Depending upon the pressure range, vacuum pumps can be separated into two general categories: rough-medium vacuum, ranging from 760 to 10 -3 torr (1 to 1.3 x 10 -6 atm), and high-ultrahigh vacuum, ranging from 10 -3 to below 10 -7 torr (1.3 x 10 -6 to below 1.3 x 10 -10 atm).
Liquid ring vacuum pumps are the most common vacuum pump design. They are non-pulsating, rotary vacuum pumps that compress gas using a liquid compression.
In a liquid ring pump, a ring of liquid, shown in blue in the animation, circulates within the impeller. Gas, shown in yellow, is admitted through a suction port. Centrifugal force separates the liquid and the pumped gas. The gas travels between the vanes, where compression takes place, and the compressed gas is released through a discharge port.
Liquid ring vacuum pumps do not require any internal lubrication because the rotors do not contact the housing. When using a coolant system, liquid ring pumps prevent the coolant from contacting and contaminating the process fluid.
As shown in the picture below, the body of the liquid pump contains a sealant fluid that, under centrifugal force, forms a ring against the inside of the casing. The source of that force is a multi-bladed, off-center impeller, contained within the pump chamber. The rotating blades propel the liquid to form a ring at the perimeter of the chamber. Gas enters through the suction port and is trapped by the liquid ring in the pockets between the blades. The gas circulates through a series of pockets of decreasing volume. The compressed gas exits through the discharge port. The primary function of the liquid ring is to act as a seal. In addition, it absorbs the heat of compression, friction, and condensation. This design allows liquid ring pumps to provide a continuous vacuum without pressure pulsations.
Liquid ring vacuum pumps can be designed with either one or two stages. Pictured to the left is a two-stage liquid ring pump. A cross-section of this kind of pump is shown on the right.
Liquid ring vacuum pumps are used in the refining industry for applications such as crude oil vacuum distillation, lube oil dryers, or asphalt production. They are also used in the power industry to evacuate steam surface condensers.
Pictured below is a system that uses a liquid ring vacuum pump in the production of biofuels. In the production process, vacuum pumps are used for methanol stripping and dehydration of methyl ester.
Other industries, such as the food, chemical, pharmaceutical, pulp, and paper industries use liquid ring vacuum pumps. The liquid ring vacuum pump pictured below is used on a vacuum dryer in a pharmaceutical company.
- Provides continuous vacuum without pressure pulsations.
- Lack of metal-metal contact reduces pump wear and eliminates the need for lubrication.
- Can handle small particles (as long as not abrasive).
- Only one rotating part.
- Can be fabricated from any castable metal.
- Minimal noise and vibration.
- Simple maintenance and rebuilding procedures.
- Slow rotational speed maximizes operating life.
- No lubricating liquid in the vacuum chamber, preventing contamination.
- Nearly isothermal.
- Can handle both condensable and non-condensable gases.
- May be damaged by liquid slugs.
- The liquid inside the pump and the process gas must be compatible to avoid contamination.
- The ultimate suction pressure of the pump is limited by the vapor pressure of the liquid inside the pump.
- Achievable vacuum is limited by the vapor pressure of sealant fluid at the operating temperature.
- High power needed to form and maintain the liquid ring requires large motors.
Also known as roots blowers and rotary-lobe blowers, roots vacuum pumps are positive displacement compressors.
Roots vacuum pumps contain two interlocking rotors that rotate in opposite directions within the pump housing to trap and compress gases, as shown in this animation.
The rotors run dry within the pump housing. The gears and rotor bearings are oil-lubricated, but they are external to the pump, as shown in the schematic below to the left. In a roots vacuum pump, two rotor lobes rotate in opposing directions to transfer large volumes of gas from the inlet to the outlet. The inlet port is isolated from the outlet by a narrow gap, which can cause a back-flow of gas.
Most roots vacuum pumps are provided with a backing pump. Common backing pumps are oil-sealed rotary piston pumps, rotary vane pumps, or liquid ring pumps. The roots vacuum pump below is backed with a rotary vane vacuum pump.
Roots vacuum pumps such as the ones shown here are widely used in the petrochemical and chemical industries. Other applications of roots vacuum pumps include food processing, plastic, semiconductor, vacuum furnace, and plywood manufacturing industries.
