PRESERVATION OF FRUITS AND VEGETABLES 15 



not be used profitably because a point is approached at which the 

 materials will be blown from the trays or at which the increased 

 speed of drying will not offset the cost of operating a larger fan. 

 Velocities of 600 to 800 feet per minute through the drying chamber 

 are satisfactory in tunnel driers; lower velocities are permissible in 

 compartment driers. 



The most practical means of removing moisture from the air, and 

 at the same time conserving heat, is through the steady discharge of 

 a portion of the air leaving the drying chamber. The rest dries 

 efficiently when mixed with fresh air from the outside and reheated. 

 All forced-draft driers, therefore, should be provided with recirculation 

 ducts connecting the air-outlet end of the drying chamber with the 

 heaters and with dampers controlling the air discharged, recirculated, 

 and drawn from the outside. 



All forced-draft driers, therefore, should be provided with recircula- 

 tion ducts connecting the air-outlet end of the drying chamber with 

 the heaters and with dampers controlling the air discharged, recir- 

 culated, and drawn from the outside. 



DETERMINATION OF AIR CONDITIONS 



Figures 2, 3, and 4 will assist in solving recirculation problems. 

 The curves in figure 4 are vapor-pressure curves, graduated and 

 expressed as pounds of water vapor per pound of dry air instead of 

 as vapor pressure. The curve corresponding to 0.020 pound of water 

 vapor per pound of dry air, for example, represents a vapor pressure 

 of 0.93 inch of mercury. These curves, therefore, can be used as 

 vapor-pressure curves in problems that do not involve the actual 

 computation of vapor pressures. 



The practical application of the charts in solving various problems 

 can best be illustrated by means of examples. The charts given 

 here serve merely to indicate the methods of computation. In 

 reaching exact results, the writers used larger charts than could be 

 reproduced here, showing more subdivisions. 



RELATIVE HUMIDITY FEOM KNOWN WET- AND DRY-BULB READINGS 



Example: Dry bulb = 150° F.; wet bulb = 100° F. 



Find in figure 2 the intersection point of the lines corresponding to a 

 dry-bulb temperature of 150° F. and a wet-bulb temperature of 100° 

 F. The line representing the relative humidity at this point reads 

 18 percent. 



POUNDS OF DRY AIR AND OF WATER VAPOR PER CUBIC FOOT UNDER KNOWN 

 CONDITIONS OF TEMPERATURE AND RELATIVE HUMIDITY 



Example: Dry bulb = 150° F.; relative humidity = 18 percent. 



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

 ity contains 0.062 pound of dry air and 0.002 pound of water vapor per 

 cubic foot. The weight of the mixture would then be the sum of 

 these weights, or 0.064 pound per cubic foot. 



CHANGE IN RELATIVE HUMIDITY PRODUCED BY A CHANGE IN TEMPERATURE 



Example: Lowering the temperature from 150° F. and 18 percent 

 relative humidity to 100° F. 



On figure 4 start at the point corresponding to 150° F. and 18 

 percent relative humidity and follow parallel to the nearest curve 



