PRESERVATION OF FRUITS AND VEGETABLES 

 Table 3. — Heat units furnished by various fuels and electricity 



11 



Source of heat 



Heat 

 units fur- 

 nished 



Fuel 



required 



to furnish 



750,000 



B.t.u. 



Fuel oil - 



B.t.u. per 



pound 



18, 500 



12, 500 



7,589 



1750 



3 3, 415 



Pounds 

 40 



Coal 



60 



Wood (air-dried, 10.3-percent moisture) ._.... . . _ . 



100 



Gas - 



2 1, 000 





* 220 







i Per cubic foot. 



2 Cubic feet. 



3 Per kilowatt-hour. 

 ■* Kilowatt-hours. 







Direct heat, direct radiation, and indirect radiation are the types 

 of heat generally employed in heating drying air. 



Direct-heating systems have the highest fuel or thermal efficiency. 

 The mixture of fuel gases and air in the combustion chamber passes 

 directly into the air used for drying. This method requires the use 

 of special burners and a fuel, such as distillate or gas, which burns 

 rapidly and completely, without producing soot or noxious fumes. 

 The heater consists simply of a bare, open firebox, equipped with one 

 or more burners, an emergency flue to discharge the smoke incidental 

 to lighting, and a main flue, through which the gases of combustion 

 are discharged into the air duct leading to the drying chamber. 



Direct-radiation systems also are relatively simple and inexpensive 

 and have a fairly high thermal efficiency. A typical installation 

 consists of a brick combustion chamber with multiple flues, which 

 carry the hot gases of combustion back and forth across the air- 

 heating chamber and out to a stack. The air is circulated over these 

 flues and heated by radiation from them. The flues are made of 

 light cast iron or sheet iron. The air-heating chamber should be 

 constructed of fireproof materials. The efficiency of the installation 

 depends upon proper provision for radiation. This is attained by 

 using flues of such length and diameter that the stack temperatures 

 will be as low as is consistent with adequate draft. 



Heating the air by boiler and steam coils or radiators is an indirect- 

 radiation system, as the heat is transferred from the fuel to the air 

 through the intermediate agency of steam. Such a system costs more 

 to install and has a lower thermal efficiency than either of the other 

 two systems. It is principally adapted to large plants operating 

 over a comparatively long season on a variety of materials where the 

 steam is needed to run auxiliary machine^ or to process vegetables. 



Air 



Large volumes of air are required to carry to the products the heat 

 needed for evaporation and to carry away the evaporated moisture. 

 Insufficient air circulation is one of the main causes of failure in many 

 dehydrators, and a lack of uniformity in the air flow results in uneven 

 and inefficient drying. 



FANS 



The fan may be installed to draw the air from the heaters and 

 blow it through the drying chamber, or it may be placed in the return- 



