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.
Rule for Estimating Horse Power of Horizontal Tubular Steam Boilers.
Find the square feet of heating surface in the shell, heads and tubes, and divide by 15 for the nominal horse power.
The office of a boiler is to make steam and its real efficiency or the measure of its utility to the purchaser is measured by the amount of water it can turn into steam in a certain length of time and the amount of coal it requires to do this work.
An ordinary 54″×16′ boiler with forty 4″ tubes, 25 sq. ft. of grate surface and 800 sq. ft. of heating surface, in a general way is a 75 h. p. boiler, but good practice will get from it 100 h. p., and the very best modern engines 200 h. p.