Overpressure turbine
A positive pressure turbine or reaction turbine is understood to mean a turbine in which the working medium in front of the impeller has a higher static pressure than behind it. The counterpart to the overpressure turbine is the constant pressure turbine or action turbine , in which the work comes only from the kinetic energy, the dynamic pressure of the working medium.
technology
The usable work for the impeller comes mainly from converting static pressure into speed according to Bernoulli's energy equation . The main feature of a reaction turbine is that only part of the pressure at the inlet to the turbine is converted into speed before the water hits the impeller . The water fills the whole space between the blades of the runner. As the water flows through the runner, the pressure in the flow channel between the individual blades decreases continuously, which is caused by the impulse effect of the water on the runner.
It makes sense to connect a diffuser to the reaction turbine, which converts the kinetic energy of the water flowing out of the turbine back into static pressure. This diffuser is called a suction pipe in water turbines .
Degree of reaction
The degree of reaction r denotes the ratio of the enthalpy converted in the rotor to the total enthalpy converted in the turbine. In the case of a reaction turbine, the degree of reaction is always greater than zero ( r > 0), in the case of a constant pressure turbine it is equal to zero ( r = 0 ) and in the case of a vacuum turbine it is less than zero ( r <0 ). The vacuum turbine is a theoretically possible constellation without practical application.
Reaction turbines implemented in practice have a degree of reaction between zero and one ( r> 0 ), which means that part of the pressure outside the rotor is reduced. For steam turbines and gas turbines, the value is close to 0.5, which means that half of the pressure reduction occurs in the tail unit and the other half in the rotor. Overpressure turbines with a degree of reaction equal to 1 ( r = 1 ) are not used in industry. In this type of turbine, all of the static pressure is converted into kinetic energy by means of a nozzle on the rotor, as is the case with the often cited textbook example.
application
The above-described applies not only to water turbines but also to steam turbines and gas turbines . Gas turbines are always designed as positive pressure turbines, steam turbines can be designed as positive pressure or constant pressure turbines. In the case of water turbines, Kaplan and Francis turbines are positive pressure turbines , while the Pelton turbine is a pure constant pressure turbine . A comparison of the properties to the constant pressure turbine can be found in the article on the constant pressure turbine.