VEGETABLE AND FRUIT DEHYDRATION 89 



driers, on the other hand, require a number of hours to start up and 

 shut down, and hence are best adapted to continuous, three-shift 

 operation. The purpose here is to describe several types of cabinet 

 driers and to discuss in greater detail suggestions for the operation 

 of these cabinets as single and multiple units. 



A typical cabinet dehydrator consists of an insulated structure, 

 square or rectangular in shape, equipped with a fan that forces the 

 drying medium (usually air) through a heating system and dis- 

 tributes it uniformly through one or more stacks of trays loaded 

 with prepared material. The walls and top can be constructed from 

 a number of materials such as plywood, masonite, transite, brick, 

 or hollow tile. Inflammable materials should be adequately pro- 

 tected around the heating system, particularly if the latter is of the 

 open-flame type. Duct turns, baffles, or other means are provided to 

 insure proper distribution of the heated air and thus prevent uneven 

 drying. Adjustable clampers are provided to admit fresh air and to 

 exhaust the moist air. By proper adjustment of the damper a defi- 

 nite percentage of the moist, hot air can be recirculated as a means 

 of conserving fuel, and in addition a control is maintained over 

 the degree of humidity in the cabinet. Dry-bulb and wet-bulb ther- 

 mometers are mounted on the exhaust side of the cabinet and a dry- 

 bulb thermometer on the intake side. 



Cabinet dehydrators differ in the manner used to heat the air and 

 in the method used to circulate and distribute this heated air through 

 the loaded trays. Some are heated by the combustion of natural gas 

 directly in the circulating air, and under controlled conditions some 

 grades of fuel oil can be similarly used. Others are heated by fin-type 

 steam coils or iron-pipe radiators, whereas still others are equipped 

 with one of several types of heat exchangers and are heated with gas, 

 oil, coal, coke, or wood as fuel. With direct -burning gas or oil heat- 

 ers, most of the heating value is utilized, whereas in the indirect heat- 

 transfer systems, only 50 to 75 percent of the heating value is made 

 available. In any case, the heating system should be of such capacity 

 that it will supply to the circulating air from 1,200 to 1,600 B. t. u. 

 per hour per square foot of active tray surface used in the cabinet. 

 A common error is the installation of a heater of insufficient capacity. 

 The actual capacity of the heating unit will depend on a number of 

 factors, such as amount of insulation in the cabinet, air flow, tempera- 

 ture ranges desired, degree of recirculation of the air, and the load to 

 be dried. Most cabinet dehydrators are equipped with centrifugal 

 or rotary fans, of which there are many types; others employ one- 

 or two-blade propeller or radial fans. These fans should be of such 

 size and should be driven at such speed as to deliver an air velocity, 

 measured between the loaded trays, of 800 to 1,000 feet per minute. 



The prepared vegetables are spread on suitable trays made from 

 either metal or wood with bottoms of hardware cloth. All-wood 

 trays are also used. The trays are usually of two sizes — a two-man 

 3 by 6-foot or the one-man 3 by 3-foot size. The loaded trays are 

 stacked one above the other, either by sliding them in on guide rails 

 arranged in a metal frame on a movable truck or by stacking them 

 directly on top of each other on the truck without any support. In 

 the smaller cabinets, individual trays are placed on guide rails one 

 above the other. The free opening between trays should be 2 to 3 

 inches. 