- Simple design
- High pumping speeds
- Low service requirements
- Dry interior minimizes back diffusion of contaminants into the process
- High heat emission
- High noise level
- Vulnerable to liquid slugs and overheating
- Choice of corrosion-resistant materials limited
- Many potential failure sites
- Nature of pumped material may adversely affect the fluid in the backing pump
- Possibility of backflow
Also known as plunger pumps, rotary piston vacuum pumps are positive-displacement compressors.
In a rotary piston pump, the pumping action is created by the moving piston. As gas enters the pump the cam rotates inside the piston. The piston moves inside the stator, pumping the gas.
The entering gas forces the eccentric cam inside the piston to rotate. The piston moves along the stator wall, pumping the gas. The piston must complete two revolutions before the operating cycle is complete: the gas fills the chamber in the first revolution and is discharged in the second.
The inlet and discharge are always separated by the “contact” point between the piston and the stator. The small clearance at this contact point is lubricated with sealing oil.
Rotary pistons use oil to seal internal components, transfer heat away from surfaces, flush moisture, and inhibit corrosion of internal components. Therefore the oil needs to be free of particulates and condensable vapors to avoid internal damage to the pump. Before being added to the pump the oil is filtered to remove these particulates and process vapors are removed by installing condensers or cold traps, such as the mechanically-refrigerated cold trap pictured below, between the process and vacuum pump. Gas ballasts are also commonly used to prevent condensation within the vacuum pump.
Rotary piston vacuum pumps are used in the semiconductor industry, in vacuum impregnations, drying, extrusion, degassing and small-batch operations.
The chemical vapor deposition reactor shown here is used in the semiconductor industry and includes rotary piston vacuum pumps.
- High pumping speed
- Relatively low price
- High heat emission
- High service requirements
- Oil and process vapors must be mutually compatible
- Oil limits process applications
- Condensation of process vapors can cause sealing oil contamination and reduce capacity
Also known as a sliding vane pump, the rotary vane vacuum pump is a positive displacement compressor.
In the rotary vane vacuum pump, an eccentrically placed slotted rotor turns in a cylindrical stator. As the rotor turns, sliding vanes are forced out against the casing wall by centrifugal force. Air or gas is drawn in, compressed, and expelled through a spring-loaded exhaust valve.
As the rotor turns in the cylindrical stator, the sliding vanes, shown in green, are forced out against the stator walls. Some rotary vane vacuum pumps have a spring between the vanes, such as the one shown in the animation below. Spring loading the vanes allows them to remain in contact with the stator. Gas is drawn in and circulates through the pump. The compressed gas is expelled through the exhaust valve.
Rotary vane pumps can be operated as one- or two-stage units. When a two-stage unit is used, as in the schematic below, the exhaust from the first stage is internally connected to the inlet of the second stage. Adding a second stage improves the ultimate vacuum achieved by the pump.
The vanes and rotor are sealed with a fluid film of oil. This limits slippage and lubricates moving parts. Some applications require such a small quantity of oil for sealing that the oil is used once and not recirculated. In other models, the oil can be separated and collected in a reservoir for reuse, as in the schematic below on the left.
The rotary vane pump can be designed to operate without oil. Such designs employ nonmetallic vanes and rotate more slowly than traditional rotary vane pumps. Dry pumps are used in applications where a clean discharge gas is required, where it is important to avoid oil backstreaming, or where available pump lubricants are not compatible with process vapors.
Rotary vane pumps are used in the semiconductor industry, in degassing processes, filtration processes, and in distillation applications. The rotary vane vacuum pump shown below to the left is used in a leak detector, and the one shown below to the right is used in an electron microscope.
- Able to achieve high vacuum levels and handle large flow rates
- Relatively low noise level
- Available in small sizes suitable for laboratory applications
- High compression ratios per stage
- Sensitive to contamination
- High service requirement
- Intolerance for liquid slugs
- Need to select a compatible lubricant
- Low tolerance for fouling
- Limited materials for construction
Diffusion vacuum pumps are designed to pump low-density gases in the high-vacuum range, 10 -3 to 10 -7 torr (1.3 x 10 -6 to 1.3 x 10 -10 atm).
