The function of the vacuum system is to expel non condensable gases and air extracted from the cold trap, in order to establish a vacuum degree that can maintain the sublimation rate of the product. High vacuum effectively reduces the resistance of water vapor airflow, while avoiding oxidation during the drying process due to the absence of air.

When vacuum freeze-drying, the pressure inside the cold trap should not be lower, but should be controlled within a certain range. Low pressure is certainly beneficial for the sublimation of ice inside the product. However, due to the unfavorable heat transfer when the pressure is too low, the product is not easy to obtain heat, and the sublimation rate decreases instead.

When the pressure is too high, the sublimation rate of ice inside the product slows down, causing the temperature of the product itself to rise. When it exceeds a certain level, the product will melt, resulting in freeze-drying failure.