Metal oil diffusion pump achieves high vacuum

The metal oil diffusion pump is a vacuum pump with low pressure, high speed and directional flow of oil vapor jet. This paper tries to further prove that the metal oil diffusion pump is the main pump equipment to obtain high vacuum from the characteristics of the metal oil diffusion pump, the working principle of the metal oil diffusion pump and how to improve the vacuum that the metal oil diffusion pump can achieve. It is widely used in vacuum smelting, vacuum coating, space simulation test and some vacuum systems which are insensitive to oil pollution.

oil diffusion pump

oil diffusion pump

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Comparison between oil diffusion pump and mercury diffusion pump

Mercury diffusion pump was invented in 1915, but since it was discovered in 1928 that mercury can be replaced by oil as working liquid, mercury diffusion pump has not been paid much attention, and oil diffusion pump has developed rapidly.

In 1938, there were three-stage metal fraction oil diffusion pumps. After the development of oil diffusion pumps, oil diffusion pumps replaced mercury diffusion pumps in most laboratory research and industrial applications. This is because the vapor pressure of oil is much smaller than mercury, so liquid air or dry ice must be used when mercury diffusion pumps are used. When using the oil diffusion pump, it can only be cooled by cold water or even air.

More importantly, mercury vapor is toxic, so it is difficult to avoid mercury vapor dispersing in indoor air in the experiment of using mercury pump. Especially, when the mercury diffusion pump of glass bursts, a large number of hot water and silver vapor emits, which is even more dangerous to people. In addition, mercury is corrosive to all kinds of non-ferrous metals, which is also a serious shortcoming. But oil also has its shortcomings. It is afraid of oxidation. Its front stage has low voltage resistance. In high voltage vacuum equipment, the vapor of oil attached to the electrodes will make the voltage drop and so on. Therefore, after the development of oil diffusion pump, mercury diffusion pump still has its special function in some laboratory applications, but most of the high vacuum work uses oil pump.

Metal oil diffusion pump advantages

The greatest advantages of metal oil diffusion pump are: metal oil diffusion pump is strong and not easy to break; secondly, large diffusion pump must be made of metal, and now the speed of glass oil diffusion pump is only 100-200 liters/seconds; and large metal oil diffusion pump can reach 20,000-40,000 liters/seconds, so all large vacuum equipments use metal oil. Diffusion pump. Because the large glass valve is difficult to make, the glass oil diffusion pump has not developed to a large scale. In addition, because metal processing is easier than glass precision, metal diffusion pump is easy to manufacture, which is also its advantage.

Metal oil diffusion pump working principle

When the metal oil diffusion pump and the air in the pumped container are pumped very thin by mechanical pump, we heat the oil of the metal oil diffusion pump with electric furnace at about 200 C (different oils). When the vapor pressure of the oil rises to 0.5-1 mm, the vapor will be ejected from the nozzles at all levels, and the direction of the ejection is downward and outward, hitting the wall of water cooling. Then condensation flow down, flow back to the boiler, continuously heating, oil vapor will be continuously ejected, the velocity of the steam molecular flow ejected.

About 200 meters per second, the original air molecule will be knocked down when it meets the steam jet, so gradually the upper pressure will be lower than the lower pressure, and the upper pressure will form a high vacuum, which is the most shallow principle of the diffusion pump.

How to improve the vacuum degree of the metal oil diffusion pump

When the metal oil diffusion pump pumps into the limit vacuum, the remaining gases include the following items: (1) air molecules of reverse diffusion; (2) air leakage; (3) air leakage; (4) rubber ring gas emission and permeability; (5) oil decomposition; (6) oil vapor and so on.

In order to improve vacuum, the following measures should be taken:

(1) using good oil without decomposing DC – 703 is the best.

(2) Strengthen fractionation, our three-stage fractionation diffusion pump, in addition to the fractionation cylinder, but also add a fractionation ring, through the hole of the oil form 180 degrees distance, so that the oil as long as possible;

(3) in order to eliminate the effect of rubber outgassing, the metal knife pump will be the main direction in the future.

(4) In order to reduce the gas absorption of oil and enhance the discharge of internal impurities, we use another oil diffusion pump as the front stage, while avoiding the possible anti-diffusion effect of mechanical pump oil.

(5) According to experience, the materials used in pump body and nozzle are iron, copper and aluminium, but brass is not available, because at high temperature, zinc will evaporate and cause oil discrimination. Theoretically, the surface of glass and enamel or vacuum evaporation may be advantageous.

(6) Reduce the temperature of the bottom of the boiler, because the temperature of oil is the highest on the surface of the boiler, so the decomposition may be mainly in this place. There are three ways to reduce the temperature: (A) reduce the area of the nozzle and add heat dissipation to reduce the burden; (B) increase the effect of heat convection to reduce the overheating of the facial oil; (C) divide the bottom of the furnace into two parts and adjust it separately. Section heating power.

(7) long time pumping and cooling technology.

In summary, the application of metal oil diffusion pump to general high vacuum technology, that is, a certain limit vacuum is easy to achieve, we can completely use domestic materials to achieve.

 

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