Buildings.Utilities.Psychrometrics

Information

The nomenclature used in this package is described at Buildings.UsersGuide.Conventions.

Package Content

Name	Description
Functions	Package with psychrometric functions
Tdp_pW	Model to compute the dew point temperature for given water vapor pressure of moist air
pW_Tdp	Model to compute the water vapor pressure for a given dew point temperature of moist air
X_pW	Humidity ratio for given water vapor pressure
pW_X	Water vapor pressure for given humidity ratio
X_pTphi	Return steam mass fraction as a function of relative humidity phi and temperature T
ToDryAir	Block to convert absolute humidity from [kg/kg total air] to [kg/kg dry air]
ToTotalAir	Block to convert absolute humidity from [kg/kg dry air] to [kg/kg total air]
Twb_TdbXi	Model to compute the wet bulb temperature
Examples	Collection of models that illustrate model use and test models
BaseClasses	Library with base classes for psychrometric classes

Buildings.Utilities.Psychrometrics.Tdp_pW

Information

Dew point temperature calculation for moist air above freezing temperature.

The correlation used in this model is valid for dew point temperatures between 0 degC and 200 degC. It is the correlation from 2005 ASHRAE Handbook, p. 6.2. In an earlier version of this model, the equation from Peppers has been used, but this equation yielded about 15 Kelvin lower dew point temperatures.

Extends from Modelica.Blocks.Interfaces.BlockIcon (Basic graphical layout of input/output block).

Connectors

Type	Name	Description
input RealInput	p_w	Water vapor partial pressure
output RealOutput	T	Dew point temperature [K]

Modelica definition

block Tdp_pW 
  "Model to compute the dew point temperature for given water vapor pressure of moist air"
  extends Modelica.Blocks.Interfaces.BlockIcon;
  Modelica.Blocks.Interfaces.RealInput p_w "Water vapor partial pressure";
  Modelica.Blocks.Interfaces.RealOutput T(start=278.15,
                                         final quantity="ThermodynamicTemperature",
                                         final unit="K",
                                         min = 0,
                                         displayUnit="degC") 
    "Dew point temperature";

equation 
 p_w = Buildings.Utilities.Psychrometrics.Functions.pW_Tdp(T=T);
end Tdp_pW;

Buildings.Utilities.Psychrometrics.pW_Tdp

Information

Block to compute the water vapor pressure for a given dew point temperature.

The correlation used in this model is valid for dew point temperatures between 0 degC and 200 degC. It is the correlation from 2005 ASHRAE Handbook, p. 6.2. In an earlier version of this model, the equation from Peppers has been used, but this equation yielded about 15 Kelvin lower dew point temperatures.

Extends from Modelica.Blocks.Interfaces.BlockIcon (Basic graphical layout of input/output block).

Connectors

Type	Name	Description
output RealOutput	p_w	Water vapor partial pressure
input RealInput	T	Dew point temperature [K]

Modelica definition

block pW_Tdp 
  "Model to compute the water vapor pressure for a given dew point temperature of moist air"
  extends Modelica.Blocks.Interfaces.BlockIcon;
  Modelica.Blocks.Interfaces.RealOutput p_w "Water vapor partial pressure";
  Modelica.Blocks.Interfaces.RealInput T(final quantity="ThermodynamicTemperature",
                                         final unit="K",
                                         min = 0,
                                         displayUnit="degC") 
    "Dew point temperature";

equation 
 p_w = Buildings.Utilities.Psychrometrics.Functions.pW_Tdp(T=T);
end pW_Tdp;

Buildings.Utilities.Psychrometrics.X_pW

Information

Block to compute the humidity ratio for a given water vapor partial pressure.

If use_p_in is false (default option), the p parameter is used as atmospheric pressure, and the p_in input connector is disabled; if use_p_in is true, then the p parameter is ignored, and the value provided by the input connector is used instead.

Extends from Buildings.Utilities.Psychrometrics.BaseClasses.HumidityRatioVaporPressure (Humidity ratio for given water vapor pressure).

Parameters

Type	Name	Default	Description
Boolean	use_p_in	true	Get the pressure from the input connector
Pressure	p	101325	Fixed value of pressure [Pa]

Connectors

Type	Name	Description
input RealInput	p_in	Atmospheric Pressure [Pa]
output RealOutput	X_w	Species concentration at dry bulb temperature
input RealInput	p_w	Water vapor pressure [Pa]

Modelica definition

block X_pW "Humidity ratio for given water vapor pressure"
  extends Buildings.Utilities.Psychrometrics.BaseClasses.HumidityRatioVaporPressure;
  Modelica.Blocks.Interfaces.RealOutput X_w(min=0, max=1, nominal=0.01) 
    "Species concentration at dry bulb temperature";
  Modelica.Blocks.Interfaces.RealInput p_w(final quantity="Pressure",
                                           final unit="Pa",
                                           displayUnit="Pa",
                                           min = 0) "Water vapor pressure";
  output Modelica.SIunits.MassFraction x_w(min=0, max=1, nominal=0.01, start=0.001) 
    "Water mass fraction per mass of dry air";
equation 
  X_w = Buildings.Utilities.Psychrometrics.Functions.X_pW(p_w=p_w, p=p_in_internal);
  x_w = X_w/(1-X_w);
end X_pW;

Buildings.Utilities.Psychrometrics.pW_X

Information

Block to compute the water vapor partial pressure for a given humidity ratio.

If use_p_in is false (default option), the p parameter is used as atmospheric pressure, and the p_in input connector is disabled; if use_p_in is true, then the p parameter is ignored, and the value provided by the input connector is used instead.

Extends from Buildings.Utilities.Psychrometrics.BaseClasses.HumidityRatioVaporPressure (Humidity ratio for given water vapor pressure).

Parameters

Type	Name	Default	Description
Boolean	use_p_in	true	Get the pressure from the input connector
Pressure	p	101325	Fixed value of pressure [Pa]

Connectors

Type	Name	Description
input RealInput	p_in	Atmospheric Pressure [Pa]
input RealInput	X_w	Species concentration at dry bulb temperature
output RealOutput	p_w	Water vapor pressure [Pa]

Modelica definition

block pW_X "Water vapor pressure for given humidity ratio"
  extends Buildings.Utilities.Psychrometrics.BaseClasses.HumidityRatioVaporPressure;
  Modelica.Blocks.Interfaces.RealInput X_w(min=0, max=1, nominal=0.01) 
    "Species concentration at dry bulb temperature";
  Modelica.Blocks.Interfaces.RealOutput p_w(final quantity="Pressure",
                                           final unit="Pa",
                                           displayUnit="Pa",
                                           min = 0) "Water vapor pressure";

  output Modelica.SIunits.MassFraction x_w(min=0, max=1, nominal=0.01, start=0.001) 
    "Water mass fraction per mass of dry air";

equation 
  p_w = Buildings.Utilities.Psychrometrics.Functions.pW_X(X_w=X_w, p=p_in_internal);
  x_w = X_w/(1-X_w);
end pW_X;

Buildings.Utilities.Psychrometrics.X_pTphi

Information

Block to compute the water vapor concentration based on pressure, temperature and relative humidity.

If use_p_in is false (default option), the p parameter is used as atmospheric pressure, and the p_in input connector is disabled; if use_p_in is true, then the p parameter is ignored, and the value provided by the input connector is used instead.

Extends from Buildings.Utilities.Psychrometrics.BaseClasses.HumidityRatioVaporPressure (Humidity ratio for given water vapor pressure).

Parameters

Type	Name	Default	Description
Boolean	use_p_in	true	Get the pressure from the input connector
Pressure	p	101325	Fixed value of pressure [Pa]
replaceable package Medium		Modelica.Media.Interfaces.Pa...	Medium model

Connectors

Type	Name	Description
input RealInput	p_in	Atmospheric Pressure [Pa]
replaceable package Medium		Medium model
input RealInput	T	Temperature [K]
input RealInput	phi	Relative humidity (0...1)
output RealOutput	X[Medium.nX]	Steam mass fraction

