INTRODUCTION
The drying of materials is often the final operation in a manufacturing process, carried out
immediately prior to packaging or dispatch. Drying refers to the final removal of water,
or another solute, and the operation often follows evaporation, filtration, or crystallisation.
In some cases, drying is an essential part of the manufacturing process, as for instance
in paper making or in the seasoning of timber, although, in the majority of processing
industries, drying is carried out for one or more of the following reasons:
(a) To reduce the cost of transport.
(b) To make a material more suitable for handling as, for example, with soap powders,
dyestuffs and fertilisers.
(c) To provide definite properties, such as, for example, maintaining the free-flowing
nature of salt.
(d) To remove moisture which may otherwise lead to corrosion. One example is the
drying of gaseous fuels or benzene prior to chlorination.
With a crystalline product, it is essential that the crystals are not damaged during the
drying process, and, in the case of pharmaceutical products, care must be taken to avoid
contamination. Shrinkage, as with paper, cracking, as with wood, or loss of flavour, as
with fruit, must also be prevented. With the exception of the partial drying of a material by
squeezing in a press or the removal of water by adsorption, almost all drying processes
involve the removal of water by vaporisation, which requires the addition of heat. In
assessing the efficiency of a drying process, the effective utilisation of the heat supplied
is the major consideration.
Classification and selection of dryers
Because of the very wide range of dryer designs available, classification is a virtually
impossible task. PARKER(18) takes into account, however, the means by which material is
transferred through the dryer as a basis of his classification, with a view to presenting
a guide to the selection of dryers. Probably the most thorough classification of dryer
types has been made by KRO¨ LL(19) who has presented a decimalised system based on the
following factors:
(a) Temperature and pressure in the dryer,
(b) The method of heating,
(c) The means by which moist material is transported through the dryer,
(d) Any mechanical aids aimed at improving drying,
(e) The method by which the air is circulated,
(f) The way in which the moist material is supported,
(g) The heating medium, and
(h) The nature of the wet feed and the way it is introduced into the dryer.
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