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Now and again those interested in the restoration and operation of antique steam engines are confronted with the problem of rating a boiler in terms of boiler horsepower. Although this term has long been abandoned in the steam power field, it is nevertheless, one used often in historical journals. Over the years there have been articles written on the history of the unit of energy, horsepower, as applied to the reciprocating steam engines. One of the best of these was done by Carl Erwin and was in the July 1977 issue of The Iron-Men Album. However, this is not the same 'horsepower' as is used in the term 'boiler horsepower.'

The history of the development of the unit of boiler horsepower follows that of the early development of the steam engine. Once someone had built an engine he must supply it with steam, and a boiler serving an engine tested to have a certain horsepower rating must certainly have the same rating. Well, as we shall see, today this does not necessarily follow.

Steam power pressures have grown over the years from New-comen's atmospheric pressure level to the super-critical pressures of 3,500 pounds per square inch used in today's electric power generation plants. Early boilers were built of copper and were not greatly different from the pot stills used to distill grain mash in the service of Demon Rum. As metallurgy of steel developed and the art of building riveted boilers progressed we find a day in which 'modern' steam pressures were in the range of 60 to 70 pounds per square inch gauge (psig). It was at about that time that the unit known as the boiler horsepower originated. One boiler horsepower was said to be the equivalent of the evaporation of 30 pounds of 100 degree feed water into saturated steam at 70 psig in an hour.

Now, water at 100 degrees Fahrenheit (F) contains (100 - 32) = 68 Btu heat per pound (Btu/lb.). Saturated steam at 70 psig contains 1183.6 Btu/lb. So, if we take the difference between these two numbers and multiply by the 30 pounds we find that one boiler horsepower is equal to 33,468 Btu per hour of heat. It is simply that and no more. But, there is more to the story. Another definition says that one boiler horsepower is equal to the evaporation of 34.5 pounds per hour of water from and at 212 degrees F. And so now we have the 'from and at' term that is so often used. But here again, it is simply a rate for producing steam heat. Let us see how this one works.

Steam at 212 F can only exist at atmospheric pressure at sea level. It contains 1150.2 Btu/lb. Water, on the other hand, at 212 degrees contains 180 Btu/lb. In other words, it takes 970.2 Btu to convert one pound of hot water into steam-- 'from and at 212.' This 970.2 Btu times the 34.5 pounds per hour in the definition of one boiler horsepower equals 33,472 Btu per hour. Generally, it has been the practice to round out the figure to33,470 Btu/hr equals one boiler horsepower.