HP to Lb/hr Conversion for Boiler

[hadams417]

It has been explained to me that to determine the steam mass flowrate (lb/hr) of a boiler you simply multiply the boiler HP by 34.5. However, no one has been able to provide me with the rational behind this conversion. Can anyone shed any light on this matter?

 

[JStephen]

"...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."

IE, it is based on a fixed pressure, fixed temperature, and wouldn't necessarily match the flow rates of your boiler at whatever pressure it operates at.

 

[TBP]

"Through long usage the term 'capacity' has come to be expressed in boiler horsepower. This term has been so generally accepted in the United States that it will probably be a long time before it can be entirely abandoned. It has no definate relation to the term 'horsepower' as applied to a steam engine, since the boiler does not perform work as normally considered.

At the Centennial Exposition in Philadelphia in 1876 a boiler horsepower was taken as the evaporation of 30 lb of water per hour into steam, at a gage pressure of 70 psi with a feedwater temperature of 100*F. This was the weight of steam used, at that time, by a simple steam engine in producing 1 hp when the steam pressure was 70 PSI. This corresponds to the evaporation of 34.5 lb of water into steam, from and at 212*F per hour, and consequently at atmospheric pressure of 14.7 psia, and is known as the 'commercial rating' as adopted by the American Society Of Mechanical Engineers."

From "Elementary Steam Power Engineering" by Edgar MacNaughton. Copyright 1923