8 CIRCULAR 619, U. S. DEPARTMENT OF AGRICULTURE 



are not sufficient to compensate for the added cost of equipment and 

 operation. Enzymes are not inactivated by evaporation alone, and 

 it will be found desirable in many cases to inactivate them by blanch- 

 ing or by other means, if vacuum drying is used. 



NATURAL-DRAFT DRIERS 



Natural-draft driers depend for their circulation upon the expansion 

 of air when heated and its consequent uplift by adjacent cooler and 

 denser air. Such driers cost less to build than forced-draft driers, 

 but their operation is less efficient and more expensive. It is difficult 

 to obtain satisfactory drying conditions within them, and a long 

 drying period is required. Recirculation is rarely practical with 

 natural draft; consequently heat is wasted unnecessarily in the dis- 

 charged air. 



FORCED-DRAFT DRIERS 



The greater efficiency and economy of forced draft combined with 

 recirculation has been clearly demonstrated by the Bureau of Agri- 

 cultural Chemistry and Engineering (12) and the Oregon Agricultural 

 College (19, 20). As a result, many operators of tunnel driers have 

 substituted forced draft and recirculation for the natural draft 

 previously used. 



The essential characteristics of all ordinary forced-draft driers 

 are: (1) A single or multiple-unit drying chamber; (2) an air-heating 

 unit; (3) a power-driven fan; (4) an air duct for conveying heated 

 air from the heater to the drying chamber; and (5) a dampered re- 

 circulation duct for returning any part or all of the used air from the 

 drying chamber to the heater. 



CONVEYOR-BELT DRIERS 



In driers of the conveyer or endless-belt type, the material is 

 carried through a drying hood or chamber on one or more moving 

 endless belts made of slats or wire mesh. These driers are used suc- 

 cessfully in completing the drying of partially dried raisins and in 

 drying sweet corn (10). This method is not suited to fruits that 

 stick to the belt or to fruits likely to be injured by dropping from one 

 belt to another when multiple-staggered belts are used. Neither 

 are such driers adapted to irregular or discontinuous loading and 

 drying operations. Because of these limitations and their small 

 loading capacity, conveyer-belt driers are less practical than tunnel 

 and compartment driers for general use. 



COMPARTMENT DRIERS 



Compartment driers (4, 5, 12) have a drying chamber divided by 

 partitions into several compartments, each holding one or two stacks 

 of trays. These trays can be handled most conveniently on trucks. 

 Air is carried from the heaters through a main duct, from which por- 

 tions are diverted to each compartment. Circulation in a vertical 

 direction through the tier necessitates a shifting of the trays during 

 drying, inasmuch as the contents of the trays farthest from the 

 source of the air supply dry more slowly than the material on nearby 

 trays. For this reason circulation of the air across the trays is prefer- 

 able. The air is discharged from each compartment into a recircu- 



