Francis turbine and Components

A Francis turbine is a type of water turbine.It is an internal flow reaction turbine combining radial flow and axial flow concepts.Francis turbines are the most commonly used turbines today, with efficiencies of over 95%.The process of realizing the modern Francis wheel design from 1848 to about 1920.It was known around 1920 as the Francis turbine, named after the British-American engineer James B.Francis who created a new turbine design in 1848.Francis turbines are mainly used to generate electricity.Generator outputs typically range from a few kilowatts to 1,000 megawatts, although smaller hydroelectric installations can have lower outputs. Performance is best when head height is between 100-300 m (330-980 ft).Steel pipe diameters between 1 and 10 m (3.3 and 32.8 ft). The speeds of the different turbine units range from 70 to 1000 rpm.Guide vanes on the outside of the turbine's rotating runner control the speed of water flow through the turbine at different power generation rates.Francis turbines are usually mounted with a vertical shaft to isolate the water from the generator.This also facilitates installation and maintenance.

Development 

Different types of water wheels have been used for over 1,000 years to power various types of mills, but they are relatively inefficient. Efficiency improvements in water turbines in the 19th century enabled them to replace nearly all water wheel applications and compete with steam engines wherever water power was available.After the development of electric generators in the late 1800's, turbines became a natural source of power for electric generators where potential hydraulic resources existed.In 1826, French engineer Benoit Fourneyron developed a high-efficiency (80%) outflow turbine.Water is directed tangentially through the turbine wheel, causing it to spin.Another French engineer, Jean-Victor Poncelet, designed an internal flow turbine around 1820 using the same principle.S. B.Howd received a U.S. patent for a similar design in 1838.In 1848, James B.Francis improved on these designs to create more efficient turbines while working as chief engineer for the Locks and Canals Company in the waterwheel textile mill city of Lowell, Massachusetts.He used scientific principles and testing methods to design very efficient turbines.More importantly, his mathematical and graphical computing methods improved the design and engineering of turbines.His analytical methods enable the design of highly efficient turbines to precisely match the water flow and pressure (head) on site.

Components

Francis turbine consists of the following main parts:

Spiral casing:The spiral casing around the turbine wheel is called the volute or volute.Throughout its length it has a number of regularly spaced openings to allow the working fluid to impinge on the blades of the runner.These openings convert the pressure energy of the fluid into kinetic energy just before the fluid hits the blades.This maintains a constant velocity as the cross-sectional area of the housing decreases uniformly around the circumference despite providing many openings for the fluid to enter the vanes.

Guide and stay vanes:The main function of the guide and stay vanes is to convert the pressure energy of the fluid into kinetic energy. It is also used to direct the flow to the runner blades at a design angle.

Draft Tube:The draft tube is the conduit that connects the outlet of the runner to the draft channel where the water exits the turbine.Its main function is to reduce the velocity of the discharged water in order to minimize the loss of kinetic energy at the exit.This allows the turbine to be positioned above the tailwater without significant drop in available head.