Rotary dryer

Classification of rotary dryer

According to the heat transfer mode between the drying medium and the wet material, it can be divided into two types: direct heat transfer rotary dryer (where the drying medium directly contacts the wet material to transfer heat) and indirect heat transfer rotary dryer (where the heat required for drying is indirectly transferred from the cylinder wall to the wet material). Due to low transfer efficiency and complex structure, indirect heat transfer rotary dryers are rarely selected and will not be introduced here.

The direct heat transfer rotary dryer is divided into two types according to the direction of flow of the drying medium and material: forward flow and reverse flow.

The main body of the direct heat transfer rotary dryer is a rotating cylinder slightly inclined to the horizontal line. The cylinder is driven by gears, with a speed of generally 2-6r/min. The inclination of the cylinder is related to its length, usually between 1 ° and 5 °. The material is fed into the higher section of the rotary drum and comes into contact with hot air. As the cylinder rotates, the material flows towards the lower end under the action of gravity and is dried before being discharged. Due to the negative pressure condition of the dryer, sealing devices are used at both the feeding and discharging ends to prevent air leakage.

The main internal structure of rotary dryer

In order to increase the uniform distribution of materials on various parts of the rotating drum section and ensure good contact with the drying medium, a lifting plate is installed inside the drum. There are several types of raised boards:

1、 Lift up lifting plate: suitable for large materials or materials that are easily adhered to the cylinder wall.

2、 Four format lifting board: suitable for materials with high density, non brittle or difficult to disperse. The lifting plate divides the cylinder into four compartments, forming a fan-shaped operating room that is not connected to each other. The contact surface between the material and the hot gas is larger than that of the lifting plate, and it can increase the filling rate of the material and reduce the falling height of the material, thereby reducing dust loss and other advantages.

3、 Cross shaped or rack shaped lifting plate: suitable for brittle and easily dispersed small pieces of material, allowing the material to be evenly dispersed throughout the entire cross-section of the cylinder.

4、 Sleeve type lifting plate: It is a lifting plate for a compound heat transfer (or half diameter heating) rotary dryer.

5、 The fan-shaped lifting plate is suitable for materials with very fine particles that are prone to powder flying. As soon as the material is fed in, it accumulates on the grid plate. When the cylinder rotates, the material is flipped and constantly comes into contact with hot gas. At the same time, the possibility of dry material being carried away by gas is reduced due to the decrease in the height of material descent.

The various forms of lifting plates mentioned above can be distributed throughout the entire cylinder. In order to ensure that the material can be quickly and evenly delivered to the lifting plate, a spiral guide plate can also be installed at a distance of 1-5 meters from the feeding end to avoid wet material sticking and accumulating on the cylinder wall. At the same time, due to the fact that the dried material is easily lifted up and carried away by exhaust gas, no lifting plate is installed at a distance of 1-2m from the discharge end.

Working principle of rotary dryer

Dry and wet materials are transported to the hopper by a belt conveyor or bucket elevator, and then enter the feeding end through the feeding pipeline through the feeding machine of the hopper. The slope of the feeding pipeline should be greater than the natural inclination angle of the material, so that the material can flow smoothly into the dryer. The dryer cylinder is a rotating cylinder slightly inclined from the horizontal line. The material is added from the higher end, and the heat carrier enters from the lower end, coming into counter current contact with the material. There are also cases where the heat carrier and the material flow together into the cylinder. As the cylinder rotates, the material moves to the lower end under the action of gravity. During the forward movement of the wet material inside the cylinder, it directly or indirectly receives heat from the heat carrier, allowing the wet material to dry, and then is sent out through a belt conveyor or screw conveyor at the discharge end. There is a copying plate installed on the inner wall of the cylinder, which is used to pick up and sprinkle the material, increase the contact surface between the material and the airflow, improve the drying rate, and promote the advancement of the material. Thermal carriers are generally divided into hot air, flue gas, etc. After passing through the dryer, the heat carrier generally needs a cyclone dust collector to capture the materials carried in the gas. If further reduction of exhaust dust content is required, it should be discharged after passing through a bag filter or wet dust collector.

Technical parameters of rotary dryer