copper tubes, heat exchanger simulations, MOGA, R290, R600a
The ultralow Global Warming Potential (GWP) of propane (R290) and isobutane (R600a) refrigerants make them highly attractive for refrigerator and freezer applications, although their flammability necessitates strict use conditions with respect to refrigerant charge. Copper tubes with smaller diameters are widely used to reduce refrigerant charge. The process of downsizing copper-tube diameters involves detailed simulations and prototype construction as well as testing and validation. A proprietary heat exchanger design and simulation software tool (Jiang et al., 2006) was used to evaluate the performance of and optimize the design of domestic refrigerator condenser coils made with 5-mm outer-diameter copper tubes. Optimization was accomplished through the use of reduced order models, meta-models and a multi-objective genetic algorithm (MOGA). Reducing refrigerant charge was the primary objective. Secondary objectives included the reduction of the total footprint and the total tube-and-fin material mass. The baseline design used 6.35-mm O.D. copper tubes with a minimum wall thickness of 0.41 mm, i.e., quarter-inch tubes with 0.016-inch wall thickness. The new designs use wavy-herringbone fins with reduced fin thicknesses as compared to the baseline design. Other variables included the horizontal and vertical spacing of the tubes; number of tubes per bank; fin density; wavy fin pattern depth; tube length; and tube circuitry. For an R600a residential application, reduced internal volume was considered to be more important than the airside pressure drop. A Pareto chart is presented of optimized values from the design space. Compared to the baseline design, the best 5-mm design reduced the internal tube volume by 41 percent, along with a 57 percent reduction in coil footprint. Additionally, test data to validate the performance of prototype coils is presented with emphasis on the design, construction and manufacture of the heat exchanger coils.