Contents - Index



Substance AIRH2O implements air-water vapor mixture (psychrometric) properties using thermodynamic data from the built-in AIR and STEAM property relations from Hyland and Wexler, "Formulations for the Thermodynamic Properties of the Saturated Phases of H2O from 173.15 K to 473.15 K, ASHRAE Transactions, Part 2A,Paper 2793 (RP-216), (1983).


Wet-bulb is approximated with the adiabatic saturation temperature.  The reference state sets the specific enthalpy of dry air to 0 at 0 F (in English units) and 0 C in (SI units) for consistency with most psychrometric charts.  Specific entropy is referenced to 0 K to be consistent with fluid AIR.  The reference state for both enthalpy and entropy of water vapor is based on the traditional steam table reference state.


The properties of air-water vapor mixtures are calculated assuming ideal gas behavior using the relation provided in Chapter 1 of the 2009 ASHRAE Handbook of Fundamentals (Eqn 28).  The property correlations are designed for use at temperatures below 100C.


Note that substance AIRH2O requires three independent properties to fix the state.  Pressure is one of the required properties.  The relative amount of moisture can be indicated by providing the humidity ratio (W), relative humidity (R), dewpoint temperature (D) or wetbulb temperature (B).   The following property functions only operate with substance AIRH2O: DEWPOINT, HUMRAT, WETBULB, RELHUM


Viscosity and thermal conductivity of air-water vapor mixture are determined using the transport property data for pure air and pure water vapor with mixing rules as described by Van den Bulck Ph.D p. 358.  


Note that all of the specific properties are defined per unit mass of dry air.  However, following the ASHRAE Handbook of Fundamentals Chapter 1, 2009, density is defined as the total (air + water vapor mass) divided by volume.



Fluid Property Information