The development overview
of molecular pumps
Momentum transfer, air exhausted to vacuum
The molecular pump is a type of mechanical vacuum pump that utilizes a high-speed rotating rotor to carry gas molecules and transfer momentum to the gas molecules, causing them to flow in a directed manner, thereby achieving high vacuum and ultra-high vacuum. Its ultimate vacuum level can reach 10-8 Pa. It has the advantages of 'low power consumption (under the same pumping speed conditions, the power is only a few tenths of that of a diffusion pump), low vibration, low noise, simple operation, convenient use, easy maintenance, quick startup, resistance to various radiation exposure, no gas storage and desorption effect, and cleanliness'. It is a future alternative product to the diffusion pump. It is widely used in high-energy accelerators, heavy particle accelerators, controllable thermonuclear reaction devices, plasma physics space research, atomic beam and molecular beam systems, surface physics and analysis instruments, advanced electronic component manufacturing, semiconductor and optical component manufacturing, vacuum coating, vacuum smelting, and other fields involving vacuum technology in electronics, metallurgy, chemical industry, and scientific research.
The main development stage of the molecular pump
Since German scientist W. Gaede invented the first molecular pump in 1913, the development of molecular pumps has lasted for more than a century. With the advancement of technology and technological innovations, the development of molecular pumps has made tremendous progress. Especially in the past 15 years, with the rapid development of computer-aided design technology, bearing technology, high-speed rotation technology, and numerical control processing technology, the development of molecular pumps has achieved unprecedented progress. Various types of molecular pumps with a pumping speed ranging from 50L/s to 60,000L/s have been successfully developed and put on the market. Magnetic levitation turbine molecular pumps and pre-stage high-pressure, wide-range composite molecular pumps have also reached the practical stage.
The development of molecular pumps has mainly gone through four stages: 'initial molecular pumps, turbine molecular pumps, molecular pumps with magnetic levitation bearings and gas static pressure bearings, and composite molecular pumps'.
Initial molecular pump
As early as 1905, German scientist W. Gaede began to research a new type of mechanical vacuum pump that was independent of the displacement principle - the molecular traction pump (referred to as molecular pump). It was based on the following assumption: Gas molecules constantly collide with the rotating solid surface at high speed. During the collision process, energy is transferred between the rotating solid and the gas molecules, causing the gas molecules to acquire momentum and move in a certain direction and be extracted. This molecular pump was successfully developed in 1913 (at the same time, based on the same assumption, W. Gaede invented the diffusion pump. In the diffusion pump, gas molecules are carried out by the high-speed sprayed steam molecules to achieve gas extraction). It was used for exhaust in vacuum tubes in the United States, but due to immature technology and frequent failures, it was replaced by the mercury diffusion pump developed by W. Gaede himself in 1915.
In 1923, F. Holweck developed a cylindrical molecular pump - the improved W. Gaede molecular pump. This pump has spiral grooves on the rotor or stator. The high-speed rotating rotor forces the gas molecules to flow along the spiral grooves to achieve gas extraction. Its pumping speed can reach approximately 4.5 L/s to 8 L/s. It has a very high compression ratio for gases and is sturdy and durable. It was used for the exhaust of naval communication triodes and 80 units were produced. In 1940, this pump was produced by a Swiss manufacturer and was used in vacuum analysis instruments, electron microscopes, and cathode ray tubes, etc.
In 1926, M. Siegbahn developed a higher-speed disc-type molecular pump in the university physics laboratory of Sweden (this pump was the predecessor of the mainstream disc-type composite molecular pumps on the market today). This pump had spiral grooves on the stator disc surface, and the rotor was a smooth disc, which was developed for use in cylindrical spectrometers. The pumping speed could reach 150 L/s. Between 1926 and 1940, the university's factory manufactured 50 units, and in 1931, it was licensed to the German Leybold company for production. In 1939, two large disc-type molecular pumps for cyclotron accelerators were manufactured.
