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124 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 Fig. 3. Phase diagram of CO2. discuss the thermodynamic and transport properties of CO2, compared to other refrigerants. Unless stated otherwise, all thermophysical properties were calculated using EES (Engineering Equation Solver), which uses the high- accuracy equation of state [22]. 2.1. Thermodynamic properties Span and Wagner [23] reviewed the available data on thermodynamic properties of CO2 and presented a new equation of state in the form of a fundamental equation explicit in the Helmholtz free energy. In the technically most important region up to pressures of 30 MPa and up to temperatures of 523 K, the estimated uncertainty of the equation ranges from ^0.03 to ^0.05% in the density, ^0.03 to ^1% in the speed of sound, and þ0.15 to ^1.5% in the isobaric specific heat. Special interest was focused on the description of the critical region and the extrapolation behavior of the formulation. Note that thermodynamic properties of CO2 in EES [22] are provided using Fig. 4. Pressure–enthalpy and temperature–entropy diagrams of CO2. (a) Pressure – enthalpy diagram, (b) Temperature – entropy diagram. critical point results in the transcritical cycle, i.e. with subcritical low-side and supercritical high-side pressure (for a single-stage cycle). The high-side pressure and tempera- ture in the supercritical region are not coupled and can be regulated independently to get the optimum operating condition. As may be observed from the phase diagram of CO2 (Fig. 3), the temperature and pressure for the triple point are 256.6 8C and 0.52 MPa, respectively, and the saturation pressure at 0 8C is 3.5 MPa. The reduced pressure at 0 8C for CO2 is 0.47 (Table 1), which is much higher than those for the conventional fluids. Owing to the low critical temperature and high-reduced pressure of CO2, the low-side conditions will be much closer to the critical point than with conventional refrigerants. Regarding transport properties (viscosity and thermal conductivity), the work by Vesovic et al. [16] is a key reference. However, improved viscosity data were published by Fenghour et al. [17]. While the earlier viscosity data were based on partly inconsistent experimental liquid viscosity data and used separate gas-phase and liquid-phase equations, the 1998 publication used new experimental data and represented the viscosity for the whole thermodynamic surface with one equation. Rieberer [14] developed the property database CO2REF for CO2, which covers both the sub- and super-critical regions. The thermodynamic and transport properties based on CO2REF were in good agreement with those from VDI [18] in spite of using different equations of state. ASHRAE [19] also presented tabular data for the thermophysical properties of CO2, which covered from the triple point to the critical point (Appendix A). Pettersen [20] presented some properties of CO2 using the program library CO2lib developed at NTNU/SINTEF, and he focused on the effect of properties on evaporating characteristics. Liley and Desai [21] also presented thermophysical properties (specific heat, thermal conductivity, viscosity, speed of sound, and surface tension) of CO2 in tabular form. The following sections willPDF Image | CO2 Vapor Compression Systems
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