Is the lower the vacuum degree of the freeze dryer, the better?

Vacuum freeze dryer is referred to as freeze dryer. Regarding the issue of vacuum degree, the past view was that the higher the vacuum degree, the better. So, is the lower the vacuum degree of the freeze dryer, the better? Answer: No. The degree of vacuum should be within a reasonable range. If the vacuum degree is too high, it is not conducive to heat transfer and the drying speed will decrease.

The vacuum degree of the freeze-drying box, that is, the pressure of the freeze-drying box, can affect the sublimation rate of the product. When the product temperature remains unchanged, if the pressure of the freeze-drying box is too high or too low, the sublimation rate will decrease. When the pressure is too high, although convection exists, which can improve heat transfer, due to the high density of the gas, the collision between gas molecules increases and the sublimation rate will decrease. When the pressure of the freeze-drying box is too low, although the gas density is small and the product is easy to sublimate, due to the lack of convection, the product cannot easily obtain heat and can only absorb its own heat, causing the product temperature to decrease and the sublimation rate to decrease.

The vacuum range of the freeze-drying box: 10% to 30% of the saturated vapor pressure corresponding to the product temperature. The gas density at the upper limit of this range does not affect the sublimation of water vapor, and convection still exists at the lower limit. The sublimation stage is a complex transfer process, including the transfer of heat from the plate layer to the product and the transfer of moisture from the product to the cold trap. The plate temperature and freeze-drying box pressure are key parameters that affect these two transfers, so the plate temperature and the vacuum degree of the freeze-drying box must be strictly controlled.