VEGETABLE AND FRUIT DEHYDRATION 



63 



the layer. A more precise measure, in view of the fall in temperature 

 of the air as it passes through the layer, is the weight of air per 

 minute per square foot. The latter is known as mass velocity. 



In general the drying rate is affected very much more by thickness 

 of layer in a cross-circulation drier than in a through-circulation drier. 

 If a very thin layer were exposed on an open-mesh support, drying 

 rates would probably be nearly the same for air flow across the layer 

 as through it. On the other hand, complete drying of a layer of vege- 

 table pieces several inches deep would be practically impossible by pure 

 cross circulation but would take place rapidly by through circulation. 



In some dehydrators of the tray type an effort is made to induce at 

 least a part of through circulation either by baffling the air stream or 

 by slanting the trays at an angle to it. Substantial savings in drying 

 time, ranging as high as 20 percent, have been reported as a result of 

 slanting trays. 



Effect of Thickness of Layer 



The slowing down of drying as the thickness of layer of wet material 

 spread on trays in a cross-circulation drier is increased is illustrated by 

 figure 30. The thickness is indicated by the values given for weight 

 of moist material spread on a square foot of tray surface. 



5 



/-% 

 >• 

 tc 



? z 



UJ 



z 

 o 



CD 



^\ 



Nc 















Tra 



V (Po 



y-Loadinq 



unds per 



Square Fo 



ot) 





CD 1 



</> 



CO 









.3.0 









-I 0.5 



K- 

 Z 

 UJ 



t- 



o 0.2 

 o 



UJ 



cc 

 ?0.l 



</> 

 O 



2 

 .05 























[V 















^^^ 



L 



lT 



i 



12 3 4 5 6 7 



TIME (HOURS) 



Figure 30. — Effect of depth of layer of material on drying time in a tray-type 



dehydrator. 



With fairly light loadings the drying time increases more slowly 

 than the rate of increase in loading ; that is, doubling the loading does 

 not double the drying time. Since the output of a given dehydrator 

 depends on the quotient of these two factors, the output may be in- 

 creased by loading trays more heavily — up to the point where a fur- 

 ther increase in loading is just compensated by a proportional increase 

 in drying time. If the loading is increased beyond this point the out- 



