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64 / January 2020 powderbulk.com DRYING DESK PBE Controlling dryer operation for product quality John J. Walsh American Drying Consultants ity, we won't discuss particle size, shape, and density as variables affecting product quality. These variables often are determined by the dryer type. For instance, a spray dryer's atomization method can affect particle size, shape, and density, greatly influencing product quality. In a milling dryer, parti- cle size reduction strongly influ- ences the product quality, and in an agglomerating dryer, particle growth is an important factor.) Controlling convection drying Cocurrent flow. In a continuous cocurrent-flow convection dryer, such as a flash dryer or spray dryer, heated air directly contacts the material as both the air and material flow in the same direc- tion through the dryer. The sim- plest control method for this dryer type is feedback control based on measuring the dryer's outlet air temperature and then adjusting the heated inlet air temperature or wet feed rate, or both, accord- ingly. (Adjusting just one variable, the heated inlet air temperature, is primary feedback control; adjusting the second variable, the wet feed rate, when primary control doesn't work is a secondary control method called cascade-loop control.) The control system may impose limits W hen sizing a continuous dryer for your applica- tion, the design engineer selects an operating design point for the dryer that corresponds to your specific feed and pro- cess requirements. These include the wet feed rate, feed moisture content, feed temperature, final product moisture content, mate- rial residence time in each drying zone, material temperature in each drying zone, and the maximum allowable heating temperature in each drying zone. Once the dryer is operating, however, some of the incoming feed's characteristics, such as wet feed rate, feed moisture content, and feed temperature, will vary. While such variation is nor- mal, it requires the dryer's control system to automatically adjust the drying operating variables so that the dryer can achieve an acceptable final product quality. In this column, we'll revisit one of our past discussions of typical control systems for continuous direct-contact (convection) and indi- rect-contact (conduction) dryers, the limitations of these control systems, and how newer technologies may overcome these limitations. Our discussion will focus on achieving an acceptable final product with the desired moisture content and no thermal degradation. (For simplic-