VEGETABLE AND FRUIT DEHYDRATION 



61 



Effect of Temperature Level 



At a given wet-bulb depression, drying will proceed faster if the 

 temperature level is high than if it is low. Figure 28 compares the 

 results of two runs, in both of which the wet-bulb depression was 

 40° F. ; in one, however, the temperatures were 90° and 130°, while 

 in the other they were much higher, 120° and 160°. In the latter 

 the rate of drying was substantially faster. Note, however, that the 

 effect was less marked than that of wet-bulb depression as shown in 

 figure 28. That is, if dry-bulb temperature were the same in two 

 runs, the one at the higher wet-bulb temperature would be the slower 

 because of the predominant effect of wet-bulb depression. 



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Figube 28. — Drying curves obtained with differing temperatures but with the 

 same wet-bulb depression. 



Effect of Air Velocity 



The higher the velocity of air over a moist surface, the more rapid 

 the evaporation from that surface. Figure 29 compares drying curves 

 from experiments with potatoes. Air velocity across the tray was 

 500 feet per minute in one case, 675 in another, and 855 in the third. 

 Drying was somewhat more rapid with higher air velocity. The 

 effect of air velocity is much more substantial in drying cabbage and 

 leafy vegetables in general than it is in drying potatoes. 



A change in the air velocity in a dehydrator has two quite inde- 

 pendent effects. One is the effect on drying rate, which has just been 

 described. The other will be considered more fully in a following 

 section, and, in brief, is a consequence of the fact that the weight of 

 air moved through the dehydrator rises in proportion to the velocity 

 of that air. The greater the weight of air circulated, the less it will 

 fall in temperature for a given amount of evaporation, and hence, 



