Steam flow diffusion pump was used to obtain high vacuum in history, and it is still widely used. It uses gas diffusion to extract gas. Diffusion pump is divided into two types according to the type of working liquid in the pump, including mercury pump and oil pump. Mercury vapor is toxic and has been avoided except for special occasions. According to the structure material, there are two kinds of glass and metal pump. Glass diffusion pumps are small, easy to manufacture, low cost, but the pump speed is small, easy to break. The metal structure has high strength and is suitable for large pump manufacture.
The bottom of the single nozzle oil diffusion pump is evaporator, and the high vacuum oil is stored in the evaporator. After heating by the electric furnace outside the pump, a certain amount of oil vapor is generated, and the pressure is about 10-1 Torr. Steam is transmitted to the upper part along the conduit and is ejected downward through the umbrella nozzle. Because the gas pressure outside the nozzle is low, about 10-2-10-3torr, the oil and steam flow can be ejected down for a long distance, which constitutes a jet moving towards the outlet.
Because the steam flow from the evaporator contains very little gas, that is, the gas density in the steam flow is low and the partial pressure of the gas in the pumped vessel above the steam flow is always large, and the gas molecules are constantly spreading from the space above the steam flow to the steam flow. The jet is supersonic, and has a high density and high molecular weight, so it has a strong capacity to carry gas. The gas molecules which spread into the supersonic jet collide with the vapor molecules to obtain downward momentum, which are compressed into a higher pressure region and finally discharged by the front pump. The jet runs into the pump wall with cooling water during the downward movement. The oil molecules are condensed down and flow back to the evaporator along the pump wall for further recycling.
In modern diffusion pumps, there are usually three to four nozzles in series, such pumps are also called three or four stage pumps. The nominal pump speed of diffusion pump is from ten to dozens of liters per second, and the pump speed of metal oil diffusion pump can reach hundreds of liters per second or tens of thousands of liters per second.
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