Thermodynamic charts. General informations on fluids

A thermodynamic fluid diagram takes the form of a planar graph showing a number of remarkable curves, in particular families of isovalues of its state functions.

By highlighting the transformations undergone by these fluids and making it possible to calculate them, at least approximately, diagrams are among the basic tools of the thermodynamicist.

For a paper diagram, the state of a point is read on the axes and by interpolation between isovalue curves. For interactive diagrams, it's a simple mouse-click away, with the precise values displayed on screen.

As we show in this article, different coordinate systems are used depending on the case, the most commonly used for pure bodies being the entropy diagram (T, s), the so-called refrigeration diagram (h, ln P) and the Mollier diagram (h, s).

Although software packages for calculating fluid properties are becoming increasingly widespread, thermodynamic diagrams continue to play an essential role.

Their appeal is threefold:

• visualize fluid properties;

• allow cycles to be represented graphically ;

• make it easier to estimate the thermodynamic characteristics of fluids.

The third interest tends to diminish, while the first two retain their relevance. In particular, visualizing on a diagram a cycle calculated using a computerized tool often even enables us to ensure that it does not include any outlier due to an error during data entry...

Because the study of thermodynamic diagrams is inseparable from that of fluids, a series of four articles is dedicated to them:

• the presentation of general information on diagrams with a reminder of the basics of thermodynamics, the subject of this article [BE 8 040] ;

• analysis of diagrams for pure fluids, azeotropic mixtures and ideal gases (article [BE 8 041] "Thermodynamic diagrams. Pure fluids, azeotropes and ideal gases");...