Dryer Handbook

In such a plant, the drying cycles
are extended with 24 to 45 hours
being quite common in certain
cases. This is a direct result of
the low evaporative rate which
normally is in the region of
0.15 to 0.25 lb/ft2hr.
Following the recent trend and
interest shown in preforming
feedstock and in particular with
regard to the design of extruding
and tray-filling equipment for
dewatered cakes (Fig. 2), it now is
possible to obtain the maximum benefit of enhanced evaporative rates by using
through-air circulation dryers when handling preformed materials. Figure 1
shows how a high performance dryer can produce 1950 pounds of dried material
in a 24 hour period at a terminal figure of 0.5% moisture when handling a
preformed filter cake having an initial moisture content of 58%. The very great
improvement in performance can readily be seen from the curve in which it is
clear that the corresponding number of conventional two-truck recirculatory
units would be between seven and eight for the same duty. The advantage is
more apparent when it is seen that respective floor areas occupied are 55 sq ft for
the Thruflo dryer pictured in Figure 3 and 245 sq ft in the case of conventional
units using transverse air flow.
Reference to the drying curves for the processing of materials in solid or filter
cake form or, in fact, in the case of wet powders, clearly indicates that the
ultimate rate-governing factor is the rate of diffusion of moisture from the wet
mass. This becomes increasingly so during the falling rate period of drying. This
situation, however, can be improved by preforming the product in order to
increase the effective surface area presented to heat and mass transfer. The
logical extension of this technique is to total dispersion drying, i.e., flash or
FIG. 2 Extruder and tray filler for Fig. 3 Thruflo dryer
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