Turbomolecular pump
Whether it is the F. Holweck pump or the M. Siegbahn pump, although they have large discharge capacity and high compression ratio for gas molecules, due to the small working gap, the centrifugal force, thermal expansion, and even the entry of tiny particle impurities into the pump can all cause the rotor to get stuck or the structure to be damaged. Therefore, their industrial applications have not been realized.
In 1945, W. Becker from the German company PFEIFFER made a new attempt to develop a mature industrial molecular pump based on the F. Holweck pump. Unfortunately, like his predecessors, it ended in failure. However, W. Becker did not give up researching other forms of molecular pumps. After more than ten years, in 1958, W. Becker designed the first commercial axial-flow turbine molecular pump in history. Thus, the turbine molecular pump came into existence. This pump adopts a horizontal structure and achieves gas extraction through the interlocking matching of the moving and stationary blades (this pump is the predecessor of the current turbine molecular pumps).
In 1966, L. Rubet from the French company SENCMA developed a vertical turbine molecular pump based on the design of W. Becker. This pump was licensed to the German company Leybold for production in 1971.
Subsequently, many countries also began to conduct research and development trials on turbine molecular pumps: In 1964, the Shanghai Vacuum Pump Factory in China successfully developed the FW-140 type horizontal turbine molecular pump; in 1977, Nan Guang Machinery (a state-owned 708 Factory) and Zhong Ke Keyi successfully developed the F-150/450 type vertical turbine molecular pump, initiating the practical application process of domestic molecular pumps, with a suction rate of 450 L/s, filling the domestic gap. The Osaka Vacuum Company in Japan successfully developed a vertical turbine molecular pump in 1971, and Russia also successfully developed the largest suction rate vertical turbine molecular pump in history - 60,000 L/s in the early 1980s.
In the early 1980s, small wide-range turbine molecular pumps appeared on the market, including standard type and chemical type. The standard type pump can operate within a pressure range of 10-7 to 200 Pa, and its flow rate can be comparable to that of mechanical booster pumps in the low vacuum range. The bearings are protected by gas cleaning measures, and there is also a structure to prevent dust and glass fragments from entering. The chemical type pump is suitable for continuous and stable exhaust of corrosive gases. The rotor is coated with a corrosion-resistant coating, and when exhausting corrosive gases, the bearings take gas cleaning measures.
Magnetic levitation bearings and gas static pressure bearings molecular pumps
In 1976, the German company LEYBOLD was the first to develop a completely contactless magnetic suspension bearing type turbine molecular pump. Its structure is that the central shaft is fixed, and the rotor with blades rotates on the outside of the shaft, thus it is called an external ring type rotation. The diameter of the emergency bearing that the pump contacts when stopped is relatively large, which is prone to causing accidents.
In 1983, the Shimadzu Company of Japan, based on the technology of Leybold Company, developed an internal-ring rotating magnetic levitation molecular pump, namely a turbine molecular pump with magnetic levitation bearings rotating on the shaft.
The turbine molecular pumps used in thermal nuclear reaction devices often employ gas static pressure bearings. This is because if oil lubricated bearings were used, tritium would cause the oil to deteriorate. If magnetic suspension bearings were used, it would be extremely difficult to maintain the stable operation of the turbine molecular pump in an environment with a strong magnetic field. When the turbine molecular pump operates in a strong magnetic field, the rotor of the pump will heat up due to eddy currents and cause the rotational speed to decrease. Therefore, a gas static pressure bearing system and a gas turbine drive system that are not affected by the magnetic field were adopted. This type of gas static pressure bearing turbine molecular pump was successfully developed in Russia in the mid-1980s.
Since 2013, domestic companies such as Zhongke Keji and Feixuan Technology have successively launched magnetic levitation bearing molecular pumps, breaking the international monopoly.
Composite molecular pump
With the rapid development of the semiconductor industry and the film industry, it is often required that molecular pumps can continuously and in large quantities discharge gas and achieve clean high vacuum. Therefore, turbo molecular pumps need to operate at pressures greater than 1 Pa. However, the pumping speed of turbo molecular pumps drops sharply within this range, far from meeting industrial demands. Thus, in order to enable turbo molecular pumps to maintain excellent pumping performance under high pressure, the R&D engineers reasonably configured F. Holweck pumps (cylindrical molecular pumps) or M. Siegbahn pumps (disk molecular pumps) at the outlet side of the original turbo molecular pumps. They combined the pumping units of the two pumps into a single unit, integrating the advantages of both, and formed the current wide-range composite molecular pump in the market. This pump has a large pumping speed and a high compression ratio within a wide pressure range, significantly improving the outlet pressure of the pump.
In 1974, the French company Alcatel successfully developed a cylindrical composite molecular pump. The radial gap was very small, and it was driven by air hydrostatic bearings and pneumatic turbines.
In 1983, the Osaka Vacuum Company in Japan adopted a new design theory, increased the radial clearance, and successfully developed a relatively reliable cylindrical composite molecular pump.
Nowadays, renowned molecular pump development companies such as PFEIFFER from Germany, Lebold from Germany, Edwards from the UK, and Shimadzu from Japan all adopt composite molecular pumps with cylindrical structures, and most of them use magnetic suspension bearings for support.
In the late 1980s, the Beijing Zhongke Keyi Institute of China began to develop disc-type composite molecular pumps. Now, the technology has become relatively mature and has reached the stage of mass production, with a large number of products being released to the domestic and international markets. Subsequently, vacuum companies such as Nan Guang Machinery, Beijing Panai, Beijing Taiyueheng, and Chengdu Wudevac also successively developed ordinary disc-shaped composite molecular pumps and released them to the market in large quantities.
In the early 1990s, the American company VARIAN began to develop a disc-type driven composite molecular pump. However, its design concept was different from that of other companies' disc-type composite molecular pumps. Due to the adoption of a novel disc-type traction structure, the pumping performance of its composite molecular pump products was superior to that of other disc-type composite molecular pumps.
The future development direction of domestic molecular pumps
With the continuous improvement of common turbomolecular pumps and composite molecular pumps, their application fields have become increasingly extensive. In some pumping systems, they have even completely replaced diffusion pumps, shortening the pumping time of the system and enabling the attainment of clean high vacuum and ultra-high vacuum environments without oil contamination.
For foreign countries, the technologies of magnetic suspension bearing-supported turbine molecular pumps and composite molecular pumps have become mature, but there are still few ultra-high flow rate molecular pumps; however, for the domestic market, there is a certain gap compared to foreign countries in terms of technology. The performance of domestic composite molecular pumps is not satisfactory, especially the safety and stability of molecular pumps installed at any angle are inferior to those of foreign products. Moreover, the product series is relatively limited and cannot fully meet the regular application requirements of industry. In the coming long period, the development of domestic molecular pumps will mainly focus on series-based composite molecular pumps and will develop in the following aspects:
(1) High-flow, high-pressure-resistant front-stage composite molecular pumps;
(2) High-stability magnetic suspension composite molecular pumps;
(3) Ultra-high and super-high flow rate composite molecular pumps;
(4) Highly integrated 'mechanical-electrical integration' intelligent control, green and energy-saving composite molecular pumps;
(5) Low-temperature composite molecular pumps for rapid exhaust of water vapor;
(6) Directly discharged to the atmosphere dry-type composite molecular pumps.
Closing Remarks
The molecular pump, as a momentum transfer vacuum pump, achieves the pumping function by imparting momentum to gas molecules, causing them to move in a directed manner. It has the advantages of higher energy efficiency, greater cleanliness, and higher efficiency. In the fields of high vacuum and ultra-high vacuum applications, it has become the recommended equipment for modern scientific research and high-end manufacturing.
Currently, the market for magnetic levitation molecular pumps is experiencing explosive growth. The global market size has reached 250 million US dollars in 2024, and is expected to exceed 500 million US dollars by 2033, with a compound annual growth rate of 8.5%. This growth is driven by the urgent demand for ultra-high vacuum environments in fields such as semiconductors, new energy, aerospace, and others.
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