Variable-Diameter Finned-Tube Heat Exchanger Optimization for R290 Split Air Conditioners

Balancing heat transfer performance with material cost and refrigerant charge remains a key challenge in split air conditioning systems. To address this issue, the present study proposes a finned-tube heat exchanger with a variable-diameter configuration, combining 5.2 mm and 7.3 mm tubes for use with R290 refrigerant. Three hybrid arrangements are examined against a conventional baseline with uniform 7.3 mm tubes, differing in the number and spacing of the 5.2 mm tubes integrated within the heat exchanger layout, thereby enabling targeted structural and thermal optimization of the indoor unit. An integrated methodology, based on a theoretical iterative algorithm and supported by numerical simulations and experimental validation, is employed to characterize heat transfer and fluid flow under rated cooling conditions. The results show that the proposed configurations achieve substantial reductions in refrigerant charge, by up to 11.5%, and copper usage, by up to 7.78%, while simultaneously enhancing the system coefficient of performance by as much as 3.75% compared to the reference design. Configurations with a higher proportion and tighter spacing of 5.2 mm tubes yield the greatest improvement in energy efficiency, whereas those maximizing the substitution of 7.3 mm tubes with 5.2 mm tubes achieve the most pronounced reductions in material usage and refrigerant charge. Overall, the findings demonstrate that variable-diameter tube heat exchangers provide an effective strategy for optimizing the trade-off between performance, cost, and environmental impact in R290-based split air conditioning systems.