Thermal performance of heat and water recovery systems: Role of condensing heat exchanger material

There is enormous potential for recovering a significant amount of latent heat at temperatures below 100°C from flue gas of combustion-based heating systems due to the presence of water vapor in their exhaust streams. However, condensation of acids along with water vapor in heat and water recovery systems makes a highly corrosive environment, which is a major challenge and a determining factor in selecting suitable materials for condensing heat exchangers. Despite the low cost and great corrosion-resistant properties of plastics, their relatively low thermal conductivities are not ideal for thermal management systems. it is still uncertain how significantly increasing thermal conductivity of the heat exchanger’s material affects thermal performance of the heat recovery systems. The present study aims to shed light on the effect of the thermal conductivity of a condensing heat exchanger’s material on the thermal performance of the unit. For this purpose, an analytical model is developed to predict the thermal performance of condensing heat exchangers, designed for recovering heat and water from wet flue gas. Further, to validate the model, a custom-designed condensing heat exchanger with replaceable tubes is designed in our lab and tested with 304 stainless-steel tubes and FEP plastic tubes under different inlet conditions. For the range of inlet conditions considered in this study, results show that there is a threshold for the thermal conductivity of the material, at which increasing the conductivity any further does not affect the condensation efficiency notably. It is worthy of note that this threshold, with respect to thermal conductivity of commonly used materials for such heat exchangers, has relatively low magnitude (e.g.~10-15 W"∙" m-1"∙" K-1 for stainless steel). This finding is significantly important as it unlocks the potential of using materials such as plastics and polymers with thermally conductive additives for latent heat recovery from flue gas.