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/ρ_{stp} =
p/p_{stp},

where *p _{std}* and

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 Buildings.Airflow.Multizone compute the mass density using the function Buildings.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 -
p_{stp} ⁄ ρ_{stp},

where *u* is the internal energy per unit mass, *h* is
the enthalpy per unit mass, *p _{stp}* is the static
pressure and

h = u + p v,

from which follows that

u = h - p v = h - p ⁄
ρ = h - p_{stp} ⁄ ρ_{std},

because *p ⁄ ρ = p _{stp} ⁄ ρ_{stp}* in this
medium model.

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

Name | Description |
---|---|

ThermodynamicState | ThermodynamicState record for moist air |

BaseProperties | Base properties |

density | Gas density |

dynamicViscosity | Return the dynamic viscosity of dry air |

enthalpyOfCondensingGas | Enthalpy of steam per unit mass of steam |

enthalpyOfGas | Enthalpy of gas mixture per unit mass of gas mixture |

enthalpyOfLiquid | Enthalpy of liquid (per unit mass of liquid) which is linear in the temperature |

enthalpyOfNonCondensingGas | Enthalpy of non-condensing gas per unit mass of steam |

enthalpyOfVaporization | Enthalpy of vaporization of water |

gasConstant | Return ideal gas constant as a function from thermodynamic state, only valid for phi<1 |

pressure | Returns pressure of ideal gas as a function of the thermodynamic state record |

isobaricExpansionCoefficient | Isobaric expansion coefficient beta |

isothermalCompressibility | Isothermal compressibility factor |

saturationPressure | Saturation curve valid for 223.16 <= T <= 373.16 (and slightly outside with less accuracy) |

specificEntropy | Return the specific entropy, only valid for phi<1 |

density_derp_T | Return the partial derivative of density with respect to pressure at constant temperature |

density_derT_p | Return the partial derivative of density with respect to temperature at constant pressure |

density_derX | Return the partial derivative of density with respect to mass fractions at constant pressure and temperature |

specificHeatCapacityCp | Specific heat capacity of gas mixture at constant pressure |

specificHeatCapacityCv | Specific heat capacity of gas mixture at constant volume |

setState_dTX | Return thermodynamic state as function of density d, temperature T and composition X |

setState_phX | Return thermodynamic state as function of pressure p, specific enthalpy h and composition X |

setState_pTX | Return thermodynamic state as function of p, T and composition X or Xi |

setState_psX | Return the thermodynamic state as function of p, s and composition X or Xi |

specificEnthalpy | Compute specific enthalpy from pressure, temperature and mass fraction |

specificEnthalpy_pTX | Specific enthalpy |

specificGibbsEnergy | Specific Gibbs energy |

specificHelmholtzEnergy | Specific Helmholtz energy |

isentropicEnthalpy | Return the isentropic enthalpy |

specificInternalEnergy | Specific internal energy |

temperature | Return temperature of ideal gas as a function of the thermodynamic state record |

molarMass | Return the molar mass |

temperature_phX | Compute temperature from specific enthalpy and mass fraction |

thermalConductivity | Thermal conductivity of dry air as a polynomial in the temperature |

- January 11, 2019 by Michael Wetter:

Reforulated assignment of`X_int`

in`setState_psX`

.

This is for #1079. - October 26, 2018, by Filip Jorissen and Michael Wetter:

Now printing different messages if temperature is above or below its limit, and adding instance name as JModelica does not print the full instance name in the assertion. This is for #1045. - November 4, 2016, by Michael Wetter:

Set default value for`dT.start`

in base properties.

This is for #575. - June 6, 2015, by Michael Wetter:

Set`AbsolutePressure(start=p_default)`

to avoid a translation error if Buildings.Fluid.Sources.Examples.TraceSubstancesFlowSource is translated in pedantic mode in Dymola 2016. The reason is that pressures use`Medium.p_default`

as start values, but Modelica.Media.Interfaces.Types sets a default value of*1E-5*. A similar change has been done for pressure. This fixes #266. - June 5, 2015, by Michael Wetter:

Added`stateSelect`

attribute in`BaseProperties.T`

to allow correct use of`preferredMediumState`

as described in Modelica.Media.Interfaces.PartialMedium. Note that the default is`preferredMediumState=false`

and hence the same states are used as were used before. This is for #260. - May 11, 2015, by Michael Wetter:

Removed`p(stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default)`

in declaration of`BaseProperties`

. Otherwise, when models that contain a fluid volume are exported as an FMU, their pressure would be differentiated with respect to time. This would require the time derivative of the inlet pressure, which is not available, causing the translation to stop with an error. - May 1, 2015, by Michael Wetter:

Added`Inline=true`

for issue 227. - March 20, 2015, by Michael Wetter:

Added missing term`state.p/reference_p`

in function`specificEntropy`

. #193. - February 3, 2015, by Michael Wetter:

Removed`stateSelect.prefer`

for temperature. This is for #160. - July 24, 2014, by Michael Wetter:

Changed implementation to use Buildings.Utilities.Psychrometrics.Constants. This was done to use consistent values throughout the library. - November 16, 2013, by Michael Wetter:

Revised and simplified the implementation. - November 14, 2013, by Michael Wetter:

Removed function`HeatCapacityOfWater`

which is neither needed nor implemented in the Modelica Standard Library. - November 13, 2013, by Michael Wetter:

Removed non-used computations in`specificEnthalpy_pTX`

and in`temperature_phX`

. - March 29, 2013, by Michael Wetter:

Added`final standardOrderComponents=true`

in the`BaseProperties`

declaration. This avoids an error when models are checked in Dymola 2014 in the pedenatic mode. - April 12, 2012, by Michael Wetter:

Added keyword`each`

to`Xi(stateSelect=...)`

. - April 4, 2012, by Michael Wetter:

Added redeclaration of`ThermodynamicState`

to avoid a warning during model check and translation. - August 3, 2011, by Michael Wetter:

Fixed bug in`u=h-R*T`

, which is only valid for ideal gases. For this medium, the function is`u=h-pStd/dStp`

. - January 27, 2010, by Michael Wetter:

Fixed bug in`else`

branch of function`setState_phX`

that lead to a run-time error when the constructor of this function was called. - January 22, 2010, by Michael Wetter:

Added implementation of function enthalpyOfNonCondensingGas and its derivative. - January 13, 2010, by Michael Wetter:

Fixed implementation of derivative functions. - August 28, 2008, by Michael Wetter:

First implementation.

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