Condensation Heat Transfer Characteristics of a Pillow-Plate Heat Exchanger for High-Temperature and High-Humidity Paper Industrial Waste Heat Recovery

Industrial waste heat discharged from paper drying processes is carried by high-temperature, high-humidity air with a large water vapor content and thus offers significant potential for both sensible and latent heat recovery. In practical applications, however, the airflow rate is typically high, and the resulting reduction in air contact time may limit condensation and, consequently, latent heat recovery. Whether condensation-dominant heat transfer persists under such high-flow conditions has not yet been sufficiently verified. In this study, experiments were conducted on a pillow-plate heat exchanger under operating conditions measured in an actual paper manufacturing process, with humid air inlet temperatures of 58-72℃ and relative humidities of 60-80%. The cooling water inlet temperature was maintained below the dew point of the humid air to ensure continuous condensation. The airflow rate was varied from 200 to 600 m³/h, corresponding to air-side Reynolds numbers of approximately 1.3×10³ to 3.9×10³. The effects of the airflow rate on the condensate mass flow rate, the latent heat transfer rate, and the latent heat contribution to the total heat transfer were systematically investigated. The condensate mass flow rate increased consistently with the airflow rate, and the latent heat transfer rate varied approximately linearly with the condensate mass flow rate. Despite the nearly threefold increase in the airflow rate, the latent heat contribution remained at 78-84% of the total heat transfer. These results indicate that, although the contact time decreases at higher airflow rates, the enhanced air-side heat and mass transfer compensates for this effect, such that the condensation-dominant heat transfer structure does not collapse even under high-velocity flow conditions.