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Crossflow Turbine

The Crossflow Turbine is also known as an Ossberger Turbine and is a good low head, high flow turbine even though it is technically an inpulse turbine. Power outputs are typically 2KW up to 100KW but small Hydro Systems can be built up to 3 MW.

Unlike most water turbines, which have axial or radial flows, in a cross-flow turbine the water passes through the turbine transversely, or across the turbine blades. The water is admitted at the turbine's edge and fter passing the runner, it leaves on the opposite side. The water flows over and under the inlet guide-vane which directs flow to ensure that the water hits the rotor at the correct angle for maximum efficiency. The water then flows ove rthe upper rotor blade, producing the torque on the rotor, then through center of the rotor and back across the low rotor blades producing more torque on the rotor. Most of the power is extracted by the upper blade (roughly 75%) and the remaining 25% by the lower blade. Obviously the rotor is rotating, so what are the upper blades one moment will be the lower blades the next. Going through the runner twice provides additional efficiency. 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. Most practical cross-flow turbines have two nozzles, arranged so that the water flows do not interfere.

One of the advantages of a crossflow turbine is that it is self-cleaning to a degree in that leaves etc. that could get pushed into and stuck on the upper blades are washed off by the exiting water on the lower blades. Also the centrifugal force tends to throw trapped debris outwards, further increasing the self-cleaning capabilities.

Micro/Mini Crossflow Turbines have just one runner but in small Hydro Systems the Cross-flow turbines are often constructed as two turbines of different capacity that share the same shaft. The turbine wheels are the same diameter, but different lengths to handle different volumes at the same pressure. The subdivided wheels are usually built with volumes in ratios of 1:2. The subdivided regulating unit, the guide vane system in the turbine's upstream section, provides flexible operation, with 33, 66 or 100% output, depending on the flow. This means that during lower flow periods the 2/3 inlet guide-vane can be completely closed allowing no water through, and the turbine will operate on just the 1/3 guide-vane which effectively means that only 1/3 of the rotor is in use. If 'average' flow rates are available the 1/3 guide-vane can close and the turbine operates on just the 2/3 side, then when the high flow rates aare available both guide-vanes can work together.

Crossflows are available in a number of rotor diameters, normally in 100 mm steps from 100 mm to 500 mm. The smalller diameters are for highr head sites. For low head sites from 2.5 to 5 meters 300 mm diameter rotors are normally used. The 400 and 500 mm rotors are used on very low head sites.

Many of the parts of crossflow turbines are standardised and only the width of the rotor is designed to match the expected range of flows at the Hydro site.

Low operating and maintenance costs are one advantage of the crossflow turbine because of its realatively simple construction, low speed operation and self-cleaning flow through design. The main rotor bearings are grease lubricated via two grease fittings, one on each side of the turbine and require greasing from a grease gun one per month. Anually the iar inlet valve position should be checked. Hydro Systems with crossflo turbines normally have 12 mm intake screens which prevent almost all but the tiniest debris entering the turbine, and this debris can pass through without any problem.

The Micro/Mini Crossfire Turbines have one runner and have manual or fixed inlet guide vanes. On the outside of the turbine the two inlet guide-vanes are moved via 'actuator arms' which are moved either by electric or hydraulic actuators. The amount and direction of actuator movement is governed by the system controller.

The regulating device controls the flow based on the power needed, and the available water. Water admission to the two nozzles is throttled by two shaped guide vanes. These divide and direct the flow so that the water enters the runner smoothly for any width of opening. The guide vanes should seal to the edges of the turbine casing so that when the water is low, they can shut off the water supply. The guide vanes therefore act as the valves between the penstock and turbine. Both guide vanes can be set by control levers, to which an automatic or manual control may be connected. The turbine rotor normally rotates more slowly than the generator, so either a belt-drive or a gearbox is used to increase the speed.

On Micro.Mini Hydro systems the drive pulley is attached directly to the turbine shaft for excellent reliability. On Small Hydro Systems a gearbox is used and conencts to the turbine with flexible coupling. The gearbox is sized based on the site specific design criteria.

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

Particularly with small run-of-the-river plants, the flat efficiency curve yields better annual performance than other turbine systems, as small rivers' water is usually lower in some months. The efficiency of a turbine determines whether electricity is produced during the periods when rivers have low flows. If the turbines used have high peak efficiencies, but behave poorly at partial load, less annual performance is obtained than with turbines that have a flat efficiency curve.

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