APV Dryer Handbook - Umbc

24
circulation. Alternatively, and where there is limited floor space or a possible
need for long residence time, multi-pass units are used with the conveyors
mounted one above the other. In similar circumstances, another special type
of multi-deck dryer can be used which employs a system of tilting trays so that
the product is supported on both the normal working and the inside of the return
run of the conveyor band. This arrangement considerably increases the residence
time within the dryer and is particularly useful where the product has poor
diffusion characteristics.
The method of airflow employed on these dryers is either vertically downward
through the material and the supporting band or alternatively upward. Sometimes
a combination of the two may be dictated by the nature of the wet feed. It
occasionally happens that extruded materials have a tendency to coalesce
when deposited on the band, in which case one or more sections at the wet end
of the dryer may be arranged for upward air flow to reduce the effect. Wherever
possible, through-air circulation is used as opposed to transverse air flow. This
results in greatly increased evaporative rates as, may be seen from Table 1.
An illustration of the relatively high performance of a band dryer operating on this
system, as compared with a unit having transverse air flow, can be cited in a case
involving the processing of a 70% moisture content filter cake. When this material
is dried in a conventional unit, the cycle time is in the region of 28 hours. This
is reduced to 55 minutes
in the through-circulation
band dryer, largely
as a result of using
an extruder/preformer
designed to produce a
dimensionally stable bed
of sufficient porosity to
permit air circulation
through the feed.
In view of this, transverse
air flow is usually used
only where the type
Figure 12. Two-stage band dryer
Photo courtesy of Mitchell Dryers Limited,
Carlisle, England