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The dryers are divided into the following four categories according to the heating method.

Update:10-11-2018
Summary:

1 convection dryer One of the most widely used dryers, […]

1 convection dryer
One of the most widely used dryers, including fluidized dryers, airflow dryers, van dryers, spray dryers, tunnel dryers, etc. The main features of this type of dryer are: 1 the hot gas flow is in direct contact with the solid, the heat is transferred from the hot gas flow to the wet solid in a convective heat transfer mode, and the generated water vapor is carried away by the air flow; 2 the hot gas flow temperature can be increased to the ordinary metal material The highest temperature that can withstand (about 730 ° C), radiation heat transfer at high temperatures will become the main heat transfer method, and can achieve high heat utilization; 3 the humidity of the air flow has a drying rate and the final moisture content of the product Impact; 4 When using low-temperature airflow, it is usually necessary to dehumidify the airflow first; 5 The energy consumption per unit mass of water is higher than that of the conductive dryer, especially when the water content of the final product is low; 6 requires a large amount of hot air to ensure The heat required for moisture vaporization, if the particle size of the material to be dried is small, the dust removal device is large and costly; 7 should be operated under conditions close to normal pressure.
2 Conductor dryer
Including spiral conveyor dryer, drum dryer, vacuum dryer, freeze dryer, etc., the main features of this type of dryer are: 1 heat through the wall (usually the metal wall), heat transfer to the wet material 2 The surface temperature of the material can be from below freezing point (when freeze-dried) to 330 ° C; 3 is convenient to operate under reduced pressure and an inert atmosphere, and the volatile solvent can be recovered. It is often used for the drying of easily oxidizable and easily decomposable materials. It is also suitable for the treatment of powdery materials.
3 radiation dryer
The wet material is heated and dried by radiation heat transfer. The electric heating radiation dryer irradiates the dried material with an infrared bulb to make the temperature of the material rise and dry. The gas heating dryer burns the gas to heat the metal or ceramic radiant panel to 400-500 ° C to generate infrared rays for heating the dried material. Radiant dryers have high production strength, compact equipment and flexible use, but they consume a lot of energy. Suitable for dry and large materials such as plastics, cloth, wood, painted products, etc.
4 dielectric dryer
The material to be dried is placed in a high-frequency electric field, and the object is heated and dried by the alternating action of the high-frequency electric field. This heating is characterized by the higher the water content of the material, and the more heat is obtained. Since the water content inside the material is higher than the surface, the energy obtained inside the material is more, and the internal temperature of the material is higher than the surface temperature, so that the temperature gradient and the water diffusion direction are consistent, which can accelerate the vaporization of water and shorten the drying time. It is especially suitable for materials that are easy to crust during the drying process and where the internal moisture is difficult to remove (such as leather). Dielectric heating and drying consumes a large amount of electric energy, and is currently mainly used in food and light industrial production. To carry out the design calculation of the dryer, you must first select the appropriate type of dryer. At present, the selection of the dryer is also very empirical. The following aspects should be considered: 1 The characteristics of the materials and products, such as the shape of the materials (such as paste, paste, powder, granules, flakes, etc.) , the particle size and strength of the solid particles, the initial moisture content and the presence of moisture, whether the material is toxic, flammable, easy to oxidize, the final moisture content required by the product, whether the product is allowed to be slightly polluted, whether the shape is allowed to change slightly, the product The maximum allowable temperature and the price of the product. 2 Conditions related to the production process, such as the amount of material to be treated, the pre-treatment and post-treatment conditions of drying, the solvent to be volatilized, whether to recover or the like. 3 dryer performance and economic indicators. After comprehensive consideration of the above aspects, after comparing and sorting various types of dryers, generally only a few dryers are left, and then a small test is conducted to find the most suitable operating parameters and structural parameters, and finally according to the equipment price. And the small test situation, decide which kind of dryer to use.

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