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Control analysis of oil return rate of oil diffusion pump

With the continuous development of vacuum technology, vacuum equipment has been widely applied in metallurgical industry. As the main pump of most high vacuum systems, oil diffusion pump has many advantages, such as simple structure, high pumping rate, easy to obtain high vacuum and so on. However, it is difficult to solve effectively in production practice for many years. In this paper, how to control the oil return rate of oil diffusion pump in order to obtain high vacuum system is discussed from the design principle of oil diffusion pump and the experience accumulated in production practice.


The structure and working principle of oil diffusion pump:

The oil diffusion pump carries the gas through the oil vapor jet to achieve the goal of vacuum pumping. Oil diffusion pump structure. It is mainly composed of pump body, draft tube, fractionating ring and heater. Oil as working fluid in the oil pan below, heated by heater into oil vapor, rises along the diversion pipe, when oil vapor to the first, second and third-class nozzle, due to the sudden reduction of the cross-section of the nozzle, the speed of steam flow increases, high-speed oil vapor flow from the nozzle ejected, forming an umbrella, as long as the gas molecules touch Oil vapor is easily carried away, and then reaches the inner wall of the pump, the pump wall is cooled by cooling water, so oil vapor is condensed into oil droplets, along the pump wall into the oil pan, gas molecules can not be condensed out; oil is heated, evaporated into oil vapor, the first level of the gas molecules from the A1 zone to A2 zone A3 It is then sent to the exhaust port and pumped by the front stage pump to achieve exhaust.

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Structural design causes the oil diffusion pump to return oil:

1.pump wall, especially the cooling capacity of the top pump wall


Improper pump wall surface temperature, so that the impact on the diffusion pump surface steam molecules may not condensate; along the pump wall downstream oil layer surface temperature than the pump wall temperature is much higher, easy to produce re-evaporation, oil reflux, so the wall cooling efficiency is higher. Full cooling of the top pump wall can greatly reduce the return flow rate caused by the re-evaporation of oil film and droplets on the surface of the primary nozzle.


2. The power of the diffusion pump is too high, the oil temperature is too high and the oil return rate is increased.


When the heating power of the oil diffusion pump is too high to exceed the optimum value, the density of the oil vapor jet increases, the exhausted gas is not easy to diffuse into the vapor molecule, and the vapor flow velocity is too high, so the impact on the pump wall can not be as high as condensation but sputtering, thus increasing the oil molecule diffusing into the pumped vessel, thereby increasing the reflux rate.


3. Diffusion pump inlet baffle prevents oil molecules from scattering


The main function of the inlet baffle of the oil diffusion pump is to prevent the irregular movement of oil vapor molecules from flowing back into the pumped vessel when the diffusion pump works normally. The structure of baffle has a great relationship with the reflux of oil molecules.


4. Deformation, tilt or position change of the jet tower


The spray tower is composed of nozzles and diversion pipes of all levels, and its deformation will directly affect the gap size and angle of the nozzle. When the diffuser pump works, the spray area and force of the oil vapor molecules are too large, the gap is too large, the speed of the steam flow is too small to touch the pump wall, the angle is too large, and the distance from the nozzle to the pump wall is too long. No, can not form a dense steam flow in the pump wall; too small opening angle, the oil molecule horizontal velocity increases, easy to cause oil vapor to reflect to the high vacuum end. The displacement of the spray tower will change the position of the nozzle, which will cause the change of the region of the gas carried by the oil vapor molecules and the change of the cooling time.


5.excessive export pressure, causing oil return


The outlet pressure of the oil diffusion pump can not exceed the maximum reverse pressure. When the outlet pressure of the diffusion pump is too high, the gas will diffuse from the front pump to the diffusion pump, changing the directional movement of the oil vapor molecules, increasing the oil vapor reflux rate.

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Conventional measures to control oil return rate of oil diffusion pump are as follows:

At present, the main measure is the application of traps. There are three types of traps: oil baffle, baffle and trap.


1. Oil retaining cap and oil retaining ring


The diffusion pump adopts the oil blocking cap and the oil retaining ring device, which can effectively prevent the oil from reflux. This structure mainly controls the re-evaporation of the oil film and droplets on the surface of the primary nozzle, and the re-evaporation of the oil film and droplets on the surface of the primary nozzle, accounting for 70% of the total return oil. The use of the oil cap reduces the main source of oil return, which can reduce the oil return rate by about 50%, but has no significant effect on pump speed. The cooling cap of water blocking cap is better than that without water. If the top nozzle, close to the pump wall with a stop ring, the effect is better, at most oil return rate can be reduced by 20%.


2. baffle


As the oil vapor flows back from the pump inlet to the pumped container, a baffle can be installed at the pump inlet to prevent the oil vapor from flowing back to reduce the oil reflux. Baffles are usually cooled by water.


In high vacuum, the vapor molecules move in a straight line, so the baffle does not allow the vapor molecules to pass through in a straight line. When the oil vapor moves toward the high vacuum end, it will inevitably collide with the baffle, so that the oil vapor molecules either condense on the baffle, or weaken its diffusivity. In the normal operation of the diffusion pump, the molecule of oil vapor is absorbed and evaporated by striking the wall of the duct, then absorbed again near the pump wall of the vacuum vessel, and then evaporated. The vapor molecule that has been collided twice continues to diffuse into the vacuum vessel, and then hits the baffle plate, thus preventing the return flow of oil vapor.

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