Banki turbine
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Crossflow Turbine
The name Crossflow Turbine stands for a water turbine developed by the Australian Michell, the Hungarian Banki and the German Ossberger. Ossberger made this turbine a serial product. The first patent for his development was granted in 1922. Nowadays the company founded by him is the leading manufacturer of this turbine type.
Contrarily to the principle of a customary turbine with an axial or a radial flow, the water passes through the turbine type transversely. Like in the case of a waterwheel the water is admitted at its periphery. After having passed the runner it leaves on the opposite side. A much better efficiency is provided with this double admission, consequently a certain self-cleaning effect, and resistance to pollution. Following its specific speed the cross-flow turbine is a low-speed machine.
Subdivided wheels are mostly built at a ratio of 1 : 2, the subdivided regulating unit (guide vane system in upstream section) provides a relatively flexible operation, with 1/3, 2/3 or 3/3 = 100% output, following the flow. Low operating costs are obtained with the relatively simple turbine construction.
Detailed description
The turbine consists of a cylindrical water wheel or runner with horizontal shaft, composed of numerous (up to 37 pcs.) blades, arranged radially/tangentially, with blade ends sharpened at both ends (because of the flow resistance), made of one half circular cross-section (pipe cut over its whole length). In order to fix the semi-cylindrical blades, arranged in the form of a cage, circular side disks at the blade ends serve for fastening the blade ends by welding. The whole construction looks a little bit like a hamster cage, instead of the bars the turbine has trough-shaped steel construction elements.
The water flows from outside to inside the turbine at first. The regulating unit, shaped in the form of a vane or tongue, modifies the flow passing through the turbine by varying the cross-section. The water jet is directed towards the cylindrical runner by a fixed nozzle; so the water enters the runner at an angle of appr. 45 degrees, transmitting some part of the kinetic energy to the active cylindrical blades.
According to the position of the regulating device (0 – 100%) water is also admitted to 0-100%*30/4 blades. Water admission is controlled by two profiled guide vanes, dividing and directing the flow in such a way that the same enters the runner free of jerks, irrespective of the opening width. Both guide vanes have been accurately adapted to the turbine casing. They are maintaining the leakage quantity so small that in case of low heads the guide vanes serve as a closing device. In this case no closing valves are required between penstock and turbine. Both guide vanes can be set up separately through control levers, to which the automatic or manual control has been connected. The turbine geometry (nozzle-runner-shaft) assures that the water jet is not made ineffective, hitting the shaft, as appr. 1/3 of the overall power is still transferred to the runner when the water is leaving the turbine. The work at the runner is therefore performed twice, at a ratio of 2:1.
The water flows through the blade channels in two directions: Outside – inside, and inside – outside. Most turbines are run with two jets, so two water jets in the runner will not affect each other. It is, however, essential that turbine, head and turbine speed are harmonised.
The cross-flow turbine is of the impulse type, so the pressure will remain constant at the runner.
A cross-flow turbine is mostly built in cell construction to improve its part load behaviour considerably. The turbine consists of two chambers, with two runners on one common shaft, the chambers being subdivided at Q*2/3 and Q*1/3. The smaller chamber is used with small flows, the larger one with medium flows, and both chambers are used with large flows as follows: Q*1/3 + Q*2/3 = Q.
Advantages
Compared with Kaplan, Francis and Pelton Turbines the peak efficiency of Cross-Flow Turbines is somewhat smaller, this is, however, compensated by the flat efficiency curve. Due to the subdivision each flow admitted between 1/1 and 1/6 of the nominal value is utilised at an optimal efficiency. For its favourable price and the simple regulation they are above all utilised with mini and micro units of up to 2.000 kW and with heads of 200 m max. Particularly with small run-of-the-river plants the flat efficiency curve yields a higher annual performance than other turbine systems as flowing waters are mostly weak for several months. It depends on the efficiency values of a turbine whether electricity is produced throughout this time or not. If turbines are used with high peak efficiencies, but an unfavourable behaviour at part load conditions, a smaller annual performance is obtained than in case of turbines with a flat efficiency curve. Due to its excellent behaviour at part load conditions the turbine is also most suitable for an autonomous electricity production. Its simple construction is advantageous, compared with all other turbine types; a maximum of two friction bearings must be exchanged, and there are three rotating elements only; the mechanical system is quite simple, repairs can therefore be performed by almost everybody.
As the water jet enters the runner, flowing out inversely from inside to outside, leaves, grass etc. will not remain in the runner, so no production losses are faced. Being a function of the relationship between part load / full load the lower nominal efficiency curve is maintained constant by the runner which always remains clean. No runner cleaning is therefore necessary, e.g. by flow inversion or variations of the speed. Other turbine types with lower outputs are polluted easily, consequently power losses are faced.
