VEGETABLE AND FRUIT DEHYDRATION 



71 



cussed. Most vegetables can be most rapidly and completely dehy- 

 drated with air of low humidity. When that is the case, the main ad- 

 vantage of recirculation lies in the saving of heat, and this saving will 

 be at least partly offset by slower drying and lower drying capacity. 



Study of typical operating conditions discloses that it is possible to 

 recirculate a substantial proportion, say 50 to 75 percent, of the air in 

 commercial tunnel dehydrators. and realize a considerable saving in 

 the cost of heat without serious loss of tunnel capacity. Figure 35 

 illustrates these relations on a skeleton humidity chart. Point S 

 represents the temperature and humidity of the fresh air. If the tem- 

 perature, designated £', is the desired hot-end temperature, then point U 

 represents the condition of the air entering the hot end; its absolute 



DRY-BULB TEMPERATURE (°F.) I f 



Figure 35. — Effect of recirculation of air on amount of heat required. 



humidity is the same as that of the fresh air. In passing through the 

 tunnel, its temperature will fall and its humidity will rise, ending at 

 the cool end of the tunnel at a point on the chart such as V. 



If there is no recirculation, all of this air will be discarded and 

 fresh air will be heated from S to U. But if, say, only one-third of 

 the exhaust air is discarded, and only that difference is made up with 

 fresh air, then humidity rises in the tunnel, and after a short time 

 new steady conditions are set up, represented by points P, Q, and R 

 on the chart. The air at the hot end of the tunnel will still be heated 

 to the same temperature as before (t') but its humidity will be higher 

 (P). In passing through the tunnel, its condition will change to 

 that indicated by Q. The mixture of one-third fresh air and two- 

 thirds exhaust air will be represented by R. The amount of heat 

 required is represented roughly by the length of the line RP, which 

 is much shorter than the one representing no recirculation, STJ. The 

 increase in wet-bulb temperature and the corresponding decrease in 

 evaporative capacity, however, are only moderate. 



