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| REFRIGERANTS Constant temperature lines lift, and so increasing lift has an increasingly significant detrimental impact on COP. Practically, the operating temperatures are driven by the application so, for example, if a direct expansion (DX) cooling coil is employed in an air conditioning unit to cool air, the cooling coil (the evaporator) will need to be at a temperature below the required air temperature (the actual temperature will be dependent on the heat transfer properties of the coil). Equally, if the condenser was rejecting heat into summer outdoor air, the temperature of the condensing refrigerant must be greater than that of the air temperature. To provide the best practicable COP, careful consideration is needed to establish, and maintain, an appropriately low temperature condenser heat sink (typically air or water), as well as establishing the highest evaporator temperature that will still provide the cooling and, often, the dehumidification required by the application. (And, of course, the efficiency that matters to the end user is the whole-year energy use, and the best way of improving that is to manage the heat load on the system so that it is used as little as possible.) So, the operating temperatures set the key conditions required of a refrigerant. In vapour compression refrigeration systems, the refrigerant is a fluid selected so that it readily evaporates and condenses at the required temperatures, at pressures that can also be practically delivered and maintained in the system. There are many fluids both synthetic and natural that can be used for this purpose, and as manufacturing and piping technology advances, the opportunity to safely operate at more extreme pressures allows novel refrigerant applications. There are a number of parameters that are considered when evaluating a potential refrigerant that, depending on the application, are likely to include: Appropriate thermophysical properties so that the system operates at acceptable pressures with low compressor input power and high refrigeration effect Environmentally benign low global warming potential (GWP) and zero ozonedepletion potential (ODP), with high COP to minimise primary energy use and indirect emissions Low or zero toxicity to occupants (directly or indirectly) Acceptably low risk of flammability. And in addition there are key operational requirements: Remains stable within the system and be compatible with system pipework and component materials (including high dielectric strength for hermeticcompressors) Pressure kPa CPD PROGRAMME Subcooled liquid 3 Critical point Superheated vapour Notional basic cycle 2a 2 Condenser Throttling device 4 Evaporator Compressor 1 Compression Cooling Enthalpy kJkg-1 Figure 2: The vapour compression cycle plotted on a pressure-enthalpy diagram. This simple basic cycle includes constant pressure evaporation, isentropic compression, constant pressure condensation and (almost) adiabatic expansion (throttling) process Self-lubricating (or at least compatible with lubricants) Easy and safe to handle and detect Acceptable cost. The evaluation of suitable substances is complex and practically includes multivariate analysis that has been led by manufacturers and research teams, including a team led by McLinden at the US National Institute of Standards and Technology (NIST).1 However, there are some fundamental properties that determine potential suitability. The basic thermodynamic properties of a potential refrigerant are the normal boiling point (NBP) and the critical temperature. A NBP the boiling temperature at atmospheric pressure below the required cold temperature will ensure that the evaporator operates at a positive pressure, so reducing the opportunity for the leakage of ambient air, non-condensable gases and water vapour into the system (which would all reduce the system performance). However, an excessively low NBP will increase the condenser pressure and density of the refrigerant vapour, which will increase the work required by the compressor. The critical temperature of a refrigerant is that above which the vapour cannot be condensed into a liquid, no matter how high the pressure. This will be constrained not only by the application but also the geographic location since, for example, warmer climes will not be able to use refrigerants with critical temperatures below that of the high ambient temperatures. However, that same equipment could operate (more efficiently) in temperate areas with different refrigerants that possess a lower critical temperature. So, the refrigerant critical temperature must be greater than the condensing temperature, and considering the critical point (as shown in Figure 2), it is evident that to gain most benefit from the rejection of heat as the refrigerant condenses, the critical pressure must be appreciably higher than the condensing pressure so as to ensure that the enthalpy difference between 2a and 3 is as large as practicable. However, an excessively high critical temperature will reduce the volumetric refrigerating capacity (refrigerating effect per specific volume of refrigerant) due to increased vapour-specific volumes, and so will increase compressordisplacement. The refrigeration effect (line 4-1 in Figure 2) is determined by the proportion of latent heat of evaporation that takes place in the evaporator. Refrigerants that have a more upright p-h characteristic curve will utilise a greater proportion of the potential latent heat of evaporation that in conjunction with the specific volume of the Flammability A Lower toxicity B Higher toxicity 1 No flame propagation (considered non-flammable) A1 B1 2L Lower flammability A2L B2L 2 Flammable A2 B2 3 Higher flammability A3 B3 Figure 3: Standardised index of refrigerant flammability and toxicity 56 June 2019 www.cibsejournal.com CIBSE Jun19 pp55-58 CPD 146 v4.indd 56 24/05/2019 16:26