110 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE 



minimum number of bins required would then be 240/80 or three bins. 

 However, since the product is to be packaged only during the 8-hour- 

 day shift, three bins are inadequate. 



For purposes of computation, let us consider the period immediately 

 after the 8 hours of packaging. At this time, at least three bins are 

 filled or partially filled because of the 12 hours required in the bin 

 drier. In addition to these three bins, four more are required to dry 

 and store the product that will be produced during the coming 16 

 hours when no packaging is done. One additional bin must also be 

 provided for use during the time the first bin is being emptied the fol- 

 lowing morning. Therefore it is obvious that at least eight bins are 

 required for this operation. 



Little is known about drying rates of various vegetables in bin 

 driers, but, as a basis for determining the approximate number of bins 

 required, it is safe to assume that the retention time will be from 6 to 9 

 hours for an air velocity equivalent to 80 or 100 cubic feet of air per 

 minute for each cross sectional square foot of active bin capacity. The 

 maximum incoming air temperature should not exceed 10° F. below 

 that recommended for the dry-end temperature in a counterflow de- 

 hydrator for the same vegetable. It may be necessary with vegetables 

 that are very sensitive to heat, to keep the temperature even lower 

 than this. 



If final results indicate that the capacity of the finishing unit is 

 below the capacity expected, the operator need only increase the re- 

 tention time in the preceding drying stages until a working balance 

 is reached. This condition was illustrated by the previous example. 



Air Desiccators 



Bin drier operation will vary considerably between humid and arid 

 regions. If high relative humidities are prevalent in the plant locality, 

 it will be almost essential to employ some means of drying the air 

 before it is introduced into the bin finisher. A unit for this purpose 

 is called an air desiccator or air dehydrator. The former is the pref- 

 erable designation, because the other may lead to confusion. 



One of the most satisfactory means of drying air is through the 

 use of a chemical adsorption or absorption drier. Units of this type 

 are capable of delivering air at fairly high temperature and low ab- 

 solute humidity. This method obviates the necessity of reheating 

 the dehumidified air, as would be required by a dehumidifying unit 

 using refrigeration. This factor may be of considerable importance 

 in the selection of an air-desiccating method. 



When an air desiccator is used with a bin finisher, it may become 

 desirable to recirculate all or part of the drying air. In multibin oper- 

 ation, the air expelled from each bin will have a different absolute 

 humidity, because the products in the bins are at various stages of 

 dryness. In practice, however, the air from each bin is discharged 

 into a common return duct. If the absolute humidity of the mixed 

 air in the return duct is less than that of the outside air, then it should 

 be recirculated; if not, it should be entirely wasted to atmosphere. 

 On "borderline" installations this condition may vary from day to 

 day ; hence repeated daily checks should be made. 



