144 STEAM BOILERS 



In the same vertical columns and opposite 130 are found 1.117 

 and 1.120; difference is .003; same as above. Hence, for an 

 increase of 10 Ib. in gauge reading, there is an increase of .003 

 in the factor of evaporation, or an increase of .0003 for 1 Ib. 

 and of .0003X2 = .0006 for 2 Ib. Therefore, for a feedwater 

 temperature of 120 and 72 Ib. pressure, the factor of evap- 

 oration is 1.128 + .0006 = 1.1286. The difference between the 

 numbers opposite 120 and 130 in the two columns headed 70 

 and 80, respectively, is 1.128- 1.117 = .011, and 1.131-1.120 

 = .011, showing that, for an increase of temperature in the 

 feedwater of 10, there is a decrease in the factor of .011, and 

 for 1 a decrease of .0011, or for 2 of .0022. Hence, the value 

 of the factor for a temperature of 122 and a gauge pressure of 

 72 Ib. is 1.1286-. 0022 = 1.126. 



Boiler Efficiency. The efficiency of a boiler may be defined 

 as the ratio of the heat utilized in evaporating water to the 

 total heat supplied by the fuel. The efficiency thus calculated 

 is really the combined efficiency of the furnace and boiler, as 

 it is not easily possible to determine separately the efficiency 

 of each. 



The amount of heat supplied is determined by first accurately 

 weighing the fuel used during the test and deducting all the 

 ash and unconsumed portions. This weight, in pounds, is 

 multiplied by the total heat of combustion of 1 Ib. of the fuel, 

 as determined by an analysis, the product being the total 

 number of heat units supplied during the test under the 

 assumption that combustion was perfect. The heat usefully 

 expended in evaporating water is obtained by first weighing 

 the feedwater and correcting this weight according to the 

 quality of the steam; the corrected weight is then multiplied 

 by the number of heat units required to change water at the 

 temperature of the feed into steam at the observed pressure. 

 The efficiency of a boiler, expressed in per cent., may be found 

 by the formula 



in which E = efficiency of boiler; 



A =heat utilized in evaporating water; 

 B = total heat supplied by fuel. 



