Multiple Plate Dryer

The free-flowing wet product passes through a drying tower with multiple vertically arranged trays where the hot gas flows over the product [5.1, 5.2]. With plate dryers, the product is moved by means of revolving wiping blades, from the inside to the outside and vice versa and from plate to plate through the dryer (Fig. 5-29). 

In a ring-plate dryer the product is evenly spread out on annular trays by means of leveling blades, to obtain a layer of uniform thickness. The product remains on the tray for one revolution of the rotor before being moved to the next tray beiow by a wiper blade. The drying gas, circulated by externally mounted fans, flows over the product. 

In a ring-plate turbo-dryer, the product flows in a manner similar to the turbo drier through the drying sections. However, the drying gas is distributed over the product by centrally mounted turbine fans. 

---

Performance Data for Direct-Heat Tray Dryers A standard two-truck diyer is illustrated in Fig. 12-48. Adjustable baffles or a perforated distribution plate is normally employed to develop 0.3 to 1.3 cm of water-pressure drop at the wall through which air enters the truck enclosure. This will enhance the uniformity of air distribution, from top to bottom, among the trays. In three (or more) truck ovens, air-reheat coils may be placed between trucks if the evaporative load is high. Means for reversing air-flow direction may also be provided in multiple-truck units. 

Transfer of heat by direct contact is accomplished in spray towers, in towers with a multiplicity of segmented baffles or plates (called shower decks), and in a variety of packed towers. In some processes heat and mass transfer occur simultaneously between phases for example, in water cooling towers, in gas quenching with water, and in spray or rotary dryers. Quenching of pyrolysis gases in transfer lines or towers and contacting on some trays in fractionators may involve primarily heat transfer. One or the other, heat or mass transfer, may be the dominant process in particular cases. 

Steam-heated air is the usual heat-transfer medium employed in these dryers, although combustion gases may be used also. Temperatures above 600 K are not usually feasible because of the problems of lubricating the conveyor, chain, and roller drives. Recirculation of air is in the range of 60 to 90 percent of the flow through the bed. Conveyors may be made of wire-mesh screen or perforated-steel plate. The minimum practical screen opening size is about 30-mesh (0.5 mm). Multiple bands in series may be used. 

Platen dryers. Drying systems have been considered that heat the surface of the veneer directly with heated platens (Loehnertz, 1988 Pease, 1980). A single veneer is placed on the lower plate in each daylight of the multiple daylight press and the platens elosed (100-200 kPa pressure on the veneer) so keeping the veneer flat... 

---