Francis Turbine of Hydro Power Plant
- It is a reaction type turbine. In such turbines, the total available head of water is partly used in the form of pressure energy and partly in the form of kinetic energy. They are mostly used for medium heads of water. The main parts of the Francis turbine are as follows :
- It is an outer ring of stationary guide blades, which are fixed to the casing. The casing distributes water evenly to the guide blades.
- In some cases, stay vanes are also provided in the casing to guide water into the guide vanes. They are casted in sections and fitted together it size is large.
- The wheel or runner carrying the rotating blades is called the inner ring. Refer Figure, the water flows radially inwards and leaves at the centre axially. The pressure of pressure of water is less than atmospheric pressure, it is required to provide a draft tube for discharge of water directly into tail race, its end is kept submerged in water to avoid entrance of air.
- The Francis turbines are constructed in two forms : (i) Horizontal type. (ii) Vertical type.
- The horizontal types are more accessible and provide higher speeds, but large capacity turbines are generally vertical types.
- The guide blades are so designed that they can be turned along the axis parallel to the turbine axis for regulating the quantity of water. They can be moved in one direction or the other direction simultaneously depending upon how the water passage is controlled.
- These type of turbines have high efficiency at 3/4 f.l. and full - load. They can be designed for high speeds, hence their diameter and weight are less. At low speed its efficiency may fall to about 65% from about 92% on f.l.