The vacuum system evacuation heating exhaust technology is different from the simple exhaust process in that the container can be heated for degassing when the air is pumped to low vacuum or high vacuum.
The heating methods of cryogenic vessels can be divided into three different forms: internal heating, external heating and internal and external overall heating. The overall heating and air extraction effect is better at low vacuum.
When this process is used for venting cryogenic vessels, the pumping time required by this process should be reduced by an order of magnitude compared with that of the simple venting process, and the adsorbent activation should be better. Therefore, it is a widely used exhaust method from the advent of multi-layer insulation to the early 1970s.
The good effect of this process is due to the temperature effect:
1. When the temperature of the material is increased, the adsorption time of molecules on the surface can be shortened, the gas diffusion coefficient inside the material can be increased, and the outgassing process of the insulation material and adsorbent can be accelerated to a large extent.
2. In addition to increasing the diffusion coefficient, increasing the temperature should also accelerate the pressure difference between layers and spaces by venting gas in the materials in the insulation layer, so as to ensure the increase of the permeability of the insulation layer and improve the “infiltration pumping” capacity.
3. When the temperature of space gas molecules rises, the molecular motion speed is constantly accelerated, and the capacity of the air extraction channel and the air extraction nozzle passing through the narrow gap of the insulation layer is constantly increased, which can slightly increase the flow conductivity of the insulation layer and the air extraction port.
The problems of this process are:
1. The heating is carried out in the process of air extraction. Due to the super thermal insulation of the vessel, heating is very difficult, and the temperature distribution is uneven, which limits the effect of temperature.
2. The problem of difficult extraction of water vapor cannot be solved.
3. The pumping time is long.
4. It is difficult to reactivate the adsorbent.
5. The influence of oil vapor backflow exists.
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