BOILER HORSEPOWER OR CAPACITY.
The capacity of a boiler is usually stated in boiler horsepower. A boiler horsepower means the evaporation of 34.5 pounds of water per hour from and at 212° F. Therefore, to find the boiler horsepower developed during a test, calculate the evaporation from and at 212° F. per hour and divide it by 34.5.
Take the test previously mentioned, for example. The evaporation from and at 212° F. or the equivalent evaporation, was 7.5 pounds of water per pound of coal. The weight of coal burned per hour was 5,000 ÷ 10 = 500 pounds. Then the equivalent evaporation was 7.5 × 500 = 3,750 pounds per hour. According to the foregoing definition of a boiler horsepower, then—
| Boiler horsepower = | 3,750 | = 109. |
| 34.5 |
The "rated horsepower" of a boiler, or the "builders' rating," is the number of square feet of heating surface in the boiler divided by a number. In the case of stationary boilers this number is 10 or 12, but 10 is very commonly taken as the amount of heating surface per horsepower. Assuming this value and assuming further that the boiler tested had 1,500 square feet of heating surface, its rated horsepower would be 1,500 ÷ 10 = 150 boiler horsepower.
It is often desirable to know what per cent of the rated capacity is developed in a test. This is found by dividing the horsepower developed during the test by the builders' rating. In the case of the boiler tested, 109 horsepower was developed. The percentage of rated capacity developed, therefore, was 109 ÷ 150 = 0.73, or 73 per cent.