boiler horsepower

Convert between boiler horsepower and other major units of power.

A unit of power, 19th – 20th centuries, defined by the Boiler Code Test Committee of the American Society of Mechanical Engineers, and used in rating a boiler’s capacity to deliver steam to a steam engine. One boiler horsepower is about 33,478.8 Btu per hour (about 9,809.5 watts).

Boilers in ships were not customarily rated in boiler horsepower.

Origin of the boiler horsepower

Every steam engine requires a boiler to produce the steam.  In the 18th and 19th centuries, buyers of steam engines, which were rated in horsepower, sought equivalent ratings for boilers that would tell them if the boiler they were buying was too big or too small for their engine.

The earliest such measure of boiler output, the “nominal or rated boiler horsepower,” was simply based on the area of the boiler's heating surface. Each 10 square feet of surface represented 1 boiler horsepower, so a boiler with 100 square feet of heating surface would be rated at 10 boiler horsepower.  Because boilers vary greatly in design and efficiency, this is a very inadequate way of rating them.

In 1876, in reporting the results of tests of boilers entered in the Centennial Exhibition in Philadelphia, the Committee of Judges adopted a unit whose magnitude approximated the capacity needed to produce a horsepower using a typical boiler and engine of the time. This “developed boiler horsepower,” was defined as the ability to turn 30 pounds of 100°F feedwater per hour into steam at 70 pounds per square inch pressure.1 

Steam engines, of course, are made for various pressures, not just 70 pounds per square inch, so using this unit required calculation. Given that, it seemed simplest to base the unit on the amount of energy required to convert water to steam at atmospheric pressure, both water and steam at 212°F. In 1884, the Committee on Boiler Tests of the American Society of Mechanical Engineers defined the boiler horsepower as that amount of power that can convert 34.5 pounds of water per hour from feedwater at 212°F to dry, saturated steam at the same temperature. The new definition expressed the same heat capacity as the previous definition in a different way. In 1899 the committee adopted the unit as the “unit of commercial horsepower.” In 1915, the Power Test Committee reaffirmed the definition by including it in the Boiler Code.

With progress in instrumentation for measuring actual steam flow it was no longer necessary to rely solely on such measurements as quantities of feedwater. The boiler horsepower could be defined purely as a quantity of power, in units of energy (Btu) per unit of time (hour), doing away with pounds of water and temperatures while retaining the original magnitude. The actual quantity (the total heat of steam, 34.5 pounds of water from and at 212°F) could be taken from readily-available steam tables.

Marks and Davis (1909) 33,479 btu per hour

1. Transactions of the American Society of Mechanical Engineers, volume 6.

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