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PHASE CHANGE HEAT TRANSFER AND HEAT EXCHANGERS :
NUSSELT THEORY OF CONDENSATION :
Condensation is a rather complicated process. It was Wilhelm Nusselt's idea to reduce the complexity of the real process to a rather simple model,namely that the only resistance for the removal of the heat released during condensation occurs in the condensate film. The following gives anexplanation of the Nusselt theory at the example of condensation on avertical wall. Condensation occurs if a vapor is cooled below its (pressure dependent) saturation temperature. The heat of evaporation which is released duringcondensation must be removed by heat transfer, e.g. at a cooled wall. Figure shows how saturated vapor at temperature Ts is condensing on a vertical wall whose temperature Tw is constant and lower than the saturation temperature.
Condensation and Boiling
Heat energy is being converted into electrical energy with the help of water as a working fluid. Water is first converted into steam when heated in a heat exchanger and then the exhaust steam coming out of the steam turbine/engine is condensed in a condenser so that the condensate (water) is recycled again for power generation. Therefore, the condensation and boiling processes involve heat transfer with change of phase. When a fluid changes its phase, the magnitude of its properties like density, viscosity, thermal conductivity, specific heat capacity, etc., change appreciably and the processes taking place are greatly influenced by them. Thus, the condensation and boiling processes must be well understood for an effective design of different types of heat exchangers being used in thermal and nuclear power plants, and in process cooling and heating systems.
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