HORSE POWER AS APPLIED TO BOILERS.
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.