Drying Desk: Controlling dryer operation for product quality
In this column, we'll discuss typical control systems for continuous direct-contact (convection) and indirect-contact (conduction) dryers, the limitations of these control systems, and how new technologies may overcome these limitations. Our discussion will focus on achieving an acceptable final product with the desired moisture content and no thermal degradation.
When sizing a continuous dryer for your application, the design engineer selects an operating design point for the dryer that corresponds to your specific feed and process requirements. These include the wet feed rate, feed moisture content, feed temperature, final product moisture content, material temperature in each drying zone, material residence time in each drying zone, and the maximum allowable heating temperature in each drying zone. However, once the dryer is operating, some of the incoming feed's characteristics -- such as the wet feed rate, feed moisture content, and feed temperature -- will vary. While such variation is normal, 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.