Influence of air thermohygrometric properties on mechanical draft evaporative towers: a calculation method to predict effects in power plants and refrigerating absorption machines

The paper describes a simulation model for the evaluation of heat exchanges occurring to a flow of water falling down in counterflow with humid air, during its path through a mechanical draft cooling tower. The contemporary heat transfer by evaporation and forced convection are separately taken into account, assuming simplified hypotheses such as water flow divided into spherical drops, constant relative speed between air and water and constant thermodynamic conditions of water and air through the tower. The model has been calibrated and validated through two existing evaporative towers: a small-size one serving a refrigerating absorption machine and a medium-size one connected with the condenser of a coal-fired power plant; the comparison with the well-known Merkel and Poppe models showed only slight differences on thermodynamic parameters involved in the process. The simulation predicts water outlet conditions starting from the air inlet temperature and relative humidity, once the power to be drained is fixed. The knowledge of outlet air thermohygrometric conditions allows the definition of water outlet temperature minimum value and the evaluation of the mass flow to be reintegrated for the evaporated water replacement. Since the outlet water temperature level affects the performances of plants connected with the evaporative towers, it is interesting to vary the air external conditions, investigating the repercussions on plants’characteristics. While the influence on power plants remains circumscribed to a light variation on the produced electric power, refrigerating absorption machines result more sensitive, showing considerable fluctuations of chilling power and Coefficient Of Performance (COP).