Conversion of cold beverage dispenser's refrigeration system to R744 refrigerant

摘要

Cold beverage dispensers belong to the group of light commercial refrigeration machines that includes beverage coolers, ice machines and storage units for food items. Beverage dispensers use a vapor compression system to charge a thermal storage, usually an ice bank, which is used to indirectly cool tap water and beverage concentrate. For these machines, R290 (propane) and R744 (carbon dioxide) are among the favorite candidates to completely replace high GWP HFC refrigerants in the future. Because of flammability and charge restrictions for R290, carbon dioxide is in some cases preferred even if some of its thermodynamic and transport properties are less favorable than those of R290, especially at high ambient temperature. This study evaluated the performance of a baseline R134a beverage dispenser machine by experimentally measuring initial pull down time and energy consumption during compressor cycling operation at 24°C and 38°C ambient temperatures, respectively. Another important performance criteria for dispensers is the burst capacity, which is the number of 350 ml drinks dispensed at the rate of 3 drinks per minute while maintaining a beverage temperature below 5°C. This test simulates the typical load profile of the machine during peak serving time in hotels and motels. Burst capacity is closely related to thermal storage capacity and therefore the ice amount was also closely monitored. The machine's refrigeration system was afterwards converted to R744 refrigerant, using a transcritical cycle. Special attention was given to optimization of capillary tube size, refrigerant charge amount and gas cooler design where the strongest impact on the performance at different ambient temperatures was expected. Appropriate instrumentation and theoretical analysis helped to identify optimum selection of capillary tube and R744 refrigerant charge. The optimized R744 system used 385g of charge and pulled down in approximately half the time and dispensed at least 25% more drinks while keeping the energy consumption comparable with its baseline R134a system. Slight increase in gas cooler cost was well overcome by the benefits of larger burst capacity and significantly shorter pull down time allowing: the redesigned beverage dispenser can be kept off longer and needs to be turned on shortly before the serving time.