Handbook of Industrial Drying

DRYER CLASSIFICATION [5–7]

1. 
   CLASSIFICATION BY MODE OF HEAT TRANSFER
   
   1. 
      Indirect Dryers
      

Indirect dryers, also called nonadiabatic units, separ- ate the heat transfer medium from the product to be dried by a metal wall. These dryers are subdivided on the basis of heat applied by radiation or through heat transfer surface and also by the method in which volatile vapors are removed.

Heat transfer fluids may be of either the condens- ing type (e.g., steam and diphenyl fluids, such as Dowtherm A) or the liquid type (e.g., hot water and glycol solutions). Because of low film coefficients of the noncondensing gaseous system, it is seldom used as the heating medium.
Indirect dryers have several distinctive operating features: (1) the risk of cross-contamination is avoided since the product does not contact the heat- ing medium; (2) since a limited amount of gas is encountered, solvent recovery is easier than with an adiabatic dryer; (3) dusting is minimized because of the small volume of vapors involved in indirect dry- ing; (4) dryers allow operation under vacuum or in closely controlled atmospheres that can avoid prod- uct degradation; and (5) explosion hazards are easier to control.
Typically, indirect dryers are used for small- or medium-size production. The product from such a unit has a higher bulk density than the same material processed in direct dryers. Particle size degradation usually can be minimized by proper selection of agi- tator speed or design. The common indirect heated dryers are tubular dryers (with or without vacuum), drum dryers (atmospheric, vacuum, horizontal or ro- tary vacuum, and others), hollow disk dryers, paddle
dryers, mechanically fluidized bed dryers, pneu- matically conveyed dyers, cone or twin-shell dryers, and others.

2. 
   Direct Dryers
   

Direct dryers or adiabatic or convective dryers trans- fer heat by direct contact of the product with the hot gases. The gases transfer sensible heat to provide the heat of vaporization of the liquid present in the solid. Direct dryers may use air, inert gas, superheated vapor, or products of combustion as the heating medium. Combustion gases are seldom used in poly- mer drying because of possible product contamin- ation. Inert gas eliminates the explosion and fire hazard and may be desirable to prevent oxidation of polymers prior to the introduction of stabilizers. Use of superheated vapor as a heat carrier is highly desir- able when solvent is vaporized in the dryer and has to

be recovered.
Commonly used direct dryers in polymer plants are rotary warm air, fluidized bed, flash, spray, tun- nel, and various vibrating and spouted bed (SB) types. All these have a common disadvantage. The amount of air or hot gas required is fairly large, which causes the auxiliary equipment needed (e.g., air heat- ers, blowers, and dust collectors) to be sized accord- ingly; the thermal efficiency is also lower than that of indirect dryers.
Although this classification of dryers has some
importance, it is quite difficult to apply it in more than a general way. Both types of dryers are commonly used in polymer-drying processes. Often a combin- ation of direct and indirect drying is economically the most efficient solution to some polymer-drying problems.

2.