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100.0 10.0 1.0 0.1 R410A R134a R513A R450A R32 R1234yf R1234ze R152a Isobutane Propane Propylene -50-40-30-20-10 0 10 20 30 40 50 60 70 80 90100 Temperature [ ̊C] Figure 2.2. Saturated vapour pressure vs. temperature Furthermore, the shape of the property diagram of a refrigerant influences the COP as it affects the values of enthalpy at the saturation points (Ekroth, 2011). As far as thermal characteristics are concerned, low vapour heat capacity, low viscosity and high thermal conductivity should be maintained. The critical and boiling point temperatures must be chosen relatively to the application. Additionally, it would be preferable to work with a refrigerant having a small specific heat compared to the latent heat of vaporisation, in order to achieve smaller losses in the expansion valve (Rothlin, 2011). Another aspect of great importance to treat when considering refrigerants is the mixtures. Their development and study is becoming increasingly important, as mixing different substances allows the creation of working mediums with more suitable properties and that can also result in the increase of the COP of the system. According to their behaviour during evaporation and condensation, the mixtures can be classified as azeotropic, near azeotropic, or zeotropic, the differences residing in if – and in what measure – a temperature glide exists. In fact, when a pure refrigerant undergoes evaporation or condensation, it will maintain a constant temperature as long as the pressure is kept constant. In general, an increased pressure results in an increased saturated vapour pressure. Instead, in the case of a mixture of two or more components, the above mentioned processes will occur with temperature varying with the liquid- vapour composition. Azeotropic mixtures In this case the glide effect is not present: the mixture acts as a single substance, thus maintains a constant boiling point, usually lower than either of the constituents. Most of these blends are binary and are most commonly used in low temperature refrigeration applications. -8- Pressure [bar]PDF Image | Next generation of refrigerants for residential heat pump systems
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