Thermodynamic-property diagrams account directly for deviations of a real gas
from ideal relationships. These diagrams are a plot of gas properties, commonly including: enthalpy, entropy, pressure, and temperature. Occasionally, a special diagram is developed for a widely used gas mixture such as a refrigerant. However, note that few charts are available for mixtures, and this method is therefore not commonly used for hydrocarbon mixtures.
When a diagram is used to predict changes of state during compression, compressibility and variance of k are not needed because these variables are already factored into the diagrams. In general, then, this method is more accurate than Equation 100-20, and when charts are available, it is certainly more convenient.
Diagrams are often used in compressor calculations for heavier hydrocarbon gases such as propane and propylene that tend to deviate considerably. Diagrams for many pure gases are well established. The Appendix includes an assortment of these diagrams.
The following equations pertain to the use of diagrams for compressor calculations. Note that for an isentropic process, there is no change in entropy, S.