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When selecting a vacuum pump, the following points should be taken into consideration: 1. The operating pressure of the vacuum pump must meet the requirements regarding the ultimate vacuum and operating pressure of the vacuum equipment. For example, vacuum coating requires a vacuum level of 110–5 mmHg; therefore, the vacuum pump used must be capable of achieving a vacuum level of at least 510–6 mmHg. Typically, the vacuum level of the pump should be half to an order of magnitude higher than that of the vacuum equipment. 2. Select the operating point of the vacuum pump correctly. Each type of pump has a certain operating pressure range; for example, the diffusion pump operates in the range of 10-3 to 10-7 mmHg. Within such a wide pressure range, the pumping speed of the pump changes with pressure, and its stable operating pressure range is 510-4 to 510-6 mmHg. Therefore, the operating point of the pump should be selected within this range, and it should not be allowed to operate at 10-8 mmHg for an extended period. For example, a titanium sublimation pump can operate at 10-2 mmHg, but it is preferable for its operating pressure to be below 110-5 mmHg. 3. At its operating pressure, the vacuum pump should be capable of removing all the gas generated during the process in the vacuum equipment. 4. Assemble the vacuum pump correctly. Since vacuum pumps have selective pumping capabilities, sometimes a single pump is not sufficient to meet the pumping requirements; it is necessary to use several pumps in combination, complementing each other, to fulfill these requirements. For example, titanium sublimation pumps have a high pumping speed for hydrogen but cannot pump helium, while triode-type sputter ion pumps (or diode-type asymmetric cathode sputter ion pumps) have a certain pumping speed for argon; by combining the two, a better vacuum level can be achieved in the vacuum system. Additionally, some vacuum pumps cannot operate at atmospheric pressure and require pre-evacuation ； Some vacuum pumps have an outlet pressure lower than atmospheric pressure, requiring a pre-pump; therefore, these pumps need to be used in combination. 5. Requirements of vacuum equipment regarding oil contamination. If the equipment requires absolutely oil-free operation, various oil-free pumps should be selected, such as water ring pumps, molecular sieve adsorption pumps, sputter ion pumps, cryopumps, etc. If the requirements are not strict, a vacuum pump can be used, along with various oil pollution prevention measures such as cold traps, baffles, and oil traps, to meet the requirements for a clean vacuum environment. 6. Understand the composition of the gas being sampled, whether it contains condensable vapors, particulate dust, or corrosive substances. When selecting a vacuum pump, it is necessary to know the gas composition and choose an appropriate pump for the gas to be evacuated. If the gas contains vapors, particles, and corrosive gases, it is advisable to consider installing auxiliary equipment such as condensers and dust collectors on the inlet pipeline of the pump. 7. What is the impact of the oil vapors emitted by vacuum pumps on the environment? If the environment does not allow pollution, an oil-free vacuum pump can be used, or the oil vapors can be vented outside. 8. Does the vibration generated by the vacuum pump during operation have an impact on the manufacturing process and the surrounding environment? If the process does not permit it, a vibration-free pump should be selected or vibration prevention measures should be taken. 9. The price of the vacuum pump, as well as the costs associated with its operation and maintenance. Selection of Water Ring Vacuum Pumps I. Determination of Pump Type The type of pump is primarily determined by the volume of air required for operation, the level of vacuum needed, or the exhaust pressure. When the pump is in operation, two aspects need to be taken into consideration: it should operate as much as possible within the high-efficiency range, that is, within the area of critical vacuum or critical exhaust pressure. Operation near the maximum vacuum level or maximum exhaust pressure should be avoided. Operating in this area not only results in extremely low efficiency, but also leads to unstable operation, with frequent vibrations and noise. For vacuum pumps with a high degree of vacuum that operate in this range, cavitation often occurs as well; obvious signs of this phenomenon are noise and vibration inside the pump. Cavitation can cause damage to components such as the pump body and impeller, resulting in the pump failing to function. Based on the above principles, when the vacuum level or gas pressure required by the pump is not high, a single-stage pump should be preferred. If the vacuum level or exhaust pressure is high, a single-stage pump often fails to meet the requirements. Alternatively, if it is necessary for the pump to handle a large volume of gas even at high vacuum levels, that is, if a relatively flat performance curve at high vacuum levels is required, a two-stage pump can be used. If a vacuum level of above –710 mmHg is required, a water ring-atmospheric pump or a water ring-Rotary vane vacuum unit can be used as the vacuum pumping device. II. Selecting the vacuum pump based on the air volume required by the system After initially determining the type of pump, it is also necessary to choose the specific model of the vacuum pump according to the air volume required by the system.