Page 274 - Chemical Process Equipment - Selection and Design

9.4. BATCH DRYERS 241

evaporation increases roughly with the 0.8 power of the linear
velocity, high velocities are desirable and are usually achieved by
Materials that require more than a few minutes drying time or are internal recirculation with fans. In order to maintain humidity at
in small quantity are treated on a batch basis. If it is granular, the operable levels, venting and fresh air makeup are provided at rates
material is loaded on trays to a depth of 1-2in. with spaces of of 5-50% of the internal circulation rate. Rates of evaporation of
approximately 3 in. between them. Perforated metal bottoms allow 0.05-0.4 Ib/(hr)(sqft tray area) and steam requirements of
drying from both sides with improved heat transfer. Hot air is 1.5-2.3 lbs/lb evaporation are realized.
blown across or through the trays. Cross velocities of 1000ft/min Drying under vacuum is commonly practiced for sensitive
are feasible if dusting is not a problem. Since the rate of materials. Figure 9.6 shows cross and through circulation tray

TABLE 9.1. Classification of Dryers by Several Criteriaa

(4 Classification of dryers based on method of operation
Dryer

Conduction Conveclion Conduction Convection
r-l
Vacuum Atmos.
Vacuum Atmos.

Classification of dryers based on physical form of feed
Wet feed
I
Evaporate Evaporate Press Grind Grind
__._ I
I\ crystalline
Pumpable slurry Soft paste Hard paste Free-nowing Fibrous Sheet
or sludne or matrix granular or solid
*,,
K
y, solid
I
Preformed
paste
I

Convection band

eSee Figure 91.4 for sketches of dryer types.
[Items [a)-(d) by Nonhebel and Moss, 1971, pp. 45,48-501.