22 CIRCULAR 619, U. S. DEPARTMENT OF AGRICULTURE 



1° F. and that required to raise the same weight of water 1°, the 

 specific heat of water being considered as 1. At constant pressure 

 the specific heat of dry air is 0.2375, or approximately 0.24, while 

 the specific heat of water vapor is 0.475. One cubic foot of a mix- 

 ture of dry air and water vapor under given conditions of temper- 

 ature and humidity will, in dropping 1°, give a number of B. t. u. 

 represented by the expression: (Pounds of dry air per cubic foot X 

 specific heat dry air) + (Pounds of water vapor per cubic foot X spe- 

 cific heat of water vapor). The number of B. t. u. given up per 

 cubic foot of mixture in dropping a given number of degrees Fahren- 

 heit would be: Drop in degrees Fahrenheit X the foregoing expres- 

 sion. Consequently a given number of B. t. u. would be given up 

 by the number of cubic feet of mixture represented by the fraction: 



Number of B. t. u. required 



Drop (°F.) X [(Pounds of dry air per cubic foot X 0.24) + (Pounds 

 of water vapor per cubic foot X 0.475)] 



In the present problem, where an average of 8,690 B. t. u. per 

 minute is required from air at 160° F. and 20 percent relative humid- 

 ity, dropping 35° in passing through the tunnel, and where the 

 weights of dry air and of water vapor in the air at 160° and 20 per- 

 cent relative humidity are 0.0598 and 0.0026, respectively, the air 

 required, in cubic feet per minute, is as follows: 



8,690 1cnnA . , ! 



35X[(0.0598X0.24) + (0.0026X0.475)] = 16 ' 000 ' ^V^xim^telj. 



VELOCITY OF AIR MOVEMENT 



As previously estimated, the tunnel will have 25 square feet of 

 free cross-sectional area. The velocity of movement of 16,000 cubic 



feet of air per minute through the tunnel therefore is — — — > or 640 



feet per minute. 



HUMIDITY AT AIR OUTLET END OF TUNNEL 



The air entering the tunnel at 160° F. and 20 percent relative 



humidity contains 0.0598 pound of dry air and 0.0026 pound of 



water vapor per cubic foot. Into 16,000 cubic feet of this mixture 



6 3? 

 is evaporated 6.32 pounds of water vapor. Thus, ' " , or 0.0004 



pound of water vapor, would be associated with the weight of dry air 

 and water vapor in 1 cubic foot of the original mixture. Disregarding 

 the leakage of air into or out of the drying chamber, the air leaving 

 the tunnel would contain 0.0026 + 0.00^04, or 0.003 pound of water 



vapor, associated with 0.0598 pound of dry air, or n ' ' > or 0.050 



pound of water vapor per pound of dry air. Air at 125° containing 

 0.050 pound of water vapor per pound of dry air has 56 percent 

 relative humidity. 



