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Steam heating transports thermal energy using water vapor, commonly allowing it to condense in a heat-emitting component. At an approximately fixed local pressure, condensation releases latent heat while the phase-change temperature remains nearly fixed. The condensate can then return to the source for reheating.

For mass flow rate $\dot{m}$ through a steady emitter, the ideal transferred power is $\dot{Q}=\dot{m}(h_{\mathrm{in}}-h_{\mathrm{out}})$, where $h$ is specific enthalpy. That difference can include both latent heat from condensation and sensible heat from cooling the condensate. The emitter transfers this energy to its surroundings through convection and thermal radiation. Measuring only the condensate temperature change would miss the latent contribution. The mechanism illustrates why a large heat transfer need not be accompanied by a large change in the working fluid's temperature.

A vapor whose partial pressure exceeds its vapor pressure at the local temperature; nucleation can permit condensation.

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.