8/15/13 Cross-flow turbine - Wikipedia, the free encyclopedia

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Cross-flow turbine. Image credit; European

Communities, Layman's Guidebook (on how to

develop a small hydroelectric site)

Cross-flow turbineFrom Wikipedia, the free encyclopedia

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A cross-flow turbine, Banki-Michell turbine, or

Ossberger turbine[1] is a water turbine developed by theAustralian Anthony Michell, the Hungarian Donát Bánkiand the German Fritz Ossberger. Michell obtained patentsfor his turbine design in 1903, and the manufacturingcompany Weymouth made it for many years. Ossberger'sfirst patent was granted in 1933 ("Free Jet Turbine" 1922,Imperial Patent No. 361593 and the "Cross Flow Turbine"1933, Imperial Patent No. 615445), and he manufacturedthis turbine as a standard product. Today, the companyfounded by Ossberger is the leading manufacturer of thistype of turbine.

Unlike most water turbines, which have axial or radialflows, in a cross-flow turbine the water passes through theturbine transversely, or across the turbine blades. As with awater wheel, the water is admitted at the turbine's edge.After passing the runner, it leaves on the opposite side. Going through the runner twice provides additionalefficiency. When the water leaves the runner, it also helps clean the runner of small debris and pollution. The cross-flow turbine is a low-speed machine that is well suited for locations with a low head but high flow.

Although the illustration shows one nozzle for simplicity, most practical cross-flow turbines have two, arranged sothat the water flows do not interfere.

Cross-flow turbines are often constructed as two turbines of different capacity that share the same shaft. Theturbine wheels are the same diameter, but different lengths to handle different volumes at the same pressure. Thesubdivided wheels are usually built with volumes in ratios of 1:2. The subdivided regulating unit, the guide vanesystem in the turbine's upstream section, provides flexible operation, with 33, 66 or 100% output, depending on theflow. Low operating costs are obtained with the turbine's relatively simple construction.

Contents

1 Details of design

2 Advantages

3 See also

4 References

5 External links

Details of design

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Ossberger turbine section

Ossberger turbine runner

The turbine consists of a cylindrical water wheel or runner with ahorizontal shaft, composed of numerous blades (up to 37), arrangedradially and tangentially. The blade's edges are sharpened to reduceresistance to the flow of water. A blade is made in a part-circular cross-section (pipe cut over its whole length). The ends of the blades arewelded to disks to form a cage like a hamster cage and are sometimescalled "squirrel cage turbines"; instead of the bars, the turbine has trough-shaped steel blades.

The water flows first from the outside of the turbine to its inside. Theregulating unit, shaped like a vane or tongue, varies the cross-section ofthe flow. The water jet is directed towards the cylindrical runner bynozzle. The water enters the runner at an angle of about 45/120degrees,transmitting some of the water's kinetic energy to the active cylindrical blades.

The regulating device controls the flow based on the power needed, andthe available water. The ratio is that (0–100%) of the water is admittedto 0-100%×30/4 blades. Water admission to the two nozzles is throttledby two shaped guide vanes. These divide and direct the flow so that thewater enters the runner smoothly for any width of opening. The guidevanes should seal to the edges of the turbine casing so that when thewater is low, they can shut off the water supply. The guide vanestherefore act as the valves between the penstock and turbine. Both guidevanes can be set by control levers, to which an automatic or manualcontrol may be connected.

The turbine geometry (nozzle-runner-shaft) assures that the water jet is effective. The water acts on the runnertwice, but most of the power is transferred on the first pass, when the water enters the runner. Only ⅓ of the poweris transferred to the runner when the water is leaving the turbine.

The water flows through the blade channels in two directions: outside to inside, and inside to outside. Most turbinesare run with two jets, arranged so two water jets in the runner will not affect each other. It is, however, essentialthat the turbine, head and turbine speed are harmonised.

The cross-flow turbine is of the impulse type, so the pressure remains constant at the runner.

Advantages

The peak efficiency of a cross-flow turbine is somewhat less than a Kaplan, Francis or Pelton turbine. However,the cross-flow turbine has a flat efficiency curve under varying load. With a split runner and turbine chamber, theturbine maintains its efficiency while the flow and load vary from 1/6 to the maximum.

Since it has a low price, and good regulation, cross-flow turbines are mostly used in mini and micro hydropowerunits of less than two thousand kW and with heads less than 200 m.

Particularly with small run-of-the-river plants, the flat efficiency curve yields better annual performance than otherturbine systems, as small rivers' water is usually lower in some months. The efficiency of a turbine determineswhether electricity is produced during the periods when rivers have low flows. If the turbines used have high peak

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efficiencies, but behave poorly at partial load, less annual performance is obtained than with turbines that have a flatefficiency curve.

Due to its excellent behaviour with partial loads, the cross-flow turbine is well-suited to unattended electricityproduction. Its simple construction makes it easier to maintain than other turbine types; only two bearings must bemaintained, and there are only three rotating elements. The mechanical system is simple, so repairs can beperformed by local mechanics.

Another advantage is that it can often clean itself. As the water leaves the runner, leaves, grass etc. will not remainin the runner, preventing losses. Therefore, although the turbine's efficiency is somewhat lower, it is more reliablethan other types. No runner cleaning is normally necessary, e.g. by flow inversion or variations of the speed. Otherturbine types are clogged more easily, and consequently face power losses despite higher nominal efficiencies.

See also

Water turbinesGorlov helical turbine, which axis is positioned perpendicular to the flowVertical axis wind turbines also have the flow pass through the turbine transversely

References

1. ^ E.F. Lindsley, Water power for your home, Popular Science, May 1977, Vol. 210, No. 5(http://books.google.com/books?id=cwEAAAAAMBAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false), 87-93.

External links

Ossberger - Ossberger Turbine (http://www.ossberger.de)JLA & Co - cross flow Turbine (http://www.jlahydro.be)

Entec T15 cross flow Turbine (http://www.entec.ch)IREM - Ecowatt Hydro Turbine (http://www.irem.it/ENG/index.php)

Cross Flow Fans and Cross Flow Wheels (http://www.crossflow-fan.com/)

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