Virial EoS

[thorade]

For the gas region (not liquid!!), a virial equation could be used to calculate start values.
See http://en.wikipedia.org/wiki/Compressibility_factor for an introduction

Add four new properties:
Compressibility factor Z
Second virial coefficient B
Third virial coefficient C
Fourth virial coefficient D

Usually, the input properties are temperature and volume.
Kabelac 1991 shows how to solve for volume, when temperature and pressure are given:
http://dx.doi.org/10.1016/0140-7007(91)90006-3 (in the appendix)

[thorade]

The virial coefficients are calculated at lim d->0
http://books.google.de/books?id=dd410GGw8wUC&pg=PA40
Therefore, f_rd, f_rdd and f_rddd should check for delta<Modelica.Constants.small.

Also see this discussion:
https://trac.modelica.org/Modelica/ticket/605
modelica/ModelicaStandardLibrary#605

[thorade]

RefProp 9.1 has two subroutines (in PROP_SUB.FOR):
subroutine VIRB uses rho=1e-8 for B and C and rho=1e-4 for D,
subroutine VIRBCD uses rho=0