Copper, Microchannel, R290, R744, Microgroove
The ongoing global effort to replace current refrigerants with zero Ozone Depletion Potential (ODP) and virtually zero Global Warming Potential (GWP) refrigerants has important implications for heat exchangers, air conditioning system design, and the materials choices in these designs. Natural refrigerants with higher flammability, CO2, new HFC blends, and new HFO’s each place different requirements on the heat exchanger design, whether it be for higher equipment efficiency, to reduce refrigerant charge, to operate to much higher operating pressures or temperatures, to prevent corrosion or to avoid leakage. This paper presents critical information that engineers need to know about how heat exchangers based on round inner-grooved small-diameter copper Microgroove™ tube and newly-developed flat copper microchannel tube can be applied in air conditioning equipment using new alternative refrigerants. Heat exchangers based on Microgroove™ tubes with 5mm or 4mm diameter provide solutions for new refrigerants. For example, Microgroove™ tube systems have been found to provide a proven and safe solution for air conditioners using new refrigerant R290 (propane). They permit significantly smaller refrigerant charge, equivalent to those used in microchannel extrusions, while providing reduced manufacturing complexity and maintaining energy efficiency similar to traditional units. These tubes have antimicrobial properties that eliminate mold growth. They have a high level of solution flexibility to provide special circuiting or enlarged coils and eliminate over-sizing for standard products. Thin wall Copper microchannel extruded tubes exhibit high burst pressure at post-brazed conditions. These tubes are especially attractive for use at the high operating temperature and pressure associated with R744 systems. Heat exchangers constructed with such tubes maintain their post braze strength and burst pressure resistance at 180°C operating temperature, have high thermal conductivity, and thus maintain a small heat exchanger size. In addition, microchannel tube produced with a copper alloy such as brass provides the additional benefits of lower cost, corrosion resistance and higher material strength that can lead to even thinner walls and reduced heat exchanger size. Examples of heat exchangers made using copper microchannel extruded tubes fabricated using Cuprobraze technology are provided in this paper. In the quest to improve the environmental performance of air conditioning systems, it is critical to examine the total Life Cycle Climate Performance (LCCP) including the effects of the refrigerant choice and the effect of energy use across the system life cycle. All-copper heat exchangers using either copper Microgroove™ or microchannel tubes suppress the growth of bacteria and mold that reduce system energy efficiency, cause product deterioration and foul odors. Mold growth studies show aluminum fin heat exchanger performance decreases up to 20% after four years of operation, where mold growth covered 60% of the fin surface. These new copper-based solutions provide an added protection against longer term potential efficiency losses in systems using new refrigerants. They are appropriate for use in air conditioning systems that achieve the highest level of Life Cycle Climate Performance.