Drying Desk: Selecting fluid-bed dryer features II
Fluid-bed dryers come in a myriad of configurations. How do you decide which features will best suit your application? In my last column [Powder and Bulk Engineering, September 2003] I discussed factors to consider when deciding between a stationary fluid-bed dryer and a vibrating unit and between a circular and a rectangular drying chamber. This month, I'll discuss using indirect heating elements in the material bed and dryer zoning, as well as selecting gas distributor screens and material distributors.
Should you use indirect heating elements?
The drying gas flowrate is determined by heat balance. To minimize the gas flowrate and, hence, the fluid-bed dryer's size [gas distributor screen area], you must use the maximum allowable temperature for the heated inlet gas, so long as this doesn't compromise the outlet gas's approach to dew point. For a temperature-sensitive material, you may need a lower inlet gas temperature, thus requiring you to dramatically increase the drying gas flowrate to provide the required heat input. The increased gas flowrate would translate into a larger fluid-bed dryer as well as higher power requirements for the drying gas blower and exhaust fan, increasing your equipment and operating costs. You can avoid these additional expenses by employing banks of indirect heating elements within the fluidized bed. These in-bed indirect heating elements, which contain steam or another heat-transfer fluid, provide conductive heat transfer to supplement the gas's direct [convective] heat transfer.