Jet pump

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Typical structure of a jet pump

A jet pump (for other designations see section "Nomenclature") is a pump in which the pumping effect is generated by a fluid jet ("propelling medium"), which sucks in another medium ("suction medium") through the exchange of impulses, accelerates and compresses / delivers, if it is under sufficient pressure .

Since this type of pump has a very simple structure and has no moving parts, it is particularly robust, low-maintenance and versatile.

Nomenclature and Classification

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The names for jet pumps are inconsistent and many different terms are used depending on the application:

  • Jet pumps , emitters and propellant pumps are common generic terms. Following the English word jet pump is sometimes also the name jet pump used.
  • An ejector ( Latin for “ejector”) or aspirator is usually a jet pump that generates a negative pressure , i.e. has a predominantly suction effect. This is very common especially in vacuum technology .
  • An injector (Latin for “thrower”) is usually a jet pump that generates overpressure , i.e. has a predominantly compressing effect. Sometimes the injector is also used as a generic term.

The classification is also inconsistent. As a pump, the jet pump is sometimes assigned to the fluid energy machines . Since the jet pump does not take up any work and since it contains no moving parts, strictly speaking it is not a machine at all , but an apparatus .

functionality

three-dimensional section through a jet pump

The picture above right shows the typical structure of a jet pump. The promotion takes place in the following steps and can be calculated quite well with a few simplifications simply by applying the laws of energy, momentum and mass conservation:

  1. The propellant (3) emerges from the propellant nozzle (2) at the highest possible speed . According to Bernoulli's law, this creates a dynamic pressure drop . For this reason, the pressure in the flow is lower than normal pressure. In the case of compressible propellants (gases or vapors), the nozzle is often designed as a Laval nozzle to maximize the speed and the propellant jet emerges at supersonic speed .
  2. In the mixing chamber (4) the propulsion jet hits the suction medium located here , which is usually under normal pressure. After exiting the nozzle, the propellant jet initially behaves like a free jet : Internal friction and turbulence create a shear stress in the boundary layer between the fast propellant medium and the much slower suction medium. This voltage causes momentum transmission , i.e. H. the suction medium is accelerated and carried away. Mixing takes place according to the principle of conservation of momentum . (The application of the Bernoulli equation, which only applies to frictionless liquids , would lead to incorrect results due to the shock losses .) The jet is slowed down by the expansion of the propulsion jet and the suction of suction means, i.e. H. dynamic pressure is converted into static pressure .
  3. Since the suction medium is accelerated in the mixing chamber, according to Bernoulli there is also a pressure drop for the suction medium, i. H. a suction effect that feeds further suction medium from the suction nozzle (1), provided that there is sufficient minimum pressure there.
  4. A diffuser (5) is often connected downstream for a further pressure increase (optional).

The higher the relative density of the propellant medium in relation to the suction medium, the stronger the pumping effect. The compression ratio can be further increased by a multi-stage arrangement as a series connection .

If the propellant is a vapor , it can condense in the mixing chamber . This increases the compression effect, which makes it even possible to achieve a higher final pressure than the propellant pressure. The opposite effect can occur in burners where ignition can occur in the diffuser.

Types and areas of application

In principle, any type of fluid (e.g. liquid , gas , vapor , flowable suspension , aerosol ) can be used both as a propellant medium and as a suction medium . This opens up a wide range of possible areas of application.

The following table shows some examples of applications, arranged according to the physical state of the motive and suction medium:

Driving medium →
suction medium ↓ 		gaseous / vaporous liquid
gaseous / vaporous
liquid
firmly
  • Gravel pump, sludge pump (water delivers suspension)

Note: The list shows just a few of the many examples. It is far from complete.

Advantages and disadvantages

Compared to other types of pumps , especially the widespread centrifugal pumps , jet pumps have some significant advantages and disadvantages:

advantages Neutral disadvantage
  • Due to the lack of moving parts and a drive , jet pumps are particularly simple, robust, and require little maintenance and wear .
  • Due to their simple construction, they are usually cheaper than other pumps.
  • Due to the simple geometry and the low mechanical stress, jet pumps made of thermally and chemically resistant special materials, e.g. B. ceramics are produced, which can not be used with other types of pumps or only with great effort.
  • With a dense propellant medium, very high compression ratios can be achieved, which is particularly important for vacuum technology .
  • Since they cannot get hot due to solid body friction, they are suitable for areas at risk of explosion .
  • Very flexible with regard to the installation position
  • Propellant and suction medium mix in the pump. This effect can be desirable if the pump simultaneously functions as a mixer or atomizer , but can also be negative if the properties of the suction medium change in an undesirable manner due to the mixed in propellant medium. Depending on the combination, it is u. U. possible to separate the media again ; however, i. d. Usually an additional thermal or mechanical separator is required.
  • Consumption of propellant medium (can possibly be separated again and at least materially recovered)
  • significantly lower efficiency than most other types of pumps
  • Significantly larger size than other pumps of the same performance, which can speak against a jet pump in a cramped installation position. However , the length can be reduced by connecting in parallel .
  • If the propellant nozzle is very small and the propellant medium is dirty, the nozzle can clog.
  • Erosion due to solid / liquid suction or propellant (take into account when choosing the material)

history

Since the pump has a very simple structure and functions, it was "invented" several times, for different applications, at different times and in different places, independently of one another. The simplest water jet pumps were already known to Ktesibios and Vitruvius in antiquity.

Some of the technicians who have decisively advanced jet pump technology in modern times were:

See also

literature

  • B. Eck: Technical fluid mechanics . Springer-Verlag, Berlin / Heidelberg / New York 1988
  • M. Wutz et al .: Theory and Practice of Vacuum Technology . Vieweg-Verlag, Braunschweig / Wiesbaden 1992
  • C. Edelmann: Knowledge storage vacuum technology . VEB Fachbuchverlag, Leipzig 1985
  • W. Pupp, KH Hartmann: Vacuum technology . Hanser-Verlag, Munich / Vienna 1991
  • N. Rao, H. Kremer: Injectors for gaseous and vaporous media . Vulkan-Verlag, Essen 1970
  • Hans Roos: Hydraulics of the water heating . 4th, completely revised Edition. R. Oldenbourg, Munich / Vienna 1999, ISBN 3-486-26399-4 .
  • Körting worksheets for jet pump applications and vacuum technology , publication from Körting Hannover AG
  • DIN24290: blasting devices . Beuth publishing house
  • MG Lotfey: Numerical simulation of gas-gas and gas-solid injectors . Karlsruhe 2002

Individual evidence

  1. ↑ a b R. Jung: The calculation and application of the jet fan , VDI research booklet 479, VDI-Verlag, Düsseldorf, 1960
  2. ↑ G. Wagner: Applications and areas of application of jet pumps , CIT 51 (1979), pp. 867-877
  3. ↑ G. wing: calculation of jet apparatus . In: VDI research booklet, 479, 1960, VDI-Verlag, Düsseldorf
  4. ↑ B. Bauer: Theoretical and experimental investigations on jet devices for compressible fluids . In: VDI research booklet, 514, 1966, VDI-Verlag, Düsseldorf
  5. ↑ H.-J. Henzler: For the design of jet suction devices for single-phase material systems . In: CIT , 54, 1982, 1, pp. 8-16
  6. ↑ H. Kremer, air intake in injector burners . In: CIT , 35, 1963 6, pp. 444-447
  7. ↑ R. Scholz, R. Jeschar, O. Carlowitz: On the thermodynamics of free rays . In: GWI , 13 (1984) 1, pp. 22-27
  8. ↑ Hans Roos: Hydraulics of the water heating . P. 235 ff.