Rotary Vane Vacuum Pumps
Rotary vane vacuum pumps are vacuum pumps that generate low-pressure zones by rotating the moving parts against the pump casing. The mating surfaces of the rotor and the housing have very...
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This article gives industry insights into vacuum systems and vacuum pumps. Read further to learn more.
A vacuum pump is a piece of equipment capable of generating a partial or low-pressure vacuum by pushing gas or air molecules out of a sealed chamber. A vacuum is a relative state at which the chamber pressure has a lower pressure than the ambient atmosphere or adjacent systems. This is different from absolute vacuum, where the pressure is at 0 Pa and devoid of gas molecules.
One of the key elements of a vacuum pump is atmospheric pressure, which is the weight of the air pressing down on the earth. This pressure is created by the weight of air molecules that decrease at higher altitudes. Air pressure or atmospheric pressure has a significant effect on the operation of machines, especially vacuum pumps. Pressure always attempts to equalize as molecules move from high to low areas to fill a space, a process that is based on the idea of pushing molecules.
The purpose of all pumps is to convert energy into pressure. The amount of energy necessary to run a pump changes in accordance with atmospheric pressure. The higher the atmospheric pressure, the more efficiently the operation of a vacuum pump. Since atmospheric pressure plays such a vital role in vacuum pump efficiency, it is an important factor in the cost of operating a vacuum pump and will vary depending on temperature, humidity, and altitude.
There are different degrees of vacuums that can be created. They can range from a low vacuum with an absolute pressure range of 1 to 0.03 bars to a high vacuum with a pressure of a billionth of a Pascal. Low and medium vacuums are commonly seen in industrial systems such as vacuum grippers, vacuum cleaners, incandescent bulbs, painting, sandblasting, vacuum furnaces, and negative pressure ventilation. Higher vacuum systems are used for laboratory applications such as particle reactors and accelerators.
There are two main categories of generating partial vacuum. One is by gas transfer or gas feeding and the other is through entrapment. Gas transfer types of vacuum pumps work by mechanically removing gasses through positive displacement or momentum transfer. Positive displacement vacuum pumps have chambers that alternately expand and contract with check or non-return valves to draw and eject flow. Momentum transfer pumps work by accelerating gasses creating a low-pressure region in its wake. Entrapment vacuum pumps, on the other hand, capture gas molecules by various principles such as condensation, sublimation, adsorption, ionization, and so on.
Vacuum ranges are characterized by the measurement of the absolute pressure of the system. Which represents the number of remaining molecules left in the system. The remaining gas molecules are normally nitrogen, oxygen, and water vapor, with traces of neon, helium, and hydrogen. As more and more molecules are removed, it becomes increasingly difficult to remove any additional ones. The fewer molecules there are to be removed, the more a vacuum is required to work harder and use more energy, since fewer molecules lowers the pressure reading.
Different vacuum ranges require different pumping techniques. Low and medium vacuum ranges can be achieved by positive displacement vacuum pumps. These are suited for most industrial systems. Achieving high and ultra-high vacuum ranges for special applications such as surface analytic techniques, microscopy, and nanolithography are achieved by both momentum transfer and entrapment pumps.
Vacuum Range | Absolute Pressure (Pa) |
---|---|
Atmospheric | 101,325 |
Low Vacuum (Rough, Coarse) | 1.01 x 10⁵ to 3.33 x 10³ |
Medium Vacuum | 3.33 x 10³ to 1 x 10⁻¹ |
High Vacuum | 1 x 10⁻¹ to 1x10⁻⁷ |
Ultra-high Vacuum | 1 x 10⁻⁷ to 1 x 10⁻¹⁰ |
The two main classifications of vacuum pumping principles are gas transfer and entrapment. Gas transfer is further divided into positive displacement and momentum transfer. To further grasp the concepts of vacuum pumps, it is best to understand the three types of flow: viscous, transitional, and molecular. Viscous or continuous flow occurs at high pressures to medium vacuum. In this type of flow, the gas is dense enough for gas molecules to collide with each other. The mean free path or the average distance traveled by a gas molecule is less than the dimensions of the chamber. When a higher vacuum is reached, the gas molecules tend to collide on the walls of the chamber more than other gas molecules. Transitional flow occurs when the viscous flow starts to change into molecular flow. Molecular flow is characterized by the random movement of gasses where their mean free path is much longer than the dimensions of the chamber.
