HUMTDITY-EEGULATED AND RECIRCULATING DRY KILN". 17 



It should now be evident that superheated vapor is the same thing 

 as moist air with the air removed. The same effects upon the mate- 

 rial to be dried are produced in both cases, as far as the vapor is 

 concerned ; but in the case of moist air, the effect of the air is added 

 to that of the vapor. The same laws apply to the vapor, whether the 

 air is present or absent. The air conveys heat, but by its presence 

 retards the diffusion of the vapor, and consequently retards the rate 

 of evaporation. 



RELATIVE HEATING CAPACITIES OF AIR AND VAPOR. 



To compare the relative heating capacities of dry air and of super- 

 heated vapor, the following deductions are made : The specific heat of 

 water vapor at a pressure of one atmosphere is 0.475 ; that is to say, 

 1 pound of superheated steam in falling 1° F. gives up 0.475 British 

 thermal unit. To evaporate 1 pound of water at 212° F., therefore, 

 will require the heat given up by 966 (latent heat at 212° F.)-7- 

 .475 ==2034 pounds of steam falling 1 degree. At 212° F. the volume 

 per pound is 26.78 cubic feet; therefore, 2034X26.78=54,470 cubic 

 feet of superheated steam falling 1 degree are required to evaporate 

 1 pound of water. The specific heat of dry air is 0.237 and the vol- 

 ume of 1 pound is 16.93 (0.05907 pound per cubic foot) at 212° F. 

 and atmospheric pressure. Therefore, to evaporate 1 pound of water 

 at 212° F. (966 B. t. u.) will require the heat given up by 



16 93 

 966 X ~^f =69,000 cubic feet of dry air falling 1 degree. Thus it 



is seen that the heating capacity per unit of volume of superheated 

 steam at atmospheric pressure is but little greater than that of dry 

 air at the same temperature and pressure, in the ratio of 69,000 to 

 54,470, or about 5 to 4. At temperatures above 212° F. and the same 

 pressure of one atmosphere a greater volume is necessary to produce 

 the same effect, since the gas and vapor expand with temperature, 

 but the ratio of the heating capacity of superheated steam and dry air 

 remains very nearly the same. The specific heat of vapor increases 

 slightly at higher temperatures. Thus, figuring in a similar man- 

 ner, it will be found that at five atmospheres pressure (59 pounds 

 gauge) the heating ratio of equal volumes of steam and air is 1.42 

 to 1, and at 1 pound absolute pressure or a vacuum of 28 inches, it 

 is 1.104 to 1. The volume of steam at five atmospheres pressure and 

 306° F. in falling 1 degree necessary to evaporate 1 pound of water 

 at this pressure and temperature is 10,336 cubic feet, and at a 

 vacuum of 28 inches at 101° F. it is 829,433 cubic feet. 



Thus it is seen that there is but little advantage, from the point 

 of view of the volume of gas to be moved, in the use of superheated 

 steam over that of dry air. 



