This medium package models moist air using a gas law in which pressure and temperature are independent, which often leads to significantly faster and more robust computations. The specific heat capacities at constant pressure and at constant volume are constant. The air is assumed to be not saturated.

This medium uses the gas law

ρ/ρstp = p/pstp,

where pstd and ρstp are constant reference temperature and density, rathern than the ideal gas law

ρ = p ⁄(R T),

where R is the gas constant and T is the temperature.

This formulation often leads to smaller systems of nonlinear equations because equations for pressure and temperature are decoupled. Therefore, if air inside a control volume such as room air is heated, it does not increase its specific volume. Consequently, merely heating or cooling a control volume does not affect the air flow calculations in a duct network that may be connected to that volume. Note that multizone air exchange simulation in which buoyancy drives the air flow is still possible as the models in IBPSA.Airflow.Multizone compute the mass density using the function IBPSA.Utilities.Psychrometrics.Functions.density_pTX in which density is a function of temperature.

Note that models in this package implement the equation for the internal energy as

u = h - pstp ⁄ ρstp,

where u is the internal energy per unit mass, h is the enthalpy per unit mass, pstp is the static pressure and ρstp is the mass density at standard pressure and temperature. The reason for this implementation is that in general,

h = u + p v,

from which follows that

u = h - p v = h - p ⁄ ρ = h - pstp ⁄ ρstd,

because p ⁄ ρ = pstp ⁄ ρstp in this medium model.

The enthalpy is computed using the convention that h=0 if T=0 °C and no water vapor is present.


ThermodynamicStateThermodynamicState record for moist air
BasePropertiesBase properties
densityGas density
dynamicViscosityReturn the dynamic viscosity of dry air
enthalpyOfCondensingGasEnthalpy of steam per unit mass of steam
enthalpyOfGasEnthalpy of gas mixture per unit mass of gas mixture
enthalpyOfLiquidEnthalpy of liquid (per unit mass of liquid) which is linear in the temperature
enthalpyOfNonCondensingGasEnthalpy of non-condensing gas per unit mass of steam
enthalpyOfVaporizationEnthalpy of vaporization of water
gasConstantReturn ideal gas constant as a function from thermodynamic state, only valid for phi<1
pressureReturns pressure of ideal gas as a function of the thermodynamic state record
isobaricExpansionCoefficientIsobaric expansion coefficient beta
isothermalCompressibilityIsothermal compressibility factor
saturationPressureSaturation curve valid for 223.16 <= T <= 373.16 (and slightly outside with less accuracy)
specificEntropyReturn the specific entropy, only valid for phi<1
density_derp_TReturn the partial derivative of density with respect to pressure at constant temperature
density_derT_pReturn the partial derivative of density with respect to temperature at constant pressure
density_derXReturn the partial derivative of density with respect to mass fractions at constant pressure and temperature
specificHeatCapacityCpSpecific heat capacity of gas mixture at constant pressure
specificHeatCapacityCvSpecific heat capacity of gas mixture at constant volume
setState_dTXReturn thermodynamic state as function of density d, temperature T and composition X
setState_phXReturn thermodynamic state as function of pressure p, specific enthalpy h and composition X
setState_pTXReturn thermodynamic state as function of p, T and composition X or Xi
setState_psXReturn the thermodynamic state as function of p, s and composition X or Xi
specificEnthalpyCompute specific enthalpy from pressure, temperature and mass fraction
specificEnthalpy_pTXSpecific enthalpy
specificGibbsEnergySpecific Gibbs energy
specificHelmholtzEnergySpecific Helmholtz energy
isentropicEnthalpyReturn the isentropic enthalpy
specificInternalEnergySpecific internal energy
temperatureReturn temperature of ideal gas as a function of the thermodynamic state record
molarMassReturn the molar mass
temperature_phXCompute temperature from specific enthalpy and mass fraction
thermalConductivityThermal conductivity of dry air as a polynomial in the temperature


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