Impulse turbine
A constant pressure turbine , also called an action turbine or impulse turbine , is a turbine in which the working medium has the same static pressure before and after the impeller . The useful work for the impeller comes solely from the conversion of kinetic energy , the dynamic pressure of the working medium, according to the Bernoulli energy equation . A constant pressure turbine has a degree of reaction of zero. The opposite term to the constant pressure turbine is the overpressure or reaction turbine .
technology
Examples of impulse turbines are the Pelton turbine , Curtis turbine , Laval turbine and the flow turbine . Constant pressure turbines are predominantly built in a chamber design, as the cross-sectional areas for leakage currents are smaller compared to the drum design . The chamber design is also possible due to the very low axial forces, because high axial forces are typical of positive pressure turbines.
The counterpart to the constant pressure turbine is the overpressure turbine , also known as the reaction turbine (degree of reaction> 0).
properties
There are properties that are common to all impulse turbines, and some that only play a role in multi-stage steam turbines. Gas turbines are not considered here because they are always designed as positive pressure turbines.
Properties of all impulse turbines
- The seal between the blade and the housing is not so important because there is no pressure difference in front of and behind the blades.
- Partial or multiple admission is possible because the working medium does not expand after leaving the guide vane.
- With the same power output, a constant pressure stage rotates 0.707 times (1 / √2) slower than an overpressure stage with a degree of reaction of 0.5, provided that the mass flow and turbine diameter are the same.
- At the same speed, a constant pressure stage delivers twice as much power as an overpressure stage with a degree of reaction of 0.5, provided that the mass flow rate and the turbine diameter are the same.
- Compared to other turbines, a turbine with constant-pressure blading has the greatest breakaway torque and is therefore of particular interest in technology where high starting torques are required at low speeds. For example, equal pressure stages are used in air starters for jet engines.
Constant pressure steam turbine
The following properties only apply to multi-stage steam turbines, since water turbines are not designed to be multi-stage.
- Smaller number of stages compared to a positive pressure turbine, because a larger pressure gradient can be used in one stage.
- The efficiency of a constant pressure turbine is somewhat lower than that of a positive pressure turbine. There are greater losses in the blading because a greater pressure gradient is reduced, which leads to higher speeds of the medium in the guide vanes. In addition, the flow is deflected more strongly in the guide vanes. In the case of partially pressurized turbines, ventilation losses occur on the blades that are not pressurized.
- Significantly more complex and robust and therefore more expensive blades than in reaction turbines.
swell
- Willy JG Braunling: Aircraft engines, Springer Verlag. Impulse turbine page 653 (3rd edition 2009) Online
Individual evidence
- ↑ Gunt (Ed.): Turbines for gaseous fluids . Hamburg ( gunt.de [PDF]).
- ^ Hermann Weihe: Machine science . Springer-Verlag, 1923, ISBN 978-3-662-34525-2 , p. 162 ( google.cz ).
- ↑ Carl Pfleiderer, Hartwig Petermann: Classic of technology: turbo machines . Springer, Berlin, Heidelberg 2005, ISBN 978-3-540-26913-7 , 10. The multi-stage flow machines, p. 468-534 , doi : 10.1007 / 3-540-26913-4_10 .
- ^ Hugo Hoffmann: Textbook of mining machines: power and work machines . Springer-Verlag, 1950, ISBN 978-3-642-52861-3 , p. 125 ( limited preview in Google Book Search).
- ↑ J. Reiser: Losses and efficiencies in steam turbines . 2000, p. 12 ( fundus.org [PDF]).