Steam turbine

A steam turbine extracts mechanical work from flowing steam as it expands through stationary passages and moving blades. The moving blades change the flow's angular momentum and receive a corresponding torque. A shaft transfers the resulting power to a generator or another load. Unlike a reciprocating steam engine, the working stream passes continuously through the machine.^[ref]

For steady approximately adiabatic operation with negligible changes in inlet-to-outlet kinetic and gravitational energy, the first law of thermodynamics giveswhere $\dot{m}$ is mass flow rate and $h$ is specific enthalpy. An actual enthalpy drop of $500\mathrm{kJ}\mathrm{k}{\mathrm{g}}^{-1}$ at $2\mathrm{kg}{\mathrm{s}}^{-1}$ therefore gives $1\mathrm{MW}$ of shaft power under these assumptions. If the reversible adiabatic drop to the same outlet pressure were $625\mathrm{kJ}\mathrm{k}{\mathrm{g}}^{-1}$, the turbine's isentropic efficiency would be $500/625=0.8$.^[ref]

A performance test measures inlet and outlet pressure and temperature, flow rate, shaft torque and rotational speed. If the exhaust is a liquid-vapor mixture, pressure and temperature alone do not determine its specific enthalpy; the vapor fraction or an additional energy balance is needed. Condensing exhaust and supplying useful process steam are different operating arrangements, so turbine efficiency and whole-plant fuel efficiency must also be distinguished.^[ref]