The whole process of freeze drying of food freeze dryers is actually the process of simultaneous heat and mass transfer (water vapor). The rate of heat and mass transfer affects the drying rate, which affects the entire freeze-drying cycle. All the factors that affect heat and mass transfer All factors will affect the drying rate, a simple analysis is as follows:
　　1. The form and composition of the material
　　is usually divided into solid and liquid according to the form of lyophilized material. The shape of the solid and the concentration of the liquid have a greater influence on the freeze-drying rate.
　　2. Pre-freezing rate
　　The size of crystals formed during freezing largely affects the rate of drying and the rate of dissolution of the product after drying. 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 conducive to sublimation, and small ice crystals are not conducive to sublimation. Rapid freezing results in a low sublimation rate and a fast desorption rate; slow freezing results in a fast sublimation rate and a slow resolution rate.
　　3, loading
　　When the material is freeze-dried, there is a certain ratio of surface area to the thickness of the material after being packed into the container, that is, freeze-drying is related to the loading amount. The small surface area and thickness are conducive to sublimation of moisture, easy to freeze-dry and ideal quality. When drying, the wet weight load per unit area of ??the tray is an important factor in determining the drying time. In general, the thinner the thickness of the material accumulation, the faster the heat and mass transfer speed, and the shorter the drying time. However, if the thickness of the material is thin, less material is dried per batch per freeze-dried area, which is not conducive to increasing the output per unit freeze-dried area and time.
　　4. Drying room pressure
　　The pressure in the drying chamber affects the rate of heat and mass transfer. In terms of mass transfer, the lower the pressure, the better, and for heat transfer, the higher the pressure, the better. The size of 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 and increase the water vapor pressure of the interface; two It is to increase the vacuum degree of the drying chamber and reduce the vapor pressure on the surface of the drying layer.
　　5. Heat transfer method Heat transfer method
　　According to the traditional classification, it can be divided into: conduction, convection, heat radiation and medium heating (microwave heating). Because the process of sublimation and drying involves the transfer of heat and mass (water vapor), which heat transfer method will be used to transfer heat to the material more effectively has a greater impact on the drying rate.