Vacuum pump
Vacuum pumps are used in technology to generate a vacuum . A distinction is made between vacuum pumps according to the vacuum they generate and according to their functional principle. Technically speaking, vacuum pumps are not , strictly speaking, pumps , but compressors . The first vacuum pump was a piston pump and was built by Otto von Guericke in 1649 (see air pump ).
Basic principle
Depending on their physical operating principle, vacuum pumps are divided into gas transfer vacuum pumps and gas-binding vacuum pumps. Depending on the vacuum level to be achieved and the required pumping capacity, different vacuum pumps or combinations thereof are used.
Gas transfer vacuum pumps transport particles either in a closed working space (displacement vacuum pumps) or by transferring impulses to the particles (e.g. through impacts). Some pumps require molecular flow , others laminar flow . Typical representatives of gas transfer pumps are diaphragm pumps , reciprocating piston vacuum pumps, rotary vane pumps , gate valve vacuum pumps, Roots pumps, screw vacuum pumps, molecular pumps, turbo-molecular pumps and liquid jet pumps.
Gas-binding vacuum pumps achieve their pumping effect by binding particles to solid surfaces - this process is generally referred to as sorption - and consequently reducing the pressure in the recipient. The gas-binding vacuum pumps include getter pumps, cryopumps and adsorption pumps.
Different pump types have different areas of application depending on their function. To achieve low vacuum pressures, two pump stages are often required ( e.g. for pressures p <10 −3 mbar). The first pump (e.g. a rotary vane pump ) creates a backing vacuum and is often referred to as the backing pump, the next pump is then connected to the recipient . Typical pump combinations consist, for example, of a displacement vacuum pump as a backing pump and a turbo-molecular vacuum pump.
Because the volume of the air decreases when the air is compressed, several pumps (of the second stage) can always be pumped by a backing pump (of the first stage) through tubes in which there is laminar flow. Automatic valves, tanks and pressure gauges ensure safety and allow the backing pump to take breaks.
Types
In the group of positive displacement pumps, for reasons of production, operating costs and reliability, variants have established themselves in some designs that use the molecular flow to seal between the rotor and stator and achieve higher speeds than other positive displacement pumps.
With propellant pumps (jet pumps) either the friction of the laminar flow is used, or one tries to create friction-increasing turbulence or exploits the fact that molecular flows penetrate one another unhindered.
Turbines with laminar flow are z. B. used in a vacuum cleaner or in the form of a spiro pump in vacuum technology. Because of its high suction power, the molecular variant is mainly used in the vacuum range.
Positive displacement pump
The common feature of all displacement machines is an encapsulated (locked) work area, the size of which changes cyclically during the work cycle. The working cycle of a displacement vacuum pump can be divided into four working cycle phases: suction, transport (compression), pushing out and pressure change. The intake and discharge phase is also referred to as the low-pressure side (LP) or high-pressure side (HP) charge exchange. In the case of positive displacement pumps, the gas contained in the recipient enters the working space formed by pistons, rotors or slides, the working space is closed, the gas may be compressed and then expelled. The mechanical elements inside wet-running pumps are sealed against each other by a liquid, usually oil. In a liquid ring pump, water is also used as a sealing medium. In recent years, however, there has been a trend towards dry-running machine types ("dry running"), in which auxiliary fluids are dispensed with in the area of the work area. This avoids contamination of the working medium. In addition, there is less maintenance and there are no costs for the disposal of the auxiliary liquids.
- The following pumps are lubricated with oil: rotary vane pump , gate valve pump , trochoid pump .
- The following pumps are sealed with PTFE : scroll pump , piston pump .
- The following pumps use an air gap: rotary lobe pump (e.g. the claw pump), screw pump .
- The following pump is sealed with an elastic membrane: the piston pump, which is then called the membrane pump .
They are mostly used to generate rough and fine vacuums .
Gate valve pump
A classic gate valve pump is a vacuum pump for generating a fine vacuum . It consists of a hollow cylinder (stator) in which another cylinder (rotor) rotates, which is guided by an eccentric along the housing wall. The piston is connected to a hollow slide which is pivotably mounted in the housing and divides the sickle-shaped working space into a suction and a pressure side. The actual pump housing is overlaid with oil and is located in an outer housing filled with oil. The oil ensures lubrication and serves to seal the pump chamber and the pressure valve.
