The Rankine cycle, explained

The four components

State 1 is saturated liquid leaving the condenser. The pump raises its pressure to state 2, using very little work because liquid is nearly incompressible. The boiler adds heat at constant pressure, turning it into superheated steam at state 3. The turbine expands the steam to state 4 and produces the work. The condenser rejects heat to cooling water and returns the fluid to state 1.

Pumping a liquid instead of compressing a gas is the cycle’s great advantage: the pump takes around 1% of the turbine output or less, where a gas turbine’s compressor takes around half.

How plants raise the efficiency

Lower the condenser pressure, which lowers the temperature heat is rejected at. Raise the boiler pressure and superheat temperature, which raises the average temperature heat is added at. Reheat the steam between turbine stages, which keeps the turbine exhaust dry enough not to erode the blades. Bleed some steam to preheat the feedwater, which is regeneration.

Each of these moves the cycle closer to the Carnot limit between the same temperatures, and that is the logic to remember rather than the list.

A worked number

A simple cycle with steam at 3 MPa and 350 °C and a condenser at 75 kPa. From the steam tables, the turbine inlet enthalpy is about 3116 kJ/kg and the isentropic exit about 2403 kJ/kg, so the turbine gives 713 kJ/kg. The pump needs about 3 kJ/kg and the boiler adds about 2729 kJ/kg.

Efficiency is (713 − 3)/2729 ≈ 26%. Real large plants with reheat, regeneration and much higher pressures reach around 40%.

Common mistakes

• Reading turbine exit enthalpy without checking quality. Below about 90% quality the blades erode.
• Forgetting the pump work, or computing it as if the water were a gas.
• Mixing up state numbering between textbooks. Write your states on the diagram first.