142 STEAM BOILERS 



EXAMPLE. A boiler generates 2,200 Ib. of dry steam per hour 

 at a pressure of 120 Ib. gauge. The temperature of the feed- 

 water being 70 F., (a) what is the equivalent evaporation? 

 <6) what is the horsepower of the boiler? 



SOLUTION. (a) According to the Steam Table, the total heat 

 H corresponding to a gauge pressure of 120 Ib. is 1.188.6 B.T.U. 

 Applying the formula, 



(b) The horsepower, which is obtained by dividing the total 

 equivalent evaporation by 34.5, the equivalent of 1 H. P., is, 

 2,621 -=-34.5 = 76 H. P., nearly 



TT _ | I OQ 



Factor of Evaporation. The quantity : - that 



96o.o 



changes the actual evaporation of 1 Ib. of water to equivalent 

 evaporation from and at 212 F. is called the factor of evapora- 

 tion. To facilitate the calculating of equivalent evaporation, 

 the accompanying table of factors of evaporation is inserted. 

 The equivalent evaporation is found by multiplying the actual 

 evaporation by the factor of evaporation taken from the table. 



EXAMPLE 1. A boiler is required to furnish 1,800 Ib. of steam 

 per hour at a gauge pressure of 80 Ib.; if the temperature of the 

 feedwater is 48 F., what will be the rated horsepower of the 

 boiler? 



SOLUTION. From the table, the factor of evaporation for 

 80-lb. pressure and a feedwater temperature of 40 is 1.214, 

 and for the same pressure and a feedwater temperature of 

 50 it is 1.203; the difference is 1.214- 1.203 = . Oil. The 

 difference of temperature is 50 40 =10, and the difference 

 between the lower temperature and the required temperature 

 is 48- 40 = 8. Then, 10: 8 = . Oil : x, or x = .009; 1.214 .009 

 = 1.205. 1,800X1.205 = 2,169 Ib., and 2,169 + 34.5 = 63 H. P., 

 nearly. 



EXAMPLE 2. What is the factor of evaporation when the 

 feedwater temperature is 122 F. and the gauge pressure 72? 



SOLUTION. In the table, under the column headed 70 and 

 opposite 120 in the left-hand column is found 1.128; in column 

 headed 80 and opposite 120 is found 1.131; difference is .003. 



