= 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.<Guide to Combined Heat and Power Systems for Boiler Owners and Operators by C. B. Oland>[§3.1.1, steam-turbine description]{ref}
For steady approximately adiabatic operation with negligible changes in inlet-to-outlet kinetic and gravitational energy, the <first law of thermodynamics> gives
$$
\dot W=\dot m(h_{\rm in}-h_{\rm out}),
$$
where $\dot m$ is <mass flow rate> and $h$ is <specific enthalpy>. An actual enthalpy drop of $500\,\mathrm{kJ\,kg^{-1}}$ at $2\,\mathrm{kg\,s^{-1}}$ therefore gives $1\,\mathrm{MW}$ of shaft power under these assumptions. If the reversible adiabatic drop to the same outlet pressure were $625\,\mathrm{kJ\,kg^{-1}}$, the turbine's isentropic efficiency would be $500/625=0.8$.<Guide to Combined Heat and Power Systems for Boiler Owners and Operators by C. B. Oland>[§3.1.2.1, actual versus isentropic enthalpy drop]{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.<Guide to Combined Heat and Power Systems for Boiler Owners and Operators by C. B. Oland>[§§3.1.1–3.1.2, condensing/backpressure arrangements and efficiency definitions]{ref}
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