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It is important to understand the function of vacuum pumps when designing or operating a vacuum system. The most common types of vacuum pumps, their principles of operation, and their applications will be discussed.
A vacuum pump is classified by its operating pressure range and can be classified as a primary pump, booster pump, or secondary pump. Each pressure range is characterized by several different pump types, each employing a different technology and offering some unique advantages in terms of pressure and flow capacities, costs, and maintenance requirements.
They all operate in the same way, regardless of their design. Vacuum pumps remove molecules of air and other gases from their chambers (or from their outlets when connected in series).
Reducing the pressure in the chamber makes removing additional molecules exponentially more difficult. A vacuum system used in industry must therefore be capable of operating over a wide range of pressures, typically 1 to 10-6 Torr.
This is extended to 10-9 Torr or lower in research and scientific applications. A typical system uses several types of pumps, each covering a portion of the pressure range, sometimes operating in series.
Vacuum systems are placed into the following broad-based grouping of pressure ranges:
The different types of pumps for these vacuum ranges can then be divided into the following:
A vacuum pump uses both gas transfer and gas capture technologies.
Gas molecules are transferred via momentum exchange (kinetic action) or positive displacement in transfer pumps.
When the gas leaves the pump, it is slightly above atmospheric pressure and contains the same number of molecules as it enters. It is the ratio between the exhaust pressure (outlet) and the lowest pressure obtained (inlet) that is called the compression ratio.
A kinetic transfer pump works by transferring momentum, directing gas toward the pump outlet in order to increase the likelihood of a molecule moving toward it. This can be achieved by introducing vapor or high-speed blades. A kinetic pump does not have a sealed volume but can achieve high compression ratios at low pressures.
Pumps that work with positive displacement trap a volume of gas and move it through them mechanically. A common drive shaft is often used to connect multiple stages of the drivetrain. Having been compressed to a smaller volume at a higher pressure, the compressed gas is released into the atmosphere (or to the next pump).
A series of two transfer pumps is often used to achieve a higher flow rate and vacuum. Using a packaged system, a turbomolecular (kinetic) pump can be used in series with a scroll (positive displacement) pump.
A capture pump captures gas molecules by capturing them on surfaces within the vacuum system. While capture pumps operate at lower flow rates than transfer pumps, they are capable of providing ultra-high vacuums, down to 10-12 Torr, and produce an oil-free vacuum. There are no moving parts in capture pumps, which operate using cryogenic condensation, ionic reactions, or chemical reactions.
It is necessary to distinguish between wet and dry pumps according to whether oil or water is exposed to the gas during pumping. The swept (pumped) gas can be contaminated by oil or water used in wet pump designs for lubrication and/or sealing.
The pumping mechanism is separated from the gas by a diaphragm or dry polymer seals (PTFE). Dry pumps do not use fluid in the swept volume. While dry pumps may contain oil and grease in the gears and bearings, gas is not exposed to them.
Wet pumps are more prone to system contamination and oil disposal than dry pumps. By simply changing the pump from a wet to a dry style, vacuum systems cannot be converted from wet to dry. The wet pump can contaminate the chamber and piping, so they must be thoroughly cleaned or replaced, or else the gas will be contaminated.
Oil Sealed Rotary Vane Pump (Wet, Positive Displacement)
Rotating vane pumps trap gas entering the inlet port using an eccentrically mounted rotor that compresses it and transfers it to the exhaust valve. Spring-loaded valves allow the gas to discharge when atmospheric pressure is exceeded.
The vanes are sealed and cooled with oil. It is a function of the number of stages used and their tolerances that determines the pressure achievable with a rotary pump. In a two-stage design, pressure can reach 1×10-3 mbar. Pumping speeds range from 0.4 to 162 feet per minute (0.7 to 275 m3/h).
An eccentrically mounted vaned impeller in the pump housing compresses the gas in the liquid ring pump. Centrifugal acceleration causes liquid to form a moving cylindrical ring inside the pump casing as it is fed into the pump.
A series of seals are formed between the impeller vanes by this liquid ring. Due to the eccentricity of the impeller’s axis of rotation, the volume enclosed by the vanes and ring varies cyclically, which compresses the gas and discharges it through the housing’s port.
Only the shaft and impeller move in this pump, which has a simple, robust design. This machine is very tolerant of process upsets and has a wide range of capacities. The pressure can be 30 mbar with 15°C (59° F) water, and lower pressures can be achieved with other liquids. The pump can pump from 25 to 30,000 m3/h (15 to 17,700 ft3/min).
Diaphram Pump (Dry, Positive Displacement)
During gas transfer, a diaphragm is rapidly flexed by a rod riding on a cam rotational by a motor. As well as being compact, it is low maintenance. Diaphragms and valves typically have a lifespan of over 10,000 hours.
