boiler horsepower
The boiler horsepower is the power to evaporate about 34.5 pounds of water an hour from and at 212 °F, taken as 9809.5 watts, or about 33,470 BTU per hour. It is the traditional rating unit for firetube boilers in North America and has nothing to do with any shaft.
Watch out: The value is conventional rather than exact and is quoted variously as 9809.5 W (the NIST figure used here) and as 33,475 BTU/h, which works out about 0.01 percent higher. It is roughly 13 times the mechanical horsepower, so a "100 hp boiler" is a megawatt of heat and not 75 kW. Boiler capacity is more often given directly in lb/h of steam or in MMBTU/h.
| 1 bhp | 9,809.5 W |
Set at the 1876 Centennial Exhibition in Philadelphia, when steam engines of the day needed roughly this much boiler to produce one horsepower at the shaft. Engines got dramatically better and the boiler rating unit never moved, so one boiler horsepower now runs far more than one horsepower of engine.