Dryers, belt tunnel

A number of factors influence the selection of a dryer from the many different types available. These factors are dominated by the nature of the feed, whether it be granular solids, a paste, a slab, a film, a slurry, or a liquid. Other factors include the need for agitation, the type of heat source (convection, radiation, conduction, or microwave heating), and the degree to which the material must be dried. The most commonly employed continuous dryers include tunnel, belt, band, turbo-tray, rotary, steam-tube rotary, screw-conveyor, fluidized-bed, spouted-bed, pneumatic-conveyor, spray, and drum dryers. 

Tunnel dryers are shown in Fig. 3.15a. Wet material on trays or a conveyor belt is passed through a tunnel, and drying takes place by hot air. The airflow can be countercurrent, cocurrent, or a mixture of both. This method is usually used when the product is not free flowing. 

Air Drying Equipment. Tunnel kiln dryers (70) are long furnaces comprised of several zones of different temperature, humidity, and air flow through which the ware travels on a moving car or belt. These kilns afford continuous processing. Periodic kiln cross-circulation dryers (70) are box furnaces in which ware is stacked on permanent racks or on a car that can be shuttled in and out of the furnace. Fans or jets are used to circulate heat uniformly through the ware. The process is not continuous, but production rates can be enhanced by shuttling multiple cars. 

Drying times range from a few seconds in spray dryers to 1 hr or less in rotary dryers and up to several hours or even several days in tunnel shelf or belt dryers. 

Drying with Changing Humidity of Air in a Tunnel Dryer A granular material deposited on trays or a belt is moved through a tunnel dryer countercurrently to air that is maintained at 170°F with steam-heated tubes. The stock enters at 14001bdry/hr with W = 1.16 lb/lb and leaves with O.llb/lb. The air enters at 5% relative humidity (Hg = 0.0125 lb/lb) and leaves at 60% relative humidity at 170°F Hg = 0.203 lb/lb). The air rate found by moisture balance is 7790 lb dry/hr ... 

Continuous Band and Tiumel Dryers This group of dryers is variously known as band, belt, conveyor, or tunnel dryers. 

The dryers generally used for the drying of piece-form fruits and vegetables are cabinet, kiln, tunnel, belt-trough, bin, pneumatic, and conveyor dryers. Among these, the cabinet, kiln, and bin dryers are batch operated, the belt-trough dryer is continuous, and the tunnel dryer is semi continuous. 

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