STEAM BOILERS 141 



shoveled in until the beam rises, and then fed directly from the 

 box to the furnace. After the test, the ashes and clinkers 

 must be raked from the ash-pit and grate and weighed. This 

 weight subtracted from the weight of the coal used gives the 

 amount of combustible. 



Measurement of Feedwater. The amount of water evap- 

 orated in a test for comparative fuel values may be taken as 

 equal to the amount of feedwater supplied without introducing 

 any serious error. The most reliable method of measuring the 

 feedwater delivered to the boilers is to weigh it. 



Standard of Boiler Horsepower. When making a horse- 

 power or an efficiency test, a more elaborate method of pro- 

 cedure is required than for a comparative fuel-value test. 

 The reason for this is that different boilers generate steam at 

 different pressures, different feedwater temperatures, and dif- 

 *erent degrees of dry ness; hence, to compare the performances 

 of boilers so as to determine their comparative efficiencies, it 

 is necessary to reduce the actual evaporation to an equivalent 

 evaporation from and at 212 F. per pound of combustible. 



A committee of the American Society of Mechanical Engineers 

 has recommended as a commercial horsepower an evaporation 

 of 80 Ib. of water per hour from a feedwater temperature of 100 

 F. into steam at 70 Ib. gauge pressure, which is equivalent to 34* 

 units of evaporation; that is, to 34 5 Ib. of water evaporated 

 from a feedwater temperature of 212 F. into steam at the 

 same temperature. 



Since 965.8 B. T. U. is required to evaporate a pound of 

 water from and at 212, a boiler horsepower is equal to 965.8 

 X 34| = 33,320 B. T. U. per hr. 



Equivalent Evaporation. The equivalent evaporation is 

 readily determined by means of the formula 



965.8 



in which W = actual evaporation, in pounds of water per hour; 

 H = total heat of steam above 32 F. at observed pres- 



sure of evaporation; 

 t = observed feedwater temperature; 

 Wi = equivalent evaporation, in pounds of water per 



hour, from and at 212 F. 

 11 



