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How does a rotary vane vacuum pump work?

Oil sealed rotary vane vacuum pump working principle

A rotary vane vacuum pump (referred to as a rotary vane pump) is an oil-sealed mechanical vacuum pump. Its working pressure range is 101325~1.33×10-2 (Pa), which belongs to the low vacuum pump. It can be used alone or as a backing pump for other high vacuum pumps or ultra-high vacuum pumps. It has been widely used in production and scientific research departments such as metallurgy, machinery, military industry, electronics, chemical industry, light industry, petroleum, and medicine.

The rotary vane pump can pump out the dry gas in the sealed container, and if it is equipped with a gas ballast device, it can also pump out a certain amount of condensable gas. But it is not suitable for pumping gas with high oxygen content, corrosive to metal, and chemical reactions to pump oil and dust particles.

The rotary vane pump is one of the most basic vacuum-obtaining equipment in vacuum technology. Rotary vane pumps are mostly small and medium-sized pumps. There are two types of rotary vane pumps: single-stage and two-stage. The so-called two-stage is to connect two single-stage pumps in a series structure. Generally, it is made into two stages to obtain a higher vacuum degree. The relationship between the pumping speed and the inlet pressure of the rotary vane pump is stipulated as follows: when the inlet pressure is 1333Pa, 1.33Pa, and 1.33×10-1 (Pa), the pumping speed value shall not be lower than 95%, 50% and 20%of the nominal pumping speed of the pump.

The rotary vane pump is mainly composed of the pump body, rotor, rotary vane, end cover, spring, and so on. A rotor is eccentrically installed in the cavity of the rotary vane pump, the outer circle of the rotor is tangent to the inner surface of the pump cavity (there is a small gap between the two), and two rotary vanes with springs are installed in the rotor slot. When rotating, relying on the centrifugal force and the tension of the spring to keep the top of the rotary vane in contact with the inner wall of the pump chamber, the rotation of the rotor drives the rotary vane to slide along the inner wall of the pump chamber.

The two rotating vanes divide the crescent-shaped space surrounded by the rotor, the pump chamber, and the two end covers into three parts A, B, and C, as shown in the figure. When the rotor rotates in the direction of the arrow, the volume of space A communicating with the suction port increases gradually, and it is in the process of suction. And the volume of space C communicating with the exhaust port is gradually reduced, just in the process of exhausting. The volume of space B in the middle is also gradually decreasing, which is in the process of compression. Since the volume of space A gradually increases (that is, expands), the gas pressure decreases, and the external gas pressure at the inlet of the pump is higher than the pressure in space A, so the gas is inhaled.

When space A is isolated from the suction port, it turns to the position of space B, the gas starts to be compressed, the volume gradually decreases, and finally communicates with the exhaust port. When the compressed gas exceeds the exhaust pressure, the exhaust valve is pushed open by the compressed gas, and the gas passes through the oil layer in the tank and is discharged into the atmosphere. The purpose of continuous pumping is achieved by the continuous operation of the pump.

One stage rotary vane vacuum pump working principle 

The single-stage rotary vane pump has only one working chamber, and the pump is mainly composed of a stator, a rotary vane, and a rotor. The rotor is installed eccentrically in the pump chamber, and two rotating vanes are installed in the rotor groove, which is close to the cylinder wall due to the elastic force of the spring (there is also a centrifugal force of the rotating vanes after rotation). The rotor and vanes divide the stator cavity into suction and discharge.

When the rotor rotates in the stator cavity, the volume on the side of the air inlet is gradually expanded periodically to inhale the gas, while the volume on the side of the exhaust port is gradually reduced to compress the inhaled gas and discharge it from the exhaust valve.

The vent valve is immersed in oil to prevent atmospheric air from entering the pump. The vacuum pump oil enters the pump chamber through the oil hole and the exhaust valve so that all the moving surfaces in the pump chamber are covered with oil, forming a seal between the suction chamber and the exhaust chamber.

Two stage rotary vane vacuum pump working principle 

In order to improve the ultimate vacuum of the pump, in addition to improving the machining accuracy of the pump body, rotor, and rotary vane, and minimizing the assembly gap and harmful space, the most effective way is to connect two single-stage pumps in series to form a two-stage pump.

The pump consists of two working chambers. The two chambers are connected in series and rotate in the same direction at the same speed. A chamber is the front stage of B chamber. A is the low vacuum stage and B is the high vacuum stage. The pumped gas enters the front stage through the high vacuum stage (B ), and is discharged out of the pump through the exhaust valve. The front stage (A) is the same as the single-stage pump, oil enters the pump chamber at any time, while the high vacuum stage (B) only has a small amount of oil when it starts to work, and no oil enters the pump chamber after working for a period of time. When the pump starts to work and the pressure of the inhaled gas is relatively high (such as starting to pump air from atmospheric pressure), the gas is compressed through the B chamber, and the pressure increases sharply, and a part of the compressed gas is directly discharged from the auxiliary exhaust valve (1) , and the other part is discharged through the front stage.

