Understanding the Processes in a Rankine Cycle

The Rankine cycle is an idealized thermodynamic cycle that describes the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat source and a heat sink. It is the fundamental operating cycle of all thermal power plants where water is used as the working fluid.

A complete Rankine cycle consists of four main processes:

• Process 1-2: Isentropic Compression - The working fluid (usually water) enters the pump as a saturated liquid and is compressed to a higher pressure. This process is considered isentropic, meaning it is both adiabatic (no heat transfer) and reversible. Work input is required for the pump.
• Process 2-3: Constant Pressure Heat Addition - The high-pressure liquid enters the boiler, where it is heated at a constant pressure. It first reaches the saturation temperature and then evaporates completely into a superheated vapor (or remains a saturated vapor, depending on the specific cycle). Heat is added from an external source.
• Process 3-4: Isentropic Expansion - The high-pressure vapor enters the turbine and expands, producing work. This process is also considered isentropic, representing an ideal, reversible adiabatic expansion. The temperature and pressure of the vapor decrease significantly.
• Process 4-1: Constant Pressure Heat Rejection - The vapor from the turbine enters the condenser, where it rejects heat at a constant pressure and condenses back into a saturated liquid. The liquid then returns to the pump to complete the cycle. Heat is rejected to a cold sink.

Looking at these processes, we can see that the Rankine cycle involves:

• Two isentropic processes: Compression in the pump (1-2) and expansion in the turbine (3-4).
• Two constant pressure processes: Heat addition in the boiler (2-3) and heat rejection in the condenser (4-1).

Therefore, a Rankine cycle consists of two isentropic processes and two constant pressure processes.