The Francis turbine is a in 1849 in the U.S. improved by the engineer James B. Francis water turbine.
When Francis spiral turbine, the water is displaced by a helical pipe, the spiral in additional twist and then directed through a stationary " stator " with adjustable vanes on the opposite curved blades of the impeller. Is the air intake area for stator not spirally formed, one speaks of a Francis turbine shaft.
By acting as a diffuser extension of the suction tube to the turbine axis, the water is discharged after passing through the impeller. With the help of the guide vanes, the speed and thus the performance of the turbine during load changes of the connected generator and changing water levels constant. The Francis turbine is a reaction turbine, the impeller inlet pressure is higher than at the impeller outlet.
Belonging to the turbine controller detects the rotational speed of the turbine to the shaft as a controlled variable, and converts the result to the servo hydraulic means in a control variable, which opens the vanes according to the rotational speed deviation or closes. The design effort for the regulation of the turbine is substantial and makes a noticeable share of investment from a Francis turbine.
The efficiency of Francis turbine varies depending on the type, age and operating point. Modern Francis turbines achieve efficiencies of over 90%.
Area of application
The Francis turbine is the most common type of turbine for hydropower plants. It comes in medium heads of the water and mean flow quantities used. It is therefore used in hydroelectric power plants and storage plants. Your range extends from 10 kW to over 700 MW.
The record for the head of Francis turbines is 695 m, and that the pumped storage power plant Häusling im Zillertal, there are 2 units of 180 MW.
As a special form of Francis turbine can view the flap turbine impeller. The flap wheel turbine has an adjustable impeller, while the fixed guide wheel is not adjustable. The impeller has a shape similar to the idler wheel of the Francis turbine. The efficiency is similar to that of Francis turbine at full load, but does not as strong at impeller increasingly closed off. The application of such a turbine is only useful for small systems up to 500 kW when the water levels fluctuate widely and only one turbine is to be used. Amounts of water from approximately 15% opening can be used economically.
Francis spiral turbine
Set tail for maximum performance
Tail set for minimum power
Closed diffuser of a Francis turbine shaft
Impeller of a turbine impeller flaps from the Saugrohrsicht
Inlet of a Francis turbine of the Grand Coulee Dam
Runner of a Francis turbine in the same power plant
Impeller for the power plant at the Three Gorges Dam
Wheel of 1930 for the power plant at Niagara Falls Chippawa