When the pump works for a period of time, when the pressure of the gas inhaled by the B chamber is low, even though it is compressed by the B chamber, the pressure cannot reach above one atmospheric pressure, and the auxiliary exhaust valve 1 cannot be discharged, and all the inhaled gas will enter The front-stage A room is discharged through the exhaust valve 3 through the continuous compression of the A room.

After the pump works for a period of time since the pressure of the high-vacuum stage air intake is greatly reduced, the outlet pressure is also very small, so the pressure difference between the inlet and outlet of the B chamber is also small, and the amount of compressed gas returned is correspondingly reduced; at the same time, the latter stage The oil molecules that are easy to evaporate in the pump are continuously sucked away by the front-stage A chamber, and the partial pressure of the oil vapor is reduced. Therefore, the oil pollution of the two-stage pump is smaller than that of the single-stage pump, and the ultimate vacuum degree will be greatly improved.

1 Stage vs. 2 Stage Rotary Vane Vacuum Pump

1 stage rotary vane pump consists of a single rotor with multiple vanes that rotates within a cylindrical chamber. As the rotor spins, centrifugal force pushes the vanes against the chamber wall, creating a seal and forming variable-volume chambers. The pumping action occurs through the expansion and compression of gas in these chambers, resulting in the generation of a vacuum.

Simplicity: 1-stage pumps have a straightforward design with fewer moving parts, making them compact, lightweight, and easy to operate and maintain.

Cost-Effective: These pumps are typically more affordable compared to 2-stage pumps, making them a cost-effective option for applications that do not require extremely low vacuum levels.

Suitable for Low to Medium Vacuum: 1-stage pumps are ideal for applications that require vacuum levels within the range of approximately 100 to 1,000 mbar (millibar).

Efficient for Low Gas Loads: They perform well when handling low gas loads, making them suitable for applications where gas flow rates are not excessive.

In a 2 stage rotary vane pump, the pumping process is divided into two sequential stages, each with its own set of vanes. The first stage, known as the high-vacuum stage, operates similarly to a 1 stage pump, creating an initial level of vacuum. The gas discharged from the first stage then flows into the second stage, where further compression occurs, resulting in even lower vacuum levels.

Higher Vacuum Levels: 2 stage pumps are capable of achieving significantly higher vacuum levels compared to 1 stage pumps. They can reach vacuum levels as low as 0.1 mbar or even lower.

Improved Gas Handling: These pumps are effective in handling higher gas loads and can efficiently evacuate larger volumes of gas.

Enhanced Backstreaming Resistance: The two-stage configuration provides better resistance to backstreaming, preventing oil or contaminants from reaching the vacuum chamber or system.

Suitable for High Vacuum Applications: 2 stage pumps are well-suited for applications requiring high vacuum levels, such as analytical instruments, vacuum coating, and semiconductor manufacturing.

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 A clean vacuum environment has always been the ideal vacuum environment pursued by scientists and enterprises. Since the creation of a vacuum environment, people have tried to use different means to achieve the effect of a clean vacuum. Up to now, clean vacuum obtaining equipment has developed to condensate vacuum pumps, plunger vacuum pumps, claw vacuum pumps, scroll vacuum pumps, and diaphragm vacuum pumps.

The development history of scroll vacuum pump

  The vortex theory was filed by Frenchman Leno Creux in 1905 for a U.S. patent on a reversible scroll expander. However, due to the limited level of processing and manufacturing at that time, the processing accuracy of the scroll profile could not be guaranteed, and the scroll pump was not manufactured for a long period of time. Since the 1970s, the aggravation of the energy crisis and the emergence of high-precision CNC machine tools have brought opportunities for the development of scroll machinery. The research report shows that the scroll compressor has incomparable advantages over other compressors, so the large-scale development and research of the scroll compressor has embarked on the road of rapid development.

  With the rapid development of semiconductors, new materials, and biopharmaceutical industries, the continuous maturity of the vortex theory, and the urgent requirement for a clean and oil-free vacuum environment, the vortex vacuum pump has emerged as the times require with its unique advantages. In the early 1980s, the scroll vacuum pump was used in the high vacuum system by Coffin Do because of its good sealing performance and low oil return rate. In 1987, Mitsubishi Electric Corporation of Japan successfully developed the rotary scroll vacuum pump for the first time, which showed absolute advantages in structure and performance. In 1988, the vertical rotary oil-lubricated scroll vacuum pump was successfully developed by Morishita E of Tokyo University in Japan.

  The difference between a dry vacuum pump and an oil-lubricated vacuum pump is that the pump chamber does not contain any oil or liquid. Therefore, solving the problems of sealing and cooling in the pump is the key to the research of dry scroll vacuum pumps. In 1990, a horizontal dry scroll vacuum pump with water cooling was successfully developed by Kushiro T. In 1998, a dry scroll vacuum pump with air cooling was successfully developed by Sawada T. Two cooling fans were installed on the main shaft, which was located at the ends of the two static discs.

