Dryer Handbook

The various batch dryers referred to operate by means of forced convection, the
transfer of thermal energy being designed to increase the vapor pressure of the
absorbed moisture while the circulated air scavenges the overlying vapor. Good
conditions thus are maintained for continued effective drying.
Alternatively and where the material is thermosensitive - implying low temperatures
with consequently low evaporative rates - some improvement can be
effected by the use of sub-atmospheric dryers, Le., by reducing the vapor
pressure. Several different configurations are in use and all fall into the category
of conduction-type dryers. The most usual type of heating is by steam although
hot water or one of the proprietary heat transfer fluids can be used.
Two particular types are the double-cone dryer (Fig. 4) with capacities up to
400 ft3 and the agitated-pan dryer not normally larger than 8 ft diameter where
average evaporative rates per unit wetted area usually are in the region of
4 lb/ft2hr. These units are comparatively simple to operate and, when adequately
insulated, are thermally quite efficient although drying times can be extended.
They are especially suitable for applications involving solvent recovery and will
handle powders and granules moderately well. They do, however, suffer from the
disadvantage with some materials that 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, quite large rotary vacuum dryers are used for pigment pastes and
other such materials, especially where organic solvents present in the feedstock
have to be recovered. These units normally are jacketed and equipped with an
internal agitator which constantly lifts and turns the material. Heat transfer here
is entirely by conduction from the wall of the dryer and from the agitator. Owing
to the nature of their construction, initial cost is high relative to capacity. Installation
costs also are considerable. In general, these dryers find only limited
application.
continuous dryers
For the drying of liquids or liquid suspensions, there are two types of dryers
which can be used: film drum dryers for duties in the region of 600 lbs/hr for
a large dryer of about 4’ diameter by 10’ face length or large spray dryers (as in
Fig. 5) with drying rates of approximately 22,000 to 23,000 lbs/hr. 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 lb steam/lb
of water evaporated and for small to medium production runs, it does have
many applications.
Drum dryers usually are steam heated although work has been done involving
the development of units for direct gas or oil heating. Completely packaged and
capable of independent operation, these dryers can be divided into two broad
classifications, i.e., single drum and double drum.
Double drum machines normally employ a “nip” feed device with the space
between the drums capable of being adjusted to provide a means of controlling the
film thickness. Alternately, and in the case of the single drum types, a variety of
feeding methods can be used to apply material to the drum. The most usual is the
simple “dip” feed. With this arrangement, good liquor circulation in the trough is
desirable in order to avoid increasing the concentration of the feed by evaporation.
Again, for special applications, single drum dryers use top roller feed. While the
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