How is the Rankine cycle improved using reheating?

Short Answer:

The Rankine cycle is improved using reheating by expanding steam in multiple turbine stages with reheating between them. In this method, steam is first expanded in a high-pressure turbine, then reheated in a reheater (usually the boiler), and finally expanded again in a low-pressure turbine. This process increases the average temperature of heat addition and reduces moisture content in the final stages, making the cycle more efficient.

Reheating helps to produce more work output, improves thermal efficiency, and protects turbine blades from damage caused by wet steam. It is commonly used in modern thermal power plants to achieve better performance and longer equipment life.

Detailed Explanation:

Rankine cycle improvement using reheating

The Rankine cycle is the most widely used thermodynamic cycle for steam power plants. It involves four main processes: pumping water, heating in the boiler, expanding steam in the turbine, and condensing steam in the condenser. One of the limitations of the basic Rankine cycle is that after expansion in the turbine, the steam becomes very wet (contains high moisture), which can reduce turbine efficiency and damage the turbine blades.

To solve this issue and increase cycle efficiency, a technique called reheating is used. This modification is known as the reheat Rankine cycle.

How Reheating is Done

  1. Initial Steam Expansion
    High-pressure steam from the boiler enters the high-pressure turbine and expands, doing work. But instead of going directly to the condenser, the steam is extracted before it gets too wet.
  2. Reheating Process
    The partially expanded steam is sent back to the boiler or reheater, where it is reheated at constant pressure. This increases the steam temperature again, without changing its pressure.
  3. Second Expansion
    The reheated steam is then expanded further in a low-pressure turbine, producing additional work and completing the expansion process with lower moisture content.
  4. Condensation
    After the second expansion, the steam goes to the condenser, where it is converted back to water for reuse in the cycle.

Benefits of Reheating in Rankine Cycle

  1. Improved Thermal Efficiency

Reheating increases the average temperature at which heat is added, which improves the Carnot efficiency of the cycle.

  1. More Work Output

Since steam is expanded in two stages, more energy is extracted from the steam, leading to higher power output.

  1. Reduced Steam Moisture

Without reheating, steam may become too wet in the later stages of the turbine. Reheating ensures the steam remains drier, which is safer for turbine blades.

  1. Extended Equipment Life

Dry steam causes less erosion and wear in turbines, which helps in maintaining performance and reducing maintenance cost.

  1. More Efficient Fuel Use

Since reheating extracts more work from the same amount of steam, less fuel is needed for the same power output.

Graphical Representation (T-s Diagram)

In a Temperature-Entropy (T-s) diagram, reheating is shown by:

  • An initial expansion line (high-pressure turbine)
  • A horizontal reheat line at constant pressure
  • A second expansion line (low-pressure turbine)

The area under the expansion curves is larger with reheating, which shows more work done.

Real-World Applications

  • Coal-fired power plants: Commonly use reheating for improved performance.
  • Nuclear power plants: Some advanced designs use reheat cycles.
  • Combined cycle plants: May include reheating stages in the steam cycle.
  • Supercritical plants: Often use multiple reheating stages for higher efficiency.

Limitations and Considerations

  • Reheating adds complexity to the system with extra piping and controls.
  • It requires more space and cost due to additional heat exchangers.
  • The reheating temperature must be controlled carefully to avoid material stress.

Despite these points, the gain in efficiency and reliability makes reheating highly valuable in large-scale power plants.

Conclusion

The Rankine cycle is improved using reheating by expanding steam in two or more stages with reheating in between. This increases thermal efficiency, produces more mechanical work, and reduces moisture content in the turbine, leading to better performance and durability. Reheating is a key modification in modern steam power plants that helps to achieve higher efficiency, fuel savings, and longer equipment life.