70 



MISC. PUBLICATION 5 4 0, XT. S. DEFT. OF AGRICULTURE 



(described in a subsequent section), the operation changes from par- 

 allel flow to counterflow at a fixed proportion of the total drying time, 

 this proportion being determined by the relative lengths of the two 

 ends of the tunnel. In a drying experiment the total time is unknown 

 until the experiment is complete, and hence the point at which the sim- 

 ulated change-over should be made is unknown. Under these circum- 

 stances it is necessary to run three experiments, with a different mois- 



80 2 4 6 8 10 



MOISTURE CONTENT ( LBS. / LB. BONE-DRY) 

 Figure 34. — Conditions for the determination of drying time. 



ture content selected as the point of change-over in each. The result 

 that would have followed at the proper ratio between times may then 

 be interpolated. 



Experimental determinations of this kind should, of course, be so 

 discounted as to allow for the usual imperfections of a commercial de- 

 hydrator, such as useless short-circuiting of air and down-time 

 while trucks are being moved. 



Effects of Recirculation of Air 



The air leaving the cool end of a commercial vegetable dehydrator 

 of the usual tunnel type is likely to have a temperature of at least 

 100° F. and sometimes even 120° or higher. A very substantial sav- 

 ing in heat can be made by letting only a part of this air escape and 

 mixing the remainder with the fresh air flowing to the heater. This 

 is "recirculation," one of only three factors which the dehydrator 

 operator can easily control; the other two are hot-end temperature 

 and the rate of charging wet material into the tunnel. 



It may seem puzzling that any of this relatively cool, moist air can 

 serve any useful purpose if returned. The fact is that addition of 

 humidity to the circulating air is essential in the dehydration of heavy 

 fruits, and may be advantageous, although to a smaller degree, in the 

 dehydration of a sweet vegetable — for example, sweetpotatoes. This 

 effect — the avoidance of case-hardening — has already been briefly dis- 



