Drying Desk: Selecting fluid-bed dryer features
Fluid-bed dryers come in a myriad of configurations. How do you decide which features will best suit your application? Should you select a stationary fluid-bed dryer or a vibrating unit? Should you use a circular or rectangular drying chamber? Should you use indirect heating elements in the material bed, and if so, what type? Should your dryer be zoned? What type of gas distributor screen and material feed should you choose? In this column and the following one I'll discuss factors that can help you answer these questions.
Let's start with the basics. A fluid-bed dryer is a vessel that allows a bed of moving particles to have intimate contact with a noncocurrent [crossflow] drying gas stream. A horizontal gas distributor screen separates the dryer's heated inlet air plenum [called a windbox] from the material bed above the screen. Wet feed material enters the dryer from the top. As the drying gas flows from the heated inlet air plenum up through the screen into the material bed, two distinct areas form - a fluidized material bed and a material-disengaging area [that is, the freeboard area above the bed and below the drying chamber's roof]. The drying gas transfers heat to the particles, and the particles' moisture is evaporated and carried away by the exiting gas stream. Dried product discharges via an overflow weir...