VEGETABLE AND FRUIT DEHYDRATION 67 



ways change materially during the course of drying any given piece 

 or tray load of vegetables. The air velocity will change correspond- 

 ingly, or it may be changed purposely as the drying progresses. Both 

 the capacity of the clehydrator and the quality of the product are 

 affected by the manner in which these changes occur. 



Cooling of Air Due to Evaporation 



The heat required to evaporate water is supplied, in all common 

 types of hot-air dehydrators, by the circulating air itself. The heat 

 absorbed by evaporation is taken away from the air ; consequently the 

 air cools. This cooling effect is very substantial; as a rough average 

 for commercial conditions, when 1.000 pounds of air takes up 1 pound 

 of water vapor by evaporation the air cools 5° F. (^) . 



For example, air at 160° F. dry-bulb and 95° F. wet-bulb tempera- 

 ture has an absolute humidity of 0.021, so that 21 pounds of water 

 vapor accompany each 1,000 pounds of dry air. The humid heat of 

 the mixture is 0.249. If one more pound of water, assumed to be at 

 the wet -bulb temperature (95°), is evaporated into this air, the latent 

 heat absorbed is 1,040 B. t. u. In addition, the water vapor will be 

 heated up to the new dry -bulb temperature of the air, and the specific 

 heat of water vapor is 0.45, so approximately 27 more B. t. u. will be 

 required, or a total of 1,067 B. t. u. The mixture will therefore cool 



down z ^^ ' ~ ~ A ~ =4.2°. If there is only 1,000 pounds of the origi- 

 1,021 X 0.249 to 



nal mixture, instead of 1,021 pounds, the cooling will be 4.3°. 7 



Evaporation of 1 pound of water into 1.000 pounds of air corresponds 

 to a rise of 0.001 in absolute humidity. If the point corresponding to 

 the new mixture, having a temperature of 155.8° F. and an absolute 

 humidity of 0.022, is spotted on the humidity chart, it will be found 

 that the wet-bulb temperature is still just 95°. This is an important 

 general rule, namely : When water evaporates into air without loss 

 of heat to, or gain of heat from, the surroundings, the wet-bulb 

 temperature of the air remains unchanged. 



In commercial dehydrators the ideal conditions of this example are 

 never fully realized ; the incoming product has to be heated up to wet- 

 bulb temperature, and trucks and trays have to be warmed up; there 

 will be losses of heat to the surroundings through the dehydrator walls, 

 and heat may be gained from an adjacent heating chamber. Neverthe- 

 less these losses and gains are usually small in comparison with the 

 main effect. Commercial dehydrators usually run with a wet-bulb 

 temperature that falls only a degree or two between the hot end and 

 the cool end. Dehydrators designed to reheat the air at intervals 

 during its passage through the equipment will, of course, exhibit a 

 rising w^et-bulb temperature, but in any section where the only process 

 is one of evaporation the wet -bulb temperature will remain substan- 

 tially constant. 



Since the fall in dry-bulb temperature of the air is proportional to 

 the weight of water evaporated into a given weight of air, there is a 

 direct parallelism between the extent of drying of the moist material 

 at any point in its passage through the dehydrator and the fall in air 

 temperature to the same point. The exact relationship will depend 



7 The 5° F. cooling given in the preceding paragraph makes a reasonable allowance for 

 losses of heat in commercial dehydrators. 



