The oil diffusion pump in the vacuum processing system is used to carry the gas through the high-speed steam jet to achieve the purpose of pumping air. It is used for working in the high vacuum area and the working pressure range is 10-2 ~ 10-6pa. Oil vapor from the umbrella nozzle (such as Class I nozzle) to the supersonic discharge, the speed increases, the pressure and density gradually decreased.
1, oil diffusion pump overview:
Oil diffusion pump works similar to the steam jet pump, but also rely on high-speed steam jet to carry gas in order to achieve the purpose of vacuum, so there will be similar characteristics of steam jet pump. The difference is that the diffusion pump works in a high vacuum area and is the main equipment for achieving high vacuum. The working pressure range of 10-2 ~ 10-6pa. Widely used in vacuum smelting, vacuum coating, space simulation tests and some of the vacuum system insensitive to oil pollution.
2, oil diffusion pump working principle and structure:
When the oil vapor is sprayed out from the bellow nozzle (such as a class I nozzle) at a supersonic speed, its speed is getting faster and faster, and the pressure and density are gradually decreased. The gas A on the jet flow is directed to the density difference The steam jet diffuses and is carried by the jet to the water-cooled pump wall B. At B, most of the working steam is condensed into oil droplets and flows back to the oil pan along the pump wall for recycling, while the evacuated gas accumulates at point B, Compressed, and finally carried away by the lower jet, so you can level by level compression, the last pump was drawn away. The pumping speed characteristic curve as shown.
3, the proliferation of oil pump development process:
History In 1915, the German physicist W. Geith published the diffusion pump report he studied. In 1916, American I. Langmuir made a condensate pump with a cooling system at the pump wall. These pumps use mercury vapor as a working medium and can eventually achieve a vacuum of 10-5 Pa. In 1928, the British CD Bolch found high-boiling petroleum derivatives. In 1936, CD Hickman and others made synthetic oil. Both oils have a very low saturated vapor pressure at room temperature, displacing mercury as the working fluid for diffusion pumps. Since then, the oil diffusion pump has been widely used in industrial production and scientific experiments in the field of high vacuum and laid the foundation of high vacuum technology. Since the 1960s, there have been new changes in oil diffusion pumps. The main improvements are: â‘ pump deflated material with a small amount of stainless steel. â‘¡ The use of saturated vapor pressure is very low, good thermal stability of the oil, such as polyphenylene ether and silicone oil for the working fluid pump. â‘¢ to change the structure, the new oil diffusion pump in the pump inlet flange constant and not excessive increase in pump size conditions, the lower part of the flange to significantly expand the pump chamber section, the pumping rate can be increased 20 ~ 40%. If installed here a large diameter diffusion nozzle and large diameter liquid nitrogen cooling oil cap and so on. The pump can pump the usual rate (that does not expand the pump section of the pump) pumping rate, and more completely overcome the pump back to get less than 10-8 Pa clean super-high. Therefore, the oil diffusion pump in the ultra-high vacuum cleaning industrial production and scientific testing has also made an important position.
4, Structure and diffusion pump Working principle The oil diffusion pump is mainly composed of pump body, diffusion nozzle, steam pipe, oil pan, heater, diffuser, cooling system and jet nozzle. When the oil diffusion pump is pre-pumped to a vacuum below 1 Pa, the pan can start to heat up. When boiling, the nozzle emits a high-speed steam flow, and the hot moving gas molecules diffuse into the vapor flow and collide with the directional oil vapor molecules. The gas molecules thus acquire momentum, resulting in the same directional flow as the molecular motion of the oil vapor. To the previous stage, the oil vapor is condensed to release gas molecules, which is pumped by the foreline to achieve the purpose of pumping.
Vacuum pump oil vapor pressure directly affects the vacuum performance of the pump. However, any pump oil used in oil diffusion pumps is a multi-component mixture of different vapor pressures. Therefore, to improve the oil diffusion pump suction and vacuum performance, the pump itself in the work of the pump oil fractionation and purification. The purpose of fractionation is to make the high vapor pressure component of the oil do not enter the high vacuum working nozzle (high vacuum end of the nozzle); purifying purpose is to make the high vapor pressure component of the oil in the work process continuously for the forepump pumping, the vacuum Oil tends to be pure.