Fluids flowing under viscous flow can be pumped mechanically by positive displacement pumps. However, molecular flow will be reached when the gas cannot be evacuated by pressure difference. At this point, another pumping system, either momentum transfer or entrapment, is used. Most high vacuum systems have two pumps in tandem. Positive displacement pumps alone are not sufficient at higher vacuum. Momentum transfer pumps will stall if the system is operated at viscous flow. Entrapment pumps will be frequently regenerated or exhausted when there is too much gas to be captured particularly at viscous flow.
Positive displacement vacuum pumps operate by expanding and contracting a sealed chamber where the flow of fluid is controlled by one-way valves. The vacuum generation process starts by expanding a sealed chamber generating a vacuum. This vacuum draws the fluid into the chamber through an intake valve. Upon reaching the maximum expansion, the intake valve closes while the exhaust opens. The fluid is ejected out of the chamber as it compresses or contracts. The cycle repeats several times per second itself creating a pulsating flow.
Like ordinary pumps, positive displacement vacuum pumps are classified according to the motion and the design of the chamber. There are two main categories: reciprocating and rotary.
Reciprocating Piston Vacuum Pump: This type of pump generates vacuum and compression through the movement of the piston sealed against a cylinder. The piston is connected to the crankshaft via a connecting rod. As the crankshaft rotates, the piston is pushed back-and-forth within the cylinder. The pistons are commonly made of cast iron, bronze, or steel.
Plunger Vacuum Pump: This type of pump operates the same way as a reciprocating piston pump. The piston or plunger of this pump is a long, solid cylinder typically made of hard-coated ceramic. The long profile of the plunger allows the high-pressure seal to be stationary relative to the cylinder, in contrast with piston pumps where the seal is attached to the piston. This enables the use of more complex sealing systems. Plunger vacuum pumps are more suited for more demanding conditions than piston vacuum pumps.
Diaphragm Vacuum Pump: Diaphragm vacuum pumps use a deformable metallic or elastomer membrane permanently joined into the chamber creating a hermetic seal. Piston vacuum pumps have the advantage when it comes to reliability and power, while diaphragm vacuum pumps are mostly suited for ejecting hazardous or corrosive substances.
Reciprocating vacuum pumps can also be classified according to the number of the chambers mainly to address the problem of pulsating flow. A pulsating flow is an undesirable characteristic of reciprocating pumps where the flow is delivered in short bursts. Adding more pistons and cylinders will create a more constant flow. This brought the development of reciprocating pumps with multiple piston-cylinder assemblies known as multiplex pumps.
In terms of the achieved cycle phase per stroke, reciprocating pumps are categorized as either single- or double-acting. Single-acting pumps create only either vacuum or compression in a single stroke. In this configuration, the piston or diaphragm is coupled to only one chamber where only one side engages the fluid. A double-acting pump, by contrast, creates both vacuum and compression in a single stroke. A common configuration is a twin piston-cylinder or twin diaphragm assembly which is actuated by a single drive rod. Other designs can feature a single piston or diaphragm serving two chambers. Double-acting pumps are more commonly used due to better efficiency, higher flow rate, and less pulsating flow.
Rotary Vane Vacuum Pump: Rotary vane vacuum pumps are the most common type of positive displacement vacuum pump. This pump has vanes inserted radially into a circular rotor. The rotor is eccentrically installed relative to the stator housing. This eccentricity is known as the stroke of the pump. The individual chambers separated by the vanes progressively become smaller as it approaches the discharge. The vanes are allowed to move radially which press against the housing mainly through the centrifugal force as the rotor rotates. A spring energizes the vanes or holds the vanes in place when the rotor is not in motion.
Rotary Piston Vacuum Pump: A rotary piston vacuum pump has an eccentric wheel as the rotor which is attached to a slide valve. Rotary piston valves can be considered as two-stroke, double-acting pumps with two separate compression chambers. As the wheel rotates during the intake stroke at the first chamber, the slide valve opens allowing the entry of the fluid. Opposite this chamber is another that undergoes an exhaust stroke. This second chamber has an exhaust valve where the compressed fluid is ejected. Like the rotary vane, the compression chamber is created by mating the rotor, in this case, the eccentric wheel, against the pump housing. This chamber progressively becomes smaller at the end of the exhaust stroke.
Screw Vacuum Pump: A rotary screw pump is one of the earliest developed positive displacement pumps which was known as the Archimedes‘ screw. In its simplest form, this pump consists of a single screw inside a hollow cylinder. Modern designs incorporate double or triple screws meshing against each other. As the fluid enters the pump, it becomes trapped in the cavities between the threads of the screw and the housing. The pressure is developed by the rotation of the screw ejecting the fluid on the other side. It is suitable in handling single and multiphase fluids and has higher tolerance in handling fluids with abrasive contaminants.
