Effective Waste Heat Recovery of Boiler Flue Gas Using a Heat Pipe Heat Exchanger: A Performance Comparison With Finned-Tube Economizer

This study evaluates the potential of a heat pipe heat exchanger (HPHX) as a boiler economizer, which recovers waste heat from exhaust flue gas for preheating feedwater, compared to the conventional one, finned-tube heat exchanger (FTHX), focusing on their waste heat recovery (WHR) capabilities. Experiments were conducted on the original economizer of an experimental liquefied natural gas (LNG) steam boiler (FTHX) and HPHX we fabricated, under actual boiler operating environments, measuring their heat transfer rate and effectiveness. The results showed that both heat exchangers exhibited comparable thermal performance. However, the HPHX achieved significantly higher performance considering heat exchanger dimensions, with a heat transfer rate per unit volume 3.24 times higher than FTHX, thanks to its working mechanism utilizing latent heat. To further evaluate HPHX’s potential, the performance of HPHX with the same dimensions as FTHX was examined based on an empirical model for HPHX. Our analysis revealed that, compared to the FTHX, the HPHX could improve heat recovery rate by up to 11.0%, increase feedwater temperature by 6.9°C, and enhance boiler efficiency by approximately 1.28%. Moreover, additional investigation based on design modifications showed that the superior thermal performance of the HPHX enables a more compact heat exchanger design, allowing a 36% reduction in length without compromising WHR performance. These findings demonstrate the HPHX’s capability to increase boiler efficiency and/or enable system compactness, suggesting that it is a promising alternative to the conventional finned-tube economizer.