Diffusion pumps create a vacuum by using a high-velocity vapor stream to entrain entering gas molecules.
As the animation demonstrates, the working fluid shown in blue vaporizes in the boiler. The vapor, shown in red, rises inside the vapor chimney and is deflected downwards by a jet assembly. This jet of vapor entrains the entering gas molecules, shown in yellow, and moves them in the direction of the pump outlet. At the pump outlet, the gas molecules are removed by a mechanical pump that is not shown. The vapor jet condenses on the pump walls and returns to the boiler, as shown by the blue arrows.
Diffusion pumps usually consist of three or more compression stages in series, as shown in the schematic. Each stage includes a jet assembly attached to the vapor chimney. Additional stages improve the ultimate pressure of the pump. Cooling coils outside the chamber ensure condensation of the working fluid.
Backstreaming, the migration of working fluid out of the pump, is a major source of contamination in diffusion pumps. To avoid this problem, traps or baffles may be used. On the left is a picture of a baffle and to the right is a picture of a water-cooled halo baffle.
Diffusion pumps are used in analytical instruments such as mass spectrometers and gas chromatography-mass spectrometers. They are also used in the semiconductor industry, vacuum furnaces, and leak detection systems.
- Inexpensive equipment
- Easy to maintain
- Excellent thermal stability and chemical resistance
- Possible backstreaming
The pumping action of a turbomolecular pump is similar to a rotary vane pump without vanes.
Turbomolecular pumps operate in molecular flow conditions. The pumping action is produced by a momentum transfer from the fast-moving surface to the gas molecules. Gas molecules collide against the inclined blades of the rotating impeller, gaining velocity in the direction of the moving surface they collide with.
The picture and schematic of the turbomolecular pump’s rotating impeller show the alternating rotating discs (rotors) and stationary plates (stators). The discs and plates are cut with slots set at an angle so that gas molecules caught in the slots of the moving discs are projected in the direction of the slots in the stationary plates. This projection of the gas molecules creates the pumping action.
Turbomolecular vacuum pumps are used in the deposition of optical and protective coatings, the production of magnetic layers, nuclear technology, analytical instrumentation, and the production of semiconductors, as shown below.
- No contaminating moving fluid
- Less chance of backstreaming
- Quick start-up/shut down
- Less need for traps
- No high voltages
- Low operating expenses
- Pumps all gases effectively
- More predictable pumping performance
- High equipment cost
- Moving parts subject to wear
- Not effective for use with light gases
In cryopumps, the pumped gas is not exhausted to the atmosphere but kept inside the pump.
The general operating principle of cryopumps is the freezing of a gas species in a closed system. Gases are condensed on a cold surface and retained within the pump.
A typical cryopump consists of two temperature stages, each refrigerated by a cryogenic refrigerator. Each stage cools a cryo panel onto which gases freeze. The first stage refrigerator cools the outer cryo panel to 50-75 Kelvin. The second stage refrigerator, also known as the cold stage, cools the inner cryo panels. This stage will freeze nitrogen, oxygen, or argon and is kept at 10-20 Kelvin. Gases not frozen on the first or second stage panels are adsorbed into charcoal located on the underside of the inner cryo panels.
Cryopumps are built as self-contained units with no outlet: once gases are frozen to the cryopump surfaces they are retained there until the pump is regenerated.
Cryopumps are used in applications requiring ultra-high vacuum with pressures below 10 -7 torr (1.3 x 10 -10 atm). Surface science applications, semiconductor production, particle acceleration, and space simulation chambers are all common uses of cryopumps. Pictured below is a physical vapor deposition machine that uses cryopumps to create a vacuum.
Pictured below are space simulating chambers. The chamber on the left is used in a laboratory to perform tests on components that will later be sent to high altitudes. The thermal vacuum chamber on the right is used by NASA to expose Hubble Space Telescope components to conditions that they will experience in space. The chamber removes all but the slightest trace of air and reduces the pressure to about a billionth of the normal atmospheric pressure of the Earth.
