Rotary Cylindrical Dryers

Rotating cylindrical dryers are suited for free-flowing granular materials that require drying times of the order of 1 hr or less. Materials that tend to agglomerate because of wetness may be preconditioned by mixing with recycled dry product. 

Diameters typically are 4-1 Oft and lengths are 4-15 diameters. The product of rpm and diameter is typically between 25 and 35. Superficial gas velocities are 5-lOft/sec but lower values may be needed for fine products, and rates up to 35 ft/sec may be allowable for coarse materials. To promote longitudinal travel of the solid, the shell is mounted on a slope of 1 in 40 or 20. 

The kind of data desirable in the design of through-circulation drying are presented for a particular case by Nonhebel and Moss (1971, p. 147). They report on effects of extrusion diameters of the original paste, the bed depth, air linear velocity, and air inlet humidity, and apply these data to a design problem. 

In a countercurrent dryer the exit temperature of the solid approaches that of the inlet gas. In a parallel current dtyer, the exit gas is 10-20°C above that of the solid. For design purposes the temperature of the exit solid in parallel flow may be taken as 100°C. 

Residence time depends on the nature of the material and mechanical features of the dryer. The performance data of Table 9.10 show a range of 7-90 min. A formula cited by Williams-Gardner (1971, p. 133) for the geometrical residence time is 

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Rotary cylindrical dryers operate with superficial air velocities of 5-10 ft/sec, sometimes up to 35 ft/sec when the material is coarse. Residence times are 5-90 min. Floldup of solid is 7-8%. 

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%-ffee 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. 

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