PRESERVATION OF FRUITS AND VEGETABLES 21 



Table 4. — Assumed thermal efficiencies of different heating systems 





Thermal efficiency 



Heating system 



Drying 

 chamber 

 (tunnel or 

 cabinet) 



Heater 



Whole 

 system 





Percent 



40-50 

 40-50 

 40-50 



Percent 

 90-100 

 80-90 

 60-70 



Percent 



36-50 





32-45 



Indirect radiation. _ 



24-35 







HEAT TO BE GENERATED BY THE FUEL 



If the heater selected is one of the direct-radiation type, burning 

 ordinary fuel oil, the thermal efficiency of the whole system, accord- 

 ing to the data in table 4, may be expected to be at least 32 percent. 

 In other words, the total heat of evaporation will equal 32 percent 

 of the total heat which must be generated. 



Total heat of evaporation 



0.32 



Total heat which must be generated. 



6,952 B. t. u. per minute 

 0T32~~ 



* or 



For the drier in question this will be 



21,725 B. t. u. per minute, on the average. One gallon of fuel oil 



yields 148,000 B.t.u. Therefore the burners must consume 



or 9 gallons of fuel oil per hour. 



148,000 



AMOUNT OF GENERATED HEAT CARRIED BY THE AIR 



Since the thermal efficiency of a heater of the direct-radiation 

 type, the one selected here, is assumed to be 80 percent, the amount of 

 generated heat carried by the air will be the total heat generated 

 multiplied by 0.80, or 21,725 B. t. u. per minute X 0.80, which is 

 equivalent to 17,380 B. t. u. per minute, on the average. 



AMOUNT OF HEAT GIVEN UP BY THE AIR 



Heat will be absorbed from the air to meet the requirements for 

 total heat of evaporation, which has been estimated in this case to be 

 6,952 B. t. u. per minute. Heat will also be lost through radiation, 

 air leakage, and absorption in heating the material, trays, and trucks. 

 On this account an allowance of 10 percent of the generated heat 

 being carried by the air will be added — in this case, 17,380 X 0.10, or 

 1,738 B. t. u. per minute. The air in passing through the drying 

 chamber will then be required to furnish, on an average, 6,952 + 

 1,738, or 8,690 B. t. u. per minute. 



VOLUME OF AIR REQUIRED 



The volume of air required to give up 8,690 B. t. u. of heat per 

 minute is calculated from the specific heats of dry air and water vapor. 

 Specific heat is the ratio between the heat required to raise (or, con- 

 versely, the heat given off by cooling) a given weight of a substance 



