The prediction and the exploitation of the thermodynamic properties of non ozone depleting refrigerants

The use of mixtures of refrigerants in refrigeration cycles can lead to improvements in cycle efficiency. The majority of refrigerant working fluids have been pure fluids. With pure refrigerants temperature profiles between the working fluid and the heat source and sink fluids may not be well matched. Mixtures of refrigerants boil and condense across a temperature range. This property can be used to reduce the mean temperature differences in the heat exchangers, and to improve the matching of the temperature profiles. This leads to higher coefficients of performance (COP). In this thesis, the improvement in refrigeration COP due to mixtures of refrigerants is investigated. Ratification of the Montreal protocol led to the phasing out of chlorofluorocarbon (CFC) refrigerants. Hydrofluorocarbon (HFC) refrigerants are the leading candidates to replace CFCs. Mixtures of HFC refrigerants are examined. An existing pilot plant refrigeration cycle was adapted and modified for use with HFC refrigerants. A binary mixture of difluoromethane (R32) and 1,1,1 ,2-tetrafluoroethane (R 1 34a) is examined experimentally. The phasing out of . CFC refrigerants means that there exists a need for methods which can predict accurately the thermodynamic properties of a proposed replacement refrigerant, from sparse amounts of data. The Cubic Chain-of-Rotators (CCOR) equation of state requires relatively little knowledge of the fluid it describes. CCOR predictions of pure and mixed HFC thermodynamic properties, were compared with published experimental data. Comparisons were also made with the more complex Carnahan-Starling-DeSantis (CSD) equation. The CCOR equation predicted saturated and superheated vapour pressure with satisfactory accuracy. Liquid density was not predicted with the same precision. Vapour density was described no worse than the CSD equation. CCOR description of binary vapour-liquid equilibrium (VLE) was superior to that of the CSD equation. Prediction of VLE data was improved by using optimal interaction constants. It was shown that if an optimal set of interaction constants was located for each experimental data point for bubble point VLE data, the interaction constants exhibited a regular dependence upon temperature and composition. The CCOR equation can be used to provide approximate preliminary thermodynamic data for a new refrigerant, for which little data exists. A simulation model of a refrigeration cycle was developed, based upon the CCOR equation. Six binary FIFC mixtures were modelled. The CON of the mixtures were compared to pure fluid COPs. Improvements in mixture COP were in the range 0.0-14.5%, depending on composition and conditions. Improvements in the UA values of the heat exchangers led to a larger mixture COP enhancement. Increased compressor polytropic efficiency had no significant effect upon mixture COP enhancement. Mixtures outperformed pure fluids when the evaporator heat source fluid temperature change was increased. Mixtures performed better than pure fluids, under conditions of higher pressure drop and decreased heat sink flow rate. Liquid-suction heat exchange affected mixtures, which performed poorly in the basic cycle, more than those that performed relatively well.