Copper tubes are well suited for use with natural refrigerants such as propane (R290) and carbon dioxide (R744), according to the International Copper Assoc They cite copper tubing's high thermal conductivity, high burst strength, and low tube volumes, especially in small diameter tubes, as their main advantages
Carbon Dioxide and Copper
In the transcritical refrigeration cycle, the refregierant travels through the the compressor but does not condense into a liquid, it just gets colder. Therefore, heat exchangers using copper tubing are called gas coolers, not condensers. Technological advancenments in high-pressure compressors and expansion valves have made R744 a feasible and relatively eco-friendly refrigerant in applications ranging from vending machines to ice-skating rinks.
When R744 is used as a refrigerant, gas cooler pressure is typically in the range of 120 bar, and burst pressure ratings are several times higher. Maximum working pressures for the evaporation phase are typically in the 45 to 60 bar range. This means traditional heat-exchanger coils with copper tubes can be used in the gas cooler and evaporator with carbon dioxide if the thickness of the tube wall and header are enough to withstand the pressures. Interestingly, burst pressure increases as tube diameter decreases, so smaller diameter tubes are especially appropriate for carbon dioxide HVAC sysgtems.
Small Tubes for Propane
Propane is another natural refrigerant being looked at as a refrigerant in HVAC equipment.. Propane is an eco-friendly hydrocarbon with thermodynamic characteristics that make it well suited for residential air conditioners. The only downside is its flammability. To reduce the risk of fires, engineers should minimize the volume of refrigerant compared to the room air volume to be cooled. It's also a good idea to add more safety features to the designs.
In a recent study, a 3D distributed-parameter model of an air conditioner was improved by installing smaller-diameter copper tubing in the heat exchanger. Simulations showed that the amount of refrigerant could be reduced significantly with thinner tubes. Tests with actual air conditioners confirmed the simulation, showing that small-diameter copper tubes can be safely used with propane-based air conditioners.