108 MISC. PUBLICATION 5 4 0, U. S. DEPT. OF AGRICULTURE 



ature of 60° F. . It is probable, then, that when the bin is filled with 

 these potatoes, the average bin temperature will be in the neighborhood 

 of 60°. Assume that air at a relatively low velocity and temperature 

 is circulated through this bed. The air will pick up moisture and 

 gradually drop in temperature until, at a certain point in the bed it 

 can no longer dry the product. From this point, the cool product will 

 gradually reduce the air dry-bulb temperature until, at a point within 

 the bed, the air becomes saturated. Further penetration of the air, 

 while in contact with the cold bed, will reduce the air temperature 



PRODUCT LOADING DRAG CONVEYOR 



PRODUCT UNLOADING CONVEYOR 



Figure 58. — Multibin finishing drier. 



below the dew point ; thus moisture will be condensed upon the product. 

 If this condition arises, the wetted product may be injured. The 

 condition can be remedied by increasing either the temperature or the 

 velocity or both, by decreasing the bed depth, or by providing auxiliary 

 internal bin heating devices. 



An operator can detect faulty bin operation b^ taking samples from 

 the uppermost surface of the product periodically throughout the dry- 

 ing cycle and analyzing them for moisture content. The time intervals 

 should be fairly close because the region of condensation, which is 

 likely to be fairly shallow in depth, might otherwise not be detected. 



Regardless of the velocity and temperature of air employed, a bin 

 drier will ultimately carry the product through the full depth of the 

 bed to a condition approximating equilibrium with the entering air. 

 This, of course, is based on the assumption that heat loss due to radia- 

 tion, convection, and conduction from the outer surfaces of the bin are 

 relatively small. 



Batch drying is intermittent in operation; therefore several bins 

 are usually provided to permit continuous drying operations. At 

 least three bins are usually employed, and while one is being loaded 

 the others may be used in drying or unloading. Figure 58 shows a 

 suitable multibin finisher design. Any number of units in addition 

 to the three shown can be provided to suit plant capacity. 



Determining Needed Capacity 



To illustrate a method of determining required capacity, we may 

 consider a small onion-drying plant producing approximately 225 

 pounds of flaked onion per hour. Laboratory tests indicate that a 

 5-foot depth of product can be dried from 12 to 4 percent moisture 



