The liquid ring vacuum pump is a crude pump which uses liquid as working medium. The water ring vacuum pump is called water ring vacuum pump, and other working media are oil, sulfuric acid and acetic acid. Industrial water ring vacuum pumps are mostly used. The liquid ring vacuum pump can be divided into single stage and double stage, as well as the difference between single and double action. Single-stage and double-stage vacuum pumps refer to the series of their impellers; single/double action refers to the form of their impellers/shells; the impellers of single-acting impeller vacuum pumps rotate once and the gas is inhaled/discharged; the impellers of double-acting vacuum pumps rotate twice a week.
When choosing vacuum pump, the following points should be paid attention to:
1. The working pressure of the vacuum pump should meet the requirements of the vacuum equipment’s limit vacuum and working pressure. 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 half to one order of magnitude higher than that of the vacuum equipment.
2. Choose the working point of vacuum pump correctly. 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 in this range, and it can not be allowed to work for a long time under 10-8 mmHg. Another example is that the titanium sublimation pump can work under 10-2 mmHg, but its working pressure should be less than 1 x 10-5 mmHg.
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. Because vacuum pumps have selective pumping, sometimes one 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 it 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. If the two pumps are combined, the vacuum device will 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 of 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 and so on. 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 choosing a vacuum pump, it is necessary to know the gas composition and select the appropriate pump for the pumped 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 polluted, oil-free vacuum pumps can be selected or oil vapor can be discharged outdoors.
8. Whether the vibration produced by the vacuum pump has an effect on the process and environment. If the process is not allowed, vibration-free pumps should be selected or anti-vibration measures should be taken.
9. Vacuum pump price, operation and maintenance costs.
The type of pump is mainly determined by the amount of air, vacuum or exhaust pressure required for work. When the pump works, the following two aspects should be paid attention to:
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 operating near the maximum vacuum or exhaust pressure. Operating in this area, not only the efficiency is very low, but also the work is very unstable, easy to produce 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 will cause damage to pump body, impeller and other parts, 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 relatively flat at higher vacuum, two stage pump can be selected. If the vacuum requirement is above -710 mmHg, water ring-air pump or water ring-Roots vacuum unit can be selected as the vacuum pumping device.
If it is only used as a vacuum pump, it is better to choose a single-acting pump. Because the single-acting pump is simple in structure, easy to manufacture and maintain, and has good cavitation resistance in high vacuum.
If it is used only for compressors with larger air volume, it is more appropriate to select double-acting pumps. 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.
Select vacuum pump:
After preliminary selection of the pump type, for the vacuum pump, the pump type should also be selected according to the volume of air required by the system.