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A steam engine is a heat engine that converts part of the energy carried by steam into mechanical work. In a reciprocating design, expanding steam pushes a piston; valves admit and exhaust the working fluid so that the motion can repeat. A crank can convert the reciprocation into rotation. A steam turbine instead extracts work from a continuously flowing stream.

The boiler supplies heat to water, but the engine's useful output depends on pressure changes during expansion and exhaust. For an idealized quasi-static piston, boundary work is $W=\int pdV$. The net work over a cycle is the signed enclosed area on a pressure-volume diagram. Some input heat must be rejected: turning boiler heating into motion does not circumvent the Second law of thermodynamics.

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]

A turbine extracts mechanical work from a flowing fluid through interaction with a rotating set of blades. Steam, hot gas or water changes its momentum while passing through the rotor, producing torque on the shaft. A steam turbine can drive an electrical generator in a thermal power station.

For steady axial through-flow in an ideal control-volume model, shaft torque is $\tau =\dot{m}(r_1{v}_{\theta 1}-r_2{v}_{\theta 2})$, the rate at which fluid angular momentum is removed. Here $v_{\theta }$ is the tangential velocity component and $r$ the radius. Shaft power is $P=\tau \Omega$ at angular speed $\Omega$. The relation connects blade-induced changes in flow direction with useful output; losses and operating conditions determine actual conversion efficiency.