vacuum pump

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vacuum pump

[′vak·yəm ‚pəmp]
(mechanical engineering)
A compressor for exhausting air and noncondensable gases from a space that is to be maintained at subatmospheric pressure.

Vacuum Pump

 

a device for removing gases and vapors from a closed space in order to create a vacuum in it. There are various types of vacuum pumps, whose operations are based on various physical phenomena: mechanical (rotary), jet, sorption, and condensate removal pumps.

The principal parameters of vacuum pumps are the ultimate pressure (also residual pressure or maximum vacuum) that can be attained by the pump, the evacuation rate—the volume of gas that can be pumped out at a given pressure per unit of time (m3/sec, l/sec), and the permissible (maximum) outlet pressure in the discharge section of the pump—which, if exceeded, would disrupt the normal operation of the pump.

Mechanical pumps. Mechanical pumps are used to create vacuums from 1 newton per sq m (N/m2), or 10-2 mm of mercury (mm Hg), to 10-8 N/m2, or 10-10 mm Hg. The piston of the simplest vacuum pump performs a reciprocating motion that forces the gas out and, during the return stroke, creates a rarefaction in the system being pumped out.

Reciprocating pumps (Figure 1) were the first mechanical pumps; they were subsequently replaced by rotary pumps. In the multiplate rotary pump (Figure 2), gas suction and ejection are accomplished by varying the volumes of the cells formed by the eccentrically mounted rotor, which has slots fitted with movable blades that press against the inner surface of the chamber and slide along it during rotation. Owing to the high rotation frequency of the rotor, these pumps have a high evacuation rate (up to 125 l/sec) considering their relatively small size. The ultimate pressure attains 2,000 N/m2 (15 mm Hg) in single-stage pumps and 10 N/m2(10-1 mm Hg) in two-stage pumps.

Figure 1. Diagram of a reciprocating pump: V„ is the volume to be pumped out; Vmin and Vmax represent the minimum and maximum cylinder volumes respectively.

The method of pumping gas with water-ring pumps (Figure 3) is similar. Upon rotation of a radially vaned wheel eccentrically positioned in a chamber, the water filling the chamber is deflected by the vanes and thrown against the casing wall by centrifugal force, forming a water ring and a crescent-shaped chamber, into which the gas being pumped flows. When the wheel rotates, the cells are alternately connected to the drain, through which the pumped gas is discharged to the atmosphere. These pumps are useful for pumping out moisture-laden and contaminated gas, oxygen, and explosive gases. The ultimate vacuum is 95 percent (in single-stage pumps) and 99.5 percent (in two-stage pumps) of the theoretically possible vacuum—for example, at a water temperature of 20° C it reaches 7.1 kN/m2 (53 mm Hg) in single-stage pumps and 3.1 kN/m2 (23 mm Hg) in two-stage pumps.

Figure 2. Diagram of a multiplate vacuum pump.

Oil-sealed rotary pumps are often used to create a medium vacuum. The pumps are either immersed in an oil bath or their working chambers are filled with oil. These pumps have an evacuation rate of 0.1-750 l/sec and an ultimate pressure of 1 N/m2 (10-2 mm Hg) in single-stage pumps and 10-1 N/m2(10-3 mm Hg) in two-stage pumps. The oil effectively seals all gaps and functions as an auxiliary cooling medium, but condensed vapors contaminate the oil during prolonged operation. To prevent the condensation of vapors, which develops under compression, the chamber is filled with a specified volume of air (ballast gas), which provides a partial vapor pressure in the vapor-air mixture at the moment of discharge without exceeding the saturation pressure. In this way, the vapor is ejected from the pump without condensation. These types of pumps are called gas-ballast pumps and are used as backing pumps (to create preliminary evacuation).

Figure 3. Diagram of a water-ring vacuum pump.