Jet pump
In jet pumps , also known as propellant pumps , steam or liquid is ejected at high speed through a suitable nozzle inside the pump. If a gas particle is captured by this particle flow, the impulse direction of the propellant flow is transferred to the particle, which is thereby transported into a zone of higher pressure inside the pump. The pump outlet is located there. If the pressure conditions require it, a backing pump is connected here, which takes over the further transport of the gas.
A water jet pump can be used for simple applications , the ultimate pressure of which is given by the vapor pressure of water .
So that the vaporous propellant itself does not get into the recipient , 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 ( oil diffusion pump ); they generate fine, high and ultra high vacuum.
Molecular pump
Molecular pumps, invented by Gaede in 1913, take advantage of the fact that a molecule falling on a wall is not immediately reflected, but rather spends a certain time on the wall between adsorption and desorption . If the wall moves within this dwell time, the circulation speed of the wall is superimposed on the isotropic speed distribution of the desorbing molecules. After leaving the wall, the particles therefore have a preferred direction, creating a flow.
The pump principle is implemented by a rigid, circular container and a rotor disc in the middle. The suction nozzle (inlet) and the fore-vacuum nozzle (outlet) are at an angle of about 90 ° on the container. The distance between the outer wall and the rotor is much smaller within this 90 ° arrangement than in the remaining 270 ° outer angle, approx. 5 micrometers, in order to avoid a backflow. The gas molecules enter the pump through the suction nozzle, adsorb on the rapidly rotating rotor, thereby gaining a preferred direction, desorb from the rotor a little later and ideally leave the pump through the fore-vacuum nozzle.
Problems with this pump principle are the frequent seizure due to the sometimes extremely thin gap between the rotor and the housing wall and the small delivery rate. These problems have been eliminated by the invention of the turbo molecular pump.
Turbo molecular pump (TMP)
The turbo- molecular pump, invented in 1956 or 1957 by Willi Becker (1919–1986) and initially also called the “New Molecular Pump ”, works according to the basic principle of the Gaede molecular pump , but is at the same time a completely new conception of the same. It consists of a single or multi-stage alternating arrangement of stators (baffles), between which rotors run similar to a compressor . The speed of the rotor blades is roughly in the order of magnitude of the mean thermal speed of the gas molecules. Unlike the compressor, the pumping action is not based on aerodynamic relationships, so the shape differs from that.
Rather, the pumping effect results from the fact that the atoms and particles are given impulses with an axial component. Whether this additional impulse is sufficient to leave the recipient depends on the particle mass and thus on the type of atom. Light molecules, for example, have a very high speed at room temperature, so that only a small additional pulse is transmitted via the pump. This is why the compressibility of hydrogen and helium in all molecular pumps is significantly worse than for the other, heavier components of the air.
Depending on the design, a distinction is made between single and double-flow turbo molecular pumps. The speed of the rotors is in the order of magnitude of a few 10,000 up to 90,000 / min, for example with a model similar to the one shown on the right a 'standby speed ' of approx. 30,000 / min and normal working speed of approx. 51,000 / min. The pumping capacity varies depending on the type from three to several thousand liters per second. Turbomolecular pumps are used with an upstream fore-vacuum to generate ultra-high vacuum, as otherwise the pump would heat up too much due to air friction or the motor output would not be sufficient.
A single-flow turbo molecular pump is shown; At the top you can see the intake, the rotor and stator blades, at the bottom left the exit to the fore-vacuum.
Cryopump
Most gases condense out on a surface that is cooled (for example with liquid helium , hydrogen or nitrogen ), which is why this pump is also called a “condensation pump”. In contrast to practically all other known vacuum pumps, the cryopumps achieve their theoretical pumping speed.
Cryopumps are only used at wall temperatures below approx. 120 K.
The cryopump is used to generate a high vacuum (p <10 −3 mbar) or ultra-high vacuum (p <10 −7 mbar).
Sorption pump
The gas is deposited on fresh, uncovered surfaces through physisorption . The area with the sorbent must be constantly renewed. Zeolites or activated carbon are used as sorbents .
