Study of steam condensation over cooled surfaces of humid non-condensable gas streams

The water consumption rate and the scarcity of water sources leads to the importance of proper water management, especially in the industrial context. Some of the many approaches that could be taken to help conserve industrial water resources include condensing part of the steam emitted in cooling towers, using finned tube heat exchangers, such as those with thermosyphons. This work aims to evaluate the performance of condensation of humid air in two different geometries, typical of these heat exchangers: fin-and-tube and parallel flat plates, using numerical simulations under steady-state conditions. To accomplish this goal, three evaluation methods were used. First, simulations were conducted using ANSYS CFX software on a section of the heat exchanger core. Then, also using CFX simulation, the condensation between parallel flat plates, which mimic tube fins, was performed. Finally, a semi-theoretical model available in the literature was employed. The findings indicate that, for a temperature difference between the heated wall and the humid air of 14.5 K, the heat exchanger could successfully recover up to 24% of steam. The results are far less favorable, when the temperature difference is only 4.5 K, with the effectiveness of the recovery of only about 2 percent. However, more water could be recovered, if the wall to humid air temperature difference was higher. More importantly, the semi-theoretical model shows that, by using a rectangular channel made up of parallel flat plates, it is possible to model the complex geometry of a finned tube heat exchanger, with results within 2% of the benchmark. The outputs obtained from the semi-theoretical model are also conservative, showing to be a good designing tool.