Study on condensing heat transfer characteristics of narrow-gap heat exchanger for flue gas in gas-fired boilers

Deep flue gas condensation is a critical technology for recovering waste heat from boilers and achieving energy conservation and carbon reduction. To simultaneously enhance condensation efficiency and reduce flow resistance, this study developed a high-efficiency narrow-gap channel condensation heat exchanger. A numerical model of flue gas heat transfer was established to analyze the performance of heat exchange channels with different fin structures under various operating conditions, including flue gas velocity, temperature, cooling water wall temperature, and vapor content. Using the optimal fin structure, response surface methodology was employed to optimize the flow channel parameters. Results indicate that flue gas velocity has the most significant impact on pressure drop and condensation rate. The braided-fin channel demonstrated optimal thermo-hydraulic performance across all operating conditions. Under 333 K flue gas temperature, the overall heat transfer coefficient reached 445.28 W·m⁻²·K⁻¹. Response surface optimization of the braided-fin channel yielded the following optimal parameters: channel length 300 mm, width 3 mm, fin height 27 mm, and thickness 5 mm.