Heat Transfer and Pressure Drop of R1123/R32 Flow in Horizontal Microfin Tubes During Condensation and Evaporation

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

Thermodynamic Assessment of High-Temperature Heat Pumps for Heat Recovery

To reduce energy consumption utilizing heat recovery systems is increasingly important in industry. The screening of refrigerants and primary energy efficiency are mainly discussed with a case study for high-temperature heat pump cycles in this study. The overall coefficients of performance of four different cycle configurations to increase temperature of compressed water up to 160 oC using waste heat of 80 oC are calculated and thermodynamically compared for selected refrigerants, R717, R365mfc, R1234ze(E), and R1234ze(Z). The calculation results suggest that a multiple stage cycle drastically reduces throttling loss in expansion valve and exergy loss in condensers, and consequently achieves the highest overall coefficient of performance (COP) among the calculated cases with a refrigerant R1234ze(Z) having the critical temperature about the target outlet water temperature. A cascade cycle using R1234ze(Z) and R365mfc performs relatively high COP and also brings many practical benefits, such as varied combination of refrigerants and lubricant oils and prevention of the liquid-back caused by cold start. At a compressor efficiency of 0.7 and an approach temperature difference in heat exchangers of 2 K, the calculated overall COP is ranging from 4.3 to 4.94. This is corresponding to the primary energy efficiency 1.62 to 1.83, when the transmission-end-efficiency of electric power generation is 0.37. Even where the compressor efficiency of 0.61 and the approach temperature difference of 8 K, the efficiency is above 1.3. As remarked above, the thermodynamic assessment demonstrated that the high-temperature heat pumps to recover waste heat are promising system to reduced primary energy consumption for industrial applications