The working pressure of the vacuum pump should meet the limit vacuum and working pressure requirements of the vacuum equipment. For example, vacuum coating requires a vacuum degree of 1 *10-5 mm Hg, and the vacuum degree of the selected vacuum pump should be at least 5 *10-6 mm Hg. Usually, the vacuum degree of the pump is higher than that of the vacuum equipment. The vacuum degree is half to one order of magnitude.
1. The working pressure of the vacuum pump should meet the requirements of the vacuum and the working pressure of the vacuum equipment. For example, vacuum coating requires a vacuum degree of 1 *10-5 mm Hg, and the vacuum degree of the selected vacuum pump should be at least 5 *10-6 mm Hg. Usually, the vacuum degree of the pump is higher than that of the vacuum equipment. The vacuum degree is half to one order of magnitude.
2, correctly select the working point of the vacuum pump. Each pump has a certain range of working pressure, such as: diffusion pump is 10-3-10-7 mmHg, in such a wide range of pressure, pump speed varies with pressure, its stable working pressure range is 5 *10-4-5 *10-6 mmHg. Therefore, the working point of the pump should be selected within this range, and it can not be allowed to work for a long time under 10-8 mmHg. For example, Ti sublimation pump can work under 10-2mmHg, but its working pressure should be less than 1 * 10-5mmHg.
3. Vacuum pump should be able to discharge all the gas produced in the process of vacuum equipment under its working pressure.
4. Correct combination of vacuum pumps. Vacuum pumps have selective pumping, so sometimes a pump can not meet the requirements of pumping. It needs several pumps to be combined to complement each other in order to meet the requirements of pumping. For example, the titanium sublimation pump has a high pumping speed for hydrogen, but can not pump helium, while the tripolar sputtering ion pump (or the bipolar asymmetric cathode sputtering ion pump) has a certain pumping speed for argon, the combination of the two will make the vacuum device get a better vacuum degree. In addition, some vacuum pumps can not work at atmospheric pressure, need pre-vacuum; some vacuum pumps outlet pressure is lower than atmospheric pressure, need the front pump, so they need to be combined to use.
5. Requirements for vacuum equipment for oil pollution. If the equipment is strictly oil-free, all kinds of oil-free pumps should be selected, such as water ring pump, molecular sieve adsorption pump, sputtering ion pump, cryopump, etc. If the requirements are not strict, you can choose to have oil pumps, plus some anti-oil pollution measures, such as adding cold traps, baffles, oil traps, etc., can also meet the requirements of clean vacuum.
6. Understand the composition of the extracted gas. The gas contains no condensable vapor, particulate dust and corrosiveness. When selecting a vacuum pump, it is necessary to know the composition of the gas and select the corresponding pump for the selected gas. If the gas contains vapor, particles, and corrosive gases, it should be considered to install auxiliary equipment such as condenser, dust collector in the intake pipeline of the pump.
7. How does the oil vapor discharged from the vacuum pump affect the environment? If the environment is not allowed to be contaminated, an oil-free vacuum pump can be selected or the oil vapor discharged to the outside.
8. Does the vibration produced by vacuum pump affect the process and environment? If the process is not allowed, no vibration pump should be selected or vibration prevention measures should be taken.
9. The price, operation and maintenance cost of the vacuum pump.
water ring vacuum pump
Determination of pump type
The type of pump is mainly determined by the amount of gas, vacuum or exhaust pressure required for the work.
When the pump works, we should pay attention to the following two aspects:
1. As far as possible, it is required to operate in the high efficiency area, that is, in the critical vacuum or critical exhaust pressure area.
2. avoid running near the maximum vacuum or maximum exhaust pressure. The operation in this area is not only inefficient but also unstable. It is prone to vibration and noise. For vacuum pumps with high vacuum, cavitation often occurs when they operate in this area. The obvious sign of this phenomenon is that there is noise and vibration in the pump. Cavitation can cause damage to parts such as pump body and impeller, so that the pump can not work.
According to the above principles, when the vacuum or gas pressure required by the pump is not high, it can be preferentially selected in the single-stage pump. If the vacuum or exhaust pressure is high, the single stage pump often can not meet the requirements, or, in the case of higher vacuum, the pump is required to still have a larger air volume, that is, the performance curve is required to be flat at higher vacuum, two stage pump can be selected. If the vacuum requirement is above -710mmHg, water ring-air pump or water ring-Roots vacuum unit can be selected as the vacuum pumping device.
If the vacuum pump is used only, the single acting pump is better. Because the single-acting pump is simple in structure, easy to manufacture and maintain, and has good cavitation resistance in high vacuum.
If the compressor is used only for a more atmospheric compressor, the double acting pump is more suitable. Because the double-acting pump has large air volume, small volume and light weight, the radial force can be automatically balanced, the shaft is not easy to produce fatigue fracture, and the service life of the pump is longer.
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