18 CIRCULAR 619, U. S. DEPARTMENT OF AGRICULTURE 



Find on figure 4 the point corresponding to 70° F. and 50 percent 

 relative humidity and follow parallel to the nearest curve to the 

 150° F. dry-bulb temperature line. The relative humidity at this 

 point reads 5 percent. 



CHANGE IN VOLUME PRODUCED BY CHANGE IN TEMPERATURE 



Example: One cubic foot of air at 150° F., IS percent relative 

 humidity, would have at 100° F. a relative humidity of 71 percent, 

 according to figure 4. 



According to figure 3, air at 150° F. and 18 percent relative humidity 

 contains 0.002 pound of water vapor and 0.062 pound of dry air, or 

 0.064 pound of mixture per cubic foot, while at 100° F. and 71 percent 

 relative humidity the mixture contains 0.002 pound of water vapor 

 and 0.068 pound of dry air and weighs 0.070 pound per cubic foot. 

 Therefore the volume at 100° F. and 71 percent relative humidity 



would be n ' n - fl or (0.914 cubic foot). 



WATER EVAPORATED DURING A GIVEN CHANGE OF TEMPERATURE 

 AND HUMIDITY 



Example: Air enters the drying chamber at 160° F. and 20 percent 

 relative humidity and leaves at 120° F. and 65 percent relative 

 humidity. 



According to figure 4, air at 160° F. and 20 percent relative humidity 

 contains 0.044 pound of water vapor per pound of dry air, while at 

 120° F. and 65 percent relative humidity it contains" 0.050 pound. 

 Then there has been evaporated into the mixture 0.050 — 0.044, or 

 0.006 pound of water vapor per pound of dry air. Since 1 cubic 

 foot of the original mixture at 160° F. contained 0.060 pound of drv 

 air (fig. 3) there has been evaporated 0.060X0.006, or 0.00036 pound 

 of water per cubic foot of the original mixture at 160° F. and 20 percent 

 relative humidity. If the air was entering the drying chamber at 

 the rate of 1,000 cubic feet a minute, theoretically the evaporation 

 would be 0.36 pound of water a minute. 



Engineering Calculations For Designing a Tunnel Drier 



The characteristics of a drier may be determined approximately 

 by calculations based on the nature and quantities of the materials 

 to be dried. When the drier is to be used for several materials 

 separate computations must be made for each, so that the drier will 

 fulfill the requirements for all. Such calculations are useful in design- 

 ing a new drier and in remedying the defects of one already in operation. 



To illustrate the computations involved, let it be assumed that a 

 tunnel drier equipped for recirculation and employing the counter- 

 current system of air circulation is to be built with a capacity for 

 drying 7 tons of fresh prunes daily. It will be assumed that the 

 temperature of the outside air is 60° F. General experience in drying 

 prunes indicates that if air is heated to 160° at about 20 percent rela- 

 tive humidity and has a temperature drop of 35° in passing through 

 the tunnel and a humidity at the discharge end not exceeding 60 to 

 65 percent, the drying period will not exceed 25 hours, aud about 



