In the previous two chapters, we studied heat transfer in forced and free convection, i.e. heat transfer with fluid motion, induced either by an external means or by density differences. In both the cases, fluid involved was homogeneous and in single phase. But, there are many important practical cases which involve heat transfer with a change of phase of the fluid, e.g. boiling where the liquid changes to vapour and condensation where the vapour condenses into a liquid. Boiling and condensation are classified under convection since there is motion of the fluid during heat transfer in these processes.
Some of the applications of boiling and condensation are: