Classification of vacuum pumps

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In recent years, new types of vacuum pumps have been developed that can achieve pressures ranging from atmospheric to high vacuum using just one pump. Generally speaking, in order to achieve high vacuum, ultra-high vacuum, and extremely high vacuum, multiple vacuum pumps are connected in series to form a pump system (such as rotary vane vacuum pumps) to carry out the pumping task. Therefore, it is very important for users to have a proper understanding of the working principle, main performance, structural features, and classification of these pumps in order to select an economical and suitable vacuum pump. A vacuum pump is a device used to create, improve, and maintain a vacuum. Based on their working principles, they are essentially divided into two types: gas transfer pumps and gas capture pumps. I. Gas transfer pumps: Gas transfer pumps are vacuum pumps that enable gases to be continuously drawn in and discharged outside the pump in order to achieve pumping purposes. These pumps are divided into two main categories: positive displacement pumps and momentum transfer pumps. 1. Positive displacement vacuum pump: A positive displacement vacuum pump is a device that utilizes the periodic variation of the volume within its pumping chamber to carry out suction, compression, and exhaust processes. The gas is compressed before being discharged; these pumps are divided into reciprocating and rotary types. (1) Reciprocating vacuum pump, which uses the reciprocating motion of a piston within the pump chamber to draw in gas, compress it, and discharge it. Therefore, it is also called a piston vacuum pump. (2) Rotary vacuum pumps, which use the rotational motion of internal components in the pump chamber to draw in, compress, and discharge gas; there are many types of pumps belonging to this category. a) Oil-sealed mechanical pump: it is a rotary positive displacement vacuum pump that uses oil to seal the gaps between various moving parts, thereby reducing unwanted spaces. Such pumps usually come equipped with an air lock, which is why they are called air-lock vacuum pumps. Based on their structural characteristics, they can be classified into rotary vane pumps, fixed-vane pumps, slide valve pumps, cycloidal pumps, and multi-chamber rotary vane pumps, among others. b) Dry vacuum pumps: The term \"dry vacuum\" generally refers to vacuum pumps that can operate within a pressure range from atmospheric pressure up to 100 ZPa. In these pumps, no oils or sealing liquids may be used in the pumping flow channels (such as the pump chamber); the exhaust port is connected to the atmosphere, allowing continuous discharge of gas into the atmosphere. Such pumps are known as dry vacuum pumps. Based on their working principle, there are also dry positive-displacement vacuum pumps, such as multi-stage Roots pumps, multi-stage piston pumps, claw pumps, screw pumps, vortex pumps, etc ； In addition, there are also advanced dry pumps such as turbine dry pumps and centrifugal dry pumps. The pumping process of these pumps is free from oil contamination, and they are among the types of pumps that have been developed quite frequently in recent times. c) Liquid ring vacuum pump: A rotor with multiple blades is mounted eccentrically within the pump casing. As the rotor rotates, it throws liquid (water or oil) towards the pump casing, thereby forming a liquid ring that is concentric with the casing. This liquid ring, together with the blades on the rotor, creates several small volumes whose sizes change periodically, which enables suction, compression, and exhaust of gas. Since the liquid ring plays a role in compressing the gas, it is also referred to as a liquid piston vacuum pump. d) Rotary vane vacuum pumps, which have two vaned or multi-ved rotor blades that rotate synchronously in opposite directions inside the pump; there is a certain gap between the rotors as well as between the rotors and the inner wall of the pump casing. It belongs to the category of vacuum pumps without internal compression. Based on their application, they are further divided into types such as wet rotary vane pumps, rotary vane pumps that discharge directly to the atmosphere, and mechanical booster pumps. 2. Momentum transfer vacuum pumps: Momentum transfer vacuum pumps use rapidly rotating blades or high-speed jets to transfer momentum to the gas being evacuated or to the gas molecules, thereby enabling the gas to be continuously transferred from the inlet to the outlet of the vacuum pump. This type of pump can be divided into the following categories. (l) Molecular vacuum pump. It is a type of vacuum pump that uses a rapidly rotating rotor to transfer momentum to gas molecules, thereby compressing and exhausting the gas. It comes in the following 3 forms: a) A molecular pump driven by traction, where the surface of the rapidly rotating rotor comes into contact with gas molecules, transferring momentum to them and thus dragging the gas molecules to the pump’s outlet for expulsion; therefore, it is a type of momentum-transfer pump. b. Turbomolecular pump, which features a rotor with multiple stages of slotted disks or blades that rotate between stator disks (or stator plates); the circumferential speed of the rotor is very high, and it typically operates in a molecular flow regime. c. Composite molecular pump: it is a type of vacuum pump that consists of a turbomolecular pump and a diffusion molecular pump, combined in an optimized manner and connected in series to operate; it can function in either molecular flow or transitional flow conditions. (2) Jet vacuum pump: It is a momentum transfer pump that uses the high-speed jet generated by the pressure drop due to the Venturi effect to convey gas to the pump outlet. It is suitable for vacuum pumps operating in viscous flow and transitional flow conditions. This type of pump can be further divided into 3 types: a) water jet pumps, which are jet vacuum pumps that use water as the working medium. b. Gas jet pump, a jet pump that uses a non-condensable gas (such as air) as the working medium. c. Steam jet pumps: Jet pumps that use steam (such as steam from water, oil, or mercury) as the working medium. Water vapor jet pumps are the most commonly used; oil vapor jet pumps are also known as oil booster pumps or oil diffusion jet pumps. (3) Diffusion pump, using oil or mercury vapor as the working medium. Mercury diffusion pumps do not have a fractionation structure, while oil diffusion pumps usually employ a fractionation structure to improve the pump’s performance. II. Gas capture pumps A gas capture pump is a vacuum pump that adsorbs or condenses the gas to be evacuated on the inner surface of the pump. It comes in two forms: (1) Adsorption pump. It is a capture-type vacuum pump that relies primarily on the physical adsorption action of adsorbents with a large surface area to evacuate air. Such as adsorption traps, getter pumps. There are also pick-up vacuum pumps that continuously form a fresh getter film, such as the shot-ion pump and the thermal evaporation sublimation pump. (2) Cryopump. A vacuum pump that uses a low-temperature surface to condense and capture gases. Such as condensation pumps and low-temperature pumps for small refrigerators, etc