What is a Francis turbine?

Short Answer:

A Francis turbine is a type of reaction turbine widely used in hydroelectric power plants. It is designed to work under medium head and medium discharge conditions. Water enters the turbine radially and leaves axially after passing through curved blades fixed on the runner. The turbine converts both kinetic and pressure energy of water into mechanical energy, which is then used to drive an electric generator.

The Francis turbine is named after James B. Francis, who developed it in the 19th century. It operates efficiently across a wide range of heads, typically from 10 to 650 meters, and is suitable for most modern hydroelectric stations due to its compact design and high efficiency.

Detailed Explanation :

Francis Turbine

A Francis turbine is a reaction type hydraulic turbine that converts the energy of water into mechanical energy by using both the pressure and velocity of water. It is an inward flow mixed turbine, meaning water enters the runner radially and exits axially. This turbine is the most commonly used type in hydroelectric power stations because of its ability to operate efficiently under varying load conditions.

It was invented by James B. Francis in 1849, and since then, it has undergone many improvements. It combines the features of both impulse and reaction turbines and works best for medium heads (10 m to 650 m) and moderate discharge rates.

Main Parts of Francis Turbine

The Francis turbine consists of the following main components:

  1. Spiral Casing:
    The spiral casing, also called a volute, is a spiral-shaped chamber that distributes the water uniformly around the turbine runner. Its cross-section gradually decreases along the flow direction to maintain constant velocity.
  2. Guide Vanes (Wicket Gates):
    These are adjustable blades fitted between the spiral casing and runner. They control the quantity and direction of water entering the runner. The guide vanes can be adjusted according to load demand, helping to regulate turbine speed and efficiency.
  3. Runner and Blades:
    The runner is the rotating part of the turbine, mounted on the shaft. It contains several curved blades designed to direct water flow smoothly from radial to axial direction. The water strikes these blades, transferring energy to the runner and causing it to rotate.
  4. Draft Tube:
    The draft tube is a gradually expanding pipe that connects the runner outlet to the tailrace. It allows the water to exit at low velocity, recovering pressure energy and improving overall efficiency.
  5. Shaft:
    The shaft transmits mechanical energy from the runner to the generator for producing electricity.

Working Principle

The working of a Francis turbine is based on the principle of energy conversion. When water flows from the head race through the penstock, it enters the spiral casing and passes through the guide vanes. The guide vanes direct the water at the proper angle onto the runner blades. As the water flows over the curved surfaces of the runner blades, its velocity and pressure both decrease, and the energy is transferred to the runner in the form of mechanical rotation.

The flow is radial at entry and axial at exit, which makes the turbine a mixed flow type. The used water then passes through the draft tube into the tailrace. The draft tube helps in recovering part of the kinetic energy by converting velocity head into pressure head, thus improving efficiency.

Operating Conditions

  • Head range: 10 m to 650 m
  • Specific speed range: 60 to 300 (metric units)
  • Type of flow: Mixed (radial and axial)
  • Discharge: Medium
  • Position of turbine: Always kept below the tailrace to ensure continuous water flow through the draft tube.

Advantages of Francis Turbine

  1. High Efficiency: Efficiency ranges between 85% and 95%, making it one of the most efficient turbines.
  2. Wide Range of Head: It can operate efficiently across medium heads.
  3. Compact Design: Requires less space compared to Pelton or Kaplan turbines.
  4. Adjustable Guide Vanes: The flow can be controlled easily to maintain constant speed under varying loads.
  5. Smooth Operation: It runs smoothly with less vibration and noise.

Applications

Francis turbines are extensively used in medium and large hydroelectric power plants across the world. They are suitable for power generation where the water head is moderate, such as rivers or reservoirs located between high- and low-head conditions. Some famous hydroelectric projects, like Bhakra Nangal (India) and Grand Coulee Dam (USA), use Francis turbines.

Example of Operation

For example, in a hydroelectric station with a head of 120 meters and water discharge of 20 cubic meters per second, a Francis turbine is ideal. It can convert this potential energy efficiently into mechanical energy to drive a generator of capacity around 20 MW.

Conclusion:

A Francis turbine is a reaction-type mixed flow hydraulic turbine suitable for medium heads and medium discharge. It works by converting both pressure and kinetic energy of water into mechanical energy through its specially designed runner blades. With high efficiency, adjustable guide vanes, and compact design, it is the most commonly used turbine in hydroelectric power generation. Its ability to adapt to different operating conditions makes it a reliable choice for sustainable power production.