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The vacuum drying operation is expensive, but vacuum drying is recommended only when the material must be dried at a low temperature or in an oxygen deficient state, or when the material is deteriorated by heating in a heating medium and at a high temperature. For a certain evaporation efficiency, the use of high temperature operation is more effective, which can reduce the gas flow rate and reduce the equipment volume. For low-temperature drying operations, an appropriate low-temperature waste heat or solar collector can be selected as the heat source, but the dryer has a relatively large volume.
Freeze-drying is a special case of drying under vacuum conditions. Here, the temperature is below the triple point of water, and the water (or ice) sublimates directly into water vapor. Although the heat required for sublimation is several times lower than that of evaporation, the power consumption of other aspects is large, so the cost of vacuum drying is still expensive.
In conductive heat transfer, heat is supplied to the dry material by jackets, agitation, heat transfer tubes, etc., and the heat medium is used instead of hot air. A typical device is a trough type cylinder dryer. Cone stirring dryer, jacket and built-in heating tube rotary dryer, drum dryer, rake dryer, vacuum belt dryer, vacuum freeze dryer and so on. In hot air drying, the temperature during constant-rate drying corresponds to the wet bulb temperature. In conducting 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 the hot air dryer.
The conductive drying method transfers the heat required for drying indirectly through metal and other surfaces. The drying rate is lower than the direct drying method. The temperature of the product during constant speed drying is not related to the temperature of the heating source and is generally the same as the saturation temperature of the gas pressure in the device. In order to increase the drying rate and prevent uneven drying, mechanical agitation is usually used or the container itself is rotated to increase or constantly update the heat transfer surface of the material. Therefore, it is necessary to further study the adhesion problem of the heat transfer mechanism. The drying device itself is expensive, but it is characterized by a small load on the dust collecting system, a high thermal efficiency, easy recovery of the solvent, and a total cost much less than the direct drying method.
Application of conductive heat transfer drying equipment (1)
Conduction drying, also known as contact drying, is very adaptable to moisture particles, and the conduction dryer has a higher thermal efficiency. Evaporated water vapor or organic solvent is withdrawn from the vacuum or discharged with a small amount of gas flow. The gas flow is the main carrier of moisture, and vacuum-operated is recommended for heat-sensitive granular materials. In conductive dryers, paddle dryers are used to dry paste materials. Rotary dryers with internal flow tubes have now been designed and put into use.