VEGETABLE AND FRUIT DEHYDRATION 93 



drying finished off at temperatures of 120° to 170° F., varying with 

 the product, to prevent scorching. 



It should be understood that when temperature or absolute humid- 

 ity of the fresh-air supply changes, both the heat supply and the set- 

 ting of the dampers must be varied accordingly if a desired tempera- 

 ture schedule is to be maintained. For example, if the temperature of 

 the fresh air supply is lowered, the heat input must be increased to 

 maintain the desired cabinet temperature. If the absolute humidity 

 of the fresh air rises, the amount of recirculation must be decreased 

 and additional heat must be supplied, since an additional quantity of 

 fresh air must then be heated up to cabinet temperature. If tempera- 

 ture and recirculation are controlled automatically, these compensa- 

 tions will be made by the mechanism. 



During the dehydration process most of the water is removed from 

 tne product during the first 2 hours, and the last portions are the 

 hardest to remove. Two methods can be used for equalizing the final 

 moisture content of the material. If single batches are used each day, 

 the simplest method is to leave the material on the trays in the 

 cabinet and continue the dehydration overnight, after lowering the 

 temperature to 120° to 125° F. and running the circulating fan at a 

 reduced speed. This practice is somewhat costly. If it is desired to 

 run the single cabinet continuously, then a small-capacity equalizing 

 bin is useful. The contents of the trays are dumped together into this 

 bin, the top is partly closed, and air heated to 120° to 125° (wet -bulb 

 not over 75° to 80°) is forced up from the bottom and through the ma- 

 terial to carry off the remaining moisture. 



Continuous Dehydration in Cabinet Driers 



Several continuous systems of dehydration have been developed by 

 the use of a series of two or more cabinet driers for handling 10 to 20 

 tons of fresh material per 24 hours, and even higher in some cases. 

 (See discussion of compartment arrangement, p. 82.) One design 

 employs a battery of four duplex cabinet dehydrators. These duplex 

 units are placed side by side in line, each fitted with two sets of doors 

 so that the trucks go in at the front and out at the back. Each unit 

 consists of two independently controlled air-conditioning systems, one 

 to provide optimum primary conditions for dehydration, the other to 

 provide optimum finishing conditions. These independent systems 

 make it possible to maintain two temperature ranges in the same unit 

 without the use of partitions. The primary chamber is designed 

 to give extremely rapid drying by the use of high temperatures and air 

 velocity during the period when the material is still wet. The finish- 

 ing chamber is designed to remove the remaining moisture from the 

 material at such temperatures as will not cause scorching or other 

 damage. 



In a suggested operation procedure, two trucks are loaded with the 

 prepared and blanched material and pushed into the primary chamber 

 of the first cabinet, and this is repeated for each unit. Initial dehy- 

 drating temperatures of 180° to 190° F. are used, and the temperature 

 is gradually reduced to 160° during the first 2 hours of drying. The 

 two trucks are then moved over into the finishing chamber for an addi- 

 tional 2 hours of drying at a temperature range of 160° down to 145°. 

 Two newly filled trucks are pushed into the empty space in the primary 