Two-lobe rotary pumps have two shaped, counterrotating rotors, creating a directional movement of the gas. These pumps have a high evacuation rate and are frequently used as intermediate (auxiliary, or booster) pumps between pre-evacuation and high-vacuum pumps. They provide vacuums of 10-2-10-3 N/m2 (10-4-10-5 mm Hg) at an evacuation rate of 15 m3/sec (Figure 4).

Figure 4. Diagram of a two-lobe rotary pump

In molecular pumps molecules pick up additional velocity in the direction of their movement during rotation of the rotor in the gas. Pumps of this type were first proposed by the German scientist W. Gaede in 1912 but were not used for a long time owing to their complexity of construction. In 1957 the German scientist W. Becker introduced the turbomolecular pump (Figure 5), with a rotor consisting of a system of disks. This type of pump attains a vacuum up to 10-8 N/m2 (10-10 mm Hg).

Figure 5. Diagram of a turbomolecular pump.

Jet pumps. In jet pumps a direct stream of the working medium carries off the gas molecules entering from the volume being pumped. Liquids or vapors of liquids may be used as the working medium. Depending on the type of medium, these pumps are called water-jet, steam-water, steam-mercury, or oil-diffusion pumps. Jet pumps are of the ejector or diffusion type, according to their principle of operation. In ejector pumps (Figure 6) the pumping operation of the jet is based on an increase of the gas flow pressure under the action of a jet with higher thrust. These pumps are used to attain a vacuum of 10 N/m2 (10-1 mm Hg). The water-jet pump, which is widely used in the chemical industry, in laboratory practice, and so on, is a simple ejector pump. The ultimate pressure of such pumps does not greatly exceed the vapor pressure of water. For example, at a water temperature of 20° C in the pump, the vacuum attained is 3,100 N/m2(23 mm Hg), but the partial pressure of the residual gases is approximately 670 N/m2 (5 mm Hg). The vortex pump, whose pumping action is based on the utilization of the negative pressure that develops along the axis of the vortex (Figure 7), can also be called an ejector pump. Pumps with water vapor as the working medium have a considerably higher evacuation rate and a lower ultimate pressure. An evacuation rate of 20 m3/sec with a vacuum of 0.7 N/m2 (5 x 10-3mm Hg) can be attained in multistage water-vapor pumps.

Figure 6. Diagram of a multijet ejector pump

The pumping operation of diffusion pumps is based on the diffusion of the pumped gas molecules in the area of action of a jet of vapor of the working medium owing to a drop in their partial pressures. W. Gaede used mercury vapor as a working medium in 1915. Mercury provides a constant vapor saturation pressure (for a given temperature) and temperature (for a given pressure), and it remains chemically inactive and is not affected by superheating; however, the mercury vapors, even in small amounts, are dangerous to the human organism.

Figure 7. Diagram of a centrifugal vacuum pump

One of the substitutes for mercury is oil. Such vacuum pumps using oil are called oil-diffusion pumps. The use of oil as a working fluid led to the widespread use of these pumps, which have evacuation rates of up to several hundred m3/sec with a vacuum of up to 10-6 N/m2 (10-8 mm Hg). In the oil-diffusion vacuum pump several pumping stages are connected in series in one casing (Figure 8). The operating pressure range of a three-stage oil-diffusion pump is 10-3-10-1 N/m2 (10-5-10-3 mm Hg).

Figure 8. Diagram of three-stage oil-diffusion pump

Sorption pumps. Sorption pumps take advantage of the capacity of several substances (titanium, molybdenum, zirconium, and others) to absorb gas. The gas being pumped settles on the surface within the vacuum system, and one of the active absorbents is continuously sprayed on the absorption surface (evaporation pump). A porous adsorbent may also be used as the absorber.

