The Rankine cycle models vapor power plants by allowing the working fluid to evaporate and condense.

The ideal cycle contains four steady-flow processes:

1. isentropic pumping of saturated liquid to high pressure;
2. constant-pressure heat addition in a boiler, producing vapor;
3. isentropic expansion through a turbine;
4. constant-pressure heat rejection in a condenser, returning to liquid.

Per unit mass,

$$w_p=h_2-h_1,$$

$$q_{in}=h_3-h_2,$$

$$w_t=h_3-h_4,$$

$$q_{out}=h_4-h_1.$$

The thermal efficiency is

$$\eta_{th}=\frac{w_t-w_p}{q_{in}}.$$

Property tables are usually required because the working fluid crosses liquid, two-phase and vapor regions where an ideal-gas model is inappropriate.

The Rankine cycle is practical precisely because it compresses a liquid rather than a vapor: pump work is small compared with turbine work. This distinguishes vapor power cycles fundamentally from gas-turbine cycles.