Analysis of the performance of direct contact heat exchange systems for application in mine waste heat recovery

Due to their relatively high thermal effectiveness and ease-of-use, direct contact heat exchange units are a favorable choice in heat recovery applications. Today, spray-based direct heat exchangers are commonly used for a variety of applications, including evaporative cooling and heat recovery. Yet in spite of their versatility, these heat exchangers are not well-understood, mainly due to their complex liquid/gas interactions, which may deem poorly-designed systems as undesirable and infeasible. Spray nozzle height and type (hollow and full cone), droplet diameter distribution, inlet air velocity and humidity, and water spray velocity are key parameters required to establish a clear understanding of direct heat exchange performance. To better understand such a multivariable phenomenon, this article offers a comprehensive study of the impact of these key factors. The main purpose of this study is to evaluate the validity of two different methods proposed for evaluating the performance of heat and mass transfer in large-scale direct contact heat exchangers, namely one-dimensional and three-dimensional approaches. The results of the present study show that while the one-dimensional approach is able to capture the experimental data within 10% of agreement with experiment, the three-dimensional approach reduces this error to 5%. In addition, this study aims to provide a framework to conduct future parametric studies with the goal of highlighting key performance factors understanding of which are a necessity for improved design of direct contact heat exchangers used in mine waste heat recovery applications. Novelty Statement To investigate the performance of an industrial-scale direct (spray) heat exchange system with a vertical configuration for application in mine waste heat recovery, two different models, namely; '1-D' and '3-D', are developed. Accuracy of two models in predicting heat transfer is evaluated through comparison with experimental data. A series of parametric studies are conducted to examine the impact of various operating parameters on the performance of heat exchange system.