STEAM 95 



additional amount of heat in falling to 130. Since the original 

 temperature of the stealn was 296.74 F. (see Steam Table), 

 each pound gives up by its fall of temperature 296.74 130 

 = 166.74 B. T. U. Consequently, each pound of the steam 

 gives up a total of 906.2+166.74 = 1,072.94 B. T. U., and 



'- = 25.16 Ib. of steam will therefore be required to 



1,072.94 



accomplish the desired result. 



SUPERHEATED STEAM 



If saturated steam is contained in a vessel, out of contact 

 with water, and heat is added to it, its temperature will begin 

 to rise and its weight per cu. ft. will begin to decrease, pro- 

 vided the pressure remains constant. As more heat is added, 

 the temperature rises farther above that of saturated steam 

 at that pressure, and the steam is then called superheated 

 steam. Superheated steam cannot exist in contact with water. 



The following distinction is usually made between saturated 

 and superheated steam: For a given pressure, saturated steam 

 has one temperature and one weight per cu. ft., neither of 

 which can change so long as the steam remains in immediate 

 contact with water. Superheated steam at the same pressure 

 has a greater temperature and less weight per cu. ft. than sat- 

 urated steam, and both the temperature and weight per cu. ft. 

 may vary while the pressure remains constant if the volume 

 increases or decreases accordingly. In other words, .both the 

 pressure and the volume of superheated steam must be con- 

 stant in order to maintain a constant temperature and a con- 

 stant weight per cu. ft. 



QUALITY OF STEAM 



Moisture in Steam. The steam furnished by the average 

 steam boiler is not dry saturated steam, but is usually wet 

 steam. A good boiler should not show more than 2 or 3% 

 of water in the steam. In a quantity of wet steam, or a mix- 

 ture of steam and water, the percentage of dry steam, expressed 

 as a decimal, is called the quality of the steam. For example, 

 suppose that a certain boiler generates wet steam that con- 

 tains 3%, or .03, of moisture; then the quality of the steam. 



