96 MISC. PUBLICATION 5 40, U. S. DEPT. OF AGRICULTURE 



The time schedule for temperature changes would then be established 

 as follows : 



Temperature changes (°F.) 

 Minutes from start : Dry bulb Wet bulb 



42 200 to 170 120 to 105 



75 170 to 155 105 to 95 



100 155 to 150 95 to 90 



During the second pilot test the product should be inspected for 

 evidence of scorching. If scorching occurs, the operator will be able 

 to adjust the temperature change in other pilot runs, which will be 

 necessary to determine suitable conditions. Let us assume that it 

 was observed that scorching occurred within 42 minutes from the 

 time of starting. This would indicate that the first lowering of 

 temperature should be made sooner but that the temperature should 

 not be lowered as much as shown above. Another pilot test would 

 be run in which the first break in temperature would be made, for 

 example, at the end of 20 minutes, from 200° to 185° F., then at 

 the end of 42 minutes from 185° to 170°. On the other hand, sup- 

 pose that it was observe'd that scorching occurred in the time range 

 from 100 minutes to completion of drying; then another pilot run 

 would be set up in which a lower finishing temperature, for example 

 145°, would be used. 



Because of the lack of complete knowledge it is difficult to set 

 up systems of temperature change in cabinet dehydration. Each 

 operator must depend upon the method of trial and error and ex- 

 perience. The method discussed above is offered for the purpose of 

 supplying a starting point for such investigations. It should be re- 

 membered that the conditions suggested may not in all cases give 

 the best results. 



SOURCES OF HEAT FOR DEHYDRATORS 



Two general systems are commonly used to heat dehydrators : The 

 direct-combustion heater and the indirect heater. With the former, 

 the fuel is burned inside the dehydrator and the gaseous products 

 of combustion are circulated along with the drying air. With the in- 

 direct system, auxiliary heating surfaces for transferring heat to the 

 drying air are used. The latter completely isolate the products of 

 combustion from the drying air and hence complete combustion is 

 not essential, so far as the product is concerned. Fuel-oil burners 

 should be installed in accordance with the recommendations of the 

 National Board of Fire Underwriters. 



Direct-Combustion Heaters 



Direct heaters are limited to liquid or gaseous fuels. The rate of 

 combustion of these fuels can be readily controlled over a wide range 

 and thus temperature can be controlled. Obviously, complete burn- 

 ing is essential with the direct heater, especially with oil, because 

 imperfect combustion might result in smoking, which would seriously 

 affect quality of product. Since there are no stack or transmission 

 losses, optimum efficiency can be obtained, and a minimum of equip- 

 ment is required for satisfactory performance. Most of the dehydra- 

 tors on the west coast use this system. 



In its simplest form, the heater consists of a "premix burner" 

 located openly in one of the air passages of the dehydrator. The in- 



