Short Answer:

The difference between impulse and reaction turbines lies in how they convert the energy of steam into mechanical work. In an impulse turbine, steam expands only in the nozzles, and the high-speed jet strikes the blades, causing rotation. In a reaction turbine, steam expands both in the nozzles and in the moving blades, and the force is produced due to both pressure and velocity changes.

Impulse turbines work on the principle of direct impact of steam, while reaction turbines use both impulse and reactive forces for rotation. Impulse turbines are simple and operate at high speed, while reaction turbines are more efficient and used for higher power applications.

Detailed Explanation:

Difference between impulse and reaction turbines

In mechanical and thermal engineering, steam turbines are classified mainly into two types—impulse turbines and reaction turbines. Both types are used for converting thermal energy of steam into mechanical energy, but they operate on different working principles and have structural differences in how the steam interacts with turbine blades.

Understanding the differences between these turbines is important for designing thermal power systems, selecting suitable turbines, and achieving high efficiency in steam-based power plants.

Working Principle

Impulse Turbine

In an impulse turbine, the entire pressure drop of steam takes place in the stationary nozzles.
These nozzles convert high-pressure steam into high-velocity jets.
The moving blades only change the direction of the steam jet and extract energy by momentum transfer.
The steam does not expand in the moving blades; pressure remains constant across them.

Reaction Turbine

In a reaction turbine, the pressure drop occurs in both fixed nozzles and moving blades.
Steam partially expands in the fixed nozzles and continues to expand in the moving blades.
The moving blades act like nozzles themselves, creating reaction forces that push the blades forward.
Energy conversion happens due to both pressure and velocity changes.

Key Differences

Aspect	Impulse Turbine	Reaction Turbine
Pressure drop	Occurs only in nozzles	Occurs in nozzles and moving blades
Blade design	Simple, symmetrical	Asymmetrical and more complex
Steam velocity	High at nozzle exit	Moderate and continuous
Blade force	Only impulse force	Impulse and reaction forces
Efficiency	Less efficient than reaction	Higher efficiency for same size
Leakage	Less because of high pressure drop in nozzle	More sealing needed due to continuous pressure drop
Construction	Simpler and more robust	More complex and precise
Example	De Laval turbine	Parsons turbine

Applications

Impulse Turbines:

Used where pressure is high and space is limited.
Common in hydroelectric plants, small steam turbines, and simple power plants.
Easier to maintain due to simpler design.

Reaction Turbines:

Used in large-scale thermal power plants and nuclear plants.
Suitable for multi-stage expansion of steam.
Ideal for applications needing high power and high efficiency.

Performance and Usage

Impulse turbines are good for low-speed and intermittent load applications.
Reaction turbines are preferred for continuous, high-load applications due to their ability to handle large steam flows and extract more energy per stage.

Real-World Example

In a thermal power station, the high-pressure steam first enters an impulse stage to reduce velocity shock, then passes through reaction stages for efficient energy extraction. This combination is called a compound turbine, utilizing the benefits of both types.

Conclusion

The main difference between impulse and reaction turbines lies in where the steam expands and how it causes blade movement. Impulse turbines convert pressure into velocity in the nozzles only and use steam jets to rotate the blades. In reaction turbines, expansion occurs in both fixed and moving blades, generating both impulse and reaction forces. Understanding these differences helps in choosing the right turbine for specific power plant designs and performance requirements.