Kaplan vs Francis Turbine: Differences, Working Principles, Types and Applications

Hydropower is one of the most important sources of renewable energy. It uses the energy of flowing or falling water to generate electricity. In a hydropower plant, turbines play a major role by converting the hydraulic energy of water into mechanical energy, which is then used to operate an electric generator.

Among the different types of hydraulic turbines, Kaplan and Francis turbines are widely used in hydroelectric power plants. Both turbines are designed for different operating conditions, depending on water flow rate and available head.

Understanding Kaplan vs Francis turbine is important for civil engineering students, mechanical engineers, and professionals working in hydraulic engineering. Although both turbines perform the same basic function, their construction, working principles, and applications are different.

In this article, we will explain the working principles, major components, types, advantages, disadvantages, and the main difference between Kaplan and Francis turbine.

1. What Is a Hydraulic Turbine?

A hydraulic turbine is a mechanical device that converts the energy of water into rotational mechanical energy.

When water flows through a turbine, it applies force to the turbine blades or runner, causing it to rotate. This rotation is transferred to an electric generator, which produces electricity.

Hydraulic turbines are commonly used in:

• Hydroelectric power plants.

• Water resource engineering projects.

• Renewable energy generation.

• Dam-based power stations.

• Small and large hydropower systems.

Hydraulic turbines are generally classified into impulse turbines and reaction turbines.

Kaplan and Francis turbines both belong to the reaction turbine category because they operate using pressure and velocity changes as water passes through the runner.

2. What Is a Kaplan Turbine?

A Kaplan turbine is a reaction-type hydraulic turbine mainly used in low-head and high-discharge hydropower plants.

It was developed by Austrian engineer Viktor Kaplan. The turbine has a propeller-shaped runner with adjustable blades, allowing it to operate efficiently under changing water flow conditions.

The main characteristic of a Kaplan turbine is that both its runner blades and guide vanes can be adjusted in many designs. This helps maintain good efficiency when the water flow rate changes.

Working Principle of Kaplan Turbine

A Kaplan turbine works on the principle of reaction. Water enters the turbine through a spiral casing and passes through adjustable guide vanes.

The guide vanes control the direction and quantity of water entering the runner. Water then flows through the runner blades, causing them to rotate.

As water passes through the runner, its pressure and velocity change, transferring hydraulic energy to the rotating shaft.

The shaft is connected to an electric generator, which converts mechanical energy into electrical energy.

Main Components of Kaplan Turbine

1. Spiral Casing

The spiral casing distributes water evenly around the turbine runner. It is designed to maintain a suitable flow velocity and reduce hydraulic losses.

2. Guide Vanes

Guide vanes control the amount and direction of water entering the runner. They help regulate the turbine's operating conditions.

3. Runner Blades

The runner consists of propeller-shaped blades that rotate when water passes through them. In a Kaplan turbine, these blades are adjustable to improve efficiency under varying loads.

4. Draft Tube

The draft tube is installed at the outlet of the runner. It helps recover part of the kinetic energy of water and allows the turbine to be installed above the tailrace water level.

5. Shaft

The shaft transfers the rotational energy from the turbine runner to the electric generator.

Advantages of Kaplan Turbine

• Highly efficient for low-head applications.

• Suitable for high water discharge.

• Adjustable runner blades improve performance.

• Performs well under changing flow conditions.

• Compact design for suitable low-head installations.

Disadvantages of Kaplan Turbine

• Construction is relatively complex.

• Initial installation cost can be high.

• Adjustable blade mechanisms require maintenance.

• Not suitable for high-head applications.

• Requires careful design to prevent cavitation.

3. What Is a Francis Turbine?

A Francis turbine is a reaction-type hydraulic turbine commonly used in medium-head and medium-flow hydropower plants.

It was developed by American engineer James B. Francis. It is one of the most widely used hydraulic turbines because of its versatility and ability to operate efficiently under different hydraulic conditions.

Unlike a Kaplan turbine, a Francis turbine generally has a runner with fixed blades. Water enters the runner radially or inward through its outer circumference and leaves in an axial direction.

