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An ideal Rankine cycle has four main steps: liquid water is pumped reversibly to high pressure; heat is added at high pressure to produce hot vapour or supercritical working fluid; the fluid expands approximately isentropically through a turbine to produce work; and low-pressure heat rejection in a condenser returns it to liquid. For steady flow, the main work and heat transfers are differences of specific enthalpy.

A steam implementation of the Carnot cycle would require compression of a wet liquid-vapour mixture and reversible isothermal heat transfer, both difficult to achieve with practical machinery. Complete condensation before the Rankine pump makes compression work small because liquid specific volume is small. High-pressure heating and superheating can then occur in a boiler without demanding isothermal addition of all the heat. Superheating also reduces turbine-exit moisture, although the calculated exit may still be wet. The pump, turbine, condenser and boiler can operate continuously as separate flow devices. The Rankine cycle sacrifices some ideal thermal efficiency in return for a practicable steam-plant configuration; no real engine can exceed the Carnot efficiency for the same extreme reservoir temperatures.

For the liquid pump take the explicit incompressible-water estimate $v_f\simeq 10^{-3}{\mathrm{m}}^3\mathrm{k}{\mathrm{g}}^{-1}$. Its work input isso $h_2=h_1+w_p=111.998\mathrm{kJ}\mathrm{k}{\mathrm{g}}^{-1}$. With $h_3=3463$ and the preceding $h_4=1838.74$ in the same units,ThusIf pump work is neglected because no liquid density was tabulated, the corresponding approximation is $(3463-1838.74)/(3463-84)=48.1\%$. Stating the approximation explains the small difference. The temperature-extreme Carnot efficiency is about $1-293.15/873.15=66.4\%$, above both results; finite-temperature-range heat addition in the Rankine cycle does not attain that limit.

W. J. M. Rankine was a Scottish engineer and physicist who contributed to thermodynamics, classical mechanics and the teaching of engineering. As professor of civil engineering and mechanics at Glasgow from 1855 to 1872, he emphasized connecting theory with practice and worked with shipbuilders on vessels and engines.^[ref]

The Rankine cycle connects his name with the analysis of heat engines using a working fluid that undergoes vaporization and condensation. Its idealized sequence of pumping, heating, expansion and cooling connects a practical steam engine to a calculation of mechanical work and thermal efficiency. The broader importance of his work lies in making quantitative physical theory part of systematic engineering design.

Institutional historical profile: Imperial College historical plaque booklet: William John Macquorn Rankine.