Explain the reheat cycle in steam power plants.

Short Answer:

The reheat cycle in steam power plants is a modified version of the simple Rankine cycle used to increase the efficiency and life of the turbine. In this cycle, steam is first expanded in a high-pressure turbine, then it is sent back to the boiler for reheating, and finally, it is expanded again in a low-pressure turbine.

This process reduces the moisture content in the final stages of the turbine and increases the average temperature of heat addition, which improves overall thermal efficiency. The reheat cycle is commonly used in large thermal and nuclear power plants for better performance.

Detailed Explanation:

Reheat cycle in steam power plants

In a typical Rankine cycle, steam is generated in a boiler, expanded in a turbine to produce work, and then condensed before being pumped back into the boiler. One major drawback of the simple Rankine cycle is that after complete expansion in the turbine, the steam often becomes wet, which can damage the turbine blades and reduce efficiency.

To overcome this, power plants use a reheat cycle, where the steam is partially expanded, reheated, and then expanded again. This method not only helps to increase the power output but also prevents erosion of turbine blades due to moisture.

Working of Reheat Cycle

  1. Steam Generation in Boiler
    Water is heated in the boiler to produce high-pressure, high-temperature steam (superheated steam).
  2. First Expansion in High-Pressure Turbine
    The steam enters the high-pressure turbine (HPT) and expands, losing part of its pressure and temperature while producing mechanical work.
  3. Reheating in Boiler or Reheater
    The partially expanded steam from the HPT is sent back to the boiler reheater section, where it is heated again at constant pressure to a high temperature (same or slightly lower than the original superheat temperature).
  4. Second Expansion in Low-Pressure Turbine
    This reheated steam then enters the low-pressure turbine (LPT) and undergoes further expansion. More work is extracted in this stage.
  5. Condensation and Pumping
    After the second expansion, the steam enters the condenser, where it turns back into water, completing the cycle. The feed pump then sends it back to the boiler.

Advantages of Reheat Cycle

  1. Improved Thermal Efficiency

By reheating the steam before the second expansion, the average temperature of heat addition increases, which improves the cycle efficiency.

  1. Reduction in Moisture Content

Reheating reduces the wetness of steam at the turbine exit, which protects the turbine blades from erosion and corrosion.

  1. Higher Work Output

Steam performs more expansion work in two stages (HPT and LPT), resulting in more mechanical power.

  1. Better Control of Turbine Performance

Dividing the expansion into two parts allows better load control and flexibility in operation.

T-s Diagram Representation

In the Temperature vs. Entropy (T-s) diagram, the reheat cycle is shown by:

  • First expansion curve (high-pressure turbine)
  • A horizontal reheating line at constant pressure
  • Second expansion curve (low-pressure turbine)

The area enclosed by the cycle increases with reheat, representing more work done.

Real-Life Applications

  • Coal-based thermal power plants using reheat cycles with pressures of 100–200 bar.
  • Nuclear power plants where reheating is essential to manage steam quality.
  • Supercritical and ultra-supercritical plants commonly employ double or triple reheat cycles for maximum efficiency.

Practical Considerations

  • Reheating requires extra boiler surfaces, increasing equipment size and cost.
  • Piping and control systems must handle high temperatures and pressure drops.
  • Proper temperature control during reheating is important to avoid thermal stress.
Conclusion

The reheat cycle in steam power plants is an important modification of the Rankine cycle that increases thermal efficiency, enhances turbine safety, and delivers higher power output. By expanding the steam in two stages with intermediate reheating, the cycle becomes more efficient and reliable for long-term operation. This is why the reheat cycle is a standard design feature in modern high-capacity power plants.