Ion pumps. The operation of ion pumps is based on the ionization of gas by a strong electric discharge and the removal of the ionized molecules by an electric field. This method is rarely used owing to complexity of construction and the large power demand, which goes mainly to create the magnetic field. At room temperature inert gases and hydrocarbons are, in practical terms, not absorbed by the sprayed-on metal films. Their removal is effected by combination getter ion (or ion-sorption) pumps, in which the sorption method of absorbing chemically active gases is combined with the ionic method of pumping out inert gases and hydrocarbons. The absorption surface is restored by the condensation of thermally vaporized titanium on the walls and also by the cathode sputtering of titanium in an electric discharge or in a magnetic field in sputter-ion or magnetic-discharge getter ion pumps (Figure 9). Getter ion vacuum pumps create vacuums up to 10-5 N/m2 (10-7 mm Hg) at a preliminary evacuation of up to 10-2 N/m2 (up to 10-4 mm Hg).

The evacuation rate depends on the type of gas. For example, the evacuation rate of hydrogen is 5,000 l/sec, nitrogen 2,000 l/sec, and argon 50 l/sec. The ultimate pressure attained in highly degassed volumes with no inleakage of gas is lower than 10-8 N/m2 (10-10 mm Hg).

Figure 9. Diagram of a magnetic-discharge getter ion pump: N and S are the north and south poles of a magnet, A is the anode, and C is the cathode.

Condensate removal (cryogenic) pumps. The operation of condensate removal (cryogenic) pumps is based on the absorption of gas by a surface that has been cooled to a low temperature (Figure 10). The hydrogen-condensation pump proposed by B. G. Lazarev and his colleagues at the Physicotechnical Institute of the Academy of Sciences of the Ukrainian SSR has a constant evacuation rate over a wide range of pressures. The cooling liquid hydrogen is produced by a liquefier located in the unit. The noncondensable gases (hydrogen and helium) are evacuated by a pump connected in parallel with, for example, a diffusion pump. A preliminary vacuum is required to engage this type of pump.

Figure 10. Diagram of a cryogenic pump

I. S. RABINOVICH

Vacuum pump

A device that reduces the pressure of a gas (usually air) in a container. When gas in a closed container is lowered from atmospheric pressure, the operation constitutes an increase in vacuum in this container.

Vacuum pumps are evaluated for the degree of vacuum they can attain and for how much gas they can pump in a unit of time. In practice, where high vacuum is required, two or more different types of pumps are used in series.

In the rotary oil-seal pump (see illustration), gas is sucked into chamber A through the opening intake port by the rotor. A sliding vane partitions chamber A from chamber B. The compressed gas that has been moved from position A to position B is pushed out of the exit port through the valve, which prevents the gas from flowing back. The valve and the rotor contact point are oil-sealed. Since each revolution sweeps out a fixed volume, it is called a constant-displacement pump. Other mechanical pumps are the rotary blower pump, which operates by the propelling action of one or more rapidly rotating lobelike vanes, and the molecular drag pump, which operates at very high speeds, as much as 16,000 rpm. Pumping is accomplished by imparting a high momentum to the gas molecules by the impingement of the rapidly rotating body.

The water aspirator is an ejector pump. When water is forced under pressure through the jet nozzle, it will force the gas in the inlet chamber to go through the diffuser, thus lowering the pressure in the inlet chamber. When high-pressure steam is used instead of water, it is called a steam ejector.

In addition, a number of pumps have been developed which meet special pumping requirements. The ion pump operates electronically. Electrons that are generated by a high voltage applied to an anode and a cathode are spiraled into a long orbit by a high-intensity magnetic field. These electrons colliding with gas molecules ionize the molecules, imparting a positive charge to them. These are attracted to, and are collected on, the cathode. Thus a pumping action takes place. Sorption pumping is the removal of gases by adsorbing and absorbing them on a granular sorbent material such as a molecular sieve held in a metal container. Cryogenic pumping is accomplished by condensing gases on surfaces that are at extremely low temperatures.

vacuum pump

A pump which produces a partial vacuum in an enclosed space; may be used to remove air or steam from a chamber or a system.

vacuum pump

vacuum pumpclick for a larger image
A typical vacuum system.
A pump used to remove air from a container. The negative pressure produced by the pump is used to drive some air-driven gyroscopic flight instruments.