78 



MISC. PUBLICATION 5 4 0, U. S. DEPT. OF AGRICULTURE 



Figuke 37. — Tunnel dehy&rator 



tables. Suggested temperatures and humidities suitable for counter- 

 flow dehydrators can be found in the later section on commodities. 



Since the rate of evaporation from the nearly dry product is very 

 low, the drop in temperature of the air near the dry end of a counter- 

 flow dehydrator must be relatively slight. At the wet end of the dehy- 

 drator the product can lose moisture rapidly, but the air will be 

 relatively cool and moist, and the evaporation rate may not be high. 

 Figure 39 illustrates the conditions of air temperature and moisture 

 content of product which would be fairly typical at points along a 

 counterflow dehydrator operating at high input of wet material. 

 Drying cannot be anything but comparatively slow near the dry end, 



HEATER 



BLOWER 



FRESH- J 

 AIR ^ 



INTAKE 



WET 



MATERIAL 



IN 



///(// 





DRY 

 + MATERIAL, 



'EXHAUST-AIR STACK TRUCKS 



Figure 38. — Counterflow arrangement in the tunnel-and-truck dehydrator. 



in spite of good drying conditions, but it is also relatively slow near 

 the wet end. Conditions like this are well adapted to the drying of 

 fruits, but are not well suited to vegetables. Vegetables could be dried 

 rapidly in this same tunnel by decreasing the total amount of evapora- 

 tion to be done (for example, by loading trays lightly) and so decreas- 

 ing the fall in temperature of the air. (See fig. 40, which may be 

 compared with fig. 39 to illustrate the change in conditions.) In the 

 example from which these figures were drawn the drying times were 

 8.2 hours to correspond to figure 39, 6.35 hours to correspond to figure 

 40; but the drying capacity of the tunnel was 30 percent less in the 

 second case than in the first. 



Parallel Flow Arrangement 



Figure 41 illustrates the arrangement of a simple parallel-flow tun- 

 nel. The direction of air flow is the reverse of that in the counterflow 



