Mitchell Rotary
Dryers provide a means of continuously handling powders and
granules. Rotary Dryers are divided into two main types dependent
on whether direct or indirect heating is employed.
Directly Heated Dryers
Wet material is fed
into a revolving drum through which hot gases are blown . The hot
gases can either be fed in the same direction as the wet material
(parallel operation), or in the opposite direction to the material
(counter-current operation). Internal lifting flights cause the
material to shower through the air stream. This promotes better
heat transfer and efficient drying. Typical applications: organic
chemicals,food applications, fertilisers and sand.
Indirectly Heated Dryers
Similar in
construction to the directly Heated Rotary Dryers. Heating of the
material is by conduction through the rotating shell which is
surrounded by an insulated shroud. The shroud can be heated by a
line of gas burners, electricity or steam.
Although flighting
is not required, disturber bars can be fitted to ensure proper
mixing in certain applications. Evaporated moisture is removed by
a small purge of air or inert gas.
In applications
where the material is easily oxidised, the shell can be enclosed
in an inert atmosphere.
Indirectly Heated
Rotary Dryers are for applications where the material must not
come into direct contact with the products of combustion, or when
the particle size of the material is small or of low density.
Steam Tube Rotary Dryers
These are used for
drying a range of materials from fine powders to large particulate
solids and sludges. The Steam Tube Rotary Dryer consists of a
horizontal rotating shell fitted with a large number of heating
tubes.
Saturated steam is
admitted to the tubes which maintains the necessary drying
temperature. Product is tumbled around the hot tubes and the
moisture is driven off as vapour.
Typical
applications: spent grains, TA and PTA, soybean, corn fibre, germ,
fish meal, bio-solids and industrial sludge.
Air Coolers
The material to be
cooled is fed into the shell, lifted and cascaded by the internal
flights through the cooling air stream.
Counter-current flow
enables the product temperature to approach closely that of the
ambient air stream. The exhausted air from the cooler can be fed
into preceding dryer stages as an economy measure.
Water Coolers
Water is normally applied externally to the shell by means of spray pipes. The cold cylinder walls in turn cool the material inside. A shroud containing the water pipes encloses the centre portion of the shell.
A cooling section is often incorporated at the end of an indirectly heated rotary calciner, a single shell serving both operations.
Leaders in Drying Technology
