Ultra high vacuum unit of characteristics and classification
The ultra-high vacuum units operating above 10-5Pa include diffusion pump, molecular pump and oil-free ultra-high vacuum unit. The unit has the following characteristics:
① The limit vacuum degree of the main pump is higher than 10-7 ~ 10-8 PA;
② It can withstand 200 ~ 450 ℃ high temperature baking;
③ The partial pressure of backflow gas in the main pump is very low;
④ For the main pump with strong gas selectivity, the auxiliary pump with enough size should be equipped;
⑤ It is required that the material outgassing rate of vacuum chamber, pipeline, valve and other parts is very low, and the air leakage rate is very small.
(1) Ultra high vacuum unit of diffusion pump
The limit vacuum degree of ultra-high vacuum unit with diffusion pump as main pump is 10-7 ~ 10-8 PA.
In order to reduce the influence of gas back diffusion from the front stage of diffusion pump and prevent the reverse diffusion of mechanical pump oil on the limit vacuum degree, a diffusion pump with low pumping speed can be applied in the front stage of diffusion pump to reduce the compression ratio and reduce the back diffusion. If a titanium sublimation trap is added on the inlet of diffusion pump, the partial pressure of nitrogen from diffusion pump can be further reduced, and ultra-high vacuum of 10-11pa can be obtained.
(2) Molecular pump ultra high vacuum unit
The extreme vacuum degree of ultra-high vacuum unit with molecular pump as main pump is 10-8 PA, which is characterized by fast start-up and less oil pollution.
The pumping speed can be increased by adding liquid nitrogen cold trap in the unit. A water cooling trap is set at the outlet of the mechanical pump to prevent oil vapor from entering the system.
(3) Oil free ultra high vacuum unit
The vacuum unit without oil as working fluid and sealed with organic materials is called oil-free ultra-high vacuum unit. The main pumps commonly used are cryogenic pump, titanium pump, sputtering ion pump, etc., which can obtain the limit vacuum degree of 10-7 ~ 10-8 PA. Compared with oil vacuum unit, it has the following advantages:
① Clean vacuum and no pollution source can be obtained;
② When the oil-free vacuum pump is started, the pre pumping pump can be isolated;
③ If the pumping stops suddenly, it will not pollute the system;
④ No oil, no sound, no vibration, improve the working environment;
⑤ High limit vacuum, high pumping speed and short pumping time.