Short Answer:

The main difference between a Francis turbine and a Kaplan turbine lies in their working heads and design. A Francis turbine operates efficiently at medium head and moderate flow, while a Kaplan turbine is suitable for low head and high flow conditions. Francis turbines have curved fixed blades, whereas Kaplan turbines have adjustable blades.

Francis turbines are typically used in hilly or dam-based hydro projects, while Kaplan turbines are ideal for rivers with large volumes of slow-moving water. Both are reaction turbines, but their internal structure and application areas are different based on site conditions.

Detailed Explanation:

Difference between a Francis turbine and a Kaplan turbine

Francis and Kaplan turbines are both reaction turbines, meaning they use both the pressure and velocity energy of water to produce mechanical energy. However, they are designed for different types of water flow and head, making their structure, working, and application distinct.

1. Working Head and Flow Conditions

The most basic difference is the range of head and flow for which they are used:

• Francis Turbine: Best for medium head (30 to 300 meters) and moderate water flow.
• Kaplan Turbine: Designed for low head (below 30 meters) and high water flow.

Francis turbines are generally installed in areas where water is stored at a higher elevation, such as mountain dams. In contrast, Kaplan turbines are preferred in flatter regions with large rivers where water falls from a low height but in large volumes.

2. Blade Design and Movement

• Francis Turbine: It has fixed curved blades on the runner. Water enters radially and exits axially. The blade shape is optimized for constant flow conditions.
• Kaplan Turbine: It has adjustable or movable blades. Water flows axially (parallel to the shaft) through the runner. The blade angle can be changed to suit different flow rates, improving efficiency under varying loads.

The adjustability of the Kaplan turbine makes it highly efficient even when the flow conditions change frequently.

3. Water Flow Direction

• In Francis turbines, the flow is mixed: water enters radially and exits axially.
• In Kaplan turbines, the flow is purely axial, meaning the water enters and leaves in the same direction along the shaft.

4. Efficiency and Usage

Kaplan turbines are more efficient at low heads and variable flow because of their adjustable blades. Francis turbines are more efficient in sites with steady medium heads. Both have high efficiency in their operating ranges, but their performance depends on matching the correct turbine to the site conditions.

5. Structure and Size

Francis turbines are more compact and can handle high rotational speeds, while Kaplan turbines tend to be larger due to the size of the runner and need more space for the adjustable blade mechanism.

Application Summary

• Francis Turbine: Used in hill-based dams or reservoirs with medium water head.
• Kaplan Turbine: Used in river-based stations with high water volume and low head.

Conclusion:

The difference between a Francis turbine and a Kaplan turbine lies mainly in their design, head and flow requirements, and blade operation. Francis turbines use fixed blades and are ideal for medium head sites, while Kaplan turbines use adjustable blades and are perfect for low head and high flow conditions. Selecting the right turbine ensures maximum efficiency and reliable power generation.