Cryogenic pump suction system and its operation flow

Cryogenic pump pumping system low-temperature pumping is to obtain a clean vacuum environment, a quick and effective method. With the development of two-stage high-reliability compact refrigerators, the cryogenic pumps of refrigerators have been rapidly developed. They have become a new type of refrigerators that are clean, pollution-free, large pumping speed, wide working pressure range, high suction efficiency, simple structure, Long-term work can be installed in any direction and vacuum access to equipment. Figure 1 helium refrigerator cryogenic pump vacuum unit Figure 1 shows a typical low-temperature gas helium refrigerator exhaust system schematic. This pumping system does not require a foreline pump and only needs to be started by a mechanical pump during roughing. It eliminates the need for liquid nitrogen cold traps to prevent cryogenic pump backflow, but cold traps, water-cooled baffles, or room-temperature baffles can be installed in the vacuum chamber to shield the thermal load during operation, but these baffles also reduce the system's Total pumping speed. Cryogenic pumps can use hydrogen vapor pressure gauge to monitor the temperature of the second-stage cold array. 1. Pressure relief valve 2. High vacuum valve 3. Vacuum chamber deflation valve 4. Ionization gauge 5. Heat conduction gauge 6. Roughing valve 7. Mechanical pump deflation valve 8. Mechanical pump 9. Cryogenic pump pre-pumping valve 10. Cryogenic pump flushing gas valve cryogenic pump pumping speed, usually used in high temperature baking degassing vacuum system. From the exhaust system structure, the cryogenic pump system is similar to the ion pump system. In order to rough pollution-free vacuum, but also with the adsorption pump. However, this combination is not suitable for pumping large amounts of inert gas, such as argon. And although cryogenic plates are easy to pump, cryogenic pumps can cause difficulties for adsorption pumps when they need to be regenerated. Because the foreline pump is only used during the initial pumping, there is very little pollution. If the foreline pump is operated in the viscous flow range, that is, the mechanical pump is roughly pumped to 200 Pa and then the adsorption pump is used to pump 1 Pa Further reduce pollution. However, in addition to the extra gas load, cryogenic pump cooling at 200 Pa results in condensable gases such as CO2 and moisture that condense on the cryopanel rather than the surrounding shield. Applying a suitable amount of activated carbon on a cryogenic plate effectively reduces this effect. Because the cryogenic pump pumping hydrogen is very small, so cryogenic pump hydrogen should be combined with the titanium sublimation pump. Cryogenic pumps can also be combined with other ultra-high vacuum pumps, such as ion pumps or turbomolecular pumps. Operation of cryogenic pump vacuum system The cryogenic pump suction system does not require a forepump and requires only a roughing pump. The cryogenic vacuuming chamber is drained with a roughing pump before reaching the switching pressure. Switch off the roughing valve and open the high vacuum valve when the switching pressure is reached. The vacuum chamber can be switched to cryogenic pump evacuation within a safe pressure range. If a mechanical pump is used for rough pumping, the vacuum chamber is switched to a cryogenic pump. The minimum allowable pressure for the pump changeover is determined by the requirement to prevent the mechanical pump from returning oil. The minimum pressure is about 10 to 20 Pa for roughing pipe with a diameter of 4 to 6 cm. For very large diameter rough pipe, the lower the pressure value of the switch is lower; but its value is greater than the pressure value when the Knudsen number is 0.01. The maximum switching pressure depends on the geometry of the cold suction surface and the capacity of the refrigerator. The low-temperature pump suction surface should have a considerable heat capacity, they can receive "sudden" gas without irreversible warming. The allowable value of the maximum gas amount Qi introduced into the cryogenic pump during a gas burst can be obtained by the manufacturer of the cryogenic pump. The amount of gas instantly introduced when the high vacuum valve is opened can be obtained from Qi = PcV. From this, it can be seen that the maximum switching Pressure is where Pc --- switching pressure, Pa V --- volume of the vacuum chamber, m3 Qi --- instantaneous burst into the maximum amount of gas in the cryogenic pump (for example, open the high-vacuum valve instantly import gas ); V is the volume of the vacuum chamber. For a typical small refrigerator cryogenic pump, switching from rough to cryogenic pump pumping pressure range of pressure if the value obtained by (4) is low to the mechanical pump before the switch will have reflux phenomenon, then It shows that compared to the vacuum chamber volume size of the pump speed is too low. By (4) obtained by the type of cryogenic pump switching pressure than the diffusion pump several orders of magnitude higher. If a burst of gas is generated, water vapor in the flash gas will reach the adsorbent at the viscous or transitional flow, overlying or saturating the adsorbent, preventing hydrogen and helium Gas pumping. In most low-temperature exhaust systems, the switching pressure is close to 100 Pa. In order to minimize backflow of oil, it is best to use a switching pressure that is as high as possible and avoids saturation of the adsorbent.