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The Francis turbine is used when the head is between about 100 feet (30 meters) and 1,000 feet (300 meters). A Francis turbine\'s rotor is enclosed in a casing. Its wheel has as many as 24 curved blades. Its axle is vertical. The wheel operates underwater. It is encircled by a ring of guide vanes, which can be opened or closed to control the amount of water flowing past the wheel. The spaces between the vanes act as nozzles to direct the water toward the wheel\'s center. The rotor is turned chiefly by the weight or pressure of the flowing water.