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.