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



The proportion of recirculation may be readily calculated by the 

 use of the following formula : 8 



a! — a 



a —a 



where r — proportion of the circulating air that is recirculated, 

 then l-r= proportion of fresh air in the mixture, 



a = absolute humidity of the incoming, or fresh air, 

 a' = absolute humidity of the mixture as it enters the 

 dehydrator, 

 and a" = absolute humidity of the air as it leaves the dehy- 

 drator, to be partly recirculated. 

 Example : Suppose that a counterflow tunnel is operating at a wet- 

 bulb temperature of 100° F., a hot-end temperature of 165°, and a 

 cool-end temperature of 137.5° F. The fresh air drawn in has a tem- 

 perature of 60° and a wet-bulb temperature of 55°. What are the 

 proportions of recirculation and fresh air intake? 



From the humidity chart, figure 23, a o = 0.0080, a' = 0.0271, a"= 

 0.0337. Then r= 0.743, or 74.3 percent, and l-r= 0.257, or 25.7 percent. 

 If air is circulating in the dehydrator at a flow of 2,000 pounds per 

 minute, the fresh-air intake will be 514 pounds per minute. 



In a complex dehydrator the formula may be used by applying it 

 separately to sections where conditions are known at both ends. For 

 example, the "fresh air" supplied to the primary end of a center- 

 exhaust dehydrator may consist partly or wholly of air exhausted 

 from the secondary section of the dehydrator. 



Control of recirculation enables the operator to maintain substan- 

 tially uniform drying conditions regardless of variations in atmos- 

 pheric conditions. If the humidity of the fresh air rises, the pro- 

 portion of recirculation can be reduced to compensate for the change. 

 Even in subtropical climates, compensation is possible without car- 

 rying a very high wet-bulb temperature in the dehydrator. For 

 example, on a very humid summer day the absolute humidity may 

 rise to 0.025; if the dehydrator hot- end temperature is 150°, some 

 recirculation will still be required to hold a wet-bulb temperature of 

 96° or above. This compensation by varying the recirculation is at- 

 tended by variation of the heat requirement of the dehydrator, as 

 will be shown in the following section. 



L 



Heat Usage in a Dehydrator 



The necessity of supplying from some source at least 1,000 B. t. u. of 

 heat for each pound of water evaporated has already been pointed out. 

 It is not necessary theoretically that this heat be supplied from an 

 artificial source; water evaporates from the washing hung on a 

 clothesline without any consumption of fuel. Even in that case, 

 however, the heat absorbed in evaporation is reflected in a cooling 

 of the air blowing past the wet clothes. 



From the practical standpoint that type of natural evaporation is 

 too slow and uncertain to be used for dehydrating vegetables, although 

 it is extensively used for drying prunes and apricots. Dehydration 

 of vegetables is speeded up and made controllable through the appli- 

 cation of artificial heat. In commercial dehydrators the heat actually 



8 The effects of leakage from the dehydrator and water vapor added by direct combustion 

 of fuel are neglected in this formula. 



