VEGETABLE AND FRUIT DEHYDRATION" 77 



square feet of conveyor surface would be required if the loading were 

 only 1:2 pounds per square foot, but would be only 625 square feet 

 if the loading could be increased to 12 pounds. 



Drying a thick layer of moist material is practical only if the flow 

 of air is through the layer. Conveyor belt driers are therefore almost 

 invariably built for "through" circulation of air. Special precautions 

 must be taken to assure uniform loading of the conveyor, and the 

 avoidance of packing, or matting, since heavily loaded areas or those 

 in which the product has packed tightly will fail to receive their share 

 of the drying air and the whole conveyor will have to be slowed down 

 to meet the abnormally slow rate of drying of those spots. 



It is usual to divide the travel of the conveyor into at least two or 

 three sections, in each of which the temperature and humidity of the 

 circulating air may be maintained at values appropriate to that stage 

 of the drying. The temperature and humidity of the air entering 

 the moist layer will be substantially the same throughout the entire 

 length of each section. This fact limits the air temperature to that 

 which may safely be used on the driest product leaving the section. 

 At the end of the first section, for example, there will ordinarily be 

 a considerable gradation of moisture content within the layer, with 

 the bottom of the layer much drier than the top ; the air temperature 

 in the entire first section must be limited to that which is safe for this 

 driest product. 



If the direction of air flow is reversed in successive sections, the 

 gradation of moisture content within the layer may be rapidly 

 smoothed out. Then if the material is repiled on a slower-moving 

 conveyor, substantial uniformity of moisture content can be achieved. 

 This repiling is desirable as a means of mixing the material prior to 

 the final stage of drying and it may also reduce the required size of 

 dehydrator materially. For example, potato strips piled on a con- 

 veyor in a layer about 4 inches deep may shrink to a layer less than 2 

 inches deep when 90 percent of the moisture has been evaporated; 

 more than half of the total drying time will still remain to be trav- 

 ersed, however. At this point the product may be firm enough to be 

 repiled in a layer 10 to 12 inches deep. If it is so repiled, the area of 

 conveyor required for the finishing stage will be only a fifth to a sixth 

 of that which would have been needed without the repiling. 



Temperatures that are suggested as suitable for three-stage conveyor 

 dehydrators will be found in this manual in the sections devoted to 

 specific vegetables. 



TUNNEL-AND-TRUCK DEHYDRATORS 



Many different arrangements of tunnel dehydrators have been 

 suggested, but most of them can be regarded as modifications or com- 

 binations of three arrangements : counterflow, parallel flow, and com- 

 partment (fig. 37). 



Counterflow Arrangement 



Figure 38 illustrates a simple counterflow tunnel dehydrator. Air 

 flows through the tunnel in the direction opposite to the travel of moist 

 material, so that the "hot end" is also the "dry end," and the "cool end" 

 is the "wet end." This is the most common type of dehydrator used 

 on the Pacific coast for drying prunes and raisins, and a number of 

 these fruit dehydrators are being used without alteration for vege- 



