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CHAPTER 2: LITERA TURE STUDY 21 - - - - - - - - - - _ ...... _ - - - - - - - - - - entropy take place near the critical point. Above the critical point pressure has an influence on the entropy and enthalpy but pressure has less effect on the entropy and enthalpy under the critical point because pressure drops may be allowed to be higher (Kim et ai., 2003). It could be seen that at higher pressures the entropy and enthalpy changes more linear because it is further away from the critical point. The change of enthalpy and entropy in the sub-critical region is represented by the 4MPa pressure line and it could be seen that change under the critical point is linear. Figure 2.8: Vapour pressure for different refrigerants. (Kim et ai, 2003). - . - R~744 _ _ R410A 8t ....,....- R-717 ' -<:- R-407C - - R...22 - - - R~134a -R-12 2 6 o 20 40 60 80 T ('Cl The vapour pressure and the slope of saturation pressure curves are shown in Figures 2.8 and 2.9 respectively for CO2 and other refrigerants. Compared to other refrigerants the vapour pressure of CO2 is between 4 and 10 times higher. Near its critical temperature (31.1DC) the vapour pressure curve for CO2 gets very steep, resulting in a smaller temperature change for a given pressure change. This means that the temperature change associated with pressure drop in the evaporator will be smaller. Because of the high vapour pressure and its closeness to the critical point, the characteristics of the liquid and vapour densities of CO2 is to a large extent different (Kim et a/., 2003). Although CO2 operates at very high pressures and the difference between the suction and discharge pressure is very high, the pressure difference to be overcome A Techno-Economical Analysis ofa CO2 Heat Pump. School ofMechanical Engineering, North-West UniversityPDF Image | CO2 HEAT PUMP Analysis
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