Modeling and simulation of a helical falling film evaporator to improve its performance and design

Abstract

This work presents a computational model of a helical falling film evaporator of 13 tubes based on physical principles. The momentum and energy transfer of the system are modeled. The work aims to understand the behavior of a horizontal falling film evaporator and to improve the evaporator design by analyzing the heat transfer. In the evaporator, the wetting efficiency varies in each tube because the evaporation of the falling film is performed gradually. Therefore, the behavior of the evaporator variables, such as the wetting efficiency in each tube, is studied. Furthermore, the model granularity based on the analysis of the subcooling and saturation zones is investigated. The effect of the film breakdown onset Reynolds number and the mass flow rate on the evaporator performance is studied. This analysis shows that the maximum heat transfer rate of the simulation is reached when the falling film Reynolds numbers of the tubes are as close as possible and pass by the film breakdown onset Reynolds number value. Additionally, the results indicate that at a low mass flow rate, the heat transfer is carried out almost completely in the first five tubes, and at a higher mass flow rate, the heat transfer is more significant in the last tubes. Finally, a comparison between the evaporator of 13 tubes and an arrangement of two evaporators of five tubes each one is performed. From this analysis, the results show that the heat transfer rate of the arrangement of two evaporators is 21.16 % and 18.69 % higher than the evaporator of 13 tubes at 0.00529 and 0.00793 kg/s⋅m, respectively.