Pressure-enthalpy diagram
A pressure-enthalpy diagram is a state diagram with the specific enthalpy on the abscissa axis and the pressure on the ordinate axis. A common form is the log-ph diagram, which scales the pressure logarithmically. Diagrams that are not logarithmically scaled are used for thermodynamic comparisons of substances. The log-ph diagram in the illustration is for R134a , a common refrigerant . Boiling line 1 marks the transition from saturated liquid to wet steam. Dew line 2 marks the transition from wet steam to saturated steam. The area between the boiling line and the dew line is called the wet steam area. In the wet steam area, the isovapores 7 show the proportion of gas in mass fractions kg gas / kg wet steam. Isovapores are lines with the same vapor content. The value 0 stands for liquid, 1 stands for gas. In European usage, the steam content is denoted by x. x = 0.75 is a wet steam with 75% gas and 25% liquid. Boiling line and dew line meet at critical point 3. The critical isotherm runs through the critical point. The isotherms 4 are shown in red in the diagram. Isotherms are lines of the same temperature. A gas can no longer be liquefied to the right and above the critical isotherm. The isentropes 6 are lines of equal entropy. Isochores 5 are lines of the same specific volume or the same density.
Scaling
A reference point for scaling enthalpy and entropy must be selected in pressure-enthalpy diagrams (Reference of state). Common here are:
- IIR: values at 0 ° C on the boiling curve, enthalpy h = 200 kJ / kg, entropy s = 1 kJ / kg / K
- NBP: values at 1 bar absolute on the boiling curve, enthalpy h = 0 kJ / kg, entropy s = 0 kJ / kg / K
- ASHRAE : values at −40 ° C on the boiling curve, enthalpy h = 0 kJ / kg, entropy s = 0 kJ / kg / K
Different diagrams that show different values for enthalpy and entropy at the same points are therefore not necessarily incorrect. Differences must always be taken into account for entropy and enthalpy. These differences must be the same for different scalings.
See also
Web links
- gunt.de: Thermodynamics of the refrigeration cycle process (PDF; 641 kB)
- Informations-gmbh.de: Log ph diagram R-407C (PDF; 2551 kB)