36 STEAM 



or the percentage of dry steam, is .97. In other words, the 

 quality of the steam is equal to 1 minus the percentage of 

 moisture, expressed decimally. This rule may be stated 

 simply by the formula 



Q=l-m, 

 in which Q = quality of the steam; 



m = percentage of moisture, expressed decimally. 



EXAMPLE. What is the quality of steam that contains 2.7% 

 of moisture? 



SOLUTION. Expressed as a decimal, 2.7% = .027. Then, 

 substituting this value for m in the formula, Q = 1 .027 = .973. 



Heat in Wet Steam. The total heat contained in 1 Ib. of 

 dry steam is the sum of the heat required to raise 1 Ib. of water 

 from 32 F. to the boiling point and the heat required to change 

 the boiling water into steam of the same temperature. That 

 is, in the Steam Table, each value given in the fourth column 

 is the sum of the values given in the third and fifth columns 

 and lying in the same horizontal row. In a mixture of 1 Ib. of 

 steam and water at the same temperature there is less heat than 

 in 1 Ib. of dry steam at the same temperature; for all the water 

 has not been changed to steam, and consequently the latent 

 heat of 1 Ib. of steam has not been utilized. Instead, there is 

 present only that part of the latent heat which is used to evap- 

 orate the portion of the mixture that is dry steam, which is 

 represented by the quality of the steam. Thus, using the 

 symbols given in the Steam Table, 



H = q+r (1) 



which is the formula for the total heat of 1 Ib. of dry steam. 

 But if the steam is wet, and Q represents the quality of the 

 steam, expressed decimally, the total heat of 1 Ib., represented 

 by//', is H l = q+Qr (2) 



EXAMPLE. What is the total heat of 10 Ib. of steam at 

 150 Ib. gauge pressure, if the steam contains 5% of moisture? 



SOLUTION. From the Steam Table, the heat of the liquid 

 of lib. of dry steam at 150 Ib., gauge, or 150+14.7 = 164.7 Ib., 

 absolute, is q = 337.84 B. T. U., and the latent heat of 1 Ib. at 

 the same pressure is r = 855.71 B. T. U. As the moisture is 5%, 

 the quality of the steam is 1.00 .05 = .95. Then, applying 

 formula 2, //' =337.84 + .95X855.7 1 = 1,150.76 B. T. U. 



