180 



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



with an outside temperature of 60° F. and 60 percent relative humidity. 

 The following assumptions have been made : Cost of fuel oil, 3 cents 

 per gallon, or coal, $5.50 per ton; cost of electricity, iy 2 cents per 

 kilowatt-hour; 60 percent fuel efficiency; constant air flow; main- 

 tenance of a constant wet-bulb depression at the cool end. Under the 

 conditions mentioned, 75 percent recirculation is most economical. 

 For any particular vegetable, and for different weather conditions, 

 similar charts can be drawn. It is possible that minimum costs may 

 not be obtainable under conditions that are optimum from the stand- 

 point of product quality. The plant operator must often determine 

 the best compromise among the three factors : Product quality, product 

 cost, and plant output. 



Discussions of operating costs usually assume a balance between the 

 preparation line and the dehydrator. Obviously, the balance is dis- 



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COST 



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30 40 50 60 70 



RECIRCULATION (PERCENT) 



80 



90 100 



Figure 76. — Cost of evaporating water in air-blast dryer at various percentages 



of recirculation. 



turbed by anything that changes the capacity of either one. For 

 example, the output of the preparation line may be increased by an 

 improvement in raw material, thus increasing the amount of material 

 fed to the drier. If the dehydrator has reserve heating capacity, it 

 may be possible to dry the increased quantity by decreasing the amount 

 of air recirculated, thus shortening the drying time. (The alternative 

 expedient of increasing tray loading may be undesirable from the 

 standpoint of quality because of increased drying time, and, for the 

 same reason, may not increase the quantity of material the dehydrator 

 will dry in a given period of time.) The increase in fuel cost due to 

 lower heat efficiency will usually be more than offset by the value oi 

 the increased output of the plant. In other words, if there is no short- 

 age of fuel, fuel economy is relatively less important than labor 

 efficiency. 



