Selection Method of Pumps for Vacuum System
Considering what kind of vacuum pump is used in a vacuum system, first of all, according to the requirements of the system, secondly, we should have a detailed understanding of the performance characteristics of the pump, after comparing the advantages and disadvantages of various pumps, we can decide which one to choose properly. In order to facilitate the selection of vacuum pumps to be used, the general situation of various types of vacuum pumps produced in China is summarized. It seems necessary to briefly compare their characteristics, advantages and disadvantages.
I. Oil Seal Mechanical Pump
The 2X type rotary vane pumps are all two-stage pumps with pumping speeds ranging from 0.5 to 150 litres per second. They have a full range of products.
Slide valve pump is divided into single stage (H type) and double stage (2H type). Pumping speed: H type is 8-600 liters per second; 2H type is 8-70 liters per second. There are also a series of products produced.
The cosycloid pump has a large pumping speed and small volume, and is currently in trial production.
These pumps are used as front-stage pumps and pre-pumps for obtaining low vacuum or high vacuum pumps. Moist air can be removed with air ballast device. But the limit vacuum is one order of magnitude lower than that of pumping dry air (without air ballast). Pumping speed refers to the pumping speed when the inlet pressure is 760 torpedoes. When the system pressure decreases, the pumping speed also decreases until it is close to the limit vacuum of the pump and the pumping speed is close to zero. The pumping coefficient has certain provisions.
2. Mechanical Booster Pump (Roots Pump)
The working pressure range is 10~5x10-4 torches, the common range is 2~5x10-3 torches, and the pumping speed range is 150~2000 liters/second. There are already a series of products in our country. The maximum pumping speed of this kind of pump is between 2 to 10-2 torches. It needs a front-stage pump and can not directly exhaust to the atmosphere. The limit vacuum varies with the front stage pump. Compared with the oil booster pump, the range of working pressure is wider, the maximum exhaust pressure is higher (10 ~ 30 tons of mechanical booster pump, 1 ~ 2 tons of oil booster pump), and the reflux oil vapor is less. It is more suitable to use it to extract condensable gases such as water vapor.
3. Oil Booster Pump
The working pressure ranges from 1 to 1x10-4 torches, the commonly used range is 0.1 to 1x10-2 torches, and the pumping speed ranges from 200 to 40000 liters/second, which makes up for the shortcomings of small pumping speed of mechanical pumps and diffusion pumps in this pressure range. The pumping speed of inert gas is the same as that of other gases.
IV. Oil Diffusion Pump
The working pressure range of ordinary oil diffusion pumps is 10-2~5x10-7 tonnes, the common range is 10-8~5x10-6 tonnes, and the pumping speed range is 20~100000 liters/second. The diffusion pumps with metal and glass structures are all produced in series. Ultra-high vacuum oil diffusion pump limit pressure is about 10-10 torr. Except for the slight difference between structure and ordinary oil diffusion pump, the main reason is that the working fluid is different and the structural material is different, i.e. 275 silicone oil is used as pump fluid and stainless steel is used as structural material. The new design of ultra-high vacuum pumps is quite different.