Thermodynamic constants — Thermodynamic data for fluids

The thermodynamic quantities of a gaseous or liquid fluid can be calculated from its p V T equation. Numerous analytical expressions can be used to represent the compressibility of fluids with acceptable accuracy, but within generally limited pressure and temperature ranges. Readers are referred to the articles in the Thermodynamics section of the Basic Sciences treatise, which give the best-known equations of state. These have analytical forms containing parameters whose numerical values are determined by fitting with compressibility tests.

Water, which exhibits singular behavior in the liquid state around 4°C, has been the subject of much experimental research, as it is the industrial energy carrier par excellence. The other fluids that have been the subject of experimentation, much less dense but weak, are relatively few in number given the variety offered by chemistry. Using the rules of dimensional analysis, we can extrapolate to any fluid what is common to known fluids belonging to the same family.

Fluid classes must therefore be defined by their essential molecular characteristics. Quantum fluids will be dealt with briefly, as they form a very restricted family, before turning our attention to non-polar or polar polyatomic fluids. Examples will show how Pitzer's approach, improved by Lee, Kesler and Wu, can be used to solve practical problems. The following text illustrates a way of calculating the thermodynamic properties of fluids using an approach that can easily be used by engineers.