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.
For air and flue gas, EBSILON has used the convention of setting the enthalpy zero point to 0 °C in the gaseous phase since the beginning. This has the advantage that enthalpy and temperature roughly correspond numerically and the enthalpy is only slightly dependent on the water content. However, if water is present in the liquid phase, this convention results in negative enthalpy contributions, which some users find disturbing.
For this reason, there are alternative values for enthalpy and also for exergy (HA and EA). The reference system for these values has the zero point for the water content in the liquid phase (as in the water/water steam table). The enthalpies HA are therefore always positive above 0°C, but strongly dependent on the water content.
HA: Enthalpy zero point of the water content: liquid
H : Enthalpy zero point of the water content: gaseous
If you prefer this reference system, you can display HA instead of H in the value crosses. However, HA and EA are only available as result values. If you want to specify enthalpies, H must 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.