APV Dryer Handbook - Umbc

14
Two particular types are the double-cone dryer shown in Figure 4 with capacities
up to 400 ft3 (11 m3 ) and the agitated-pan dryer not normally larger than 8 ft
(2.4 m) in diameter, where average evaporative rates per unit wetted area usually
are in the region of 4 lb/ft2hr (25 kg/m2h). These units are comparatively simple
to operate and when adequately insulated are thermally efficient, although drying
times can be extended. They are especially suitable for applications involving
solvent recovery and will handle powders and granules moderately well.
However, with some materials, the tumbling action in double-cone dryers and the
action of the agitator in agitated-pan machines can produce a degree of attrition
in the dried product which may prove unacceptable.
Similarly, large rotary vacuum dryers are used for pigment pastes and other
such materials, especially where organic solvents present in the feedstock need
to be recovered. These units are normally jacketed and equipped with an internal
agitator, which constantly lifts and turns the material. Here, heat transfer is entirely
by conduction from the wall of the dryer and from the agitator. Due to the nature
of their construction, initial cost is high relative to capacity. Installation costs also
are considerable. In general, there are limited applications for these dryers.
CONTINUOUS DRYERS
For the drying of liquids or liquid suspensions, the evaporator of choice is usually
either a drum dryer or a spray dryer.
A typical film drum dryer with a 4 ft (1.2 m) by 10 ft (3 m) long drum will
evaporate about 600 lb/h (270 kg/h) of water. A typical spray dryer as shown
in Figure 5 will evaporate 22,000 lb/h (10,000 kg/h).
Where tonnage production is required, the drum dryer is at a disadvantage.
However, the thermal efficiency of the drum dryer is high in the region of 1.3 to
1.5 mass units of steam per mass unit of water evaporated (65 to 75% efficiency)
and for small to medium production runs, it does have many applications.
Drum dryers are usually steam heated, although work has been done to develope
units for direct gas or oil heating. Completely packaged and capable of
independent operation, these dryers can be divided into two broad classifications:
single drum and double drum.