Condensation of pure and blended HFC refrigerants

An experimental analysis to investigate the heat transfer performance of HFC refrigerant mixtures when condensing inside a horizontal smooth tube is being carried out at the Dipartimento di Fisica Tecnica of the University of Padova. HFC fluids, pure or blended, are considered as possible substitutes for R22, whose phase out is underway in Europe. As first choice, pure fluids and near-azeotropic HFC mixtures (such as R410A) can be used. Some of the fluids under consideration may display a high operating pressure. When the substitution for traditional ozone-depleting refrigerants involves the use of high pressure fluids, this usually implies a reduction of friction losses, with higher energetic performance, and a reduction of the size of the plant. R410A is a binary mixture with a very low boiling range (around 0.1°C) obtained by blending R125 and R32 at 50/50 % mass composition. Alternatively, highly zeotropic mixtures can be used as substitutes, above all when searching for new fluids to be utilised in high temperature compression cycles. Zeotropic mixtures with high temperature glide could be used for developing new Lorentz-cycle machines. In fact, optimization of the temperature profiles in the heat exchangers and higher efficiency in some industrial processes, such as drying and heating of water with compression heat pumps, are eligible to be obtained by developing an inverse Lorentz cycle machine. On the other hand, employing zeotropic mixtures as alternative working fluids may introduce some heat transfer problems in the practical use, namely the degradation of heat transfer coefficients caused by the zeotropic characteristics of the mixture. The heat transfer penalty is mainly due to the mass transfer thermal resistance build-up. The investigation is carried out by blending (at three different mass compositions) two pure HFC fluids characterised by having far different pressure at the same saturation temperature. R125 in the present case is the “high pressure” fluid, while R236ea can be considered a “low pressure” fluid. Mixing of those two HFC fluids results in a high temperature glide mixture. The mixture tested is a refrigerant blend of R125 and R236ea, of 46/54 % mass composition, characterised by a temperature glide of 21°C at a nominal saturation pressure of 1000 kPa.