14 MISC. PUBLICATION' 5 2 4, U. S. DEPT. OF AGRICULTURE 



There are several types of tunnel driers. In the parallel-flow, the 

 counter-flow, and the center-exhaust types the food, piled on trays, 

 is moved on trucks through the tunnel. In the parallel-flow type, 

 the air current moves in the same direction as the trucks. The hot 

 air enters at the same end as the food — the "wet" end — and is re- 

 moved at the "dry" end, so that the fresh, damp material is in contact 

 with the hottest and driest air. At the dry end, however, the air 

 is not only cooler but is more or less saturated with the moisture 

 taken up from the product. In the counter-flow drier these con- 



I 



Figure 7. — Tunnel dehydrator used for experimental studies in the Western 

 Regional Research Laboratory, Albany, Calif. Trucks carrying trays of pre- 

 pared food move on the track through the center. This dehydrator is equipped 

 with instruments that permit studies of such factors as relative humidity, 

 velocity, and temperature of the air. 



ditions are reversed : the hottest and driest air enters at the dry end, 

 and the wet end is in contact with the cooler and partly saturated 

 air. The center-exhaust type is superseding the other two. In this 

 drier, two streams of air are used, one moving from each end. Usually 

 a partition of canvas or other material separates the two ends and 

 is lifted only when the trucks are moved forward. This arrange- 

 ment permits closer regulation of temperature and humidity. 



In another type of tunnel drier, a large moving belt conveys the 

 food from one end to the other, and the circulating air passes through 

 the belt and the layer of food. 



The cabinet dehydrator is a single compartment or series of com- 

 partments in which the trays remain stationary during drying. It 



imemm 



