Explain the construction and working of Francis Turbine.
Layout & Component:- The general layout of hydroelectric power plant using a rection turbine-like Francis turbine.
A) Penstock:- It is a large sie conduit or pipeline which conveys water from the reservoir to the turbine runner.
B) Volute:- It is a closed passage between the spiral casing and runner whose cross-sectional area gradually decreases so as to increase the velocity.
C) Guide Vanes:- The guide vanes guide the water into the runner at an angle appropriate to the design of the runner blades. The guide vanes can be adjusted from the outside.
D) Governing Mechanism:- Governing mechanism can adjust the guide vanes to affect a variation in water flow rate according to the changes in loading on the turbine.
E) Runner and Runner Blades:- The force for the motion of the runner is developed both due to impulse and reaction effects on blades in which pressure energy of the water is utilized.
F) Draft Tube:- The draft tube is a gradually expanding tube that is used and incorporated to discharge water from the outlet of the runner to the tailrace. The draft tube permits a negative or suction head at the runner so as to increase the power developed by the runner. It is able to convert a large portion of the velocity available in the discharge water from the runner into useful pressure energy which enables the water reaching the tailrace to attain the atmospheric pressure.