 Dry scroll vacuum pump working principle  

  The scroll vacuum pump is mainly composed of orbiting scroll, fixed scroll, anti-rotation mechanism, shell, and other components. Its basic structure is shown in the figure below;

The orbiting scroll and the fixed scroll rotate 180° relative to each other and are mutually staggered by a certain distance (the distance is the radius of the crankshaft). With the rotation of the crankshaft, the movable scroll is moved in translation to realize the meshing of the scroll teeth of the movable scroll and the fixed scroll. The meshing of the scroll will generate multiple meshing points, forming multiple crescent-shaped closed working chambers. With the rotation of the crankshaft, the meshing moves from the outside to the inside along the spiral tooth wall, so as to realize the crescent-shaped working cavity from large to small gradually to form a periodically changing working cavity volume. At the same time, the gas pressure in the working chamber increases continuously with the rotation of the crankshaft, and finally is discharged from the exhaust port at the center of the fixed scroll, so as to realize the suction, compression, and discharge of the gas, and complete the exhaust process of the scroll vacuum pump. 

The pumping process of the scroll vacuum pump is realized by the meshing motion of the movable scroll and the stationary scroll at all times. Therefore, the design of the scroll is one of the important steps in the development of the scroll vacuum pump, which determines the suction of the scroll vacuum pump. The most important design of the scroll is the design of the scroll profile. In general, the scroll profile of the orbiting scroll and the fixed scroll is the same.

Scroll vacuum pump types and structure

  Scroll vacuum pumps can be divided into two types according to the different scroll motion modes, revolution type, and rotary type. One scroll of the pump is fixed and is called a fixed scroll, and the other scroll is an orbiting scroll. The motor drives the crankshaft to rotate, and the crankshaft pushes the center of the base circle of the movable scroll around the base circle of the stationary scroll to make a circular motion of radius r. The anti-rotation structure restricts the movable scroll from rotating. 

  The two scrolls in the rotary scroll vacuum pump are movable scrolls, and they rotate in the same direction around their own base circle center.

  a. Revolution type

     The revolution-type scroll vacuum pump is one scroll that is stationary (fixed scroll) and the other that revolves around it and translates (orbiting scroll). The orbiting scroll is driven by the crankshaft, and the position of the sealing point rotates synchronously with the main shaft. It has a simple overall structure and few parts. The rotary speed of the vortex is small, and the mechanical wear is small, but it needs a dynamic balance design. The scroll vacuum pump uses the outermost scroll ring to contain gas to form a closed suction chamber. In order to reduce the flow conductance between the scroll and the air inlet, the air inlet is set near the end of the outer ring of the scroll. The port is located near the center of the stationary scroll.

  b. Rotary type

      The rotary-type scroll vacuum pump consists of two scrolls mounted on the bearings on both sides, one of which is directly driven by a motor, and the other is driven by a cross-slip ring mechanism to rotate in the same direction. Its sealing position forms a line, the direction is always the same, and the pump adopts a vertical structure. The drive motor is in the upper part of the casing, and the scroll is in the lower part. Its overall structure is complex, with many parts and high mechanical wear.

  c. Difference between two types

      ◆Different way of turning

      ◆Different seal positions and orientations

      ◆The direction of the gas radial and tangential forces on the scroll is different

      ◆Different balance

      ◆Overturning torque and axial force are different

Scroll vacuum pump vacuum range

The vacuum range of a scroll vacuum pump refers to the range of pressures that the pump is capable of achieving and maintaining. The vacuum range is determined by the pump's design, construction, and operating conditions.

Scroll vacuum pumps can work from atmospheric pressure. The ultimate vacuum of different brands of scroll vacuum pumps is different, but usually, it can reach 10Pa to 0.5Pa. However, it is important to note that the vacuum range of a scroll vacuum pump can vary depending on factors such as the size of the pump, the type of scroll pump, and the operating conditions (temperature, oil-free operation, etc.). Additionally, the vacuum range can be affected by factors such as leaks, blockages, or contaminants in the pump or the vacuum system.

Features of scroll vacuum pumps

◆ Small gap, less leakage, and high vacuum degree.

◆ Simple structure and few parts.

◆ The working pressure range is wide. Because the volume of the working chamber changes continuously, the change in driving torque and power is small.

◆ Low vibration noise and high reliability.

◆ Due to the limitation of its own characteristics, the scroll vacuum pump is not easy to be made into a pump with a large pumping rate. At present, the pumping rate of the scroll vacuum pump of some manufacturers can reach 16L/s.

Application of oil-free scroll vacuum pumps

◆Semiconductor industry - thin film preparation equipment, semiconductor packaging equipment;

◆Scientific instrument industry - synchrotron radiation beamlines, electron microscopes, analysis, and testing instruments;

◆Mechanical equipment industry - material preparation equipment, vacuum testing, material purification equipment;

◆Chemical industry;

◆Medical equipment - dental instruments, dialysis machines, biological products, and drug preparation;

◆ Packaging industry - packaging equipment for food, medicine, biological products, etc.

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