APV Dryer Handbook - Umbc

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whether there is any need to re-slurry or dilute, in order to make the feed suitable
for spray drying. As a rule of thumb, the limiting viscosity for good atomization
is 250 – 300 Centipoise.
The economic viability of a drying process ultimately depends on the cost per
pound of the dried product. The spray dryer usually has a greater amount of
water to remove by thermal methods than other types. For example, to dry
20,000 lbs/hr (9,000 kg/h) of a 30% solids/water slurry to 0.5% moisture,
the spray dryer would have an approximate diameter of 30 ft for the evaporation
of 14,000 lbs/hr (6,360 Kg/h). If, however, the feed solids were increased to
50% by evaporation, the hourly evaporation rate would decrease to 6,000 lbs/h
(2,700 Kg/h) and the chamber diameter would be about 20 ft (6 m), with
a corresponding decrease in thermal input and air volume. The former system
would, as a result, also require larger fans and product collection systems.
The overall thermal efficiency would remain substantially constant at 76% with
increasing feed solids. However, the cost per pound of dry product is significantly
lower when drying from the higher feed solids.
Spray drying does have many advantages, particularly with regard to the final
product form. This is especially so where pressing grade materials are required,
i.e., in the production of ceramics and dust-free products such as dyestuffs.
With the introduction of new geometries and techniques, there has been further
development into areas such as foods, and in the production of powders which
may be easily reconstituted. These spray dryers usually incorporate one or two
fluid beds – static and vibrating – for the final drying and cooling of the
agglomerated powder.
ROTARY DRYERS
Another type of dryer, popular in the chemical and process industries, is the
continuous rotary dryer. This machine is generally associated with tonnage
product, and as a result of its ability to handle products having a considerable
size variation, can be used to dry a wide range of materials. The principle
sources of thermal energy are oil, gas and coal. While typical inlet temperatures
for direct-fired dryers using these fuels is in the order of 1200°F (650°C), they