This type of turbine is also known as a mixed-flow turbine because water changes its direction while passing through the runner.

Working Principle of Francis Turbine

A Francis turbine operates by converting the pressure and kinetic energy of water into mechanical energy.

First, water enters the spiral casing, which distributes it around the runner. It then passes through the stay vanes and guide vanes.

The guide vanes direct water at the correct angle toward the runner blades. As water enters the runner, its pressure and velocity change, producing a force that rotates the runner.

After passing through the runner, water moves into the draft tube and finally discharges into the tailrace.

The rotating runner transfers mechanical energy through a shaft to an electric generator.

Main Components of Francis Turbine

1. Spiral Casing

The spiral casing surrounds the turbine runner and distributes water uniformly around its circumference.

2. Stay Vanes

Stay vanes are fixed components that support the guide vane assembly and help direct water toward the runner.

3. Guide Vanes in Francis Turbine

Guide vanes in Francis turbine play an important role in controlling the flow of water entering the runner.

They are adjustable and can be opened or closed to regulate the quantity of water supplied to the turbine.

Their main functions include:

• Controlling the water discharge.

• Directing water at the correct angle toward the runner.

• Regulating turbine speed and power output.

• Improving hydraulic efficiency under different operating conditions.

The guide vanes are usually connected to a regulating mechanism that allows them to move together.

4. Runner

The runner is the rotating component of the Francis turbine. It contains curved blades that convert hydraulic energy into mechanical rotational energy.

5. Draft Tube

The draft tube is fitted at the outlet of the runner. It helps recover kinetic energy and directs water toward the tailrace.

6. Shaft

The shaft connects the runner to the generator and transfers mechanical power.

Advantages of Francis Turbine

• Suitable for medium-head applications.

• Provides high hydraulic efficiency.

• Can operate over a useful range of flow conditions.

• Widely used in hydroelectric power stations.

• Reliable design with established construction technology.

Disadvantages of Francis Turbine

• More complex than simple impulse turbines.

• Requires proper maintenance.

• Cavitation may occur under unsuitable operating conditions.

• Not the best choice for very low-head and high-discharge applications.

• Performance can decrease when operating far from its design conditions.

4. Kaplan vs Francis Turbine: Main Differences

Kaplan and Francis turbines are both reaction turbines, but their designs and operating conditions are different.

Understanding the difference between Kaplan and Francis turbine is important when selecting a suitable turbine for a hydropower project.

1. Type of Water Flow

A Kaplan turbine uses axial-flow water movement through the runner. Water enters and leaves the runner mainly in the axial direction.

A Francis turbine uses mixed-flow movement. Water enters the runner radially inward and leaves in an axial direction.

2. Operating Head

A Kaplan turbine is generally used for low-head hydropower applications.

A Francis turbine is commonly used for medium-head applications, although its operating range can extend beyond typical medium-head conditions depending on its design.

3. Water Discharge

Kaplan turbines are suitable for high water discharge.

Francis turbines are commonly used for medium discharge, depending on the available head and turbine size.

4. Runner Design

The Kaplan turbine has propeller-shaped runner blades that are adjustable in most conventional designs.

The Francis turbine generally has curved runner blades that are fixed in position.

5. Efficiency

Kaplan turbines maintain good efficiency over changing flow conditions because their runner blades can be adjusted.

Francis turbines also provide high efficiency, particularly when operating near their design conditions.

6. Construction

Kaplan turbines have a more complicated runner blade adjustment mechanism.

Francis turbines have a different runner design and generally use adjustable guide vanes to regulate water flow.

7. Applications

Kaplan turbines are commonly used in low-head rivers, canals, and run-of-river hydropower plants.

Francis turbines are widely used in medium-head dams, reservoirs, and large hydroelectric power stations.

5. Vertical Francis Turbine

A vertical Francis turbine is a type of Francis turbine in which the main shaft is installed in a vertical position.

This arrangement is commonly used in large hydroelectric power stations where the turbine and generator are installed in a vertical configuration.

The water enters the spiral casing, passes through the guide vanes, and reaches the runner. After transferring energy to the runner, water flows through the draft tube toward the tailrace.

