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



Closed-Cycle Arrangement 



Any type of dehydrator can be modified so that water vapor is con- 

 densed from the exhaust air, which is then returned and used over 

 again as "fresh" air. In this way it becomes possible to use an inert 

 gas, such as carbon dioxide or scrubbed flue gas, without an insup- 

 portable loss of the gas. While this process, as applied to vegetable 

 dehydration, is being investigated in several quarters, its cost and 

 complexity have discouraged wide application. The dehumidifica- 

 tion of the exhaust gas can be accomplished by passing it through 

 pipes cooled externally by trickling cold water or a refrigerant, or, 

 more simply, by direct contact of the exhaust gas with a spray of water 

 at a temperature lower than the dew point of the gas. In the com- 

 partment type of tunnel some of this dehumidification can be accom- 

 plished in each compartment by means of a water spray in advance 

 of the heater. 



Combined Blanching and Predrying 



Some development work has been done on very brief predrying at 

 very high wet-bulb temperature, for the purpose of producing a 

 blanching action simultaneously with the removal of a substantial 

 part of the easily evaporated water. For example, exposure of cut 

 vegetables to a current of air at a temperature of 250° F. or even 

 300°, and a wet-bulb temperature of 200° for from 6 to 10 minutes 

 may evaporate as much as half of the total moisture, and at the same 

 time blanch the material. While the process has interesting possibili- 

 ties it has not yet reached commercial development. 



STARTING AND STOPPING THE TUNNEL DRIER 



Under normal conditions, the operation of the tunnel drier pre- 

 sents few difficulties ; the beginning and ending of a dehydration run, 

 however, require certain modifications of conditions and these are dis- 

 cussed below. 



Simple Counterflow Tunnel 



Unless corrective measures are applied, the first truckload of prod- 

 uct introduced into a counterflow drier will be subjected to a hot, un- 

 tempered blast of air during its entire time in the tunnel. Each suc- 

 ceeding truck, until the tunnel is filled and stabilized, will be sub- 

 jected to abnormal, nonuniform drying conditions. The variation 

 from normal drying conditions will, however, decrease as the tunnel 

 becomes filled. As a result, the product dried during the starting-up 

 period may be scorched and ruined. 



In theory at least it is possible to schedule the drying in accordance 

 with a predetermined time-temperature drying curve and adjust the 

 recirculation of air to produce a constant wet-bulb temperature 

 throughout the initial charging. The curves could be established 

 from similar dehydrator runs or from published data or pilot tests. 

 A typical time-temperature drying curve is shown in figure 47. This 

 precise method is, however, impracticable, because it is impossible to 

 adjust tunnel temperatures rapidly to the desired levels. Theoreti- 

 cally, as many temperature adjustments should be made as there are 

 cars in the tunnel. 



