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Difference between Francis and Kaplan Turbine
A turbine is a device which converts the energy of some fluid into rotational mechanical energy. There are a number of turbines available based on the type of input fluid used such as steam turbine, water turbine, gas turbines, etc.
In this article, we will discuss about Francis turbine and Kaplan turbine and the key differences between them. Both Francis turbine and Kaplan turbine are types of water turbines, where the hydraulic energy of water is converted into mechanical energy (rotation of turbine). Basically, both Francis and Kaplan turbines come under the category of reaction turbine, which means in both cases, kinetic and pressure energy is required for operation.
The Francis turbine is an old water turbine, while the Kaplan turbine is the upgraded version of the Francis turbine which means the limitations of the Francis turbine are rectified in Kaplan turbine.
Therefore, we will differentiate the Francis turbine and Kaplan turbine by considering various parameters such as definition, construction, working, operating conditions, etc. But, before going into the differentiation, it is advantageous to know about the basics of Francis turbine and Kaplan turbine.
What is a Francis Turbine?
A Francis turbine is a type of water turbine which converts the flow of water into rotational mechanical energy. It was invented by James Bicheno Francis, a British-American civil engineer.
In electric power generation, the Francis turbine is mainly used for low to medium water heads. It consists of an outer ring of stationary guide blades fixed to the turbine casing and an inner ring of rotating blades forming the runner. The flow of water to the turbine is controlled by the guide blades. In case of the Francis turbine, the water flows radially inwards and then its direction changes to downward while passing through the runner.
Since in the Francis turbine, the water passes over the rotating blades of the runner which reduces the pressure and velocity of the water. This results in a reaction force which spins the turbine.
What is a Kaplan Turbine?
A Kaplan turbine is a propeller type water turbine invented by an Australian Engineer Viktor Kaplan. In hydro-electric power plants, the Kaplan turbine is used for low heads and large quantities of water.
The Kaplan turbine consists of automatically adjustable propeller blades. In case of Kaplan turbine, the runner of the turbine receives water axially, i.e. the water flows radially inwards through the regulating gates all around the sides and then changing the direction in the runner of the turbine to axial flow. This results in a reaction force which spins the turbine.
Difference between Francis Turbine and Kaplan Turbine
Both Francis turbine and Kaplan turbine are reaction-type water turbines. However, there are many differences between them which are highlighted in the following table
| Basis of Difference | Francis Turbine | Kaplan Turbine |
|---|---|---|
| Definition | The Francis turbine is a reaction type water turbine in which water flows radially inwards and changes to a downward direction while passing through the runner. | The Francis turbine is a propeller-type water turbine which has adjustable blades and the runner receives water axially. |
| Water head | Francis turbine is used for low to medium water heads. It generally varies from 100 m to 600 m. | Kaplan turbine is used for low water heads, generally 100 meters. |
| Construction | The construction of the Francis turbine is simple. | The Kaplan turbine is relatively complicated in construction. |
| Rate of water flow | Francis turbine is used for medium rate of water flow. | Kaplan turbine is used for high rates of water flow. |
| Water flow type | In Francis turbine, the water enters to the runner radially and leaves axially. For this reason, it is also called mixed flow turbine. | In Kaplan turbine, water flows axially, which means the water enters and leaves the runner in axial direction. |
| Number of runner blades | The number of blades in the runner of the Francis turbine are generally between 16 and 24. | In Kaplan turbine, the number of runner blades is generally between 3 and 8. |
| Size | The size of the Francis turbine is larger. | The size of the Kaplan turbine is relatively smaller than the Francis turbine. |
| Losses | Francis turbine has large number of blades which causes large energy loss due to friction. | In Kaplan turbine, number of blades are less, hence the energy loss due to friction is also less. |
| Efficiency | Francis turbine is less efficient than Kaplan turbine. | The efficiency of Kaplan turbine is high compared to Francis turbine. |
| Direction of shaft | In Francis turbine, the mounting of shaft may be horizontal or vertical depending upon the requirement. | The mounting of shaft in Kaplan turbine is always in vertical direction. |
| Speed of rotation | For the Francis turbine, the speed of rotation of the turbine is less. The specific speed of turbine varies from 60 RPM to 300 RPM. | The speed of rotation of the Kaplan turbine is high. The specific speed varies from 600 to 1000 RPM. |
| Type of runner vanes | Francis turbine has fixed runner vanes. | Kaplan turbine has adjustable runner vanes. |
| Speed governing system | The governing system of Francis turbine is simple. | The governing system of Kaplan turbine is complex. |
| Applications | Francis turbine is used in hydro-electric power plants to produce electricity, and also used in irrigation pumping systems. | Kaplan turbine is widely throughout the world to generate electricity in hydro-electric power plants. |
Conclusion
The most significant difference between a Francis Turbine and a Kaplan Turbine is that the Francis turbine is a mixed flow turbine suitable for low-to-medium water heads, while the Kaplan turbine is an axial flow turbine which is suitable for very low water heads.
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