An experimental investigation of propane/isobutane mixtures as alternative refrigerants in compression refrigeration machines

Due to political regulations, the currently used refrigerants (e.g., R134a) have to be replaced by fluids with reduced greenhouse warming potential (GWP) in future. Natural refrigerants such as isobutane (R600a) or propane (R290) are such fluids; they are environmentally friendly, inexpensive, available in large quantities and have good thermodynamic properties. If isobutane and propane are mixed, a zeotropic mixture results. Based on basic thermodynamic theory, the non-isothermal phase change of zeotropic mixtures leads to reduced exergy losses in the heat exchangers and thus, to an increased coefficient of performance (COP). Apart from the changes in the heat exchangers, all other process components are also influenced by the mixture composition. This usually remains unconsidered in the large number of theoretical studies. To improve and validate theoretical models, a vapour-compression heat pump test rig was set up. The test rig allows the systematic investigation of different pure fluids and fluid mixtures. In addition, extensive measurement equipment is installed to measure mass flows rates, temperature profiles and pressure levels. Based on these measured values, all important process variables can be derived, such as the COP and the compressor power input as a function of the mixture composition. In this study, the pure fluids isobutane and propane as well as their mixtures are investigated. Discussed parameters are the COP, the isentropic compressor efficiencies, the specific compressor work as well as the temperature glide as a function of the evaporation temperature (0 to 6 degrees C), for a constant condensation temperature of 30 degrees C, at different compressor power and rotation speeds (1050, 1500 and 2100min-1). For steady state conditions the results show that the use of a zeotropic mixture increases the COP compared to the respective pure fluids. The highest COPs are achieved at a composition of xPropane = 0.732; xIsobutane = 0.268 and are between 3.3 and 5.4 depending on the operating conditions. Compared to theory, the experimental results for the mixture xPropane = 0.732; xIsobutane = 0.268 show only slight improvements of the COPs by 5% compared to pure propane. A more detailed analysis shows that mainly two aspects influence the process efficiency. On the one hand the isentropic efficiency of the compressor, which depends strongly on the composition and on the other hand, the pressure losses in the heat exchangers reduce the efficiency. Especially the pressure losses in the evaporator reduces the actually usable temperature glide. These effects are usually not taken into account in simple theoretical approaches.