What are the heat transfer methods in the freeze dryer?

What are the heat transfer methods in the freeze dryer?
The heat transfer modes in the freeze dryer are mainly solid heat conduction heating, radiation and microwave heating, and convection heating.
1. Solid heat conduction heating
The material is placed on the tray, the tray is in direct contact with the heating shelf, and the heat passes through the frozen layer of the material tray and the material to reach the sublimation interface. In the contact-type conduction heating lyophilization equipment, not only the heating plate and the tray have good thermal conductivity, but also the contact between the heating plate, the tray and the material is good, so that the heat transfer effect can be improved and the heat can be evenly distributed. .
2. Radiation and microwave heating
Radiation is the transfer of energy by an object through electromagnetic waves, which can be carried out in a vacuum. The material is suspended between the upper and lower shelves, and is not in direct contact with the shelf; the heat is radiated to the surface of the material through the upper and lower shelves, and then reaches the sublimation surface through the heat conduction of the dried layer.
In the radiant-heated freeze-drying equipment, the material and the container (dray) are not required to be in close contact with the shelf, which facilitates the material entering and leaving the drying chamber. This heating method is now widely used in large-scale freeze-drying equipment.
Microwave heating also belongs to electromagnetic wave heating. However, unlike radiant heating, microwave heating is the substance itself directly acting as a heating element. There is no need to transfer heat from the outside to the inside, so the material center heats up quickly. Since the microwave heating is basically unaffected by the drying layer, the heat can directly reach the sublimation surface of the material, so that the temperature difference between the material surface and the sublimation interface is small, so that the freezing layer can be maintained below and close to the maximum temperature allowed by the material.
3. Convection heating
Since the freeze dryer is operated under conditions close to vacuum. Therefore, the effect of convective heat transfer is small, but in recent years, the cyclic pressure method is often used to achieve forced convection heat transfer to accelerate the process of one drying. The so-called cyclic pressure method, that is, the pressure of the control vacuum system fluctuates up and down within a certain range to improve the effect of heat and mass transfer.