In the low temperature pump is arranged in the refrigerator or by liquid helium cooled to very low temperature cold plate. It condenses the gas and keeps the steam pressure of the condensate below the limit pressure of the pump, so as to achieve the pumping effect. The main functions of cryogenic pumping are low temperature condensation, cryogenic adsorption and cryogenic capture. Cryogenic condensation: condensation of gas molecules on the surface of the cold plate or condensation on the condensed gas layer, the equilibrium pressure is basically equal to the vapor pressure of the condensate. The temperature of cold plate must be lower than 25K when pumping air, and the temperature of cold plate is lower when pumping. The condensing layer thickness of cryogenic condensation is about 10 millimeters. Cryogenic adsorption: gas molecules adsorbed on the surface of adsorbents coated on cold plates with a single molecular layer thickness (centimeter order). The equilibrium pressure of adsorption is much lower than that of vapor at the same temperature. As in the 20K when the vapor pressure is equal to atmospheric pressure of hydrogen, the hydrogen absorption of activated carbon for 20K adsorption equilibrium pressure is taken low. So it is possible to exhaust at low temperature through cryogenic adsorption. Cryogenic trapping: gas molecules that cannot be condensed at the temperature of suction are buried and adsorbed by the increasing condensable gas layer.
Generally speaking, the limit pressure of the pump is the vapor pressure of the condensed gas under the cold plate temperature. When the temperature is 120K, the vapor pressure of water is lower than that of PP. When the temperature is 20K, the vapor pressure of other gases is lower than that of PA except helium, neon and hydrogen. But because by pumping the container and low temperature cold plate temperature, pressure pump is higher than the vapor pressure of the condensate. For containers at room temperature, low temperature plate is 20K, 4 times the limit pressure of pump about condensate vapor pressure.
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