VEGETABLE AND FRUIT DEHYDRATION 109 



in approximately 8 hours. The plant will accommodate a bin not 

 over 10 feet wide. What should be the bin proportions ? 



The bulk density of nearly dried flaked onions is found to be ap- 

 proximately 15 pounds per cubic foot, Since one continuous bin is to 

 be installed, it must have sufficient volume to accommodate the entire 

 output of the plant for an 8-hour period. Its capacity can be com- 

 puted by multiplying the retention time in hours by the plant output 

 per hour and dividing by the bulk density of the product. Thus the 

 volume of the bin equals 8X225/15 or 136 cubic feet. With a bed 

 depth of 5 feet, the cross section of the bin must be slightly over 27 

 sqoiare feet. Since the bin is to fit into a 10-foot-wide space, a bin 

 4X7X5 feet deep may be satisfactory. 



Let us suppose that a unit of this size has been built, and that sub- 

 sequent tests show that the 8-hour drying time is too short. If the 

 highest permissible air temperature for drying onions is already in 

 use, the bin capacity can be further increased by resorting to higher 

 air velocities. If, however, the fan is operating at full capacity, the 

 only remaining alterable condition is to reduce the moisture of the 

 entering product by increasing the retention time in the preceding 

 stages. 



Increased drying in the preceding stage affects the bin capacity 

 in either of two ways. First, the longer retention period in the pre- 

 ceding stage decreases the input to the bin drier. Second, the reduc- 

 tion in total moisture to be removed by the bin drier shortens retention 

 time here. Hence, under a particular ratio of primary to secondary 

 retention time, a balance will be reached. Although predictable with 

 a fair degree of accuracy, the optimum ratio can best be found by 

 trial and error. 



The balance established under one set of drying conditions will not 

 necessarily hold for all other drying conditions. For example, the 

 operating balance established with daytime initial drying and night- 

 time bin drying may not be suitable if the drying cycle is reversed. 

 This change in retention ratio from day to day is obviously undesir- 

 able. However, a safe working ratio can be established, based on the 

 most unfavorable conditions, and thus a workable solution can be ob- 

 tained. The effect is to produce extra drying some of the time. How- 

 ever, the ease of operation on a fixed schedule will in all probability 

 offset the expense of the additional heat. The loss of product weight 

 due to extra drying will be negligible. 



As an example of multibin driers, let us assume that the manage- 

 ment of a medium-sized plant is considering the installation of a mul- 

 tibin finisher to increase the plant capacity. The plant is to be able 

 to process 300 pounds of nearly dry potatoes per hour, having a mois- 

 ture content of 12 percent. Packaging is to be done only during one 

 8-hour shift. How large a multibin finisher will this plant require? 



The bulk density of partially dried potatoes is found to be 15 pounds 

 per cubic foot. Material will then be delivered to the bins at a rate 

 of 300/15 or 20 cubic feet per hour, or 160 cubic feet per shift. Assume 

 that previous experience indicates that 12 hours are required to re- 

 duce the moisture content of potatoes from 12 to 4% percent. The 

 bins, therefore, must have a minimum total volume of 12 X 20 or 240 

 cubic feet if packaging operations are to be continuous. The bins 

 selected for use are to have a working volume of 80 cubic feet. The 



