Conductive drying, also known as contact drying, is very suitable for wet particles, while conductive dryers are more thermally efficient. Evaporated water vapor or organic solvent is withdrawn by vacuum or with a small amount of gas stream, which is the main carrier of moisture, and vacuum operation is recommended for heat sensitive granular materials. In a conduction dryer, a paddle dryer is used to dry the paste material. Rotary dryers with internal flow tubes have been designed and put into use.

Vacuum drying is expensive, and vacuum drying is recommended only when the material must be dried under low temperature or anoxic conditions, or when the material is dried under heating medium and high temperature. For a certain evaporation efficiency, it is more effective to use high temperature operation, which can reduce the gas flow rate and reduce the volume of the equipment. For low temperature drying operations, a suitable low temperature waste heat or solar collector can be selected as the heat source, but the dryer is relatively bulky.

Freeze drying is a special case of drying under vacuum conditions. Here, the temperature is below the three-state point of water, and water (or ice) is directly sublimated into water vapor. Although sublimation requires several times less heat than evaporation, other aspects of power consumption are greater, so vacuum drying costs are expensive.

In the conduction heat transfer, heat is supplied to the dry material by a jacket, a stirring, a heat transfer tube, etc., and the heat medium is used instead of the hot air. A representative device is a trough-type cylinder dryer. Conical stirring dryer, jacket and built-in heating tube rotary dryer, drum dryer, rake dryer, vacuum belt dryer, vacuum freeze dryer, etc. In hot air drying, the temperature during the constant-speed drying period corresponds to the wet bulb temperature. In conduction heat transfer drying, it is necessary to artificially create a heating surface, so that the heat transfer area is small and the heat transfer coefficient is increased. In this way, the structure is more complicated and the equipment investment is larger than that of the hot air dryer.

The conduction drying method indirectly transfers the heat required for drying through a surface such as a metal. The drying rate is lower than the direct drying method. The product temperature during constant-speed drying has no relationship with the temperature of the heating source, and is substantially the same as the saturation temperature of the gas pressure in the apparatus. In order to increase the drying rate and prevent uneven drying, it is usually necessary to mechanically stir or rotate the container itself to increase or continuously update the heat transfer surface of the material, so it is necessary to study the adhesion problem of the heat transfer mechanism. The drying unit itself is expensive, but it is characterized by a small load on the dust collecting system, high thermal efficiency, easy recovery of the solvent, and the total cost is much cheaper than the direct drying method.

(1) Attachment dryer

The attached dryer is also called a drum dryer. The main structure is an internally heated horizontal drum, which is partially immersed in the trough. When rotating, the material adhering to the drum wall is turned and dried by the heating of the drum wall, and the dry material is scraped off by the scraper. It has high thermal efficiency and can also be operated under vacuum, which is suitable for materials that are not easily damaged and re-dissolvable. This method involves attaching a solution, slurry or paste material to a heated drum for drying. The heating time is short and the thermal efficiency is high, which is suitable for the processing amount below medium.

(2) Vacuum drying is a process of heating a material under vacuum or adding a certain amount of energy to the material to diffuse, internally evaporate, sublimate, and evaporate the moisture, thereby performing low-temperature and low-pressure drying. The utility model has the advantages of low heating temperature, good oxidation resistance, uniform water content of the product, superior quality, easy control of the process and wide application.

(3) Vacuum oven

Since the evaporation temperature of the volatiles contained in the evaporation of the material can be reduced after decompression, it is suitable for drying various heat sensitive and easy oxide materials. The device is usually a cylinder or other vacuum-operable outer casing, which is heated by electric heating or hot water or heat-conducting oil through a heating plate or a heating pipe, and is suitable for batch production in small batches.

(4) Double cone rotary vacuum dryer

The body is slightly olive-like, with a cover at both ends, and two shafts in the middle to support the body. The body has a jacket to heat, and the body can be rotated when dry, so that the material and the wall are frequently replaced, which overcomes the disadvantage that the material in the vacuum oven mainly passes through the heating cylinder and has low thermal efficiency. Rotary vacuum dryers have been widely used in fine chemicals and medicines, and are not suitable for materials with high viscosity or strong adhesion during the turning process.

(5) Freeze dryer

The liquid is first cooled and frozen, and then the pressure is reduced to sublimate the ice to obtain a dry material. Since the entire process is carried out below freezing point, it is often used for drying of heat sensitive articles. It is difficult to heat the frozen material under vacuum, and a large refrigeration system is required to condense the sublimated water vapor under reduced pressure, so the drying cost is large. The moisture in the highly heat sensitive material is frozen and will be maintained at a temperature below freezing below under high vacuum. The water is sublimed to separate from the material. The loss of active ingredients in the material is small, but the drying rate is low.

These are the basic types of several common drying units, and many variations and newly developed models are further described in the following sections.