Analytical solutions of heat and mass transfer process in combined gas-water heat exchanger applied for waste heat recovery

The main task of a combined gas-water heat exchanger (CGHE) in a waste heat recovery system is to transfer heat from flue gas to boiler backwater. A spray tower and an indirect contact heat exchanger are key components of the CGHE. Analytical solutions of the heat and mass transfer process for the overall system are presented in this paper. Analytical solutions of the air and water parameters agree well with the results gained from both experiment results and numerical solutions. The result developed from the analytical method has been used for optimizing the thermal performance of the CGHE. It's indicated the heat recovery efficiency is determined by CGHE structure characteristics and spray water flow rate, while inlet states of the flue gas and the backwater have limited effect. In a specified system, there is an optimal flow rate of the spray water corresponding to the minimum thermal resistance of the CGHE. This is mainly because the increase in the spray water flow rate is beneficial to the contact area in the spray tower, but adverse to the matching performance between the fluids. When the CGHE is optimized, the maximum heat recovery efficiency and the maximum heat transfer rate are obtained. (C) 2020 Elsevier Ltd. All rights reserved.