Vacuum pump

Vacuum serve the purpose, to produce a technical vacuum. We distinguish the vacuum after the vacuum created by them and by their technique. Technically vacuum pumps are not really pumps but compressor. The first vacuum pump was built in 1649 in the form of a piston pump by Otto von Guericke.

Basic principle

Vacuum pumps are classified according to their physical working principle in gas transfer pumps and vacuum pumps in gas- binding.

Gas transfer vacuum transport particles either in a closed working chamber ( positive displacement ) or by pulse transmission to the particles (e.g. by collisions ). Some pumps require molecular flow, other laminar flow. Typical representatives of the gas transfer pumps, for example, diaphragm pumps, Hubkolbenvakuumpumpen, rotary piston pumps, sliding vane pumps, Roots pumps, screw pumps, molecular pumps, turbo molecular pumps or liquid jet pumps.

Gas -binding vacuum pumps achieve their pumping action by binding to solid surfaces bind particles - this process is commonly referred to as sorption - and thus reduce the pressure in the vacuum chamber. The gas-binding vacuum pumps include getter pumps, cryopumps and adsorption pumps also.

Different types of pumps have functionally in various application areas. To achieve low vacuum pressures, it is often needed (e.g., for pressure p < 10-3 mbar ) has two pumping stages. The first pump (eg, a rotary vane pump ) generates a pre-vacuum and is often referred to as a backing pump, the next pump is then connected to the receptacle. Typical pump combinations consist for example of a positive displacement pump as backing pump and a turbomolecular vacuum pump. Because the compression of the air, its volume is reduced, a plurality of pumps ( the second stage) can still be pumped through pipes in which there is laminar flow of a backing pump ( the first step). Automatic valves, boiler and pressure gauge provide one with security and enable the backing pump breaks.

Types

Displacement in the group have been established for the sake of manufacturing, the cost and reliability for some types variants, using the molecular flow to the seal between the rotor and stator and to achieve higher speeds than other positive displacement pumps.

In propellant pumps ( ejectors ) used either the friction of the laminar flow, and every effort is made to friction or turbulence uses the fact of that penetrate molecular flows unhindered.

Turbines with laminar flow are used, for example, to suck the air out of a PC case or in the form of a spiro- pump in vacuum technology. Because of their high suction molecular variant in the vacuum region but mainly used.

Displacement pump

Common feature of all positive displacement is an encapsulated ( completed ) work space whose size varies cyclically during the working cycle. The operating cycle of a positive displacement pump can be divided into the four working phases of play: sucking, transporting ( compacting ), extensions and pressure changes. The intake and exhaust phase is also called a low-pressure side (LP ) or high-pressure side ( HD ) charge exchange. In positive displacement pumps, the gas contained in the recipient enters the work space formed by pistons, rotors, or a slide, the working space is complete, the gas is compressed and then eventually expelled. The mechanical elements within wet running pumps are against each other, sealed by a liquid, usually oil. At a liquid ring pump, water is also used as a sealing medium. In recent years, however, there is a trend to dry-running mechanical types ( "dry runners " ) was recorded, which is omitted in the area of the work space on auxiliary fluids. This contamination of the working medium is avoided. In addition, a lower maintenance cost arises and the cost of disposal of the auxiliary liquids dispensed with.

• The following pumps are lubricated with oil: rotary vane pump, sliding vane pump, trochoid.
• The following pumps are sealed with PTFE: scroll pump, piston pump, screw pump.
• The following pumps use an air gap: rotary piston pump (eg the claw pump ).
• The following pump is sealed with an elastic membrane: the piston pump, which then is diaphragm pump.

They are mostly used for the production of coarse and Feinvakua.

Gate valve pump

A conventional blocking slide pump is a vacuum pump for generating a fine vacuum. It consists of a hollow cylinder ( stator) in which a further cylinder (rotor) is rotating, is guided by an eccentric along the housing wall. The piston is connected to a hollow pusher which is pivotably mounted in the housing and divides the crescent shaped working space into a suction side and a pressure side. The actual pump housing is overlaid with oil and is in an oil-filled outer housing. The oil to ensure the lubrication, serving for sealing the pump chamber and the pressure valve.

