Working Principles of Common Vacuum Pumps

3D structure diagram of oil-free vacuum pump

Figure 1: Oil-free vacuum pump structure diagram.

The vacuum pump is a device that vacuumizes the pumped container under the action of electromagnetic force to maintain the vacuum. Vacuum pumps are usually widely used in low temperature equipment, papermaking machinery, vacuum equipment, and petroleum, mining, foundation treatment and other fields. Different vacuum pumps have completely different working principles and applications.

Water Ring Vacuum Pump

The eccentric rotor with fixed blades in the pump throws water to the stator wall where the water forms a liquid ring concentric with the stator, and the liquid ring and the rotor blades together form a variable volume rotary vacuum pump. The rotary vane is divided into single-stage and double-stage.

The principle is to utilize the eccentric rotor to form a volume change by means of rotation in the pump chamber and discharge the gas out of the pump. Mainly during the suction process in which the volume of the suction chamber expands and the degree of vacuum decreases, the gas in the container is sucked into the pump cavity. During the exhaust process in which the volume becomes smaller and the pressure increases, the sucked gas is discharged out of the pump through the oil seal.

The impeller is eccentric with the pump case. The pump case is filled with a certain amount of water. The rotation of the impeller makes the water form a water ring

Figure 2: The sketch and internal structure of working principle of water ring vacuum pump.

Roots Vacuum Pump

There are two “8-shaped" rotors installed on a pair of parallel shafts perpendicular to each other in the pump chamber of the Roots vacuum pump, where a pair of gear belts with a transmission ratio of 1 act to reverse each other's synchronous rotation. There is a certain clearance between the rotors and between the rotor and the inner wall of the pump casing, which can realize high-speed operation.

It is due to the vacuum pump without internal compression that Roots vacuum pump has low compression ratio, so that the medium and high vacuum pumps need a backing pump. The maximum vacuum of the Roots pump depends not only on the structure and manufacturing accuracy of the pump itself, but also on the maximum vacuum of the backing pump.

The Roots vacuum pump can be used in series to enhance its maximum vacuum degree. Its working principle is similar to the Roots blower’s. Due to continuous rotation of rotors, the pumped gas is sucked into the space between the rotor and the pump housing from the air inlet, and then discharged through the exhaust port. Since the space is fully closed after suction, the gas in the pump chamber is not compressed and expanded. However, when the top of the rotor turns around the edge of the exhaust port and the space is connected with the side of exhaust port, due to the high gas pressure at the side of exhaust port, some gas will recoil into the space, resulting in a sudden increase in the gas pressure. When the rotor continues to rotate, the gas is discharged out of the pump.

With the continuous rotation of the rotor, the extracted gas is sucked into the space between the rotor and the pump case from the air inlet, and then discharged through the exhaust port

Figure 3: The sketch and internal structure of working principle of roots vacuum pump.

Rotary Vane Vacuum Pump

Rotary vane vacuum pump (abbreviated rotary vane pump) is an oil-sealed mechanical vacuum pump. Its working pressure is ranging from 101325~1.33x10-2(Pa), belonging to low vacuum pump. It can be used solely, or as a backing pump of other high vacuum pumps or ultra-high vacuum pumps. It can pump the dried air in the airtight container, and might pump a certain amount condensable air with air ballast devices. However, it is not suitable for pumping gas with high oxygen content, corrosive to metal, chemical reaction to pump oil and gas containing particulate dust. Rotary vane pump is one of the basic vacuum acquisition equipment in vacuum technology. Rotary vane pumps are mostly small and medium-sized pumps.

An eccentric rotor is installed in the pump case. The rotor is provided with a plurality of slots in which sliding plates can slide. The sliding plate in the rotor rotating groove extends all around and does not contact the pump case

Figure 4: The sketch and internal structure of working principle of rorary-vane vacuum pump.

Rotary vane pump is mainly composed of pump body, rotor, rotary vane, end cover, spring, etc. A rotor is eccentrically installed in the chamber of the vane pump, whose surface is tangent to the outer circle of the rotor (there is a small clearance between them), and two vanes with springs are installed in the rotor slot. During rotation, the top of the rotary vane is kept in contact with the inner wall of the pump chamber by centrifugal force and spring tension, and the rotation of the rotor drives the rotary vane to slide along the inner wall of the pump chamber.

The two rotary vanes divide the crescent-shaped space enclosed by the rotor, the pump chamber and the two end covers into three parts, A, B and C. When the rotor rotates in the direction of the arrow, the volume of the space A connecting with the suction inle is gradually expanding, which is in suction process. And the volume of the space C connecting with the exhaust port is gradually shrinking, which is in the process of exhausting. The volume of the central space B is also gradually reducing, which is in the process of compression.

Since the volume of space A is gradually increased (expanded), the gas pressure is reduced, and the external gas pressure at the inlet of the pump is stronger than the pressure in the space A, so the gas is sucked into. When space A is isolated from the suction port, which means it turns to the position of space B present, the gas begins to be compressed, the volume gradually shrinks, and it connects 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 is discharged into the atmosphere through the oil layer in the oil tank. The purpose of continuous pumping is achieved by the continuous operation of the pump. If the discharged gas is transferred to another stage (low vacuum stage) through the air passage, it is pumped away by the low vacuum stage, then compressed by the low vacuum stage and then discharged into the atmosphere, which constitutes a two-stage pump. At this time, the total compression ratio is borne by two stages, thereby enhancing the maximum vacuum.