Short Answer:

The processes in the Rankine cycle include four main steps—pumping, heating, expansion, and condensation. In this cycle, water is pumped to high pressure, heated in a boiler to produce steam, expanded in a turbine to generate work, and then condensed back to water in a condenser.

In simple words, the Rankine cycle converts heat energy into mechanical work using water as the working fluid. The cycle repeats continuously, ensuring a steady generation of power. These four processes form the foundation of energy conversion in all thermal power plants.

Detailed Explanation :

Processes in the Rankine Cycle

The Rankine cycle is the fundamental thermodynamic cycle used in thermal power plants to convert heat into work. It describes how water and steam circulate through a system to produce energy. The cycle consists of four main processes—isentropic compression (in the pump), constant pressure heat addition (in the boiler), isentropic expansion (in the turbine), and constant pressure heat rejection (in the condenser).

These processes together form a closed loop system, where the working fluid (water or steam) is continuously circulated to maintain continuous power generation.

1. Process 1–2: Isentropic Compression (in the Pump)

In the first process of the Rankine cycle, the condensate (water) from the condenser is at low pressure and must be pumped into the boiler at high pressure.

• This is achieved by the feedwater pump, which increases the water pressure.
• The process takes place isentropically, meaning it occurs with no change in entropy (no heat exchange with surroundings).
• Since water is nearly incompressible, only a small amount of work is needed to raise its pressure.

Process Description:

• The pump receives water at point 1 (low pressure).
• The pump adds mechanical energy to the water, increasing its pressure to the boiler level (point 2).
• The work input to the pump is very small compared to the work produced by the turbine.

Effect:
The water at the pump outlet (point 2) has high pressure and is ready to enter the boiler for heating.

Mathematical Expression:

Where,
= Pump work
= Specific volume of water
and  = Boiler and condenser pressures respectively.

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

In this process, water is heated in the boiler at constant pressure until it turns into steam.

Process Description:

• The high-pressure water from the pump enters the boiler.
• It absorbs heat from the combustion of fuel (coal, oil, or gas).
• The temperature of water gradually increases until it reaches its boiling point, after which it starts changing into steam.
• Depending on the design, the steam may become saturated (just dry steam) or superheated (steam at higher temperature than saturation).

Effect:
This process adds thermal energy (Q_in) to the working fluid. The heat supplied in the boiler is the main energy input for the cycle.

Mathematical Expression:

Where,
and  = Enthalpies at points 3 and 2 respectively.

3. Process 3–4: Isentropic Expansion (in the Turbine)

In the third process, high-pressure and high-temperature steam from the boiler is expanded through the steam turbine, producing mechanical work.

Process Description:

• The steam enters the turbine at high pressure (point 3).
• As it expands through the turbine blades, it does work on the rotor, causing it to spin.
• The entropy remains constant (isentropic process), but the steam’s pressure and temperature decrease during expansion.
• The mechanical energy produced by the turbine is used to drive an electric generator for electricity production.

Effect:
This is the power-producing stage of the Rankine cycle, where thermal energy is converted into mechanical work.

Mathematical Expression:

Where,
= Turbine work
and  = Enthalpies before and after expansion.

The efficiency of the turbine depends on how effectively it converts steam energy into mechanical energy.

4. Process 4–1: Constant Pressure Heat Rejection (in the Condenser)

After expansion in the turbine, the steam enters the condenser, where it is cooled and converted back into water at constant pressure.

Process Description:

• The low-pressure exhaust steam from the turbine flows into the condenser.
• It comes in contact with cooling water flowing through tubes.
• The steam loses its latent heat of vaporization and condenses into water (condensate).
• The condensed water is then collected in a tank and sent back to the pump, completing the cycle.

Effect:
This process rejects waste heat (Q_out) to the environment through cooling water. The condenser operates at very low pressure to improve cycle efficiency.

Mathematical Expression:

Where,
= Heat rejected
and  = Enthalpies before and after condensation.

5. Summary of the Four Processes

Process	Type of Process	Main Equipment	Function
1–2	Isentropic Compression	Pump	Increases water pressure
2–3	Constant Pressure Heating	Boiler	Converts water into steam
3–4	Isentropic Expansion	Turbine	Produces mechanical work
4–1	Constant Pressure Condensation	Condenser	Converts steam to water

(Note: This table is for reference understanding only, not for marks presentation as per instruction.)

Each process is interlinked, forming a closed thermodynamic cycle where the working fluid is reused continuously for efficient power generation.

6. Representation on T–S Diagram

In a Temperature–Entropy (T–S) diagram:

• 1–2: Vertical line showing isentropic compression.
• 2–3: Horizontal line indicating heat addition at constant pressure.
• 3–4: Downward curve showing isentropic expansion.
• 4–1: Horizontal line representing heat rejection at constant pressure.

This graphical representation helps visualize how temperature and entropy change during each process of the Rankine cycle.

Conclusion

The processes in the Rankine cycle—pumping, heating, expansion, and condensation—together describe how thermal energy from fuel is converted into mechanical and electrical energy. The cycle operates in a closed loop, ensuring efficient use of water and heat. The two isentropic (adiabatic) processes and two constant pressure processes make the Rankine cycle practical for real-world applications like thermal and nuclear power plants. Its simplicity, reliability, and adaptability make it one of the most important cycles in mechanical and power engineering.