If the layer is formed by vapor deposition of a metal, one speaks of “getter pumps”. In the ion getter pump , the gas is ionized by electron impact and driven to the sorbent by an electric field. These pumps require a good fore-vacuum and are used to generate an ultra-high vacuum.
A widely used variant of the ion getter pump is the orbitron pump ; In order to ionize the largest possible number of residual gas particles, the electrons circulate around a centrally arranged, rod-shaped anode, which is surrounded by a cylindrical cathode.
Generation of an ultra-high vacuum
In applied physics, several types of pumps are used to create an ultra-high vacuum. First, mechanically operating pumps ( rotary vane pump , diaphragm pump , scroll pump ) are used to generate a pre-pressure in the recipient in the range of 10 −2 to 10 −3 mbar. Depending on the size of the recipient and the pumping capacity of the pumps, this normally takes a few minutes. Next, turbomolecular pumps generate a high vacuum in the pressure range of around 10 −7 mbar in a process that takes at least several hours . Due to the constant desorption of adsorbed water and other compounds with low vapor pressure within the chamber, this pressure can no longer be reduced without further aids, even with an infinitely long-lasting pump output. These desorption processes are accelerated if the chamber is brought to a temperature by direct heating of the chamber walls and indirect heating of the inner surfaces, which is at least above the boiling point of water, but if possible significantly higher. The most important criterion for the choice of temperature is the temperature resistance of the built-in components such as the valve seals, the transfer systems, the electrical connections and the viewing windows. Usual bakeout temperatures are between 130 and over 200 ° C. Most of the water, which desorbs to a high degree, is pumped out during the bake-out process using the turbo molecular pumps, as is any carbon contamination. This process takes at least 24 hours; In the case of chambers with complex internal surfaces due to attached apparatus, the heating is usually shut down after two to three days.
Non-mechanical pumps are used to achieve the ultra-high vacuum. An ion getter pump pumps through ionization and trapping of the residual gas molecules in titanium tubes in a pressure range of 10 −7 mbar to 10 −10 mbar. The pumping power is only sufficient if the heating has previously reduced the residual gas pressure sufficiently. A titanium sublimation pump works with titanium vapor that is thermally distributed in the chamber, which is characterized by high chemical reactivity and binds residual gas atoms to itself and the (cold) chamber wall, so that the residual gas pressure is further reduced. The lowest residual gas pressure that can be achieved in this way is in the range of 10 −11 mbar.
By using cold traps at the lower part of the chamber, a further part of the residual gas can be temporarily bound and the chamber pressure can be reduced to around 10 −12 mbar - for a short time with optimal functioning of all components involved.
Application examples in practice
- The sample space in an electron microscope is evacuated so that the electrons are not scattered by air molecules on the way from the electron source to the sample to be examined and on to the screen, which is a prerequisite for its function. The same applies to the inside of a cathode ray tube (e.g. in televisions, other electron beam monitors, etc.).
- Mass spectrometers require a high vacuum to operate.
- For the molecular ultra high vacuum is required so 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 bulbs are evacuated before the protective gas is filled.
- With vacuum infusion , epoxy resin is drawn into a mold with the help of a vacuum pump and the mold is pressed.
- Numerous cockpit instruments in aircraft are based on gyro technology . Since gyroscopes have to be brought to considerable speeds in order to generate the highest possible stability, on the one hand, and, on the other hand, the drive should take place without being impaired by reaction forces, a vacuum is used for driving. For reasons of safety, two independent vacuum pumps are often installed in aircraft. The failure of a single existing vacuum pump can lead to the dangerous situation in flights with no view to the outside (instrument flight) that instruments that are mandatory for safe flight operations are incorrectly displayed. This state can creep in. In modern aircraft, a vacuum pump is often not used as a centrifugal drive. Instead, electric drives are used.
Web links
Individual evidence
- ↑ Wolfgang Demtröder : Experimentalphysik. Volume 1: Mechanics and Warmth. 4th, revised and updated edition. Springer, Berlin et al. 2006, ISBN 3-540-26034-X , p. 264.
- ↑ Introduction to high and ultra high vacuum generation. (PDF file; 864 kB) Pfeiffer Vacuum, September 2003.