Working Principle of Vacuum Pumps

Working Principle of Water Ring Vacuum Pumps / Liquid Ring Vacuum Pumps

A water ring vacuum pump (abbreviated as water ring pump) is a type of rough vacuum pump, capable of achieving an ultimate vacuum of 2000–4000 Pa. When connected in series with an atmospheric ejector, it can reach a vacuum level of 270–670 Pa. The water ring pump can also be used as a compressor, known as a water ring compressor, which falls into the category of low-pressure compressors with a gauge pressure range of 1–2×10⁵ Pa.

Initially designed as a self-priming water pump, the water ring pump has gradually found applications in numerous industrial sectors including petroleum, chemical engineering, machinery, mining, light industry, pharmaceuticals, and food processing. It is widely used in many industrial processes such as vacuum filtration, vacuum priming, vacuum feeding, vacuum evaporation, vacuum concentration, vacuum conditioning, and vacuum degassing. With the rapid development of vacuum application technology, the water ring pump has long been valued for rough vacuum generation. Since gas compression in the water ring pump is an isothermal process, it can extract flammable and explosive gases, as well as dusty and moist gases, leading to its growing range of applications.

A proper amount of water is filled in the pump casing as the working fluid. When the impeller rotates clockwise as shown in the diagram, the water is thrown to the periphery by centrifugal force, forming a closed circular ring of approximately uniform thickness that conforms to the shape of the pump cavity. The inner surface of the lower part of the water ring is exactly tangent to the impeller hub, while the inner surface of the upper part of the water ring is in contact with the tips of the impeller blades (in practice, the blades have a certain insertion depth into the water ring). At this point, a crescent-shaped space is formed between the impeller hub and the water ring, and this space is divided into several small chambers equal in number to the impeller blades.

Taking the lower 0° position of the impeller as the starting point:

As the impeller rotates through the first 180°, the volume of each small chamber increases from small to large and connects to the suction port on the end face, drawing gas into the chamber. At the end of the suction stroke, the small chamber is isolated from the suction port.

As the impeller continues to rotate, the volume of each small chamber decreases from large to small, compressing the gas inside.

When the small chamber connects to the exhaust port, the compressed gas is discharged out of the pump.

Working Principles of Rotary Vane Vacuum Pumps and Roots Pumps

To sum up, the water ring pump achieves suction, compression and exhaust by virtue of changes in the volume of the pump chamber, so it is classified as a positive-displacement vacuum pump.

Working Principle of Roots Pumps

Inside the pump chamber of a Roots pump, two figure-8-shaped rotors are mounted perpendicularly on a pair of parallel shafts, and driven by a pair of gears with a transmission ratio of 1 to rotate synchronously in opposite directions. A certain clearance is maintained between the rotors, as well as between the rotors and the inner wall of the pump casing, enabling high-speed operation.

As a vacuum pump without internal compression, the Roots pump usually has a very low compression ratio, so it requires a backing pump when used as a high or medium vacuum pump. The ultimate vacuum of a Roots pump depends not only on its own structure and manufacturing precision, but also on the ultimate vacuum of the backing pump. To improve the ultimate vacuum of the pump, multiple Roots pumps can be used in series.

The working principle of a Roots pump is similar to that of a Roots blower. As the rotors rotate continuously, the gas to be pumped is sucked from the inlet into the space v₀ between the rotors and the pump casing, and then discharged through the outlet. Since the space v₀ is completely sealed after suction, there is no compression or expansion of the gas inside the pump chamber.

However, when the top of the rotor passes the edge of the exhaust port and the space v₀ is connected to the exhaust side, part of the gas will rush back into the space v₀ due to the higher pressure of the gas on the exhaust side, resulting in a sudden increase in gas pressure. As the rotor continues to rotate, the gas is discharged out of the pump.

Working Principle of Rotary Vane Vacuum Pumps

A rotary vane vacuum pump (abbreviated as rotary vane pump) is an oil-sealed mechanical vacuum pump. Its operating pressure range is 101325 ~ 1.33×10⁻² Pa, falling into the category of low-vacuum pumps. It can be used independently, or as a backing pump for other high-vacuum or ultra-high-vacuum pumps. It has been widely applied in production and scientific research fields such as metallurgy, machinery, military industry, electronics, chemical engineering, light industry, petroleum and pharmaceuticals.

A rotary vane pump can extract dry gas from sealed containers; if equipped with a gas ballast device, it can also extract a certain amount of condensable gas. However, it is not suitable for extracting gases with excessively high oxygen content, gases that are corrosive to metals, gases that can chemically react with pump oil, or gases containing particulate dust.

The rotary vane pump is one of the most basic vacuum acquisition devices in vacuum technology. Most rotary vane pumps are small and medium-sized pumps, which are divided into single-stage and two-stage types. The so-called two-stage pump is structurally formed by connecting two single-stage pumps in series. Most rotary vane pumps are manufactured as two-stage pumps to achieve a higher degree of vacuum.

Relationship Between Pumping Speed and Inlet Pressure of Rotary Vane Pumps & Its Structural Operating Principle

The relationship between the pumping speed and inlet pressure of a rotary vane pump is specified as follows:At inlet pressures of 1333 Pa, 1.33 Pa, and 1.33×10⁻¹ Pa, the pumping speed shall not be lower than 95%, 50%, and 20% of the pump’s nominal pumping speed, respectively.

A rotary vane pump is mainly composed of a pump body, rotor, vanes, end covers, springs, and other components. A rotor is eccentrically installed in the pump chamber, with the outer circle of the rotor tangent to the inner surface of the pump chamber (with a very small clearance between them). Two vanes fitted with springs are placed in the slots of the rotor. During rotation, the tips of the vanes are kept in contact with the inner wall of the pump chamber by centrifugal force and spring tension, and the rotating rotor drives the vanes to slide along the inner wall of the pump chamber.

The two vanes divide the crescent-shaped space enclosed by the rotor, pump chamber, and two end covers into three parts: A, B, and C. When the rotor rotates in the direction indicated by the arrow:

The volume of space A, which is connected to the suction port, gradually increases, and the pump is in the suction phase. As the volume of space A expands, the gas pressure inside decreases. The external gas pressure at the pump inlet is higher than the pressure in space A, so gas is drawn into the pump.

The volume of space C, which is connected to the exhaust port, gradually decreases, and the pump is in the exhaust phase.

The volume of the middle space B also gradually reduces, and the pump is in the compression phase. When space A is isolated from the suction port and moves to the position of space B, the gas inside starts to be compressed, with its volume shrinking continuously until space B is connected to the exhaust port.

When the pressure of 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 oil tank and is discharged into the atmosphere. Through the continuous operation of the pump, continuous gas extraction is achieved.

If the discharged gas is transferred to another stage (low-vacuum stage) through an air passage, pumped away by the low-vacuum stage, and then compressed by the low-vacuum stage before being discharged into the atmosphere, a two-stage pump is formed. In this case, the total compression ratio is shared by the two stages, thereby improving the ultimate vacuum level of the pump.