A vacuum pump is a piece of equipment capable of generating a partial vacuum or a low-pressure space by drawing gas 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.
Note that this is different from absolute vacuum in which the pressure is at 0 Pa absolute and is completely devoid of gas molecules.
There are different degrees of vacuum that can be created. It can range from being a low vacuum with an absolute pressure range of 1 to 0.03 bars to an extremely high vacuum with a pressure of a billionth of a Pascal.
Low and medium vacuum 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 gases 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 gases 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 pumps should be regularly serviced and maintained in order to avoid breakdowns and to keep them working at their ultimate efficiency and performance.
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Vacuum ranges or regimes are classifications of the quality of vacuum characterized by the measurement of the absolute pressure of the system.
The absolute pressure represents the amount of remaining matter inside the system, mostly composed of gas molecules such as nitrogen, oxygen, water vapor, and trace gases such as neon, helium, and hydrogen.
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.
Absolute Pressure (Pa)
Low Vacuum (Rough, Coarse)
1.01 x 10⁵ to 3.33 x 10³
3.33 x 10³ to 1 x 10⁻¹
1 x 10⁻¹ to 1×10⁻⁷
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 travelled 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 gases 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 when 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.
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.
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 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.
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.
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.
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.
Lobe vacuum pumps deliver fluid the same way as gear pumps. But instead of having mating gear teeth, lobe pumps have two or more meshing lobes.
These two-lobe rotors are both driven and timed with one another. This allows the rotors to rotate with no contact. Thus, high rotating speeds are possible with less wear to the rotor.
This feature of timed rotation is available in gear pumps as well. Moreover, the lobes also allow a continuous fluid seal contact across the surfaces of the lobes. Gear pumps have a fluid seal that jumps discontinuously from teeth to teeth.
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 gases 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.