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 or disturbers 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.
The material to be cooled is fed into a rotating shell similar to a rotary dryer , 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 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.