A heat engine that extracts thermal energy from pressurized steam by expansion over several stages to do mechanical work on a rotating output shaft. The basic operation of a steam turbine is similar to that of the gas turbine.

The ideal (theoretical) steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine, with isentropic efficiencies ranging from 20–90%.

3 stage steam turbine

Thermodynamics of a steam turbine
The steam turbine operates on basic principles of thermodynamics using the part 3-4 of the Rankine cycle. Superheated steam (or dry saturated steam, depending on application) leaves the boiler at high temperature and high pressure. At entry to the turbine, the steam gains kinetic energy by passing through a nozzle (a fixed nozzle in an impulse type turbine or the fixed blades in a reaction type turbine).

When the steam leaves the nozzle it is moving at high velocity towards the blades of the turbine rotor. A force is created on the blades due to the pressure of the vapor on the blades causing them to move and rotating the shaft. The steam leaves the turbine as a saturated vapor (or liquid-vapor mix depending on application) at a lower temperature and pressure than it entered with and is sent to the condenser to be cooled.

The Rankine cycle (ideal)
The four processes states of the Rankine cycle.

Process 1-2
The water (working fluid) is pumped from low to high pressure.

Process 2-3
In the boiler the high-pressure liquid is heated at constant pressure to become a dry saturated vapour.

Process 3-4
The vapour (dry saturated) expands through the turbine, making the turbine rotate and thereby generating power. The temperature and pressure of the vapour decreases, and some condensation occurs.

Process 4-1
Wet vapour is condensed at a constant pressure to become a saturated liquid.

Actual power cycle
An actual vapor power cycle differs from the ideal Rankine cycle because of fluid friction and heat loss to the surroundings making the processes irreversible. The fluid friction causes pressure drops in the boiler, the condenser, and the piping between the components, therefore the steam leaves the boiler at a lower pressure. The heat loss reduces the net work output. To maintain the same level of net output as for the ideal cycle heat needs to be added to the steam in the boiler.

In a real power plant cycle, the compression and the expansion are not isentropic. Therefore these processes are non-reversible and entropy is increased during the two processes, increasing the pump power abnd decreasing the power generated by the turbine.

Water droplet formation will reduce the efficiency of a steam turbine. When the water condenses, water droplets will hit the turbine blades at high speed and may cause pitting and erosion, decreasing the life of turbine blades and efficiency of the turbine. One way to ocercome this by superheating the steam.

Work per unit of mass assuming no heat transfer to the surroundings.
 
\dfrac{W}{m} = h_3-h_4
 
W = The rate at which work is developed per time unit.
m = The of mass flow through the turbine

h₃ = The specific enthalpy at stage three of the Rankine cycle

h₄ = Thespecific enthalpy at stage 4 of the Rankine cycle

Isentropic efficiency – Actual work divided by the ideal work.
 
\eta_t = \dfrac{h_3-h_4}{h_3-h_{4s}}
 
h₃ = The specific enthalpy at stage three of the Rankine cycle
h₄ = Thespecific enthalpy at stage 4 for the actual turbine
h_4s = Thespecific enthalpy at stage 4 for the isentropic (Ideal) turbine