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



process 5 tons of raw product per 24 hours. The dehydrator is a 

 nine-truck tunnel of small cross section, and it is assumed that one 

 truck will be loaded every 40 minutes. The preparation line will 

 probably be operated only one or two shifts per day ; the drying will 

 therefore continue only until all the product in the tunnel has been 

 dried. 



The operation of plants much smaller than those handling 25 tons 

 per day is likely to be intermittent, and batch-type driers or tunnels 

 smaller than the usual commercial type may, therefore, be preferable. 

 The use of tunnel-type driers in a discontinuous operation is feasible 

 only if close control of temperature and humidity is maintained dur- 

 ing the starting-up and shutting-down periods. 



The choice of drier may be influenced by the amount of labor 

 required. If tray-type driers are used, all practical labor-saving 

 methods and devices should be installed. Conveyor driers require 

 less labor. Tray handling and washing may entail a considerable 

 amount of hand labor, whereas belt cleaning may be almost entirely 

 automatic. Where labor rates are high, the rehandling costs involved 

 in multistage drying may be sufficient to cause a reconsideration of 

 the system to be installed. Automatic movement of the cars in and 

 between the tunnels may overcome this disadvantage. 



The lower labor cost in operating a conveyor drier may offset the 

 higher initial capital cost. The output per dollar of investment for 

 a conveyor dehydrator is generally less than for a tunnel drier. It 

 may not be possible, however, to determine which type of dehydrator 

 is preferable on the basis of cost alone. It is probable that the choice 

 will be determined mainly by technological factors. It may depend 

 also upon the availability of construction materials. Availability 

 of operating labor and materials must also be considered. 



The upkeep of the drier is important. The cost of maintaining the 

 trays in proper condition can be balanced against the upkeep of a 

 large and costly belt or conveyor. 



Ample capacity in the dehydrator is usually a good investment. 

 Since the fuel and power costs are relatively low, an increase or 

 decrease of even a substantial percentage does not seriously affect 

 the total processing cost. Increased labor costs due to inefficient use 

 of labor in the preparation line, when the dehydrator is unable to 

 handle the output of the line, usually amounts to far more than any 

 additional drying cost resulting from the use of a slightly oversized 

 dehydrator. 



Finishing bins used in conjunction with the dehydrator increase 

 the capacity of the dehydrator proper by shortening the time of the 

 main drying operation. This shortening of drying time may result 

 in an improvement in product quality. The over- all cost per unit of 

 drying capacity will usually be less when finishing bins are used. 



Loading and stacking trays. — One tray line should ordinarily be 

 adequate for plants handling up to 100 tons per day. Proper timing 

 of tray loading, stacking, drying, and tray scraping is essential for 

 efficient operation. This is especially true for large plants. A mini- 

 mum of about 10 to 12 seconds should be allowed for handling each 

 tray at the loading point, although the actual operation of taking the 

 tray from the loading table and placing it on the truck requires 

 somewhat less time. On this basis, the 100-ton plant is near the upper 

 limit for one tray line. It should be borne in mind that if the rate is 



