Short Answer:

The Kaplan and Francis turbines are both reaction turbines, but they differ mainly in the direction of water flow and their operating heads. The Kaplan turbine is an axial flow reaction turbine used for low heads and high discharge, while the Francis turbine is a mixed flow reaction turbine used for medium heads and moderate discharge.

In the Kaplan turbine, the blades are adjustable, which allows it to maintain high efficiency under varying loads. In contrast, the Francis turbine has fixed blades and works efficiently only at a particular design condition. Both are widely used in hydroelectric power generation systems.

Detailed Explanation :

Differences Between Kaplan and Francis Turbines

Both Kaplan and Francis turbines belong to the category of reaction turbines, where energy conversion occurs due to both pressure and velocity changes of water. However, they differ in design, operating conditions, and applications. These differences make each turbine suitable for a specific range of heads and discharges.

1. Direction of Flow

• Kaplan Turbine:
  The water flows axially, meaning parallel to the turbine shaft. The flow enters and exits the runner in the same direction as the axis of rotation.
• Francis Turbine:
  The water flow in the Francis turbine is mixed, entering radially and exiting axially. This mixed-flow nature helps achieve efficient energy conversion under medium head conditions.

2. Type of Blades

• Kaplan Turbine:
  It has adjustable blades, which can change their angle according to the water flow and load requirements. This feature allows the Kaplan turbine to maintain high efficiency over a wide range of flows.
• Francis Turbine:
  The Francis turbine has fixed blades, designed for a specific flow rate and head. Therefore, its efficiency reduces if the load or water flow changes.

3. Operating Head

• Kaplan Turbine:
  It is designed for low heads ranging from 2 to 40 meters. It works efficiently in areas where a large quantity of water is available but with a small height difference.
• Francis Turbine:
  It operates under medium head conditions, typically between 40 and 600 meters, making it suitable for a wide range of power stations.

4. Discharge Rate

• Kaplan Turbine:
  It works with high discharge rates, as it is suitable for low-head and high-flow water sources such as rivers and tidal streams.
• Francis Turbine:
  The discharge rate is moderate, suitable for medium-flow conditions typically found in dam-based hydro projects.

5. Efficiency and Load Variation

• Kaplan Turbine:
  The adjustable runner blades allow it to maintain high efficiency (up to 90%) even when the water flow and load vary.
• Francis Turbine:
  Efficiency decreases when operating away from the design point, as the fixed blades cannot adapt to changing flow conditions.

6. Runner Design

• Kaplan Turbine:
  The runner of the Kaplan turbine resembles a ship’s propeller, having 3 to 6 adjustable blades attached to a hub. The flow of water is axial through the runner.
• Francis Turbine:
  The runner of the Francis turbine has a crown-shaped wheel with fixed curved blades, where water flows radially inward and exits axially.

7. Specific Speed

• Kaplan Turbine:
  The specific speed is very high, typically ranging from 300 to 1000, which makes it suitable for low head and high flow.
• Francis Turbine:
  It has a medium specific speed ranging from 60 to 300, appropriate for medium heads and moderate flows.

8. Cavitation Tendency

• Kaplan Turbine:
  It is more prone to cavitation because of high flow velocity and low head. Proper design of the draft tube is necessary to minimize cavitation.
• Francis Turbine:
  It has less cavitation tendency due to higher head and lower discharge velocity compared to Kaplan turbines.

9. Speed of Operation

• Kaplan Turbine:
  It runs at high rotational speed, which allows the use of smaller generators.
• Francis Turbine:
  It runs at medium speed, requiring slightly larger generators compared to Kaplan turbines.

10. Application

• Kaplan Turbine:
  Used in low-head, high-flow hydroelectric plants, tidal power stations, and run-of-river projects.
• Francis Turbine:
  Used in medium-head hydro plants such as dam-based stations and pumped storage plants.

11. Example Installations

• Kaplan Turbine:
  Installed in projects like the Bhakra Nangal (India) and Grand Coulee Dam (USA) for low-head conditions.
• Francis Turbine:
  Widely used in the Tehri Dam (India) and Hoover Dam (USA) for medium-head applications.

12. Maintenance and Cost

• Kaplan Turbine:
  Due to the adjustable blades and complex mechanism, maintenance is comparatively higher.
• Francis Turbine:
  Having fixed blades and a simpler structure, it requires less maintenance and is less costly to construct.

Summary of Key Differences:

Feature	Kaplan Turbine	Francis Turbine
Flow direction	Axial	Mixed (radial to axial)
Blade type	Adjustable	Fixed
Head range	2–40 m (Low)	40–600 m (Medium)
Specific speed	High	Medium
Efficiency under varying load	High	Moderate
Discharge	High	Medium
Runner design	Propeller-like	Crown-like
Cavitation tendency	High	Low

Conclusion:

The Kaplan turbine and Francis turbine are both efficient reaction turbines, but their applications depend on water head and discharge conditions. The Kaplan turbine, with adjustable blades and axial flow, is ideal for low-head and high-flow situations, whereas the Francis turbine, with fixed blades and mixed flow, suits medium-head conditions. Each type has its unique design and performance advantages. Therefore, the selection between the two depends on the site’s hydraulic conditions, desired efficiency, and maintenance requirements.