ABSTRACT
Boiling and condensation are essentially convection heat transfer processes involving phase change. In many types of power and refrigeration cycles, phase changes from vapor to liquid or liquid to vapor are involved, depending on the particular part of the cycle considered. These are accomplished by boiling or condensation. From thermodynamics we know that when the temperature of a liquid at a given pressure is raised to the saturation temperature Tsat corresponding to that pressure, the liquid will boil. Similarly, when the temperature of a vapor is lowered to Tsat, condensation occurs. Even though boiling and condensation are forms of convection heat transfer, they depend on the latent heat of vaporization hfg of the fluid and the surface tension σ at the liquid-vapor interface, in addition to the properties of the fluid at each phase. Furthermore, since under equilibrium conditions the temperature remains constant during a phase-change process at a fixed pressure, large amount of heat can be transferred during boiling and condensation, essentially at constant temperature. However, in actual processes, it is necessary to maintain some difference between the surface temperature Ts and saturation temperature Tsat for an effective heat transfer. Heat transfer coefficient h associated with boiling and condensation are several times higher than those associated with other forms of convection heat transfer processes that involve only a single phase.
