to
know in determining the commercial rating of boilers.
But it is a fact that, without an engine attached, there is no
such thing as calculating the horse power of a boiler upon general
principles. A well constructed engine with a given pressure of steam
upon a piston of a given area and moving at a certain velocity in feet
per minute, will always and under all conditions develop the same power
so long as the boiler is able to furnish a sufficient quantity of steam
to keep up that pressure; and it matters not whether the steam is taken
from a boiler rated at 60 horse power or 30.
An evidence of the fact that there is no standard rule for calculating
the horse power of boilers that can be depended upon, is that no two
engine builders send out the same sized boilers with the engine of the
same rated power. Experience has taught them that to furnish steam
sufficient to work their engines up to their ratings that a certain
sized boiler is required, and what would be considered 30 horse power
by one manufacturer might be considered 35 or more by another—the
difference being in the economy of the engine of using the steam, and
not in the boiler for making it.
Then, again, a boiler that might furnish a sufficient quantity of steam
to work a certain type of engine up to 40 horse power without forcing
the fire might, with another style of engine, in order to generate
the same power and perform the same duty, require to be forced beyond
the limits of safety or economy. Therefore, considering the varying
conditions under which all steam boilers are placed, there is no such a
thing as any reliable standard rule for calculating the horse power of
boilers, but only an approximate one at the best.
Hence it is best to select an engine of a certain power, and then let
the same manufacturers furnish a boiler to correspond with it; and so
long as the two are adapted to each other and the boiler of sufficient
capacity to work the engine up to its full ratings, it matters but
little whether the boiler figures the same horse power or not.
It has been found in practice that it is not good economy to carry
pressure higher than eighty pounds in single cylinder automatic cut off
engines.
As pressures increase, it becomes possible to use more economical
engines, reducing water consumption per horse power per hour, thus
requiring a smaller amount of heating surface and grate surface, that
is to say, a smaller boiler and furnace for a given power.
For pressure between eighty and one hundred and twenty pounds, the
compound engine gives the best results, while for higher pressures
triple and quadruple expansion engines are the most economical.