On isentropic lines and isentropic exponents

Highlights: ► We identify the indicators to characterise isentropic lines. ► Fluids with an isentropic index k < 1 behave differently than those with k > 1. ► The critical molar volume allows to classify fluids with k > 1 and k < 1. ► The engineering implications of the above findings are analysed. - Abstract: The three indicators of isentropic lines, namely, the isentropic index, the ratio of pressure and density p/ρ and the derivative (∂p/∂ρ)s are investigated for all of the fluids in the RefProp 9.0 program. The behaviour of these three entities is evaluated along the saturated vapour line as well as in the superheated vapour region. There is a distinct demarcation of fluids whose isentropic indices can be less than 1 and others for which this behaviour is absent. The critical molar volume is found to be the characterizing feature. Several other interesting features of those three thermodynamic properties are also highlighted. It is observed that most practical engineering compression and expansion processes occur along the decreasing direction of the sound speed.