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



chamber, and this procedure is repeated in each unit. At the end of 

 the dehydration period, the trucks are removed from the finishing 

 chamber and the dried product equalized in moisture content in a fin- 

 ishing bin, and then packaged. 



Establishing a Time and Temperature Schedule for Cabinet 



Dehydration 



As drying in a cabinet progresses it is necessary to lower the dry-bulb 

 and wet-bulb temperatures by steps until the appropriate finishing 

 temperature is reached. These changes in temperature should proceed 

 in accordance with a schedule that must be determined by experimental 

 pilot tests. Each schedule is of course applicable only to uniform 

 conditions of air flow, tray loading, and shape and size of pieces. 



Since it is usually impractical to make moisture analyses of the 

 product as drying progresses, changes in moisture content can be esti- 

 mated by weighing at frequent intervals. The temperature changes 

 are made when the reduction in weight reaches certain values deter- 

 mined by multiplying the estimated dry weight of the product by the 

 factors indicated in table 8 for each product. 



At least two pilot tests are required to establish a time schedule for 

 temperature change as dehydration progresses. The first provides an 

 estimate of the amount of water lost from the product during dehydra- 

 tion, or conversely the yield of dry product from prepared. This is 

 accomplished by loading the drier with prepared material, recording 

 the weight on one or more trays, drying under constant temperature 

 at or near that suggested as finishing temperature for the product, and 

 recording the weight and percentage of yield of dry product. 



Let us assume that the drier has a capacity of 1,000 pounds of pre- 

 pared potatoes and that it is provided with three trays that can be 

 readily removed for weighing. One of the trays is located halfway 

 between the top and middle, one in the middle, and one halfway from 

 the middle to the bottom of the tray stack. When the cabinet is fully 

 loaded, it is found that the net \veight of product on each of the three 

 trays is 20 pounds and that after dehydration at 150° F. dry-bulb 

 and 90° wet-bulb the average weight of the trays of product is 4.2 

 pounds. Then the yield of dry product is 21 percent. 



The second pilot test is made to determine the time schedule for re- 

 ducing the temperature during the regular run and is carried out as 

 indicated in the following example : The cabinet is loaded with 1,000 

 pounds of prepared potatoes. The three trays mentioned above are 

 each loaded with 20 pounds of raw material. In the first test it was 

 determined that the drying yield for these potatoes was 21 percent, so 

 that from each of the trays a yield of 0.21 X 20=4.2 pounds of dry mate- 

 rial can be expected. Dehydration is started at 200° F. dry-bulb and 

 120° wet-bulb temperature. At frequent intervals the trays are 

 weighed. When the average weight of a tray of material drops from 

 20 to 3 times the expected dry weight, or 12.6 pounds, the time for this 

 drop in weight is recorded and the temperatures dropped to 170° F. 

 dry-bulb and 105° wet-bulb as indicated in table 8. When the average 

 weight is reduced from 12.6 pounds to 2 times the expected dry weight 

 or 8.4 pounds, the time for this drop in weight is recorded and the 

 temperature dropped to 155° dry -bulb and 95° wet -bulb as indicated 

 in table 8. The time required for the weight of the product to be re- 

 duced from 8.4 to VA times the expected dry weight or 6.3 pounds is 



