Convective heat and mass transfer. Flows with phase change

Convective heat and mass transfer. Flows with phase change 

Some of the convective heat and mass transfer processes with phase change that we will deal with in the following have already been explained in the previous chapters. This includes the evaporation of a liquid at the interface between a gas and a liquid or the sublimation at a gas-solid interface. They can be described using the methods for convective heat and mass transfer. 

However in many heat and mass transfer processes in fluids, condensing or boiling at a solid surface play a decisive role. In thermal power plants water at high pressure is vaporized in the boiler and the steam produced is expanded in a turbine, and then liquified again in a condenser. In compression or absorption plants and heat pumps, boilers and condensers are important pieces of equipment in the plant. In the separation of mixtures, the different composition of vapours in equilibrium with their liquids is used. Boiling and condensing are, therefore, characteristic for many separation processes in chemical engineering. As examples of these types of processes, the evaporation, condensation, distillation, rectification and absorption of a fluid should all be mentioned. 

In order to vaporize a liquid or condense a vapour, the enthalpy of vaporization has to either be added to or removed from it. Under the precondition of thermodynamic equilibrium, the phase change demands that there is no difference in the temperatures of the two phases. However, in reality an imbalance is necessary for the phase change to occur, even if it is only a small temperature difference. 

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