NEWS & BLOG
Selection of Vacuum Pumps
Category: Company News
Time: 2021-01-18
Summary: When selecting a vacuum pump, the following points should be taken into account: 1. The operating pressure of the vacuum pump must meet both the ultimate vacuum requirement and the working pressure requirement of the vacuum equipment. For example, if a vacuum coating process requires a vacuum level of 1×10^-5 mmHg, the vacuum pump selected must have an ultimate vacuum capability of at least 5×10^-6 mmHg. Generally, the vacuum pump’s ultimate vacuum should be half to one order of magnitude higher than the vacuum equipment’s required vacuum level. 2. Select the operating point of the vacuum pump correctly. Each type of pump has a specific working pressure range. For instance, diffusion pumps operate within a range of 10^-3 to 10^-7 mmHg. Within this wide pressure range, the pump’s pumping speed varies with pressure; its stable operating pressure range is typically between 5×10^-4 and 5×10^-6 mmHg. Therefore, the pump’s operating point should be chosen within this range and should not be allowed to operate continuously at pressures below 10^-8 mmHg. Similarly, titanium sublimation pumps can operate at 10^-2 mmHg, but it is best if their working pressure remains below 1×10^-5 mmHg. 3. At its operating pressure, the vacuum pump must be able to remove all the gases generated during the vacuum equipment’s process. 4. Combine vacuum pumps correctly. Since vacuum pumps have selective pumping capabilities, sometimes using just one pump may not meet the pumping requirements. In such cases, several pumps need to be combined and used in tandem to complement each other and achieve the desired pumping performance. For example, titanium sublimation pumps have a high pumping speed for hydrogen but cannot pump helium. On the other hand, triode sputtering ion pumps (or asymmetric cathode sputtering ion pumps) have a certain pumping speed for argon. Combining these two types of pumps will enable the vacuum system to achieve a better vacuum level. Additionally, some vacuum pumps cannot operate at atmospheric pressure and thus require a preliminary vacuum stage; others have outlet pressures lower than atmospheric pressure and therefore need a backing pump. Hence, it is essential to combine pumps appropriately. 5. Consider the oil contamination requirements of the vacuum equipment. If the equipment strictly demands oil-free operation, various oil-free pumps should be selected, such as liquid-ring pumps, molecular sieve adsorption pumps, sputtering ion pumps, and cryogenic pumps. If the requirements are less stringent, oil-lubricated pumps can be chosen, provided that appropriate measures are taken to prevent oil contamination—for example, by installing cold traps, baffles, or oil traps—to ensure clean vacuum conditions. 6. Understand the composition of the gas being pumped—whether the gas contains condensable vapors, particulate dust, or corrosive substances. When selecting a vacuum pump, it is crucial to know the gas composition and choose a pump that is suitable for the specific gas being pumped. If the gas contains vapors, particles, or corrosive gases, consider installing auxiliary equipment, such as condensers or dust collectors, on the pump’s inlet pipeline. 7. Assess the environmental impact of the oil vapor emitted by the vacuum pump. If the environment cannot tolerate any pollution, opt for an oil-free vacuum pump or direct the oil vapor outside the facility. 8. Determine whether the vibrations generated by the vacuum pump during operation will affect the process or the surrounding environment. If the process is sensitive to vibration, select a pump that produces no vibration or take measures to mitigate vibration. 9. Consider the cost of the vacuum pump, as well as its operating and maintenance expenses.
When selecting a vacuum pump, the following points should be noted:
1. The working vacuum pressure of a vacuum pump should meet both the *limiting vacuum* and the operating vacuum pressure requirements of the vacuum equipment. For example, if vacuum coating requires a vacuum level of 1×10⁻⁵ mmHg, the vacuum pump selected must have a vacuum capability of at least 5×10⁻⁶ mmHg. Generally, the vacuum capability of the pump should be one to two orders of magnitude higher than the vacuum requirement of the equipment.
2. Select the operating point of the vacuum pump correctly. Each type of pump has a specific working pressure range. For example, diffusion pumps operate within a range of 10⁻³ to 10⁻⁷ mmHg. Within this broad pressure range, the pump’s pumping speed varies with pressure; its stable operating pressure range is between 5×10⁻⁴ and 5×10⁻⁶ mmHg. Therefore, the pump’s operating point should be chosen within this range and should not be allowed to operate continuously at pressures below 10⁻⁸ mmHg. Similarly, titanium sublimation pumps can operate at pressures as high as 10⁻² mmHg, but it is best if their working pressure remains below 1×10⁻⁵ mmHg.
3. Under its operating pressure, the vacuum pump should be able to exhaust the total amount of gas generated during the vacuum equipment’s process.
4. Properly combine vacuum pumps. Since vacuum pumps exhibit selective pumping characteristics, sometimes a single pump cannot meet the required pumping capacity. In such cases, several pumps must be combined and used in tandem to complement each other and achieve the desired pumping performance. For example, titanium sublimation pumps have a very high pumping speed for hydrogen but cannot pump helium. On the other hand, triode sputter ion pumps (or asymmetric cathode sputter ion pumps of the diode type) have a certain pumping speed for argon. Combining these two types of pumps can help attain a better vacuum level in the vacuum system. Additionally, some vacuum pumps cannot operate at atmospheric pressure and thus require a preliminary vacuum stage; others have outlet pressures lower than atmospheric pressure and therefore need a forepump. Hence, it is often necessary to combine different pumps for optimal operation.
5. Requirements for oil contamination in vacuum equipment: If the equipment has strict requirements for oil-free operation, it is advisable to select various oil-free pumps, such as liquid-ring pumps, molecular-sieve adsorption pumps, sputter ion pumps, and cryogenic pumps. If the requirements are not stringent, oil-lubricated pumps can be used in combination with certain measures to prevent oil contamination—for example, installing cold traps, baffles, or oil traps—to still meet the clean vacuum requirements.
6. Understand the composition of the gas being pumped, whether the gas contains condensable vapors, particulate dust, or corrosive substances. When selecting a vacuum pump, it is essential to know the gas composition and choose a pump that is suitable for the specific gas being pumped. If the gas contains vapors, particles, or corrosive gases, you should consider installing auxiliary equipment—such as condensers or dust collectors—in the pump’s inlet pipeline.
7. What is the environmental impact of oil vapor discharged by vacuum pumps? If the environment cannot tolerate any pollution, you can opt for an oil-free vacuum pump or route the oil vapor outdoors.
8. Does the vibration generated during the operation of the vacuum pump affect the process and the environment? If the process does not permit vibration, a pump that produces no vibration should be selected, or vibration-reducing measures should be implemented.
9. The price of the vacuum pump, as well as its operating and maintenance costs.