Diaphragm pumps are used for backing small compound turbo-molecular pumps under high vacuum conditions. Research and development labs widely use this pump for sample preparation. Using a diaphragm pump to back a compound turbo-molecular pump can typically achieve a pressure of 5 x 10-8 mbar. The pump can pump 0.6 to 10 m3/h (0.35 to 5.9 ft3/min).
There are two scrolls in the scroll pump. The outer one orbits and traps a volume of gas; it compresses the gas in a spiral until it reaches maximum pressure and volume at the spiral’s center.
As a result, the gas stream is sealed between the two scrolls by a spiral polymer (PTFE) tip seal. It is possible to achieve a typical ultimate pressure of 1 x 10-2 mbar. With a pumping speed range of 5.0 to 46m3/h (3.0 to 27ft3/min), it can pump water quickly.
Roots Pump (Dry, Positive Displacement)
Roots pumps are designed to remove large volumes of gas as vacuum boosters. The pump’s two lobes mesh without touching and rotate counter-clockwise to continuously transfer gas in one direction.
In addition to increasing the speed of a primary/backing pump by about seven times, it also improves the pressure by about ten times. There can be two or more lobes on roots pumps. The ultimate pressure of primary pumps can typically reach 10-3 Torr. Pumping speeds of 100,000 m3/h (58,860 ft3/min) are possible.
The claw pump has two counter-rotating claws that transfer gas axially rather than top-to-bottom like the Roots pump. A Roots pump is commonly used in conjunction with it, which is a combination of Roots and claw stages on a common shaft.
It provides a high flow rate and is designed to withstand harsh industrial environments. It is possible to achieve an ultimate pressure of 1 x 10-3 mbar. Pumping speeds range from 100 to 800 m3/h (59 to 472 ft3/min).
There are two rotating screws in the screw pump, one left-handed and one right-handed. The screws do not touch each other during rotation. During rotation, gas moves from one end to the other. As the gas passes through the screws, the space between them shrinks, and it becomes compressed, causing a reduction in pressure at the entrance.
There are several benefits to this pump, including high throughput capacity, efficient liquid handling, and the ability to tolerate dust and harsh environments. The ultimate pressure can typically be reached at 1 x 10-2 Torr. Pumping speeds range from 450 feet/minute to 750 meters per hour.
High-speed rotating, angled blades in turbomolecular pumps transfer kinetic energy to gas molecules and propel them at high speeds: the blade tip speed is usually 250 – 300 m/s (670 miles/hr). Molecular movement is increased by transferring momentum from the rotating blades to the gas.
Neither of these systems provides high pressures or rapid transfer rates. An ultimate pressure of less than 7.5 x 10-11 Torr is typical. The pumping speed ranges between 50 and 5000 liters per second. Turbomolecular pumps exhaust to higher pressures when they are combined with bladed pumping stages (above 1 Torr).
The vapor diffusion pump applies kinetic energy to gas molecules by dragging the gas from the inlet to the outlet with a high-velocity heated oil stream. In most cases, dry turbomolecular pumps have been replaced these pumps with newer technologies.
With no moving parts, they provide high reliability at a low price. It is possible to achieve an atmospheric pressure of less than 7.5 x 10-11 Torr. Pumping speeds range from 10 – 50,000 l/s.
By capturing and storing gases and vapors rather than transferring them through the pump, the cryopump operates. At 10°K to 20°K (minus 260°C), they freeze or trap the gas at a very cold surface (cryocondensation or cryosorption). The gas storage capacity of these pumps is limited, but they are very effective.
Pumps need to be periodically regenerated by heating the surface and pumping away collected gases/vapors. The surfaces of cryopumps need to be cooled by a refrigeration compressor. There is a range of pumping speeds between 1200 l/s and 4200 l/s for these pumps, which can achieve a pressure of 7.5 x 10-10 Torr.
Sputter ion pumps capture gases by combining chemically active materials with gas molecules and ionizing them (making the molecules electrically conductive and capturing).
When a high magnetic field is combined with a high voltage (4-7kV), an electron-positive ion cloud (plasma) is deposited on a titanium cathode and sometimes a second tantalum cathode as well. As a result of the cathode capturing gases, a thick film is formed. Sputtering is the name given to this phenomenon.
Cathodes need to be replaced periodically. Pumps such as these have no moving parts, require little maintenance, and can reach pressures as low as 7.5 x 10-12 Torr. The pumping capacity is 1000 liters per second.
In summary…
There has been a brief description of various types of vacuum pumps, but for a better understanding of their advantages and limitations, a more detailed examination is necessary.
A vacuum pump is one of, if not the most important component on a vacuum furnace. We achieve quality and run processes based on how these systems work.
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