EBSILON®Professional Online Documentation
For enthalpies and entropies, not the absolute values but only differences are physically relevant. This is why reference statuses have to be specified for each material value library. Regarding the libraries contained in Ebsilon, these are programmed (fixed). Only for the Refprop library the reference status can be changed by the user.
From the beginning Ebsilon has used the convention to set the enthalpy zero point for air and flue gas to 0° C in the gaseous phase. This has the advantage that enthalpy and temperature roughly coincide numerically and that the enthalpy depends only little on the water fraction. If, however, water is present in the liquid phase, this convention will lead to negative enthalpy contributions, which some users find annoying.
For this reason, there are alternative values for enthalpy and also for the exergy (HA and EA). The frame of reference for these values has the zero point for the water fraction in the liquid phase (as also in the water-/steam table). The enthalpies HA are thus always positive above 0°C, however strongly dependent on the water fraction.
HA: Enthalpie-Nullpunkt des Wasseranteils: flüssig
H : Enthalpie-Nullpunkt des Wasseranteils: gasförmig
If you prefer this frame of reference, you can have HA instead of H displayed in the value crosses. However, HA and EA are only available as result values. If one wants to specify enthalpies, H has to be used.
Enthalpy "per dry air" is used with the cooling tower diagrams of components 111 and 112.
Air: name of an air pipe
Enthalpy per moist air : Air.H
Enthalpy per dry air : Air.HA / (1.0 - Air.XH2O)
with Air.HA = Air.H * (1.0 - Air.XH2O) + (Air.H + 2500.0) * Air.XH2O
HA can be read directly from the pipe too.
For these libraries the zero points for enthalpy and entropy are defined individually for each library. You find these definitions in the chapter "Reference states" in the description of each library. These descriptions are not integrated in the Ebsilon online help system, but they are delivered with Ebsilon. You can find them in the subdirectory "Docu\Fluids" below the Ebsilon installation directory. Unfortunately, this documentation is not completely available in English.
For the REFPROP fluids, the zero points for enthalpy and entropy can be specified by the user in component 1 or 33 within the universal fluid dialog. The default settings for each fluid can be found in the fluid files (".fld") in the subdirectory "Data\Refprop\Fluids". In these files, the following abbreviations are applied:
NBP: h,s = 0 at normal boiling point(s)
ASH: h,s = 0 for sat liquid at -40 C (ASHRAE convention)
IIR: h = 200, s = 1.0 for sat liq at 0 C (IIR convention)
OTH: other, as specified by h0, s0, t0, p0 (real gas state)
OTO: other, as specified by h0, s0, t0, p0 (ideal gas state)
In the last two cases, the corresponding values for h0, s0, t0 and p0 are specified in the .fld file.
Hint: Unfortunately, it is not possible to extend the list of available substances by simply adding new .fld files. An extension of fluids requires a new Ebsilon release.