VEGETABLE AND FRUIT DEHYDRATION 



79 



tunnel; the "hot end" is therefore the "wet end," and the "cool end" is 

 the "dry end." 



This arrangement leads to a kind of exaggeration of good and bad 

 drying conditions. At the wet end of the tunnel, where the product 

 will lose moisture most readily, the air is hottest and driest ; evapora- 

 tion will be exceedingly rapid because both conditions favor it. At 

 the dry end, on the other hand, the natural slowness of loss of 

 moisture is reinforced by the low temperature and high humidity 

 of the air. The equilibrium moisture content of the product will be 

 correspondingly high. It may be impossible to reach the desired 



o 10 20 30 



DISTANCE FROM WET END (FEET) 



5 



a 

 i 



o 



CD 



.2 



m 

 _) 

 ,")< l 



V 

































m 











D 1 



2 



3 







DISTANCE FROM WET END (FEET) 



Figure 39. — Drying conditions in a heavily loaded counterflow tunnel drier. Time, 

 8.2 hours ; relative capacity, 1.00 ; final moisture content, 5 percent. 



final dryness. Figure 42 illustrates the change in air temperature 

 and moisture content through such a tunnel at high input of wet 

 material. Under these conditions the product would not be dried 

 below about 10-percent moisture even if the tunnel were indefinitely 

 long. 



If the rate of input of wet material to this tunnel were decreased 

 (as by loading trays lightly), so that the fall in air temperature 

 would not be so great, drying conditions at the dry end would 

 not be so unfavorable. In fact, it is obvious that the performance 

 of a parallel-flow tunnel would be indistinguishable from that of a 



