default compressibility factors

[jkisse]

I would like to discuss the default-compressibility factors that are stored in properties/[fluid name]/compressibility.txt in the format slope, offset.
For H and L natural gas, the slope (=der_comp) is currently -0.0022, i.e. Z=0.78 @ 100bar. According to this figure on ResearchGate this means that we are on the 0° C isothermal curve. At 25 °C, Z would be 0.85, according to the chart.

We could also choose the 10 °C curve (-0.0019) as default since it seems closer to real operational conditions. In the long term, we could also implement different factors for a range of temperatures. What do you think?

This is in particular relevant for the calculation of high pressure networks, as the error is proportional to the pressure.

[jkisse]

This could be solved by nested properties (der_compr = f(p,T))

[jkisse]

We could also consider a generic description of Z, for example by applying the CNGA-method that calculates Z from pressure, temperature and relative density. An example is [Liu et al., 2020]

[dlohmeier]

We will compare our results to other tools and if we find that it makes sense, we change the approach, otherwise we stay with der_comp=-0.022 which is at 0°C.

[jkisse]

According to Clees et al., 2021, the GERG-2008 EoS [Kunz & Wagner, 2012] yields best results for mixtures and pure hydrogen. An implementation would be way more complex than the linear approach.