What is HP to the Boiler

Boiler Horsepower
The term Horsepower is attributed to James Watt, the inventor of the steam engine (and the term “Watt” as a unit of power) as a way to help sell his engines. Watt knew he had to have a way to communicate the power of his steam engines and the best way was to equate it the most common form of power at the time - the horse which his engines would replace.
He noted that horses were typically used in grinding mills where they would walk in a 24 foot diameter circle (75.4 feet circumference). He estimated that the typical horse pulled with the equivalent force of 180 pounds and further observed that the horse would make 144 trips around the circle in an hour or 2.4 laps per minute. This means that the horse traveled at a speed of 180.96 feet per minute. Rounding up to 181 feet per minute and multiplying that by 180 pounds of force he arrived at 32,580 ft-lbs./minute which was rounded up to 33,000 ft-lbs./minute or 1 Horsepower.
Boiler Horsepower was calculated in a much different manner. Prior to 1876 Boiler Horsepower was based upon the heating area of a boiler with every 10 square foot of heating surface equaling 1 boiler horsepower. This would mean that a boiler with 200 square feet of heating surface would be rated at 20 boiler horsepower. Not a very exact way to measure the output of a boiler to say the least, especially given the different designs and efficiencies of boilers available.
All this changed at the 1876 Centennial Exhibition held in Philadelphia, PA. There judges decided to use “developed horsepower” which they defined as the ability to turn 30 pounds of 100 degree F feedwater per hour into steam at 70 psi. In 1884, ASME defined Boiler Horsepower as the amount of power required to convert 34.5 pounds of water per hour from feedwater at 212 degree F to dry, saturated steam at the same temperature. Today 1 boiler horsepower is equal to 33,475 Btu/Hour which is the amount of energy required to evaporate 34.5 pounds of water at 212 degree F in one hour. (disadur dari blog boiler Inggris_02/12/2008)