Pump oil fractionation is mainly the use of pump oil evaporation temperature of each component is different, so that they were in different boiler area evaporation. When working, the condensed pump oil flows back to the wall of the pump with the heat shield and the oil is preheated and then flows to the perimeter of the pan. Heated by the bottom ring heater, the light fraction of the oil reaches the evaporation temperature Evaporation into low-vacuum working nozzles - nozzles close to foreline pumps; some of the non-evaporated light ends also float to the oil surface because of their small specific gravity compared to other distillates. Due to the restriction of the outer vapor conduit, this fraction still evaporates into the low vacuum working nozzle. Heavy fraction of the oil in the outer vapor conduit did not reach the evaporation temperature and did not evaporate, so from the outer layer of the gap between the bottom of the pan flow evaporation pan, into the high vacuum nozzle. Light, heavy fraction of the oil vapor in such a separate way to supply low vacuum nozzle and high vacuum working nozzle this process is called fractionation. The oil vapor ejected from the jet nozzle can not be sufficiently cooled at the anterior stage, and the light fraction in the vapor is not completely condensed and is removed by the anterior stage pump. This cycle of work, less and less light pump oil, heavy fraction of the proportion will become larger and larger, pump oil will gradually become pure. This process is called decontamination. Pump oil fractionation and purification, to improve the ultimate vacuum pump has an important role.
5, Diffusion pump oil In the pump structure and without leakage under the premise of the oil diffusion pump suction characteristics and ultimate vacuum pump oil is mainly to see the performance. Many types of pump oil, the table for several types of major pump oil and its characteristics.
Diffusion pump oil is heated by the electric furnace, the oil vapor generated along the diversion tube through the umbrella nozzle to the bottom of the spray. Because of the vacuum (Pa) provided by the mechanical pump on the outside of the nozzle, the oil vapor stream can be ejected for a long distance to form a jet that moves toward the air outlet. Jet finally hit by the cooling water cooling pump wall condenses into liquid flow back to the evaporator, that is, by the oil evaporation jet condensation cycle to achieve exhaust. The gas molecules entering the pump from the air inlet drop down into the vapor stream and then get downward momentum. Since the jet has a high flow rate (about 200 m / s), a high vapor density, and a large molecular weight of the diffusion pump oil (300 to 500), gas molecules can be effectively taken away. Gas molecules are taken to the exit and then pumped by a mechanical pump.
6, oil diffusion pump maintenance and care (1) oil diffusion pump installed vertically to the pump core components in the normal working condition. All rubber seals should be sealed and reliable.
(2) The oil diffusion pump must ensure that the cooling water is clear before it is heated. The pump can be heated under a vacuum of about 1 Pa.
(3) oil diffusion pump stop working, the pump should be avoided into the atmosphere to prolong the life of the pump oil and reduce pump suction, in particular, should stop the pump just stopped working before cooling into the atmosphere, in order to avoid oil oxidation.
(4) long-term storage of oil diffusion pump, the best pumping into a vacuum to prevent pump oil pollution and corrosion of parts, and should purge the cooling water pipes and water jacket surplus water.
(5) was pumping gas should be dry, non-corrosive, dust-free gas at room temperature state.
(6) sudden deterioration of performance during normal pump operation, should check the heater is short circuit, open circuit, and three-phase current is balanced.
(7) After prolonged work, the performance of the pump is getting worse and worse. Check whether the amount of oil is reduced or oxidized so as to increase the amount of oil or dry the new oil after cleaning.
(8) Once loading and unloading, if the performance is still up, we must check whether the seal is reliable or improper assembly.
(9) During the operation, pay attention to whether the work of the front-stage mechanical pump of the diffusion pump is normal. The ultimate pressure of the general mechanical pump should be lower than the pressure of the previous stage of the pump by an order of magnitude. When the performance of the mechanical pump deteriorates, it directly affects the oil diffusion The pump is working.
(10) For small and medium-sized pump formed by pulling core with pump core, the first-stage nozzle cap should be gently tightened, and the pump core should be prevented from being subject to heavy-weight deformation.
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