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Vacuum drying is to heat the material under vacuum or add a certain amount of energy to the material, so that the internal moisture diffusion, internal evaporation, sublimation, surface evaporation, and thus low temperature and low pressure drying process. It has the advantages of low heating temperature, good oxidation resistance, uniform moisture content, superior quality, easy process control, and wide application.
(2) Adhesion dryer.
Adhesion dryer, also known as drum dryer. The main structure is the internal heating of the level of the drum, partially immersed in the trough, when the rotation by the drum wall heating will be attached to the drum wall of the liquid side to dry, the dry material was scraped shovel. Its thermal efficiency is higher and it can also be operated under vacuum, which is more suitable for materials that are not easy to be destroyed and can be re-dissolved. This method is to allow the solution, slurry or paste material to adhere to a heated roller for drying. Short heating time, high thermal efficiency, suitable for medium or lower processing volume.
(3) Freeze dryer.
The liquid is cooled and frozen first, and then the ice is sublimated to obtain a dry material. Since the whole process is performed below the freezing point, it is commonly used for the drying of heat-sensitive articles. Due to the difficulty of heating the frozen material under vacuum, and the need to condense and sublimate the water vapor under reduced pressure, a relatively large refrigeration system is required, and thus the drying cost is large. The moisture in the highly heat-sensitive material is frozen and will be maintained under a high vacuum to a temperature below the freezing point. Sublimates moisture and separates it from the material. There is little loss of active ingredients in the material, but the drying rate is low.
(4) Vacuum oven
Due to the decompression, the evaporation temperature of the volatile matter contained in the evaporation of the material can be reduced, which is suitable for the drying of various heat-sensitive and easily oxidized materials. This device is often a cylinder or other vacuum-operated housing. It is heated by electric or hot water and heat-conducting oil through a heating plate or a heating tube. It is suitable for small batch batch production.
(5) Double cone rotary vacuum dryer
Its body is slightly olive-like, with both ends covered, and two shafts in the middle to support the body. The body has a jacket for heating, the body can be rotated when dry, so that the material and the wall are often replaced contact, overcome the shortcomings of the vacuum oven material mainly rely on the heating tube conduction and low thermal efficiency. Rotary vacuum dryers have been widely used in fine chemicals, medicines, etc., and are not suitable for materials with high viscosity or strong adhesion during rotation.
The above are the basic types of several common drying devices. There are many variants and newly developed models that will be further described in the following sections.
Application of conductive heat transfer drying equipment (2)
(1) Vacuum dryer.