Short Answer:

A reheat Rankine cycle is a modified form of the simple Rankine cycle used to increase efficiency and reduce moisture in the turbine exhaust. In this cycle, the steam is expanded in a turbine in two stages. After partial expansion, the steam is sent back to the boiler for reheating and then expanded again in another turbine stage.

In simple words, the reheat Rankine cycle improves the performance of a power plant by reheating the steam before sending it for the final expansion. This reduces energy losses, prevents turbine blade erosion, and increases the overall efficiency of the system.

Detailed Explanation :

Reheat Rankine Cycle

The reheat Rankine cycle is an improved version of the simple Rankine cycle that is commonly used in thermal power plants to enhance efficiency and avoid excessive moisture formation in the steam at the turbine outlet. In a simple Rankine cycle, the steam expands in a single turbine, resulting in high moisture content in the exhaust, which reduces efficiency and damages turbine blades.

In the reheat Rankine cycle, this problem is overcome by dividing the turbine expansion into two or more stages. The steam is partially expanded in the high-pressure turbine (HPT), then reheated in the boiler, and again expanded in the low-pressure turbine (LPT). This reheating process increases the average temperature at which heat is added, thus improving the thermal efficiency of the cycle.

1. Principle of Reheat Rankine Cycle

The reheat Rankine cycle works on the same basic principle as the simple Rankine cycle — converting heat energy into mechanical work — but with an added reheating process between two turbine stages.

The main idea is to keep the steam dry during expansion and increase efficiency by raising the mean temperature of heat addition. By reheating, the steam’s energy level is restored before it undergoes further expansion, allowing more work to be extracted from the turbine without increasing moisture content.

2. Processes in the Reheat Rankine Cycle

The reheat Rankine cycle consists of the following main processes:

Process 1–2: Isentropic Compression (in Pump)

The condensed water from the condenser is pumped to a high pressure by the feedwater pump. This process is nearly isentropic, meaning there is no change in entropy.

• The pressure of water increases to the boiler pressure.
• The pump work is very small compared to the turbine work.

Process 2–3: Constant Pressure Heat Addition (in Boiler)

In the boiler, the high-pressure water absorbs heat from fuel combustion and changes into superheated steam at constant pressure.

• The steam generated is at a high temperature and pressure and is then sent to the high-pressure turbine.

Process 3–4: Isentropic Expansion (in High-Pressure Turbine)

The superheated steam expands in the high-pressure turbine (HPT), producing mechanical work and partially losing pressure and temperature.

• The expansion is nearly isentropic.
• After partial expansion, the steam leaves the turbine at an intermediate pressure.

Process 4–5: Reheating (in Reheater or Boiler)

The partially expanded steam is returned to the boiler and reheated at constant pressure.

• The reheating process increases the steam temperature to almost the initial turbine inlet temperature.
• This ensures the steam remains dry during further expansion.

Process 5–6: Isentropic Expansion (in Low-Pressure Turbine)

After reheating, the steam expands again in the low-pressure turbine (LPT) to produce additional work.

• This expansion occurs until the steam reaches condenser pressure.
• The reheating helps prevent condensation and increases efficiency.

Process 6–1: Constant Pressure Heat Rejection (in Condenser)

The low-pressure steam leaving the turbine enters the condenser, where it is condensed into water at constant pressure.

• The heat is rejected to cooling water, and the condensed water is then sent to the pump, completing the cycle.

3. Representation on T–S Diagram

In the Temperature–Entropy (T–S) diagram, the reheat Rankine cycle is represented by two expansion lines instead of one.

• 1–2: Pumping (isentropic compression)
• 2–3: Heating in the boiler (constant pressure heat addition)
• 3–4: Expansion in the high-pressure turbine (isentropic)
• 4–5: Reheating (constant pressure heat addition)
• 5–6: Expansion in the low-pressure turbine (isentropic)
• 6–1: Condensation (constant pressure heat rejection)

The area enclosed by the curve on the T–S diagram represents the net work output of the cycle.

4. Components of the Reheat Rankine Cycle

The major components used in the reheat Rankine cycle are:

1. Boiler: Generates and reheats the steam.
2. High-Pressure Turbine (HPT): Expands steam partially to intermediate pressure.
3. Low-Pressure Turbine (LPT): Further expands the reheated steam.
4. Condenser: Condenses the exhaust steam to water.
5. Feedwater Pump: Pumps the water from low to high pressure.
6. Reheater: A part of the boiler that reheats the partially expanded steam.

Each component works together to ensure efficient energy conversion and reduced moisture content in the steam.

5. Advantages of Reheat Rankine Cycle

1. Increased Efficiency:
   Reheating increases the average temperature at which heat is added, improving the thermal efficiency of the cycle.
2. Reduced Moisture Content:
   The steam remains dry during the final stages of expansion, preventing turbine blade erosion and improving turbine life.
3. Higher Work Output:
   The total work output of the turbines increases due to two expansion stages.
4. Better Control of Steam Conditions:
   The process allows control of pressure and temperature at intermediate stages for stable operation.
5. Improved Reliability:
   Reduces mechanical wear and corrosion in turbine components.

6. Disadvantages of Reheat Rankine Cycle

1. Increased Complexity:
   The system requires additional equipment like reheaters, control valves, and piping, which increases system complexity.
2. Higher Cost:
   The additional reheating stage increases the initial cost of installation and maintenance.
3. Slight Increase in Fuel Consumption:
   Some extra fuel is required for reheating the steam.
4. Longer Start-Up Time:
   The presence of multiple stages and components increases the time needed to reach full operation.

Despite these drawbacks, the efficiency gains make the reheat Rankine cycle highly beneficial for modern thermal power plants.

7. Efficiency of Reheat Rankine Cycle

The thermal efficiency (η) of the reheat Rankine cycle is given by:

where,

•  = Total turbine work (sum of work from high and low-pressure turbines)
•  = Pump work
•  = Total heat added in the boiler and reheater

Because of reheating, the mean temperature of heat addition increases, leading to higher efficiency compared to the simple Rankine cycle.

8. Applications of Reheat Rankine Cycle

The reheat Rankine cycle is widely used in:

• Thermal power plants (coal, oil, and gas-fired)
• Nuclear power plants
• Industrial steam power systems

It is especially useful in high-capacity power plants where efficiency and turbine protection are major concerns.

Conclusion

The reheat Rankine cycle is an advanced form of the simple Rankine cycle designed to improve efficiency and reduce moisture in turbine exhaust steam. It involves reheating the steam between two stages of turbine expansion, which raises the average temperature of heat addition and enhances overall performance. Although it increases system complexity and cost, the efficiency improvement and longer turbine life make it highly effective for modern thermal power plants. Thus, the reheat Rankine cycle is an essential modification for achieving high-efficiency power generation.