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



guide vanes at one end of the cabinet. The air travels through one 

 or more stacks of trays and is turned back toward the fan by means of 

 another series of similar vanes at the opposite end of the cabinet. 

 Suitable dampers are provided for air recirculation and humidity 

 control. 



Preparation Equipment for Use With Cabinet Driers 



The amount of preparation equipment required depends on the ex- 

 tent of operation. For small batches, hand preparation methods can 

 be used, but if several runs of 500 to 1,000 pounds of prepared material 

 are to be made each day, then supplementary preparation equipment 

 will be required. It will be necessary to set up a regular preparation 

 line for the handling of 15 to 20 tons of raw material per 24 hours in 

 a series of cabinets. 



In general the following procedure may be used with a single- 

 cabinet dehydrator. Fresh vegetables are prepared and properly 

 blanched. For small-batch operations where a limited supply of 

 steam is available, a cabinet-type steam blancher holding 3 or 4 indi- 

 vidual trays can be conveniently used. The loaded trays are slid into 

 the cabinet one above the other on supports made from several pieces 

 of perforated pipe, all connected to the steam supply. 



After loading, the steam is turned on and the blanching time 

 started after the temperature around the trays has reached 200° to 

 212° F. One or more trucks of trays loaded with the prepared and 

 blanched vegetables, or individual trays in the small units, are pushed 

 into the cabinet, the doors fastened, and the fan and heater started. 

 It is good practice to preheat the cabinet in order to cut down on 

 the time required for the temperature to gain its maximum point after 

 loading. The heating capacity should be high enough to maintain 

 the dehydration temperature at the desired point when the load of 

 wet material is placed in the cabinet. 



Operation of the Cabinet Drier 



The proper adjustment of the intake-exhaust damper must be de- 

 termined by experience with a particular type of cabinet dehydrator. 

 and with the products to be dried. In most cases it will be necessary 

 to close the damper at the beginning of the drying process in order to 

 allow the inside temperature to reach the desired point as quickly 

 as possible after loading. The damper should then be opened and 

 adjusted to give a high rate of fresh-air intake with little or no re- 

 circulation, in order to remove the water vapor as rapidly as possible. 

 High dry-bulb and low wet-bulb temperatures are maintained to dry 

 the product in the shortest length of time. At this point the large- 

 capacity heating unit is a necessity in order to insure the maintenance 

 of a high dry-bulb temperature. The rate of drying will be very 

 rapid at first because of the high rate of water evaporation from the 

 product, large air circulation, high temperature, and low humidity. 

 As drying progresses, the rapid evaporation of water is materially 

 reduced, being replaced in part by a slow diffusion process. When this 

 takes place, lesstheat and air circulation are required, and the damper 

 can be partly closed to increase the amount of recirculation and thus 

 avoid excessive heat losses. As the damper is closed, the dry-bulb 

 temperature on the intake side should be lowered in steps and the 



