Tray and Conveyor Belt Dryers

Krauss Maffei Corporation manufactures an indirectly heated plate dryer with arms and plows that transfer the material to be dried from the top of the unit to the discharge at the bottom, being conveyed downward in a spiral fashion by a rotating shaft to which stationary plates are attached. An illustration of this dryer is found 

TABLE 9.2. Evaporation Rates and Thermal Efficiencies of Dryers 

TABLE 9.3. Comparative Performances of Basic Dryer Types  

Product filter cake clay sand Ti02 spent grain coal 

Product dispersion in gas slab packed bed gravity flow spray dispersed fluid bed 

Characteristic product shape thin slab extrusion granules spherical drops grains -in. particles 

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CONTINUOUS TRAY AND CONVEYOR BELT DRYERS 243 TABLE 9.2. Evaporation Rates and Thermal Efficiencies of 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. 

Figure 9.8. Rotary tray, through-circulation belt conveyor, and heated screw conveyor dryers, (a) Rotary tray dryer (Wyssmont Co.), (b) Action of a rotating tray and wiper assembly (Wyssmont Co.), (c) A single conveyor belt with air upflow in wet zone and downflow in dry (Proctor and Schwartz Inc.), (d) A two-stage straight-through belt conveyor dryer, (e) A three-belt conveyor dryer as the material becomes dryer, the loading becomes deeper and the belt longer (Proctor and Schwartz Inc.), (f) Screw conveyor dryer with heated hollow screw (Bepex Corp.).

More than one kind of dryer may be applicable to a particular product, or the shape and size may be altered to facilitate handling in a preferred kind of machine. Thus, application of through-circulation drying on tray or belt conveyors may require prior extrusion, pelleting, or briquetting. Equipment manufacturers know the capabilities of their equipment, but they are not always reliable... 

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. 

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