Gear Vacuum Pump: This type of reciprocating pump has rotors in the form of two meshing gears where one gear drives the other. Gear pumps can be either external or internal. An external gear pump has two mating external gears. External gear pumps operate by creating an expanded cavity as the teeth come out of mesh when rotating towards the inlet. The fluid is drawn into this cavity due to the vacuum generated. As the gears rotate, fluid is trapped between the teeth and the pump housing. The fluid is ejected to the other side of the chamber. On the other hand, internal gear pumps have rotors composed of a driven external gear and an internal gear. Pumping is achieved the same way as external gear pumps where the fluid is drawn from the expanding cavity as the gear teeth come out of mesh.
Scroll Vacuum Pump: This type of pump is composed of two co-wound spirals or scrolls with one acting as the rotor while the other as a stator. The rotor does not rotate but moves eccentrically relative to the other. Scroll pump works by drawing fluid from the periphery of the scrolls. The fluid trapped between the scrolls is transported towards the center where the volume is progressively decreased.
Momentum transfer pumps operate by inducing the movement of gas or liquid molecules through kinetic energy transfer. This happens at the molecular flow, in contrast to the viscous or continuous flow occurring in positive displacement pumps. The uniform velocity distribution of the molecules is altered continuously to a preferred direction by the fast-moving surfaces hitting them. These surfaces are not only limited to impeller surfaces, but to other liquids as well. An example is a diffusion pump where high-speed jets of motive fluid impart momentum to the gasses to be drawn from the inlet. Momentum transfer pumps are suited for creating a high vacuum. However, to create a molecular flow, low pressure must exist throughout the system. The exhaust cannot be directly released to the atmosphere or at pressures where backstreaming can occur. To solve this problem, a backing pump is installed in tandem with the vacuum pump. The backing pump can be a positive displacement pump that operates at a lower vacuum level which can directly discharge to the atmosphere.
Turbomolecular Vacuum Pump: A turbomolecular vacuum pump has multiple stages of rotating and stationary turbine blades. The rotating blades are angled in such a way that it transfers sufficient momentum to the gas molecules moving them axially towards the subsequent stages until it reaches the exhaust. The stator has angled blades as well and ensures the correct direction of the gas. Since the mass of the gas is very small, the rotors must rotate at very high speeds. Friction heat build-up and rotor deflection limit the design of turbomolecular pumps.
Diffusion Vacuum Pump: As mentioned earlier, a diffusion pump works by using a motive fluid used to transfer momentum to the gas molecules. The motive fluid is usually oil or steam. The general design of an oil diffusion pump involves a heater to heat the oil and is ejected to nozzles on top of the boiler or vaporizing chamber. The vaporized oil leaves the nozzles at supersonic speeds which collects randomly flowing gasses drawn from the low-pressure chamber. Cooling coils are present to condense the vaporized oil which then returns to the boiler. The collected gas molecules continue to flow towards the exhaust. Steam or hydrocarbon gas ejectors work similarly. But these types do not need a boiler since the steam or motive fluid is already vaporized and has sufficient speed.
Entrapment vacuum pumps employ multiple physical and chemical phenomena to capture gas molecules. The working principle is different from each type. Common to almost all entrapment pumps is their ability to operate at high vacuum regimes without any oil contamination. Entrapment vacuum pumps do not rely on rotors or other moving parts. The downside, however, is that it cannot operate continuously since it needs to be regenerated once the surface or material capturing the gasses is full. Moreover, they cannot remove lighter gasses such as hydrogen, helium, and neon. Below are some of the common entrapment vacuum pumps.
Cryogenic Vacuum Pump: This type of vacuum pump works by cooling the gas down to its condensation or freezing point. It captures gasses such as nitrogen and oxygen below 20 K at the high vacuum regime. For capturing lighter gasses such as helium and hydrogen, they must be cooled down to 8 to 10 K. The typical design of a cryogenic pump is a two-stage cooler. The first stage is for removing water vapor and oil by cooling at around 70 to 80 K. The second stage is for removing gasses that cool at around 10 to 20 K. At this stage, an adsorbent such as activated charcoal is integrated to capture the cooled gasses.
