R1123 (tri fluoro ethylene; CF2 = CHF) has a GWP of approximately 1, a normal boiling point of -59 °C and a critical temperature of 59 °C, those are considered to be desirable for air condition systems. However, the concentration is limited below 60% because of the instability. Therefore, using this substance as a mixture coupled with R32 is attempted. In this study, heat transfer characteristics in a horizontal microfin tube are assessed for the new candidate of low GWP refrigerant mixture R1123 / R32 (40 / 60 mass%). Of the tested microfin tube, an outer diameter, an equivalent inner diameter, a number of fins, and a helix angle are respectively 6 mm, 5.21 mm, 60, and 18 deg. The heat transfer coefficient and pressure drop during condensation and evaporation process are experimentally quantified at saturation temperatures of 40 °C and 10 °C, respectively, and compared to those of R32 alone. The condensation heat transfer coefficient of R1123 / R32 (40 / 60 mass%) was somewhat lower than that of R32; while, the evaporation heat transfer coefficient was almost comparable to R32. The temperature glide of R1123 / R32 (40 / 60 mass%) is approximately 1 K. Consequently, the mass transfer resistance caused by volatility difference was not obvious in heat transfer coefficient. On the other hand, the pressure drop during both process of condensation and evaporation was obviously lower than that of R32 because vapor density of R1123 / R32 (40 / 60 mass%) is higher than that of R32 alone. From the aspect of heat exchanger designing, the tested mixture offers good solution to shifting lower GWP
