The working principle of intermittent exhaust

    In obtaining the vacuum of the cryogenic container, the pumping is usually carried out continuously. The study found that continuous pumping for a long time has no obvious effect on accelerating the material outgassing process. Therefore, an intermittent pumping method is used.

    The main method of intermittent exhaust is to suspend the pumping under the condition of obtaining high vacuum in the container, especially when the gas molecules in the insulating layer are in the state of molecular flow. After the pressure of the container rises to a certain value, use the unit to release the material again The gas is pumped away. Then stop pumping and repeat until the material outgassing volume is reduced to the allowable range.

    The main purpose of intermittent pumping is to work intermittently for the pumping unit to achieve the same effect of continuous pumping with very little actual pumping time, while reducing the impact of oil vapor backflow. Compared with continuous pumping method, intermittent pumping is the most economical and reasonable.

    1) Stopping air extraction will not affect the material deflation process. The characteristic of material outgassing is that once the material, structure and temperature are determined, within a certain pressure range, the reduction of material outgassing rate mainly depends on the outgassing time, and has nothing to do with the pumping speed and the degree of vacuum. Therefore, continuous evacuation does not promote a faster reduction in the outgassing rate. In the case of intermittent evacuation, because the "certain pressure range" that does not affect the outgassing of the material in the insulation layer is maintained during the stop of pumping, the outgassing rate can continue to decrease with time without affecting the outgassing process.

    2) During the stop of pumping, the effect of gas diffusion and permeation pumping still exists. Due to the influence of flow conductance during pumping, the outgassing rate of the material in the insulation layer is not uniformly reduced. During the pumping stop, the outgassing rate of the material in the insulation layer is the highest, the volume between the layers is small, and the pressure rises the fastest. However, the volume of the interlayer space is large, the outgassing rate of the surrounding materials is low, and the pressure rises the slowest. As a result, there is always a pressure difference between the insulation layer and the interlayer space. According to tests, this differential pressure can be as high as tens or even hundreds of times when pumping, and several to ten times when pumping is stopped. When the pumping is stopped, under the action of the pressure difference, the gas in the insulation layer can continuously reach the interlayer space through diffusion and permeation. At this time, the space always has a certain pumping effect for the gas in the insulation layer.

    3) The interlayer space has a large gas storage effect. The interlayer space has a large volume and the surrounding materials have the lowest outgassing rate, so a large amount of gas released by the materials in the insulation layer can be stored.

    4) The stored gas can be removed quickly. When re-pumping, the gas stored in the space can be quickly pumped away by the pump.

(Huiputech)