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56 / January 2021 powderbulk.com PBE DRYING DESK John J. Walsh, American Drying Consultants Drying with particle size control: A review O ne factor that can greatly influence your dryer selec- tion is the need to maintain or reduce your material's parti- cle size or shape, or both, during drying. In past columns, we've discussed different types of dry- ers and their ability to do this. In this column, we'll review some of that information. Dryers that minimize particle attrition To maintain the size and shape of very fragile particles, you must avoid or at least minimize parti- cle attrition during drying. This requires a dryer that can gently handle the material. The following are typical batch and continuous dryers that meet this requirement. Tray dryer (batch). A tray dryer consists of a housing enclosing shelves or hollow plates that sup- port multiple drying trays. The dryer operates in batch mode. For each batch, workers spread wet feed material as a shallow layer across each tray, load the trays into the shelves or plates in the dryer, and, after drying, unload the trays. For drying under atmo- spheric conditions, the dryer uses convection — circulating heated air over the top surface of the material on the trays — and this air sweep removes evaporated moisture. For drying under vacuum, the dryer uses conduction — circulating an indirect heat-transfer fluid, such as steam, hot water, or hot oil, through the hollow plates under the trays — and a vacuum pump removes evaporated moisture. In either operation, the material remains stationary during drying, elimi- nating particle attrition. However, the dryer's manual loading and unloading steps are labor intensive. Another limitation is that a mois- ture gradient can develop across the material's depth on the trays, resulting in nonuniform drying of the final product. Tray dryer (continuous). The typical continuous tray dryer incorporates a vertical cylindri- cal housing containing a stack of several circular trays and a ver- tical shaft mounted with arms and plows that rotate at the cen- ter of the trays. Wet feed material is continuously fed onto the top tray, where it forms a shallow layer, and, as the shaft rotates, the arms and plows slowly move the material across each tray in a spiral flow path. The material cascades from the top tray to the tray below, where the spiral flow path reverses, and the final dried product exits the dryer from the bottom tray. The dryer operates by convection under atmospheric conditions, with heated air flowing across the material surface on each tray. The material is continuously turned over by the plows, exposing new material surfaces to the heated airstream for moisture evapo- ration. This material turnover improves air-to-material contact, which minimizes moisture gradi- ents in the final dried product. The plowing action is slow enough to gently handle fragile particles with very little attrition. The continuous operation also eliminates manual loading and unloading. Plate dryer. Similar to the con- tinuous tray dryer, the plate dryer uses circular hollow plates instead of circular trays and also operates continuously. The plate dryer is a conduction dryer with steam, hot water, or hot oil flowing through the hollow plates. The dryer can operate under atmospheric con- ditions, removing evaporated moisture with an air sweep, or the unit can operate under vacuum, removing evaporated moisture with a vacuum pump. The rotating shaft's plows move the material as a thin layer across the plates in a spiral flow path, and the material cascades from the top tray down to the next until exiting from the bottom tray. The dryer's gentle plowing action handles fragile par- ticles with very little attrition. Belt conveyor dryer. The belt con- veyor dryer operates continuously and consists of a long tunnel-like housing containing a moving belt conveyor equipped with a porous belt. Wet feed material is contin-