Selection of vacuum pumps

When selecting a vacuum pump, the following points should be noted:

1. The working vacuum pressure of the vacuum pump should meet both the ultimate vacuum requirement and the operating vacuum pressure requirement of the vacuum equipment. For example, if a certain vacuum drying process requires an operating vacuum of 10 mmHg, the selected vacuum pump must have an ultimate vacuum no lower than 2 mmHg—and ideally, it should be able to reach as low as 1 mmHg. Generally, the ultimate vacuum rating of the pump should be half to one order of magnitude higher than the operating vacuum required by the vacuum equipment.

2. Select the operating point of the vacuum pump correctly. Each type of pump has a specific working pressure range. For example, the 2BV series liquid-ring vacuum pumps have a working pressure range of 760 mmHg to 25 mmHg (absolute pressure). Within this wide pressure range, the pump’s pumping speed varies with pressure (for detailed variation patterns, refer to the pump’s performance curve). The pump’s stable operating pressure range is between 760 and 60 mmHg. Therefore, it is advisable to select the pump’s operating point within this range rather than allowing it to operate continuously at pressures below 25 to 30 mmHg.

3. Under its operating pressure, the vacuum pump should be able to exhaust the entire volume 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 sputtering ion pumps (or diode asymmetric cathode sputtering ion pumps) have a certain pumping speed for argon. Combining these two types of pumps can significantly improve the vacuum level achieved by 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 multiple 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 instead, combined with certain measures to prevent oil contamination—for example, installing cold traps, baffles, or oil traps—to still meet the cleanliness requirements for vacuum systems.

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 specifically suited to the 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 choose an oil-free vacuum pump or direct 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.

Selection of Liquid Ring Vacuum Pumps

I. Determination of Pump Type

The type of pump is primarily determined by the required air volume, vacuum level, or exhaust pressure for the application.

When the pump is operating, you need to pay attention to the following two aspects:

As much as possible, operate within the efficient range—that is, within the region of critical vacuum or critical exhaust pressure.

Operation near the maximum vacuum level or maximum exhaust pressure should be avoided. Operating within this range not only results in extremely low efficiency but also leads to unstable operation, making the pump prone to vibration and noise. For vacuum pumps with higher vacuum levels, operating within this range often triggers cavitation—a phenomenon clearly indicated by noise and vibration inside the pump. Cavitation can cause damage to components such as the pump body and impeller, ultimately rendering the pump inoperable.

Based on the principles outlined above, when the required vacuum level or gas pressure is not high, a single-stage pump should be preferred. However, if the vacuum level or exhaust pressure is relatively high, a single-stage pump often cannot meet the requirements. Alternatively, if the pump is expected to deliver a large volume of gas even at higher vacuum levels—meaning that the performance curve should remain relatively flat at higher vacuums—a two-stage pump would be more suitable. If the required vacuum level exceeds -710 mmHg, a liquid-ring-atmospheric pump or a liquid-ring-Rotary vane vacuum unit can be selected as the vacuum extraction system.

2. Select a vacuum pump based on the air volume required by the system.

After initially selecting the pump type, for vacuum pumps, you must also choose the pump model based on the required gas flow rate of the system.

For guidance on selecting the pumping speed of vacuum pumps and calculating the pumping time, please refer to our company’s webpage, “Vacuum Calculation Formulas.”

In response to the various types of liquid-ring vacuum pumps and compressors, we have specially compiled their respective characteristics as follows, to help users make the right selection.

3. The water temperature significantly affects the performance of liquid-ring vacuum pumps. Since the performance curves of liquid-ring vacuum pumps are all measured under water temperatures of 15°C, when selecting a liquid-ring vacuum pump for actual applications, it is necessary to adjust the pump’s pumping rate accordingly.

The actual pumping speed of a liquid-ring pump can be calculated by applying a correction factor based on the formula Qt = Q15K, where K = (P1 - Pt) / (P1 - P15). Here:

Qt—Gas flow rate at water temperature t℃, in m³/min; Q15—Gas flow rate at water temperature 15℃, in m³/min.

P1—Suction pressure of the liquid-ring pump, in mmHg; Pt—Saturation vapor pressure of water at temperature t℃, in mmHg.

P15—Saturation vapor pressure at 15℃, in mmHg; K—Gas flow correction factor

The K-values of the liquid-ring vacuum pump under various temperatures and suction pressures can be referred to the table below.