Sorption Vacuum Pump: This type of pump uses adsorbents such as activated charcoal, zeolite, or other types of molecular sieves for capturing gas molecules. It is usually paired with cryogenic pumps for condensing the gasses or lowering the gas vapor pressure.
Sputter Ion Vacuum Pump: Sputter ion vacuum pump, also known as ion getter pump or ion pump, operates by ionizing the incoming gasses by an anode. The ionized gas then binds to a cathode or a getter, typically made of titanium. The binding may be through chemical or physical means depending on the type of gas present. As the ionized gas impacts the cathode, some atoms or electrons of the cathode are ejected from the surface, known as sputtering.
Titanium Sublimation Vacuum Pump: In this type of vacuum pump, an electric current is periodically introduced through a titanium filament. This heats the titanium and directly vaporizes it inside a chamber. The gasses flowing through or present in the chamber are captured by the vaporized titanium by bonding while in transit or upon developing a film on the chamber wall. Once the titanium film is consumed, the remaining titanium filament is again vaporized to create another layer.
Centrifugal Pumps: Centrifugal pumps use velocity and momentum to move fluids using a fan and impellers to create fluid velocity. The working principle is based on the concept of a forced vortex, which means when a mass is rotated by external force, there is an increase in pressure. The mounting and growing pressure causes a fluid to be transferred. Centrifugal pumps are used for large volumes of liquids with high flow rates where flow rates can be easily adjusted.
The main part of a centrifugal pump is its impeller that accelerates the movement of a liquid and is attached to the shaft that transmits torque inside the shaft sleeve.
Aside from the pumping principles, vacuum pumps can be categorized according to the type of lubrication and sealing system. Vacuum pumps can be wet or dry lubricated. Choosing between the two mainly affects other performance factors such as wear resistance, pumping speed, fluid contamination, and so on.
The Venturi principle is used by vacuum ejectors and Venturi vacuum pumps. With vacuum ejectors, a Venturi nozzle is used to move materials at high speeds. Vacuum ejectors and Venturi vacuum pumps do not have any moving parts and work based on Bernoulli’s principle.
A vacuum ejector, or venturi vacuum pump, operates using the Venturi effect that is based on Bernoulli's principle. The physical concept is the law of conservation of energy applied to fluids and states the inverse relationship of kinetic energy and pressure. When a fluid‘s velocity increases, its pressure decreases and vice versa. Vacuum ejectors use compressed air as their energy source as opposed to electricity.
Inside a vacuum ejector is a venturi, which is a jet nozzle that shoots high pressure air across a chamber and out through a receiver nozzle. The venturi nozzle narrows into a smaller orifice and then gradually expands to speed up the air and decrease the pressure. The fast moving stream of air between the two nozzles has lower pressure due to the increased velocity. Outside air is drawn into the chamber and out through the receiver nozzle along with the compressed air.
Venturi pumps are placed inside a vacuum ejector housing with a port between the nozzles that can supply vacuum to a variety of applications.
The advantages of a vacuum ejector:
On the other hand, the disadvantage of using a vacuum ejector is the inevitable mixing of the motive fluid and the fluid from the vacuum connection. If the intention is to recover the fluid drawn from the evacuated chamber, special separation techniques must be performed.
Vacuum ejectors are usually used in applications for drawing liquids such as water and steam where mixing the motive and vacuum streams poses no negative issues. They are commonly seen in power plants, petroleum and petrochemical plants, and water treatment facilities.
Venturi vacuum pumps have an inlet and outlet with a nozzle in between the inlet and outlet. The nozzle restricts the flow of fluids, increases the fluid’s velocity, and decreases its pressure. A vacuum is created by the pressure drop that pulls fluid into the nozzle and then forces it out the other side.
Unlike other vacuum pumps, venturi vacuum pumps do not need a power source but do need access to compressed air. They are tiny, lightweight and can operate continuously for years. Since they do not require a power source, venturi vacuum pumps do not generate heat and do not overheat. Venturi vacuum pumps can be customized to the needs of an application by changing the diameter of the nozzle to maximize pressure loss and the generated differential pressure to meet the needs of an application.
Advantages of a Venturi Vacuum Pump:
Venturi vacuum pumps are used in applications where accuracy is required and pressure loss is not acceptable. They are capable of moving wet or dry materials and fluids through a pipeline. As with vacuum ejectors, venturi vacuum pumps are added to regular vacuum pumps to assist in moving materials long distances.
Rotary vane vacuum pumps are vacuum pumps that generate low-pressure zones by rotating the moving parts against the pump casing. The mating surfaces of the rotor and the housing have very...
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