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Diffusion pump

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  • Diffusion pump

    Diffusion pump

    Ideal diffusion pump fluid should be stable, low vapor pressure, low gasification heat, and should be safe to take, use and pour. It should not decompose, absorb gases, and react with surrounding substances. Unfortunately, there is no pump fluid to meet these requirements. Mercury was first used as the only liquid element in the diffusion pump fluid. Mercury has now been replaced by fractionated carbon-grade compounds and synthetic pump fluids in almost all applications. Mercury vapor pressure is high and must be trapped in a cold trap during operation. It is also poisonous. Even so, it is still used in some special applications, such as mass spectrometry, because it does not decompose and dissolve gases, and because its mass spectrometry is easy to identify.

    Before concluding the discussion on the subject of mercury as pump fluid, it should be noted that mercury and oil can not be replaced in conventional oil diffusion pumps. Mercury requires a lower degree of depression than oil, and reacts with some structural materials of the oil diffusion pump. Unless a baffle is installed between the liquid nitrogen cold trap and the mercury diffusion pump, the mercury stored in the boiler will be depleted after several days of continuous operation of the liquid nitrogen cooling above the mercury pump.

    Most diffusion pumps use various fractionated hydrocarbons, vinegars, polysiloxanes or ethers. It has been proved that none of the pumping fluids can be well applied to all applications. The pumping speed of light oil is higher than that of heavy oil, and the vapor pressure of heavy pump liquid is lower, which is necessary to obtain the required low limit pressure in high-demand applications. Carbon-nitrogen compound oil was the first organic pump fluid to replace mercury. These pumps decompose when heated, thus limiting the ultimate pressure they can obtain. The light fraction of the pump liquid will flow back into the vacuum chamber, while the heavy fraction will accumulate on the pump. A fractionation pump is introduced to make the light fraction evaporate preferentially to the bottom nozzle after condensation, so that it can be pumped out by the front pump. It is possible to degasse the pump oil by keeping the wall of a section of ejector at a high temperature. Combining with the above and other design advances, the diffusion pump of heavy oil made by molecular distillation of selected hydrocarbons can reach 5*10-5Pa (without cold trap), and 5*10-7Pa with liquid nitrogen cold trap.

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