Rotary dryers are classified as direct or indirect (air heating
method), current or counter current (material air contact).
Direct industrial rotary dryer operates on the principle of lifting and showering the
product through a hot gas stream moving either in parallel or counter flow.
The indirect is more useful to small dusty materials. In indirect dryer,
there is little or no contact between the product and drying gases as they
are heated from the outside the chamber through a stationary jacket fitted
with either multiple burners or other external heat source.
Rotary dryer not only reduces moisture content in the product but is also
able to heat up the material, stimulating a reaction between two or more
materials. This type is known as Rotary Kiln or Rotary Calciner.
The rotary drum dryer is driven through a girth gear and tyre arrangement.
Seales are provided at the ends to ensure minimum air leakage.
In rotary vacuum dryer which are jacketed cylindrical vessel, wet
feed is charged as a batch and is subjected to indirect heating while
undergoing agitation due to the action of paddle mixing. These rotary vacuum dryer have
specially designed scraper blades which scrap the entire internal surface of
the dryer and continuously move and rotate the material, thereby precluding
the possibility of material remaining in contact with hot surfaces for
protracted periods of time. The operation is normally carried out in vacuum.
Recovery of solvent is possible by condensing the vapours generated during
the drying operation.
Special Features of Rotary Vacuum Dryer
Rotary vacuum dryer has some typical applications in
pharmaceuticals, organic chemicals, pasty materials, insecticides,
pesticides, solvent recovery, dye intermediates, dyestuffs, etc.
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