88 MISC. PUBLICATION 5 4 0, TJ. S. DEPT. OF AGRICULTURE 



dry-bulb temperature is decreased, to simulate normal drying condi- 

 tions. This is achieved by dropping the hot-end temperature in suc- 

 cessive steps equal to the average temperature drop across a truck each 

 time an empty truck is placed in the tunnel and until the unit is emptied 

 of all products. The average temperature drop across the truck may 

 be computed by dividing the difference between the hot-end and cold- 

 end tunnel temperatures by the number in the tunnel. Similarly, the 

 wet-bulb temperature can be maintained substantially at a constant 

 level by adjustment of the recirculation damper as each succeeding 

 truck is removed. By reference to figure 48, it will be noted that a 

 straight line between the upper and lower temperatures in the parallel- 

 flow drying range will not be at great variance from the curve that 

 represents temperature. This fact suggests the reliability of the 

 method. 



Counterflow Tunnel as Second-Stage Drier 



In multistage dehydration, the counterflow tunnel is almost invari- 

 ably used as the secondary or intermediate stage. The entering 

 product has been predried and usually contains about 50 percent 

 moisture (T=1.0). There is little danger of case hardening; there- 

 fore exposure to normal dry-end tunnel temperatures over prolonged 

 periods of time will result only in drying to lower moisture levels, 

 approaching the equilibrium point as a limit. 



Since the moisture removal in a secondary stage is comparatively 

 small, the temperature drop in the tunnel is also small. Thus, when 

 the predried product is first placed in the counterflow tunnel, which 

 has been brought up to normal operating temperature, the product 

 will be subjected to a temperature slightly higher than the average 

 for its normal retention time. The result will be a satisfactory 

 product that has been slightly overdried. In the event that excep- 

 tional operating precautions are required with a temperature-sensitive 

 product, the procedure previously outlined for the counterflow tunnel 

 operation can be used. For most if not all products, this procedure 

 will be found unnecessary. 



No special precautions are suggested for shutting down a counter- 

 flow secondary drier. It is operated at normal temperatures until 

 the product has been cleared from the tunnel. Some fuel may be 

 saved by readjustment of the recirculation damper to maintain 

 normal wet -bulb temperatures. Here again, it is possible to shorten 

 the retention time of the last trucks as a means of preventing ex- 

 cessive drying. 



CABINET DEHYDRATORS 



Cabinet driers serve two main functions: (1) In single or multiple 

 units they are suitable for capacities such as 1 to 20 tons of fresh 

 vegetables per day, and particularly when different vegetables are 

 to be handled at the same time, and (2) they are useful for pilot- 

 scale and experimental operations as a means of obtaining drying- 

 rates and other data applicable to tunnel drying. 



Batch dehydrators, of which cabinet driers are an example, thus 

 have a special field of usefulness quite distinct from that of con- 

 tinuous dehydrators. Batch driers can be built as small as desired, 

 for community, institution, farm, or even family use. Since the 

 drier can be shut down completely when a batch has finished drying, 

 it is well adapted to daytime or one-shift operation. Continuous 



