CO2, commercial refrigeration, efficiency, bottle cooler
Glass door merchandisers are used in grocery and convenience stores to display chilled beverages or refrigerated foods. Among other possible choices for this application, carbon dioxide (R744, CO2) is seen as a promising low-GWP refrigerant alternative that is non-flammable and non-toxic. While R744 itself is less costly than synthetic alternatives, successful implementation of high-performance, low-cost transcritical R744 technology is challenging. This paper summarizes important R744-specific design issues and differences in comparison to conventional R134a bottle coolers. Due to cost reasons, it is highly desired to use relatively conventional components, including round-tube-plate-fin heat exchanger designs, fixed geometry capillary tubes (instead of variable geometry expansion valves), and single-speed compressors. While a conventional round-tube-plate fin evaporator designs deliver acceptable results, transcritical R744 systems require substantially different heat exchanger designs in order to deliver suitable performance when used as gas cooler. Internal heat exchange, which in conventional R134a systems is often achieved by wrapping the capillary tube around the compressor suction line, plays a much more important role in transcritical R744 systems, and shows large optimization potential. Experiments also show that proper capillary tube sizing and refrigerant charge optimization have much bigger impact on transcritical R744 systems in terms of cooling performance and energy efficiency in comparison to R134a systems. Presented is an example of a successful R744 bottle cooler design that is on par with a comparable R134a system in terms of performance and cost. This low-cost, high performance design has been implemented and experimentally validated.