The oil in the diffusion pump is heated and boiled in vacuum (about 200 C) to produce a large amount of oil vapor. The oil vapor rises through the diversion pipe and is ejected from the nozzles at high speed. Since the partial pressure of the sucked gas in the ejected oil vapor is less than that of the sucked gas at the inlet of the pump, the molecule of the sucked gas is continuously diffused into the vapor stream. Oil vapor impinges on the absorbed gas molecules, causing them to flow along the direction of oil vapor flow to the pump wall at a high speed, and the gas molecules are reflected back from the pump wall by the impact of oil vapor and re-flow along the direction of steam flow to the pump wall. After several collisions, the gas molecules are compressed to the low vacuum end, and then the vapor from the lower nozzles is compressed in multiple stages, and finally pumped out by the front stage pump. At the same time, the oil vapor is condensed on the cooling pump wall and then returned to the oil pan to be reheated, thus circulating.
Oil used in oil diffusion pump should be saturated vapor pressure is very low, good thermal stability of oil, in order to prevent the oxidation of pump oil, oil diffusion pump must be pre-pumped to a certain vacuum with Roots vacuum pump, oil pan can start heating. Diffusion pump should be used in series with the front-stage pump, the front-stage pump generally uses oil-sealed rotary mechanical vacuum pump, such as rotary vane vacuum pump, slide valve vacuum pump. The oil diffusion pump system with high vacuum or ultra-high vacuum can be obtained by combining diffusion pump with different front-stage pumps.
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