Optimization considerations in the design of shell and tube condensers implementing enhanced tubes: Experimental and theoretical analysis

Passive heat transfer enhancement technique, which includes modification of surfaces by various shapes and structures, has been a topic of research during the last decade. The helical microfin tubes, which are deemed promising in improvements of heat transfer, are often implemented in the context of flow condensation. While research regarding their impact on the two-phase flow of refrigerants is accruing, a systematic assessment is required to ensure their promised claims to increase the efficacy of thermal systems. To further expand the database within the literature, two performance criteria, Performance Index (PI) and Penalty Factor (PF), are evaluated for a database comprised of 1192 data during flow condensation of low global warming potential HydroFluoroOlefins (HFOs) inside microfin tubes ranging from 3 to 7 mm outer diameter (OD). The tube of 5 mm OD demonstrated the highest efficacy by the method of PI, while also demonstrating undesirable high PF values. In microfin tubes, despite enhanced heat transfer due to augmentation of turbulence and shear-stress forces, higher frictional pressure drops could nullify the positive effects depending on the circumstances. In a shell and tube condenser with condensation on the tube side, for fixed input and output temperatures on the coolant-side, the pressure drops could cause a noticeable drop in logarithmic mean temperature difference (LMTD). Therefore, an optimal design mass flux must exist to maximize heat transfer coefficients while avoiding adverse effects on LMTD. In some cases, due to lower frictional pressure losses for a smooth tube, it is better to avoid using a microfin structure all together. The article proposes a design procedure to correctly identify and assess the performance by conducting a preliminary simulation of a condenser.