Advantages of Vertical Francis Turbine

• Suitable for large hydropower installations.

• Requires relatively less horizontal floor space.

• Commonly used with large generators.

• Allows a compact vertical arrangement of major components.

• Suitable for installations where the turbine and generator are positioned at different elevations.

Applications of Vertical Francis Turbine

Vertical Francis turbines are commonly installed in:

• Large hydroelectric power plants.

• High-capacity dam projects.

• Reservoir-based power stations.

• Large water power generation facilities.

6. Horizontal Francis Turbine

A horizontal Francis turbine is a Francis turbine in which the main shaft is installed horizontally.

This arrangement is generally suitable for smaller or certain medium-sized hydropower installations, depending on the turbine design and available space.

Its working principle is the same as that of a vertical Francis turbine. Water enters through the spiral casing, passes through the guide vanes, rotates the runner, and exits through the draft tube.

The main difference is the position of the shaft and the arrangement of the turbine and generator.

Advantages of Horizontal Francis Turbine

• Convenient arrangement for certain small and medium installations.

• Can provide easier access to some mechanical components.

• Suitable for layouts where horizontal space is available.

• Can simplify maintenance in some installation configurations.

Applications of Horizontal Francis Turbine

Horizontal Francis turbines may be used in:

• Small hydropower plants.

• Mini-hydropower installations.

• Certain industrial hydropower systems.

• Power stations with suitable horizontal layouts.

7. Vertical Francis Turbine vs Horizontal Francis Turbine

Both vertical and horizontal Francis turbines work on the same hydraulic principle. The main difference is their shaft orientation and installation arrangement.

A vertical Francis turbine has its shaft positioned vertically, while a horizontal Francis turbine has its shaft positioned horizontally.

Vertical arrangements are frequently preferred for large-capacity hydroelectric installations because they can accommodate large turbine-generator units and save horizontal floor space.

Horizontal arrangements are useful in smaller installations or locations where the available building layout makes a horizontal shaft more practical.

The selection depends on turbine capacity, powerhouse dimensions, maintenance requirements, installation cost, and project conditions.

8. Applications of Kaplan and Francis Turbines

Both turbines are widely used in hydroelectric power generation, but their applications depend mainly on the available water head and discharge.

Kaplan Turbine Applications

• Low-head hydropower plants.

• River-based electricity generation.

• Irrigation canal power projects.

• Run-of-river hydroelectric stations.

• Locations with high water discharge and low elevation differences.

Francis Turbine Applications

• Medium-head hydropower plants.

• Large dams and reservoirs.

• Hydroelectric power stations.

• Water storage and power generation projects.

• Installations requiring efficient mixed-flow turbines.

9. Which Turbine Is Better: Kaplan or Francis?

The selection between a Kaplan and Francis turbine depends on the hydraulic conditions of the hydropower project.

If a project has a low water head and a high discharge rate, a Kaplan turbine is generally a suitable choice because of its axial-flow design and adjustable runner blades.

If the project has a medium water head and requires an efficient turbine for power generation, a Francis turbine is usually more appropriate.

Engineers should also consider the following factors before selecting a turbine:

• Available water head.

• Water discharge.

• Required power output.

• Overall turbine efficiency.

• Construction and installation costs.

• Maintenance requirements.

• Risk of cavitation.

• Operating conditions.

There is no single turbine that is best for every hydropower project. The most suitable option is the one that matches the project's hydraulic and technical requirements.

Conclusion

Kaplan and Francis turbines are two important types of reaction turbines used in hydroelectric power generation. Although both convert the energy of flowing water into mechanical energy, they have different designs and operating characteristics.

A Kaplan turbine is mainly suitable for low-head and high-discharge applications, while a Francis turbine is commonly used in medium-head hydropower plants.

Guide vanes in Francis turbine control the quantity and direction of water entering the runner, helping regulate the turbine's operation. Francis turbines can also be installed in vertical or horizontal arrangements depending on the design and requirements of the power station.

Understanding the difference between Kaplan and Francis turbine, their working principles, and their applications helps civil and mechanical engineering students develop a better understanding of hydraulic machines and hydropower systems.