Freeze Drying Process Of Vegetable Drying Equipment

• Posted by guanfeng gfmachine 6 May 2020 - Filed in Business - #Freeze Dry Machine，Vegetable Drying Equipment  - 30 views

  The whole freeze-drying process of Freeze Dry Machine is actually a simultaneous process of heat and mass transfer (water vapor). The heat and mass transfer rates jointly affect the drying rate, thus affecting the whole freeze-drying cycle. All factors affecting the heat and mass transfer will affect the drying rate. Simple analysis is as follows:

  1, the material form and composition
  According to the morphology of freeze-dried materials, they are usually divided into solid and liquid.  The shape of solid and the concentration of liquid have great influence on freeze drying rate.

  1. Pre-freezing Rate
     The crystal size formed during freezing greatly affects the drying rate and the dissolution rate of the dried product.  The process of quick freezing and slow freezing has the following differences: the ice crystals produced by quick freezing are smaller, and the ice crystals produced by slow freezing are larger.  Large ice crystals are favorable for sublimation, while small ice crystals are unfavorable for sublimation. Rapid freezing results in low sublimation rate and fast desorption rate.  Slow freezing results in fast sublimation rate and slow resolution rate.
  2. Loading capacity
     When materials are freeze-dried, there is a certain ratio of surface area to material thickness after being split into containers, i.e. freeze-drying is related to the loading amount.  Small surface area and thickness are conducive to moisture sublimation, easy freeze-drying and ideal quality.  During drying, the wet weight load per unit area of the tray is an important factor that determines the drying time. In general, the thinner the material accumulation thickness, the faster the heat and mass transfer speed, and the shorter the drying time.  However, if the material thickness is thin, there will be less dried materials per batch per unit freeze-drying area, which is unfavorable for improving the unit freeze-drying area and the output per unit time.

  4, drying indoor pressure
  The pressure in the drying chamber affects the rate of heat and mass transfer. For mass transfer, the lower the pressure, the better, while for heat transfer, the higher the pressure, the better.  The mass transfer rate is mainly determined by the temperature and pressure of the sublimation interface and the surface of the drying layer. To increase the escape rate of water vapor in the drying layer, one is to increase the temperature of the sublimation interface so as to increase the water vapor pressure at the interface.  The second is to improve the vacuum degree of the drying chamber and reduce the vapor pressure on the surface of the drying layer.

  5, heat transfer mode heat transfer mode
  According to the traditional classification, it can be divided into conduction, convection, heat radiation and medium heating (microwave heating).  The sublimation drying process involves the transfer of heat and mass (water vapor), therefore, the way of heat transfer to transfer heat to materials more effectively has a greater impact on the drying rate.

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