Thermo-Hydraulic Performance of Additively Manufactured Compact Shell-and-Tube Heat Exchangers Fabricated from PLA and PLA–Copper Composites

Additive manufacturing enables the fabrication of compact polymer heat exchangers, although their performance is constrained by low wall thermal conductivity and manufacturing-induced surface characteristics. This study experimentally compares two geometrically identical shell-and-tube heat exchangers fabricated by fused filament fabrication from pure PLA and PLA containing 25 vol.% copper particles (PLA + Cu). The combined influence of effective thermal conductivity, surface topography, and hydraulic resistance was investigated under water–water counterflow conditions. The measured thermal conductivity increased from 0.243 ± 0.018 W·m−1·K−1 for PLA to 0.422 ± 0.008 W·m−1·K−1 for PLA + Cu, corresponding to an approximately 74% improvement. The composite also exhibited higher surface roughness (Sa = 17.0 μm versus 13.5 μm) and a developed interfacial area ratio (Sdr = 43.5% versus 19.7%). The PLA + Cu exchanger achieved a maximum heat duty of approximately 289 W and an overall heat-transfer coefficient of 151 W·m−2·K−1, compared with approximately 208 W and 93 W·m−2·K−1 for PLA. However, the thermal enhancement was accompanied by increased hydraulic losses, with tube-side pressure gradients reaching approximately 5.7 kPa·m−1. Thermal-resistance decomposition identified shell-side convection as the principal limitation of the investigated configuration. Effective convective coefficients estimated using the thermal-resistance model showed approximate consistency with Chilton–Colburn analogy estimates, with mean absolute percentage errors of 7–13%. The findings demonstrate that improved material conductivity does not necessarily translate into proportional overall performance gains and highlight the need to optimize shell-side convection and hydraulic resistance alongside material selection.