What is reheating?

Short Answer:

Reheating is a process used in multi-stage turbines to increase the efficiency and output of the power plant. After the steam expands partially in the high-pressure turbine, it is taken back to the boiler or reheater, where it is heated again at constant pressure before being sent to the low-pressure turbine for further expansion.

In simple words, reheating means heating the steam again between two turbine stages to increase its temperature and energy. This reduces moisture content in the later stages of the turbine, increases work output, and improves the overall efficiency and life of the turbine system.

Detailed Explanation :

Reheating

Reheating is an important process used in multi-stage steam turbines and sometimes in gas turbines to enhance the performance and efficiency of the power cycle. In a turbine, steam expands from a high pressure to a low pressure, producing work. However, during this expansion, the steam temperature and pressure decrease, and the steam becomes wet. The presence of moisture reduces turbine efficiency and may damage the blades.

To overcome this problem, the partially expanded steam is taken back to the boiler and heated again at constant pressure before being expanded further in the next stage. This process is called reheating, and the device used is known as a reheater. The process increases the average temperature of heat addition, improves efficiency, and reduces turbine blade erosion due to wet steam.

Process of Reheating

The reheating process takes place in two main stages of expansion in a turbine:

  1. First Expansion (High-Pressure Stage):
    • Steam from the boiler first enters the high-pressure (HP) turbine.
    • It expands partially in this stage, producing work, but its temperature and pressure fall significantly.
    • The steam leaving the HP turbine becomes partially wet.
  2. Reheating Stage:
    • The wet steam is sent back to the reheater section of the boiler.
    • It is heated again at constant pressure to increase its temperature to nearly its original superheated condition.
    • The steam’s pressure remains the same, but its enthalpy (energy) increases.
  3. Second Expansion (Low-Pressure Stage):
    • The reheated steam then enters the low-pressure (LP) turbine, where it expands again and produces additional work.
    • Because of reheating, the steam entering the LP turbine is dry and has higher energy, reducing losses and increasing power output.

This two-stage expansion with reheating significantly improves the efficiency of the turbine cycle.

Working Principle of Reheating

Reheating is based on the principle of increasing the mean temperature of heat addition in the thermodynamic cycle.

In a Rankine cycle (used in steam power plants), the thermal efficiency depends on the temperature at which heat is added to the working fluid. Higher mean heat addition temperature means higher efficiency.

In the simple Rankine cycle (without reheating):

  • Steam expands completely in a single turbine, leading to high moisture at the outlet.
  • Efficiency decreases due to condensation and losses.

In the reheat Rankine cycle:

  • Steam expansion occurs in two or more stages.
  • Between these stages, the steam is reheated at constant pressure.
  • The average temperature of heat addition increases, improving cycle efficiency.

Mathematical Representation:
The total work done in the reheat cycle is:

Where,

  •  = Enthalpy of steam at turbine inlet
  •  = Enthalpy after first expansion
  •  = Enthalpy after reheating
  •  = Enthalpy after second expansion

The efficiency of the reheat cycle is higher because the average temperature of heat addition is greater than that in a simple Rankine cycle.

Advantages of Reheating

  1. Increases Efficiency of the Cycle:
    • Reheating increases the mean temperature of heat addition, thereby improving the thermal efficiency of the power plant.
  2. Increases Work Output:
    • Since the steam expands twice, the total work obtained from the turbine is greater than that of a single expansion cycle.
  3. Reduces Moisture Content in Low-Pressure Stages:
    • By heating the steam again, moisture in the later turbine stages is reduced, preventing erosion and corrosion of turbine blades.
  4. Improves Turbine Life:
    • The turbine operates under better conditions with dry steam, which increases its life and reliability.
  5. Better Fuel Utilization:
    • By reusing heat from the same boiler for reheating, the plant utilizes fuel energy more efficiently.
  6. Higher Average Temperature of Heat Addition:
    • The process ensures that heat addition occurs at a higher average temperature, which is thermodynamically favorable.

Disadvantages of Reheating

  1. Increased Cost and Complexity:
    • The system requires additional equipment such as reheaters, pipes, and control valves, increasing cost and maintenance.
  2. Pressure Losses:
    • During the transfer of steam from the HP turbine to the reheater and back to the LP turbine, some pressure losses occur.
  3. Requires More Space:
    • The addition of reheating equipment increases the size of the power plant layout.
  4. Fuel Consumption May Increase Slightly:
    • Though efficiency improves, reheating consumes extra heat, which may slightly increase fuel usage if not optimized.

Despite these disadvantages, the overall gain in performance and turbine life justifies the use of reheating in modern thermal power plants.

Effect of Reheating on Rankine Cycle

Reheating modifies the simple Rankine cycle into the Reheat Rankine Cycle, which has the following effects:

  1. The average temperature of heat addition increases, improving thermal efficiency.
  2. The moisture content at turbine exhaust decreases, improving the quality of steam.
  3. The expansion process becomes smoother, distributing mechanical stresses more evenly across the turbine stages.
  4. The output per kg of steam increases, meaning more energy is produced for the same mass flow.

Graphically, on the T–S (Temperature–Entropy) diagram, the reheat process appears as a horizontal line (constant pressure heating) between two expansion curves, showing that heat is added between turbine stages.

Applications of Reheating

  • Thermal power plants using high-capacity steam turbines.
  • Nuclear power stations where high efficiency and low moisture are important.
  • Combined-cycle plants where reheating is used between turbine stages.
  • Marine propulsion systems and industrial turbines for high efficiency.
Conclusion

In conclusion, reheating is a process of increasing the temperature of steam after partial expansion in a turbine before it enters the next stage. It is used to increase efficiency, reduce moisture, and improve the life of turbine components. Reheating raises the average temperature of heat addition and provides more useful work output from the same amount of steam. Although it adds cost and complexity, the reheating process is essential in modern power plants for achieving higher efficiency and safe turbine operation.