Jet pump

In the jet pumps, also known as propellant pumps, vapor or liquid to be ejected through an appropriate nozzle in the interior of the pump at high speed. A gas particles covered by this particle, the impulse direction of the propellant stream is transmitted to the particles, which is thereby transported into a zone of higher pressure within the pump. There, the output of the pump is located. If required by the pressure conditions, here, a backing pump is connected, takes care of the further transport of the gas.

For simple applications, a water-jet pump can be used, the discharge pressure given by the vapor pressure of water.

Thereby vapor blowing agent itself does not enter into the container, it is condensed on the cooled outer walls of the pump. This structure is usually implemented with oil jet pumps, in which the oil is either liquid or vapor form (oil diffusion pump) is; they produce fine, high and ultra high vacuum.

Molecular pump

Invented in molecular pump, Gaede of 1913, one uses the fact that a falling of a wall molecule is not reflected immediately, but spends between adsorption and desorption of a certain residence time on the wall. Then moves the wall within this dwell time, the rotational speed of the wall of the isotropic velocity distribution of the desorbing molecules is superimposed. Therefore, the particles are after leaving the wall is a preferred direction, thus, it results in a flow.

The pump principle is realized by a rigid circular container and a rotor disk in the middle. The intake ( inlet ) and the side arm (outlet) are at an angle of about 90 ° to the container. The distance between the outer wall and the rotor is within the 90 ° arrangement much smaller than in the remaining 270 ° external angle, about 5 microns, in order to prevent a backflow. The gas molecules pass through the intake manifold to the pump, adsorb on the rapidly rotating rotor to get in one preferred direction, a little later desorb from the rotor and leave the pump ideally through the side arm.

Problems of this pump principle are the frequent seizure because of the sometimes extremely thin gap between the rotor and the housing wall and the small capacity. These problems have been overcome by the invention, the turbo molecular pump.

Turbo -molecular pump (TMP)

The turbo molecular pump, 1956 or 1957 by Willi Becker ( 1919-1986 ) invented and initially called " New molecular pump " works according to the basic principle of Gaede 's molecular pump, but is simultaneously also a complete redesign of the same. It consists of a one or more stages alternating arrangement of stators ( baffles ) between which rotors similar to run at a turbine. The speed of the blades is approximately in the order of the average thermal velocity of the gas molecules. The pumping action is based not as with other turbines on aerodynamic contexts, therefore, differs from the shape of those.

Rather, the pumping action results from the fact that the atoms and particles of pulses are added with an axial component. Whether this is sufficient additional impulse to leave the recipient, depends on the particle mass and thus on the type of atom. Light molecules have a very high speed, so that via the pump, only a small additional pulse is transmitted, for example at room temperature. Therefore, the compression capacity for hydrogen at all -molecular pumps is significantly worse than for the other heavier components of the air.

Depending on the design, a distinction between one-and two-flow turbomolecular pumps. The speed of the rotors is of the order of some 10,000 to 90.000/min, for example, with a similar model as shown at right an ' willingness speed ' of about 30.000/min and normal operating speed of approximately 51.000/min. Pump capacity varies depending on the type of three to several thousand liters per second. Turbomolecular pumps are utilized with an upstream pre-vacuum for the production of ultra-high vacuum, since the pump would otherwise heat up too much because of the air friction or engine power would not be enough.

Shown is a single-flow turbomolecular pump; above you can see the intake manifold, the rotor - stator blades and, at the lower left exit for pre-vacuum.

Cryopump

To a cooled (e.g., liquid helium, hydrogen, or nitrogen) surface condense out most of the gases and therefore this pump will be referred to as " condensation pump ." Unlike virtually all other known vacuum the cryopumps reach their theoretical pumping speed.

Only at wall temperatures below about 120 K is referred to as cryopumps.

The cryogenic pump is used to generate high-vacuum (P < 10-3 mbar) and Ultrahochvakua (p < 10-7 mbar).

Sorption

Through the gas physisorption of fresh, uncovered surfaces separates. The area with the sorbent must be constantly renewed. As sorbent zeolites or activated carbon.

If the layer is formed by vapor deposition of a metal, it is called " getter ". In the ion pump, the gas is ionized by electron impact and driven by an electric field to the sorption agent. These pumps require a good pre-vacuum and used to generate a ultra-high vacuum.

