Equations of State

Gases can be treated individually or as mixtures by the methods just outlined for most applications including evaluation of vendor proposals. More sophisticated equations of state can be used for real gas applications when large deviations from the perfect gas law are anticipated. For mixtures, more sophisticated mixing rules can be paired with the equation of state when required. For hydrocarbons, the most widely used equation of state is the Benedict-Webb-Rubin (BWR) equation [2]. For a gas mixture, the pseudocritical constants used in the BWR equation may be developed using Kay's mixing rule. If the application is outside Kay's Rule guidelines, a more complex rule such as Leland-Mueller may be substituted [3]. An alternate approach is the Starling BWR implementation [4]. Starling includes gas mixing in formulation of the equation of state. Another, the Redlich-Kwong equation, is widely used because of its simplicity. Finally, for chlorinated compounds and halocarbon refrigerants, the Martin-Hou equation yields results generally superior to the previously mentioned equations, which were developed primarily for hydrocarbons [5]. The equations of state discussed are by no means a complete list, but they have proved to be especially accurate in direct application.

The equations of state will not be further described or presented in more detail as they are unfortunately somewhat difficult to solve without the use of a computer. Full details are available in the referenced material for those wishing to pursue this subject further. In the past, these equations required the use of a mainframe computer not only to solve the equations themselves, but to store the great number of constants required. This has been true particularly if the gas mixture contains numerous components. With the power and storage capacity of personal computers increasing, the equations have the potential of becoming more readily available for general use.

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