VEGETABLE AND FRUIT DEHYDRATION 99 



temperature within the dehydrator can be accomplished by a relatively 

 simple method, which is applicable over a complete range of dehy- 

 drator sizes. On the other hand the equipment is considerably in 

 excess of that required by other methods, and the over-all efficiency is 

 considerably below unity because of boiler and transmission losses and 

 will in all probability be less than 60 percent. Initial cost, upkeep, 

 and overhead are correspondingly high. 



Heat exchange surfaces are normally copper-finned coils. Rela- 

 tively large amounts of heat can be transferred to air in motion by a 

 compact heat-exchange surface of this type. However, because of 

 wartime restrictions, copper heating coils cannot be procured at this 

 time and, as an alternate, steel-finned coils are being manufactured. 

 Their procurement is difficult, and the operator may find it expedient 

 to improvise some other surface, such as steel pipes arranged to form a 

 suitable coil. The amount of unfinned pipe required to effect a given 

 heat transfer will be considerably in excess of that required for finned 

 pipes. The size of coil thus formed might well exceed the normal space 

 usually allotted to the heating unit, and may therefore be a limiting 

 factor in the design of the steam-heated dehydrator. 



A second limitation would be an increase in static pressure caused by 

 pipes or coils, which must be overcome by the fan or blower. To keep 

 the face velocity of the air and hence the static pressure down, the pipes 

 must be properly spaced. 



If a coil is to be built for use in a dehydrator, it is suggested that 

 care be taken in the design to provide ample surface for adequate heat 

 transfer, because the performance of the dehydrator is at stake. Eapid 

 and complete elimination of the condensate and air is also necessary 

 to promote coil efficiency. 



Some large steam-heated dehydrators have been successfully de- 

 signed to use steam for driving the blowers and to utilize the exhaust 

 steam in the heating system. The blowers are driven by bleeder-type 

 turbines, operating at a back-pressure of from 10 to 50 pounds per 

 square inch. Such an arrangement may reduce power costs Very con- 

 siderably. Each turbine may be designed and operated so that the 

 steam flow through it will normally be less than the minimum steam 

 demand of the corresponding heaters, so that additional high-pressure 

 steam will always have to be bled into the heating system. Automatic 

 controls will then adjust the auxiliary steam supply to satisfy the 

 heater demand without affecting the operation of the blower. 



TEMPERATURE CONTROLLERS 



There are two general basic types of temperature controllers — the 

 off-on controller and the modulating controller. The choice between 

 these two is determined by the type of heating system used in the 

 dehydrator. Essentially, the piping arrangement is the same and is 

 independent of the type of controller used, as well as the kind of 

 fuel or heating medium employed. (See figure 55 for a typical 

 arrangement.) 



As its name implies, the off-on controller operates a motorized valve 

 in such a way that it is either fully opened or fully closed. There is 

 no intermediate valve position. For this reason, all off-on controllers 

 are essentially instruments with high sensitivity. This type of con- 

 troller is relatively simple to operate, and is inexpensive. It is avail- 



