VEGETABLE AND FRUIT DEHYDRATION 73 



supplied to the circulating air varies between about 1,500 and 6,000 

 B. t. u. per pound of water evaporated. The requirement varies both 

 with the type of dehydrator and with the conditions of operation ; it 

 is partially within the control of the operator. 



The heat consumption of most types of dehydrators may be esti- 

 mated approximately from the following formula : 9 



5=l,250[r+(l-r)/y^] 



where B = heat that must be supplied, B. t. u. per pound of water 

 evaporated, 

 t = temperature of the incoming, or fresh air, in ° F., 

 t' = temperature of the mixture as it enters the drying section 



of the dehydrator, and 

 t" = temperature of the air as it leaves the drying section of 

 the dehydrator, to be partially recirculated. 

 Example: In the simple counterflow tunnel used in previous ex- 

 amples, r=0.743, £ o = 60°, £' = 165°, £" = 137.5°. If these values are 

 substituted in the formula, Z?= 2,150 B. t. u. per pound of water evapo- 

 rated. If the evaporation is 11.1 pounds of water per minute, as in 

 preceding examples, the necessary heat input is 1,430,000 B. t. u. per 

 hour. 



In a complex dehydrator this formula can be used to estimate the 

 heat requirement of separate sections, provided the necessary infor- 

 mation is available for each section. 



Even though this estimate makes some allowance for ordinary heat 

 losses, the designed heating capacity of a dehydrator will normally 

 be increased generously over the estimate in order to allow for air 

 leakage and unforeseen exigencies of operation. Excess heating ca- 

 pacity will allow the operator a desirable degree of flexibility in 

 choosing conditions under which the dehydrator shall work, particu- 

 larly the wet-bulb temperature level. 



Consideration of the formula for heat requirement brings out the 

 following relationships : 



1. The higher the proportion of recirculation, other things being 

 equal, the lower will be the heat consumption. 



Example : If the dehydrator in the previous example were operated 

 without recirculation, but at the same rate of evaporation and same 

 hot-end temperature as before, the heat usage would be 4,750 B. t. u. 

 per pound of evaporation, or 3,180,000 B. t. u. per hour. The wet- 

 oulb temperature in the tunnel would be lowered from 100° F. to 86°. 



2. For given conditions inside the dehydrator, the heat con- 

 sumption will be higher if the outside air is colder or if its 

 humidity is higher. 



Example : Consider conditions in the counterflow dehydrator main- 

 tained at 100° F. wet-bulb temperature, hot-end temperature 165°, 

 cool-end temperature 137.5°. 



a. The heat consumption when the outside air has a temperature 

 of 60° F., wet-bulb temperature of 55°, has already been calculated as 

 2,150 B. t. u. per pound of evaporation, 1,430,000 B. t. u. per hour. 

 What will it be if the outside temperature is 5°, wet-bulb 4° ? From 



9 The coefficient 1,250 is the product of 5° F. (change in temperature per 0.001 increase 

 in absolute humidity), divided by 0.001 (to convert to pounds of water evaporated per 

 pound of air), multiplied by 0.25 (average humid heat of air in the dehydrator). It makes 

 a reasonable allowance for ordinary heat 



