What are the quick freeze methods for freeze dryers?

What are the quick freeze methods for freeze dryers?
After the quick-frozen raw materials are selected, rinsed, shaped, and placed in the previous process, they can enter the freeze-drying stage. The first step of lyophilization is to freeze the lyophilized material, and place the material tray in the lyophilizer for quick freezing (some lyophilizers do not have a quick-freezing function, and a quick-freezer is required). The quick freezing process has the following types:
Immersion method: The material is immersed in medium such as liquid nitrogen, liquid freon, low temperature brine for rapid freezing. This method freezes fastest.
Air-cooling method: blowing cold air to the material to freeze the material to a certain temperature within a specified time.
Direct cooling method: The refrigerant or refrigerant is placed in the evaporation tube to absorb the heat of the material for refrigeration purposes.
Vacuum refrigeration method: vacuuming is used to evaporate water in the material to remove heat, and the cooling material is cooled.
Regardless of the method adopted, it should be ensured that the material is frozen to a certain temperature within a specified time without substantial deformation.
In order to ensure that the ice in the material is insolubilized and directly vaporized (sublimation), and the vapor is continuously removed to ensure normal sublimation, the vacuum in the tank must be lower than the saturated vapor pressure of ice at a certain temperature (below 610 Pa).
The cold trap is used to capture water vapor and reduce the load of the vacuum pump. The design of a reasonable cold trap will not cause the vacuum pump to fail, and the energy consumption is low. However, the design requirements of the refrigeration system and the cold trap are strict.
As the ice continues to sublimate, the vapor is removed, and the heat of the material is taken away, so that the temperature of the material (ice temperature) continues to decrease, so that the saturated vapor pressure of the steam is reduced, and the sublimation speed is slowed down. In order to maintain a certain rate of sublimation, it is necessary to continuously replenish the material with heat. Too little heat supplement will reduce the rate of sublimation. If the heat is added too fast, it will cause the ice to melt, causing the material to collapse and freeze.
Since the freeze-drying process of the freeze dryer is complicated and the working state range is narrow, the quality of the freeze-dried product is degraded with a slight carelessness, so an automatic control system is required. The design of the freeze-drying machine automatic control system requires simple operation, starting from the material into the button, quick-freezing vacuum sublimation and drying in one go, prompting the shutdown after the freeze-drying is completed. Relevant temperature and pressure control points should be carefully selected and properly controlled. The correlation between quick freezing, vacuum holding and heating should be properly controlled, and the process curve should be set for different materials.