# Comparison of Hydroelectric turbines

Pelton, Francis and Kaplan are the most commonly used turbines in the production of hydroelectricity. Each of the turbine is unique in its characteristics and must be used in suitable operating conditions. In this video we will compare these three turbines and see when to use which turbine.

Detailed webpage version of the video is given below.

## Nature of the Prime force

When we move from Pelton to Kaplan the force producing mechanism changes from impulse to reaction.

Fig.1 In Pelton turbine the force producing mechanism is pure impulse force; while in Kaplan it is pure reaction

## Suitability of Pelton, Francis and Kaplan turbines

In Pelton pure impulse force of the water jet is responsible for the rotation of the impeller. Water stored at high altitude can produce high impose force due to its high velocity. So Pelton turbines are suitable for operation when water energy is available at high head and low flow rate.

Fig.2 Impulse action in produced in Pelton bucket due to the jet impingement.

Whereas Kaplan turbines are suitable for the reverse case. In Kaplan reaction force is produced by airfoil action of the runner blades. A high water flow guarantees efficient production of reaction force. So when water is available and high flow rate and low head, Kaplan turbine is the best choice. Following is the operating condition preferable for Kaplan turbines.

• Flow rate = 70-800 m3/s

Fig.3 Flow around the runner blades produce a reaction force due to the airfoil action

Francis turbine comes in between for medium head to medium flow rate applications. Following is the operating condition Francis turbine is suitable for.

• Flow rate = 10-700 m3/s

Francis turbine is not a pure reaction turbine a portion the force comes from the impulse action also. The specially curved impeller blades of Francis turbine extract energy from the water via reaction and impulse action.

Fig.4 In Francis turbine major amount of the force is induced to airfoil reaction, but a portion of force comes due to the impulse action also

## Turbine Selection graph

The following graph gives a clear idea about when to use which turbine depending upon the available water head and flow rate. It is obvious that Francis turbine covers a wide range of operating conditions or they can work efficiently under wide range of operating conditions.

Fig.5 Head verses Flow rate graph where each turbine is suitable for operation

This is why Francis turbine is the most preferred hydraulic turbine. The following graph shows variation of turbine efficiency as a function of specific speed of the turbine. If you use a turbine out if it’s recommended operating range it will work but less efficiently.

Fig.6 Variation of each turbine efficiency as a function of the speicif speed

It is also interesting to note the direction a fluid flow with respect to the axis of rotation in each case. In Pelton the jet flow is tangential to the axis of rotation. In Kaplan fluid flow through the runner is axial in nature. While in Francis, fluid enters radially and leaves axially; they are also referred as mixed flow turbines.