Refrigerant charge reduction, propane R290, efficiency, light commercial, microchannel
The majority of beverage manufacturers prefer natural refrigerants over synthetic refrigerant options as the working fluid for their beverage display cooling equipment. The two major refrigerants considered for these light commercial applications are CO2 (carbon dioxide) and R290 (propane). CO2, used in a transcritical cycle, offers advantages in terms of flammability, however, reduced performance at high ambient temperatures and safety issues related to the high working pressures need to be addressed in order to design efficient and cost-competitive systems. The challenges encountered with CO2 are typically no concern for R290; however, the major risk associated with propane is the flammability of the refrigerant. One way to mitigate the risk is rigorous reduction of refrigerant charge to levels that are substantially below legal limits. This task has been accomplished by employing experimentally validated simulation models that can be used to reliably predict the refrigerant charge in each component at different ambient conditions. Design and optimization efforts have to be focused on the compressor oil type and charge amount which dissolves large quantities of propane that does not contribute to generating the desired cooling effect. The other component that bears significant potential to reduce refrigerant charge is the condenser; low internal volumes are desired that can provide an optimal balance between required refrigerant charge, heat transfer, and pressure drop. A serpentine style microchannel heat exchanger design featuring low fin density on the air side was developed and implemented. The reduced number of fins on the air side reduces maintenance requirements and allows the system to be used in dusty environments. The improvements along with further optimizations were implemented into a beverage cooler holding 700 cans with a volume of 355mL per can. The achieved cooling capacity was on the order of 1 kW featuring a propane refrigerant charge of less than 50 g. The performance of the redesigned beverage display cooler was experimentally validated at an ambient temperature of 40oC and it was found that all of the manufacturers’ pull-down and energy consumption test requirements were successfully achieved.