- Clean pump, free from hydrocarbons
- High speed
- Periodic regeneration or degassing is necessary
- Possibility of desorption of previously pumped gases if temperature increases due to overloading or other misoperation
Ion pumps capture and store molecules of gas. The schematic below shows a diode ion pump. A gas molecule collides with high-energy electrons, loses one or more of its own electrons, and is left with a positive charge. The resulting ion is under the influence of a strong electric field and is therefore accelerated into a titanium cathode. The force of the collision is great enough that the ions and some titanium ions are ejected from the cathode and collide with the adjacent walls of the pump, where they accumulate.
In the triode design shown below to the left, the cathode is built with slits. These slits prevent the ions from being implanted in the pump walls to any large extent and also prevent previously buried ions from being unburied. This design is used when noble gases are being pumped because of the tendency of previously buried noble gas ions to be released, causing instability in diode ion pumps. A cutaway view of an ion pump is shown below to the right.
Ion vacuum pumps operate in the ultrahigh vacuum pressure range, at pressures below 10 -7 torr (1.3 x 10 -10 atm). They are used in analytical instruments, such as mass spectrometers and electron microscopes. They are also used in linear accelerators and power tube devices.
- Complete isolation from the atmospheric environment
- Capable of pumping any gas, including noble gases, hydrocarbons, and chemically inert gases
- Pump must be reconditioned or replaced periodically
A dry vacuum pump is a positive-displacement pump in which there are no lubricants or sealing liquids and which continuously discharges to the atmosphere.
Dry pumps are compact and energy-efficient and are unique in the CPI (chemical process industries) because they do not require fluid, lubricating, or sealing, for operation.
Dry pumps, like the Roots blower, operate with two interlocking rotors on two parallel shafts that are synchronized by timing gears and rotate in opposite directions to trap and transport gases.
There are three main types of dry vacuum pumps: rotary claw, rotary lobe, and rotary screw.
The geometric shape of the rotary claw dry pump allows a greater compression ratio across the rotors at higher pressures. As shown in the picture below, two claw rotors rotate in opposite directions, synchronized by timing gears. The rotors do not touch each other. After the gas has been uncovered, it enters the pump through an inlet port and fills the void space between the rotors and pump housing. On the next rotation, that same trapped gas is compressed and discharged as the discharge port opens.
The rotary-lobe vacuum pump contains two symmetrical two-lobe rotors mounted on separate shafts in parallel as shown in the picture below. These rotate in opposite directions to each other at high speeds. Timing belts are used to synchronize the rotation between the two rotors, and to provide constant clearance between the two.
In the rotary screw vacuum pumps, two long helical rotors in parallel rotate in opposite direction to each other. Timing gears synchronize the rotation and keep the rotors from touching each other. Gas flows along the screw with no compression from suction to discharge. Pockets of gas that are trapped between the rotors and casing are transported to the discharge port, where compression occurs and the gas is discharged against atmospheric pressure.
Dry vacuum pumps can be used in all situations, particularly when environmental efficiency is a priority. They can be used in place of almost all of the other pumps in chemical and pharmaceutical companies. Pictured below to the left is a dry screw vacuum pump used in recovering acetone vapors. The vacuum pump shown below to the right is used in a leak detector.
- Energy efficient
- Low service requirements
- Dry interior minimizes back diffusion of contaminants into the process
- Does not contribute to air pollution
- High rotational speed reduces the ratio of gas slip to displacement, increases net pumping capacity, and reduces total pressure
- Can be constructed of standard, inexpensive cast iron because there is no condensation involved
- More expensive than other pumps
- May require a silencer
- May discharge gases at high temperatures
- May require a gas purge for cooling, or to protect the seals and bearings from the process gas
- Most difficult to repair or rebuild
- Some gases may polymerize due to high operation temperatures
- Agilent Technologies, Santa Clara, CA
- Angstrom Engineering, Kitchener, Ontario
- Becker Pumps Corp., Akron, OH
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- CVD Equipment Corporation , Ronkonkoma, NY
- Dekker Vacuum Technologies, Inc., Michigan City, IN
- Ebara Technologies, Sacramento, CA
- Gardner Denver Nash, Charleroi, PA
- GE Energy, Houston TX
- Graham Corporation, Batavia, NY
- Kurt J. Lesker Company, Clairton, PA
- Oerlikon Leybold Vacuum GmbH, Germany
- Quincy Compressor, Quincy, IL
- Schoonover, Inc., Canton, GA
- Wintek Corporation, Flanders, NJ
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