Heat recovery characteristics of a pillow-plate heat exchanger for low- and high-velocity exhaust in paper processes

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The drying section of the paper industry discharges large volumes of high-temperature and humid air containing substantial sensible and latent heat, while conventional fin-and-tube heat exchangers are susceptible to fouling and corrosion under these conditions. Existing air-side Nusselt number correlations for pillow-plate heat exchangers (PPHE) have primarily been developed for single-phase, non-condensing flow, leaving their heat transfer characteristics under condensing paper-exhaust conditions insufficiently characterized. To address this gap, this study investigates condensing PPHE performance in the low-Reynolds-number and transitional regimes and develops a geometric correction approach to account for changes in flow cross-sectional area. A PPHE was experimentally evaluated under simulated paper-exhaust conditions at inlet temperatures of 70–90 °C, relative humidity of 25–45%, and air flow rates of 200–600 m 3 /h. Two test sections with different plate numbers (7 and 3 plates) were used to cover Reynolds numbers of approximately 700–3600. The high-velocity configuration exhibited an overall heat transfer coefficient up to 2.3 times higher, whereas the low-velocity configuration achieved up to 1.8 times greater total heat recovery owing to its larger condensing area, with latent heat accounting for up to 77% of the recovered heat. A Nusselt number correlation with a Reynolds number exponent of 0.48 was developed and extended to both geometries using an empirical flow-area correction factor, reducing the mean absolute percentage error from approximately 16% to 3.4%. These results demonstrate that the overall heat transfer coefficient and total heat recovery should be evaluated independently under condensing conditions and provide a practical correlation for designing PPHE for humid-exhaust heat recovery.