VEGETABLE AND FRUIT DEHYDRATION 107 



pletely filled. A draft of heated dry air is forced through the louver 

 inlets at the bottom and the product dries progressively upward ; there- 

 fore, the dried product at the bottom is removed at the same rate as 

 the bin is loaded from the top. 



The batch-process bin finisher is somewhat similar in design. A 

 gate is located at the bottom to permit dumping of the entire bin. 

 Three or more bins are usually required, as will be explained later. In 

 operation, the entire bin is filled with the partially dried product; the 



PRODUCT LOADING 

 - DOOR 

 - 



XHAUST- 

 AIR DUCT 



PARTIALLY 

 DRIED PRODUCT 



HOT DRY AIR 

 SUPPLY DUCT 



Figure 57. — Continuous-bin finisher. 



material dries progressively upward, and, as soon as the material near 

 the top is dried to the required moisture content, the entire bin is 

 dumped and a new cycle is commenced, 



Operation of Bin Driers 



During normal operation, the product, as loaded into the bin drier, 

 contains not over 15 percent moisture. This partially dried product 

 is of sufficient stiffness so that it will not crush, and will form a solid 

 mat. Therefore care should be taken not to tamp the product during 

 loading operations. The charging depth may range between 2 and 6 

 feet. Under this condition of loading, air can be readily forced 

 through the material by standard-design blowers. 



As a product loses its moisture, its rate of drying becomes slower 

 and slower. The latent heat necessary for evaporation will be required 

 at a proportionally slower rate. Since the air is the source of heat 

 supply and transfer medium, the required quantity and velocity of 

 air become correspondingly smaller. The air bathes the product and 

 carries away the diffused moisture as it is released. The principles 

 involved are discussed in publications by Furnas (15) and Gamson, 

 Thodos, and Hougen (16). 



The depth of bed will determine to a large degree the batch retention 

 time. Usually bins are operated at the upper limit of the usable 

 temperature range. If excessive bed depths are used, the dried product 

 on the bottom may be injured by prolonged exposure to maximum 

 temperature. Therefore, to reduce the batch-retention time, relatively 

 high velocities should be used. 



An undesirable condition that may arise is illustrated by the follow- 

 ing example : Let us assume that a deep-bed bin drier is being used to 

 finish potatoes. Assume further that the potatoes have been scraped 

 from drying trays of the preceding stage in a room having a temper- 



