The design of countercurrent evaporative condensers with the hybrid method

Abstract

The heat and mass transfer in evaporative condensers are complex to model analytically and numerical simulations, when applied to multi-phase fluid dynamics in complex paths, often involve too high computational costs. Experimental campaigns at full scale of different heat transfer geometries and tube arrangements involve long lead times and high costs as well. The aim of the present work is to overcome the present limitations and to apply a new method to evaluate the overall performance of the countercurrent evaporative condensers, starting from the experimental, numerical or analytical data with a small scale approach. A test bench has been purposely designed and built up in order to reach and keep constant all the parameters determining the evaporative condenser heat transfer performance. In previous experimental contributions available in the literature, the air conditions were not controlled: here, an air handling unit placed before the evaporative condenser inlet allows to set up temperature and relative humidity of air in large ranges. An extended experimental campaign has been carried out to get affordable data to be used to find a relationship correlating the dry bulb temperature and relative humidity of air after its interaction with water and the condenser tubes surfaces, while all the parameters were set up and controlled. The regression function fits well the experimental data as the predicted values of temperature and relative humidity are characterized by a maximum percent deviation lower than 2.5% and 4% respectively. An iterative procedure was then implemented to determine the conditions of air going through the evaporative condenser in order to extend small scale results to full scale performance according to real geometries. The effect of the water flow rate on the cooling capacity was investigated and the results show that an increase of 50% of the sprayed water leads to an increase of 14% of the performance.