A much used variant of the ion pump is the Orbitronpumpe; to ionize the largest possible number of residual gas particles, the electrons rotate a centrally disposed, rod-like anode surrounded by a cylindrical cathode.

Generation of an ultrahigh- vacuum

In Applied Physics, use is made of several types of pumps in order to produce an ultra-high vacuum. First ( rotary vane pump, diaphragm pump, scroll pump ) is generated form in the recipient in the range of 10-2 to 10-3 mbar with mechanical acting pumps. Depending on the size of the receptacle and of the pump power of the pump, this usually takes several minutes. Next, create turbo-molecular pumps in a process lasting at least several hours, a high vacuum in the pressure range of about 10-7 mbar. This pressure can no longer be reduced without further aids due to constant desorption of adsorbed water and other compounds with low vapor pressure within the chamber, and that too at infinitely long periods of pump power. This desorption can be accelerated as well, the chamber is brought to a temperature by direct heating of the chamber walls and indirect thermal heating of the inner surfaces, which is at least above the boiling point of water, but preferably much higher. The most important criterion of the level of temperature is the temperature resistance of the installed components, such as valve seals, transfer systems, the electrical connections and the viewing window. Typical annealing temperatures are between 130 and 200 ° C. Desorbing the now highly water is largely evacuated during baking by means of the turbomolecular pump, as well as any carbon contamination. This process takes at least 24 hours; in chambers with relatively complex inner- surface grown by the heating apparatus is shut down normally after two to three days. To achieve the ultra-high vacuum mechanical pumps not now be put to use. An ion getter pumps by ionization and trapping of residual gas molecules in titanium tubes in a pressure range from 10-7 mbar to 10-10 mbar. This shows that the pump power is only sufficient if the baking has previously reduced the residual gas pressure is sufficient. A titanium sublimation pump operates via thermally distributed in the chamber titanium vapor, which is characterized by a high chemical reactivity and residual gas atoms binds to and the (cold ) chamber wall, so that consequently further reduced the residual gas pressure. The minimum achievable with this method, the above-described residual gas pressure in the range of 10-11 mbar. Through cold traps at the lower part of the chamber, a statistically significant portion of the residual gas can be temporarily bound and the chamber pressure to approximately 10-12 mbar now also - be reduced - in the short term with optimal functioning of all components involved.

Examples in practice

• In many aircraft cockpit instruments are based on gyroscope technology. Since gyro to produce the highest possible stability must be brought to considerable speeds on the one hand, on the other hand the drive without impairment should be done by the reaction forces, one uses to drive a vacuum. For safety reasons often two independent vacuum pumps are installed in aircraft. A failure of a single existing vacuum pump for flights without outward visibility ( instrument flight ) lead to the dangerous situation that display incorrectly mandatory instruments for safe flight. This condition can occur insidiously. In modern aircraft is often dispensed with a vacuum pump and centrifugal drive. Instead, one uses electric drives.
• As a further application of vacuum technology in the aviation industry, the wastewater system has been established in modern passenger aircraft. Wastewater from the toilets be removed by a vacuum. These evacuated a typically configured as a radial blower vacuum pump all wastewater system as required. This is usually only near the ground because at cruising altitude, the cabin is held by branched turbine air to the pressure of the earth's surface. The difference between the prevailing low outside atmospheric pressure and the cabin pressure then compresses the waste water into the sewage system.
• The sample chamber is evacuated with an electron microscope, so that the electrons are impeded in the path from the electron source to the sample to be analyzed is not of air molecules. The same applies to the inside of a cathode ray tube (for example in TV sets, CRT monitors, etc.).
• Mass spectrometer need to operate a high vacuum.
• For an ultra high vacuum molecular beam epitaxy is needed to ensure that the molecular beam is not deflected by collisions with residual gas atoms, and to avoid contamination of the produced layers.
• In the manufacture of incandescent lamps, the glass bulb is evacuated before the inert gas is introduced.
• As a braking system on the railroad
• Generation of vacuum for the brake booster of cars and light trucks.
• Adherence and / or transport of flat workpieces or components by means of a suction pad
• Further application examples, see: characteristics of the vacuum