This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Horsepower Of A Boiler. This phrase was originally intended to mean that a boiler having a certain stated horsepower would furnish all the steam which was required to develop that amount of power in an ordinary engine, the standard engine at that time being a plain slide-valve engine, using about 30 pounds of steam at 70 pounds gauge pressure per horsepower per hour. Subsequent improvements in engines, however, have greatly reduced the amount of steam required, being as low as 13 pounds in some of the best engines. The term has thus lost its original meaning. To remedy this defect, the American Society of Mechanical Engineers has adopted a new definition, which is, that one boiler horsepower equals 33,330 B. T. U. per hour, absorbed and transmitted from the fuel to the steam or water.
Boilers are also rated by their ability to evaporate water from, and at, a temperature of, 212°, 30 pounds per hour being reckoned as 1 horsepower. This method, however, is merely an indirect way of arriving at the number of heat units transmitted through the boiler, and has no particular advantage. The method given, of rating by the number of heat units actually transmitted per hour, enables all varieties of steam and hot-water boilers and hot-air furnaces to be compared directly, without further computation.