Kaplan vs Francis Turbine
Water, always in motion, carries energy with it. Humans have always wondered at the tremendous power of it and often made use of the power. But, only in the wake of the 19th century, engineers developed machinery to harness this energy with efficiency. Turbines are the machines designed to capture the energy from the fluid flow and turn it into mechanical energy.
Francis turbine and the Kaplan turbine are two types of reaction turbines used in hydro power plants for driving the generator. They are the most common types of turbines that are used in the modern power plants.
Francis Turbine
Francis turbine was developed by Englishman James B. Francis in 1849 while working as head engineer of the Locks and Canals Company. The turbine was designed for powering the textile factory machines using the river nearby. Using scientific methods and experimentation he was able to develop the design to achieve up to 90% efficiency. Today Francis turbines are the most widely used turbines in the world.
Francis turbine is enclosed in a casing, and the blades have special curved features designed to gain optimal performance from the turbine. Francis turbines operate under a water head of 10-650 meters and the generator driven by the turbine can give a power output up to 750 MW. The turbines have a speed range from 80 to 100 revolutions per minute.
Francis turbine has a vertical shaft assembly, and a horizontally oriented rotor assembly, called the runner, which operates under water. The water inlet is also vertical and is directed towards the runner by controllable guide vanes. The runner rotates mainly due to the weight / pressure of the water.
Kaplan Turbine
Kaplan turbine was developed in 1913 by the Austrian professor Viktor Kaplan. It is also known as the propeller turbine because its runner resembles the propeller of a ship. It has adjustable blades and wicket gates to obtain the optimal efficiency in different pressure/head conditions. Therefore, Kaplan turbine can gain efficiencies up to 95% and operate under low head condition which is not possible in Francis turbines.
In Kaplan turbine also the runner is driven by the pressure and the water input level is controlled by the guide vanes. The water head of the Kaplan turbine ranges from 10-70 meters and the generator power output can be from 5-120 MW. The runner diameter is about 2-8 meters and delivers 80-430 revolutions per minute. Since the Kaplan turbines are able to operate under low head conditions they are used in high-flow, low-head power production throughout the world.
What is the difference between Kaplan and Francis Turbines?
• In Kaplan turbine water enters axially and leaves axially, while in Francis turbine water enters the runner radially and exits axially.
• The Kaplan turbine runner has 3-8 blades while Francis turbine runner has 15-25 blades in general.
• Kaplan turbine has a higher efficiency than Francis turbine.
• Kaplan turbine is smaller and compact compared to Francis turbine.
• The rotation speed (RPM) is higher than that of the Francis turbine.
• Kaplan turbine has lesser friction loss and higher efficiency.
• Kaplan turbines can operate under a wide range of head conditions, but Francis turbine requires relatively higher head conditions.
• Kaplan turbines are used in small hydro power plants.