Initially, Francis turbine was designed as a pure radial flow reaction turbine by an American engineer James B. Francis. Modern Francis turbine is mixed flow reaction turbine as water enters the turbine in radial direction and exits in axial direction. It operates under medium heads and also requires medium quantity of water. It is employed in the medium head power plants. It covers wide range of heads from 30 to 150 m. Water is brought down to turbine from reservoir and directed to a number of stationary orifices fixed all around the circumference of runner, which is known as guide vane.
Water head is partly converted into kinetic head and rest remains as pressure head. There is difference in pressure between guide vane and runner, called as reaction pressure. The reaction pressure is responsible for motion of the runner. Therefore, a Francis turbine is called a reaction turbine.
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