|Drying and Solids
- Rotary, cylindrical dryers employ superficial air velocities of 5-10 ft/s, sometimes to 35 ft/s when the material is coarse. Residence times are 5-90 min. Holdup of solids is 7-8%. An 85% free cross section is taken for design purposes. In countercurrent flow, the exit gas is 10-20°C above the solid temperature; in parallel flow, the temperature of exit solids is 100°C. Rotation speeds of about 4 rpm are used, but the product of speed (rpm) and diameter (ft) is typically between 15 and 25.
- Drying times range from a few seconds in spray dryers, to one hour or less in rotary dryers, and up to several hours (or even days) in tunnel shelf or belt units.
- Continuous tray or belt dryers for granular materials of natural size or pelleted to 3-15 mm have 10-200 min drying times.