1.5 Properties of Working Fluids in Thermal Energy Systems

The Importance of Thermophysical Properties

Properties are at the core of thermal energy system design and analysis. In the modeling of thermal energy systems there are many assumptions made that simplify the model. However, at the same time, these assumptions tend to make the model a bit more questionable concerning its accuracy. Using high accuracy thermophysical properties in a system model takes out one level of uncertainty and results in a more accurate result.

By their very nature, fluid properties are uncertain in the sense that they are determined from empirical equations fit to experimental data. Therefore, it is incumbent that the most accurate thermohysical property model for a fluid is used when during design and analysis work on a thermal energy system. The developers of the software packages mentioned in Section 1.2 strive to maintain the most current and accurate property formulations for the fluids included in the package.

Some industries develop their own thermophysical property models based on more comprehensive models. For example, suppose the enthalpy of a fluid is needed as a function of pressure and temperature in a limited range. Pseudo-data can be generated that represents h=f(P,T) from a software package over the limited P and T range needed. These pseudo-data can then be fitted to a three-dimensional equation to determine the h=f(P,T) empirical equation. While thermodynamically inconsistent, this approach is acceptable so long as the P and T range is not exceeded in any calculations.