An iterative approach for optimum sizing and off-design performance analysis of water-cooled brazed plate heat exchanger as supercritical CO₂ gas cooler.

Brazed plate heat exchangers (BPHXs) are widely used as gas coolers (GCs) for providing hot water in transcritical CO₂ heat pumps. Dynamic building loads require varying GC operating conditions. Despite their widespread use, methodologies for optimal sizing and off-design performance studies of BPHXs as GCs are rare. This study presents an iterative, segment-based finite-volume model to size and predict BPHX performance under various scenarios, including changes in entering supercritical CO₂ (s-CO₂) mass flowrate, temperature, pressure, and water side inlet temperature and mass flowrate. Fifteen combinations of five different heat transfer coefficients (HTCs) and three pressure-drop (PD) correlations are investigated, and the model is validated against experimental data. Building on this framework, a multi-criteria Overall Performance-Cost Indicator, encompassing energetic performance, exergy destruction, cost, CO₂ side PD, and desired water outlet temperature, is introduced to support optimal GC selection. The effects of the number and length of plates, and water to s-CO₂ mass flowrate ratio, and inlet s-CO₂ temperature and pressure variations on GC load, thermal performance, PD on each side, capital cost, outlet temperatures, and total exergy destruction are detailed. The presented model contributes practical insights into geometry-performance trade-offs and decision-making strategies for GC design and selection under variable operating conditions.