ROTARY DRYERS

Rotary Dryers

Rotary Dryers have been used for many decades for drying a wide variety of materials. The Rotary Dryer has become firmly established in the building materials, minerals and raw materials industries but the fluidized bed dryer has also been in use for quite some time, especially in the chemical industry as well as in the foodstuffs and pharmaceutical industries. A number of special applications have been developed for fluidized-bed dryers, such as the spray granulation dryer, the suspension and paste dryer, the fluidized bed dryer/cooler with heat exchangers integrated into the fluidized bed, and the entrained bed dryer, etc

Operating Principles of Rotary Dryers

In Rotary Dryers the material is moved by the rotation of the rotary tube and the action of the internal fittings. The older rotary dryers were usually built with a slight slope to assist the transport of the material but modern dryers built on the MOZER® system are now almost always set up horizontally. Lifting scoops pick up the moist material from the bottom of the rotary tube and then let it fall again so that there is intensive contact between the hot drying air and the moist material. In the majority of drying applications there is concurrent transport of the material, i.e. it moves in the same direction as the gas flow. The material is conveyed by guide vanes so that there is a combination of parallel flow and cross flow between the drying gas and the material. Counter current applications are used mainly in the asphalt industry and in high temperature applications, e.g. for calcination.

As a rule the drying air for applications in the minerals industry is heated by gas or light oil burners. The combustion gases are mixed with ambient air to achieve average drying air temperatures of between 600 and 900 °C. For materials like quartz sand that are not sensitive to temperature the flame can burn directly inside the rotating tube. Combustion chambers that ensure that the hot combustion gases in the combustion chamber are adequately mixed with ambient air to achieve the required average drying air temperature before they enter the rotary tube are used for drying temperature-sensitive materials such as limestone, clay, bentonite, recycled plastics or organic residues and waste materials. Combustion chambers are also employed when using fuel oil (diesel) to ensure low-pollution soot-free burn-out of the fuel oil.

The total engineering outlay for heating rotary dryers is low. Modern burners have relatively small combustion air fans. As a rule there is no need for fans for conveying the drying air or for expensive piping. The moist exhaust air is extracted from the dryer by an exhaust air fan, passed through a bag filter to collect the entrained dust and released to the surroundings through a chimney.

Rotary Dryers are generally built for material through puts of between 5 and 150 t/h. One particular advantage of the rotary dryer is that it is not at all sensitive to fluctuations in the input moisture of the materials to be dried or to fluctuations in the throughput or particle size composition of the material. Even if there is a power failure the operation can in most cases be resumed immediately.

Rotary Dryers are suitable for fine sands and are also particularly suitable for coarse and very coarse bulk materials. If there is a change in product it is not essential to adjust the amount of air. Even if the supply of drying air fails the material is transported onwards by the rotation of the rotary.

Rotary Dryers are not at all sensitive to operating errors and are therefore ideal for installation in areas with less well developed economies. The expenditure required for automation of the dryer control system is comparatively low.