VEGETABLE AND FRUIT DEHYDRATION 51 



dry air, what is the humid volume of the mixture? Humid vol- 

 ume = (0.0253 + 0.0405X0.27) (220+460) =24.6 cubic feet per pound 

 of dry air. 



In rough calculations of the drying capacity of common types of 

 hot-air dehydrators it is often sufficiently accurate to assume that the 

 humid volume of air is 16 cubic feet per pound. This approximation 

 should not be used in calculations where precision is essential. 



Specific Heat of Air- Water Vapor Mixtures 



The specific heat of any substance is substantially equal to the 

 amount of heat, in B. t. u., required to raise the temperature of 1 pound 

 of it 1° F. The specific heat of dry air in the usual range of dehy- 

 drator temperatures is 0.24. That is, 0.24 B. t. u. applied to 1 pound 

 of dry air will raise its temperature 1°. 



The humid heat of air containing water vapor is defined as the 

 quantity of heat required to raise the temperature of 1 pound of dry 

 air, plus whatever water vapor accompanies it, 1° F. Since the specific 

 heat of pure water vapor is approximately 0.45, the humid heat may 

 be calculated by using the following formula if the absolute humidity 

 is known: Humid heat =0.24+ 0.45 X absolute humidity. 



Example: 1,000 pounds per minute of dry air with an absolute 

 humidity of 0.05 pound of moisture per pound of dry air is flowing 

 through a heater. How much heat must be supplied per minute to 

 raise the temperature of the air 150° F. ? Humid heat = 0.24 + 

 (0.45 X 0.05) = 0.262. Heat required = 1,000 X 150 X 0.262 = 39,300 

 B. t. u. per minute. 



Wet-Bulb Temperature 



When a moist object is exposed to a current of air, the evaporation 

 of water from the object cools both the air flowing past it and the object 

 itself. The drier the air, the greater will be the amount of cooling. 

 If the air is saturated with water vapor, no evaporation will take place 

 and no cooling will occur. These facts are the basis for the use of the 

 wet-bulb thermometer for measuring and controlling the humidity of 

 air in a dehydrator. The temperature of the air, as read on an 

 ordinary thermometer, may be distinguished by calling it the "dry- 

 bulb temperature." The difference between dry-bulb temperature and 

 wet-bulb temperature is called the wet-bulb depression. At any given 

 temperature, the greater the wet-bulb depression the drier the air. A 

 wet-bulb depression of 0° F. indicates saturated air. Wet-bulb de- 

 pressions of as much as 100° are encountered in some commercial 

 dehydrators. 



The directions given in this manual for dehydrating various fruits 

 and vegetables are expressed in terms of dry -bulb and wet-bulb temper- 

 atures. While these values can be translated into terms of percent 

 relative humidity or percent absolute humidity if desired, nothing 

 important from the operative viewpoint is gained by doing so. Direct 

 use of the temperatures, which are read from the instruments, has the 

 great advantage of simplicity. 



Calculations of dehydrator capacity and heat requirement, on the 

 other hand, do require translation of these temperatures at least into 

 terms of absolute humidity. This may be accomplished most easily 

 by means of a humidity chart. 



