Cross-flow turbine
1 - ventilation valve
2 - diffuser
3 - housing
4 - impeller
5 - corner box
6 - blades
7 - water flow
8 - shaft
The cross- flow turbine , also known as cross- flow turbine or, after the developers' names, Bánki turbine , Michell turbine or Ossberger turbine , is a water turbine in which the water flows transversely through the turbine runner, unlike a conventional turbine with an axial or radial flow . Similar to a water wheel , the water enters the circumference and, after passing through the inside of the impeller, exits again on the opposite side. The double impact results in a comparatively better effect and thus a certain self-cleaning effect or dirt resistance. The cross-flow turbine is one of the slow runners based on its specific speed .
The turbine was developed in parallel by the Hungarian Donát Bánki , the German Fritz Ossberger and the Australian Anthony George Maldon Michell. Ossberger brought this type of turbine to series production. In 1922 he patented a "free jet turbine" and in 1933 the "cross-flow turbine". The company he founded is now a leading manufacturer of this type of turbine.
The cell wheel is usually divided in width by one or two thirds of the width, the water regulation by the equally divided regulating device (flap system in the upper water) allows a three-stage operation; Depending on the amount of water, with a third (small width part), two thirds (large width part) or three thirds (both width parts) of the full power. The comparatively simple turbine construction enables low operating costs.
Detailed description
The turbine consists of a cylindrical water wheel or impeller with a horizontal shaft, consisting of numerous (up to 37 pieces), radially / tangentially arranged blades with blade ends pointed on both sides (because of the flow resistance), consisting of a halved circular ring cross-section (pipe cut lengthwise ). In order to weld the semi-cylindrical blades attached in the form of a cage, circular edge disks are arranged at both blade ends. In terms of appearance, the comparison with a hamster running wheel is obvious: The round running bars there are channel-shaped at the turbine.
The turbine is initially loaded from the outside to the inside. The flap-shaped and / or tongue-shaped regulating device varies the flow rate of the turbine by changing the cross section. The direction of the water jet towards the imaginary cylindrical outer surface of the impeller is constructed by means of a fixed nozzle, so that the water enters the impeller at about 45 degrees relative to the entry tangent and releases part of the kinetic energy to the currently active cylinder blades.
Depending on the position of the regulating device (0-100%), 0-100% * 30/4 blades are also applied. The headwater supply is controlled by two force-balanced profile guide vanes. The guide vanes divide the water flow, direct it and allow it to enter the rotor without bumps, regardless of the opening width. Both rotating blades are precisely fitted into the turbine housing. They keep the amount of leakage so small that the guide vanes can serve as a shut-off device at low heads. Gate valves between the pressure pipe and the turbine can then be omitted. Both guide vanes can be adjusted separately from one another using control levers to which the automatic or manual control is connected. The turbine geometry (nozzle-impeller-shaft) thus ensures that the water jet does not break up or atomize on the shaft (through contact) inside the impeller, as approx. 1/3 of the total output still exits the impeller from the inside to the outside is to be delivered. This means that work is done twice on the impeller in a ratio of 2: 1. From the perspective of the blading, the water flows bidirectionally through the blade channels, outside – inside and inside – outside. Because most turbines work with two jets, two water jets can be guided inside the impeller without interfering with one another. However, this is only ensured if the turbine, gradient and turbine speed are coordinated with one another.
The flow turbine is a constant pressure turbine , i. H. the pressure on the impeller does not change.
A flow-through turbine is usually built in a cellular design in order to decisively improve its overall partial load behavior. The turbine consists of two chambers with two impellers on a common shaft, the chambers being divided for 1/3 and 2/3 of the volume flow . The smaller chamber is used with small water flows, the larger one with medium water volume and with large water volume both chambers are flowed through.
Comparison with other turbines
Compared to Kaplan , Francis and Pelton turbines , the flow-through turbine has a slightly lower peak efficiency, which is offset by the flat profile of the efficiency. Due to the division, every amount of water from 1/6 to 1/1 is processed with optimal efficiency. Due to the low price and the simple adjustability, it is mainly used in small and micro power plants with a nominal output of up to approx. 5,000 kW and heads of up to 200 meters. Especially with small run-of-river power plants , the flat efficiency curve results in higher annual work than with other turbine systems, as run-of-river waters often have poor water flow for several months. Whether electricity is generated during this time depends on the efficiency characteristics of the respective turbine. Turbines with high peak efficiency, but unfavorable partial load behavior, generate a lower annual yield in run-of-river power plants with fluctuating water flow than turbines with a flat efficiency curve. Due to the robust part-load behavior, the turbine is also very suitable for autonomous power generation. One advantage over all other turbine types is the simplicity of the construction, there is a maximum of two roller bearings to be replaced and only three rotating parts, the mechanics are very simple and can therefore be repaired by almost anyone without special tools.
Because the water jet enters the inside of the impeller and immediately reverses its flow direction from the inside to the outside, leaves, grass, etc. cannot get stuck inside the impeller and thus generate flow losses. The lower nominal efficiency curve as a function of the part load / full load ratio is kept constant by the constantly clean impeller. Other types of turbines with low output are more at risk of pollution when the water is unidirectional and lose output. Here, an impeller cleaning process must be carried out by reversing the flow or disrupting the flow by changing the speed, which is not required in the case of cross-flow turbines.
literature
- Philipp Oppermann, Torsten Rüdinger: Little mill knowledge : German history of technology from the friction stone to the industrial mill , published by the German Society for mill knowledge and mill maintenance eV , Terra Press, Berlin / Potsdam 2010, ISBN 978-3-9811626-7-7 .
Individual evidence
- ↑ Patent DE361593 : free jet turbine. Registered on February 14, 1922 , published October 16, 1922 , applicant: Fritz Ossberger.
- ↑ Patent DE615445 : Cross- flow turbine. Registered on March 23, 1933 , published on July 5, 1935 , applicant: Fritz Ossberger.
