A Thermal-hydraulic assessment of condensing tube bank heat exchangers for heat and water recovery from flue gas

Abstract

Condensing heat exchangers are capable of recovering a significant amount of latent heat at temperatures below 100 °C from the flue gas of combustion-based heating systems due to the presence of water vapor in their exhaust streams. However, the condensation of acids along with water vapor creates a highly-corrosive environment in these heat exchangers. As such, a large majority of these heat exchangers are made from corrosion-resistant alloys, such as stainless steel. Polymer-based materials are cost-effective and have great corrosion-resistant properties. Since the performance parameters of a heat and water recovery unit depend on the size and compactness of their heat exchangers, it is challenging to compare the overall performance of stainless-steel condensing heat exchangers with polymeric ones based on the data available in the literature. The main goal of the present study is to develop and assess the thermal–hydraulic performance of a proof-of-concept condensing heat exchanger made of fluorinated ethylene propylene compared to the same condensing heat exchanger made of stainless steel for heat and water recovery from flue gas. For this purpose, an in-depth parametric study is conducted experimentally to evaluate the water recovery efficiency, total heat recovery rate, and pressure drop in the flow paths. The results revealed that the water recovery efficiency of the unit with a specific size, declines when the mass flow rate of the gas increases, although it enhances the total heat recovery of the unit. Moreover, increasing the volumetric flow rate of the heat transfer fluid flow slightly increases the total heat recovery of the stainless-steel condensing heat exchanger, but has a negligible impact on the total heat recovery rate of the polymer-based heat exchanger. Increasing the humidity ratio of the flue gas or the inlet temperature of heat transfer fluid does not have any significant effect on the flue gas pressure drop. These findings are significantly important and novel as they unlock the potential of using polymer-based materials with thermally conductive additives for latent heat recovery from flue gas.