1. The working pressure of the vacuum pump should meet the ultimate vacuum and working pressure requirements of the vacuum equipment. For example, the vacuum coating requires a vacuum of 1×10-5 mmHg, and the vacuum of the vacuum pump selected must be at least 5×10-6 mmHg. The vacuum of the pump is usually chosen to be half an order of magnitude higher than the vacuum of the vacuum equipment.
2. Correctly select the working point of the vacuum pump. Each pump has a certain working pressure range, such as: diffusion pump is 10-3~10-7mmHg. In such a wide pressure range, the pumping speed of the pump varies with pressure, and its stable working pressure range is 5×10. -4~5×10-6mmHg. Therefore, the working point of the pump should be selected within this range, and it should not be allowed to work for a long time at 10-8mmHg. Another example is that the titanium sublimation pump can work at 10-2mmHg, but its working pressure should be less than 1×10-5mmHg.
3. Under the working pressure, the vacuum pump should be able to remove all the gas generated during the vacuum equipment process.
4. Combine the vacuum pump correctly. Since the vacuum pump has selective pumping, sometimes a pump cannot meet the pumping requirements, and several pumps need to be combined to complement each other to meet the pumping requirements. For example, a titanium sublimation pump has a high pumping speed for hydrogen, but it cannot be pumped, and a three-pole sputter ion pump (or a two-pole asymmetric cathode sputtering ion pump) has a certain pumping speed for argon. When combined, the vacuum device will have a better vacuum. In addition, some vacuum pumps can't work under atmospheric pressure, and need pre-vacuum; some vacuum pump outlet pressure is lower than atmospheric pressure, and the foreline pump is needed, so it is necessary to combine the pumps.
5. Requirements for oil pollution from vacuum equipment. If the equipment is strictly required to be oil-free, you should choose various oil-free pumps, such as: water ring pump, molecular sieve adsorption pump, sputter ion pump, cryopump, etc. If the requirements are not strict, you can choose to have an oil pump, plus some oil pollution measures, such as adding cold traps, baffles, oil traps, etc., can also achieve clean vacuum requirements.
6. Understand the composition of the gas to be pumped. The gas contains no condensable vapor, no particle dust, and no corrosiveness. When selecting a vacuum pump, you need to know the gas composition and select the appropriate pump for the pumped gas. If the gas contains vapors, particulates, and corrosive gases, consider installing auxiliary equipment such as condensers, precipitators, etc. on the inlet piping of the pump.
7. How does the oil vapor from the vacuum pump affect the environment? If the environment does not allow contamination, you can choose an oil-free vacuum pump or discharge the oil vapor to the outside.
8. The vibration generated when the vacuum pump works has no effect on the process and environment. If the process does not allow, choose a vibration-free pump or take anti-vibration measures.
9. The price, operation and maintenance cost of the vacuum pump.
Selection of water ring vacuum pump I. Determination of pump type The type of pump is mainly determined by the amount of air, vacuum or exhaust pressure required for the work.
When the pump is working, you need to pay attention to the following two aspects:
1. As far as possible, it is required to operate in the high efficiency zone, that is, in the region of critical vacuum or critical exhaust pressure.
2. Avoid operating near maximum vacuum or maximum exhaust pressure. Operating in this area is not only extremely inefficient, but also unstable and prone to vibration and noise. For vacuum pumps with a high degree of vacuum, cavitation often occurs in this area, and the obvious sign of this phenomenon is noise and vibration in the pump. Cavitation can cause damage to parts such as the pump body and impeller, so that the pump cannot work.
According to the above principle, when the vacuum or gas pressure required by the pump is not high, it can be selected firstly in the single-stage pump. If the degree of vacuum or exhaust pressure is high, the single-stage pump is often unable to meet, or the pump is required to have a large volume at a higher vacuum, that is, the performance curve is required to be flat at a higher vacuum, and two are optional. Stage pump. If the vacuum is required to be above -710 mmHg, an optional water ring-atmosphere pump or a water ring-Roots vacuum unit is used as the vacuuming device.
If it is only used as a vacuum pump, it is better to use a single-acting pump. Because the single-acting pump is simple in construction, easy to manufacture and maintain, and has good cavitation resistance under high vacuum conditions.
If only a large volume compressor is used, it is more suitable to use a double acting pump. Because the double-acting pump has a large gas volume, small volume, light weight, the radial force can be automatically balanced, the shaft is not prone to fatigue fracture, and the service life of the pump is long.
Second, according to the amount of gas required by the system, after the vacuum pump is initially selected, the type of pump is selected. For the vacuum pump, the model of the pump should be selected according to the amount of gas required by the system.
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