When steam pushes on a piston of an engine, it expands (decreases in pressure) and in doing so decreases in temperature and heat content. 778 foot-pounds is the mechanical equivalent of heat for one BTU. Using this, the formula for calculating the ideal or thermal horsepower of an engine is

778 M (H1 - H2) / 33,000

where H1 is the heat content in BTU's of the steam entering the engine. H2 is the heat
content in BTU's of the steam exhausting the engine. M is the mass or pounds of steam per
minute passing through the engine.To find the heat content you need to know the
temperature and pressure of the steam. This would be established by the boiler and
piping. Then find a thermodynamics book with steam tables in it, and they should list the
corresponding BTU's.

Offered by Gary.