Modelica definition

block X_pTphi 
  "Return steam mass fraction as a function of relative humidity phi and temperature T"
  extends Buildings.Utilities.Psychrometrics.BaseClasses.HumidityRatioVaporPressure;
 replaceable package Medium =
      Modelica.Media.Interfaces.PartialCondensingGases "Medium model";

public 
  Modelica.Blocks.Interfaces.RealInput T(final unit="K",
                                           displayUnit="degC",
                                           min = 0) "Temperature";
  Modelica.Blocks.Interfaces.RealInput phi(min = 0, max=1) 
    "Relative humidity (0...1)";
  Modelica.Blocks.Interfaces.RealOutput X[Medium.nX](min = 0, max=1) 
    "Steam mass fraction";
protected 
  constant Real k = 0.621964713077499 "Ratio of molar masses";
  Modelica.SIunits.AbsolutePressure psat "Saturation pressure";
 parameter Integer i_w(min=1, fixed=false) "Index for water substance";
 parameter Integer i_nw(min=1, fixed=false) "Index for non-water substance";

initial algorithm 
  i_w :=1;
    for i in 1:Medium.nXi loop
      if Modelica.Utilities.Strings.isEqual(Medium.substanceNames[i], "Water") then
        i_w :=i;
      end if;
    end for;
  i_nw := if i_w == 1 then 2 else 1;
algorithm 
  psat := Medium.saturationPressure(T);
  X[i_w] := phi*k/(k*phi+p_in_internal/psat-phi);
  //sum(X[:]) = 1; // The formulation with a sum in an equation section leads to a nonlinear equation system
  X[i_nw] := 1 - X[i_w];
end X_pTphi;

Buildings.Utilities.Psychrometrics.ToDryAir

Information

Block that converts humidity concentration from [kg/kg total air] to [kg/kg dry air].

Extends from Modelica.Blocks.Interfaces.BlockIcon (Basic graphical layout of input/output block).

Connectors

Type	Name	Description
output RealOutput	XiDry	Water vapor concentration in [kg/kg dry air]
input RealInput	XiTotalAir	Water vapor concentration in [kg/kg total air]

Modelica definition

block ToDryAir 
  "Block to convert absolute humidity from [kg/kg total air] to [kg/kg dry air]"
  extends Modelica.Blocks.Interfaces.BlockIcon;
  Modelica.Blocks.Interfaces.RealOutput XiDry 
    "Water vapor concentration in [kg/kg dry air]";
  Modelica.Blocks.Interfaces.RealInput XiTotalAir 
    "Water vapor concentration in [kg/kg total air]";

equation 
  XiDry = XiTotalAir / (1-XiTotalAir);
end ToDryAir;

Buildings.Utilities.Psychrometrics.ToTotalAir

Information

Block that converts humidity concentration from [kg/kg dry air] to [kg/kg total air].

This block may be used, for example, to convert absolute humidity that is received from EnergyPlus to [kg/kg total air], which is the convention used by Modelica.Media.

Extends from Modelica.Blocks.Interfaces.BlockIcon (Basic graphical layout of input/output block).

Connectors

Type	Name	Description
output RealOutput	XiTotalAir	Water vapor concentration in [kg/kg total air]
input RealInput	XiDry	Water vapor concentration in [kg/kg dry air]
output RealOutput	XNonVapor	Mass fraction of remaining substances

Modelica definition

block ToTotalAir 
  "Block to convert absolute humidity from [kg/kg dry air] to [kg/kg total air]"
  extends Modelica.Blocks.Interfaces.BlockIcon;
  Modelica.Blocks.Interfaces.RealOutput XiTotalAir 
    "Water vapor concentration in [kg/kg total air]";
  Modelica.Blocks.Interfaces.RealInput XiDry 
    "Water vapor concentration in [kg/kg dry air]";

  Modelica.Blocks.Interfaces.RealOutput XNonVapor 
    "Mass fraction of remaining substances";
equation 
  XiTotalAir = XiDry / (1+XiDry);
  XNonVapor = 1 - XiTotalAir;
end ToTotalAir;

Buildings.Utilities.Psychrometrics.Twb_TdbXi

Information

Given a moist are medium model, this component computes the states of the medium at its wet bulb temperature.

For a use of this model, see for example Buildings.Fluid.Sensors.WetBulbTemperature

Extends from Modelica.Blocks.Interfaces.BlockIcon (Basic graphical layout of input/output block).

Connectors

Type	Name	Description
input RealInput	Tdb	Dry bulb temperature [K]
input RealInput	p	Pressure [Pa]
output RealOutput	Twb	Wet bulb temperature [K]
input RealInput	Xi[Medium.nXi]	Species concentration at dry bulb temperature

Modelica definition

block Twb_TdbXi "Model to compute the wet bulb temperature"
  extends Modelica.Blocks.Interfaces.BlockIcon;
  replaceable package Medium =
    Modelica.Media.Interfaces.PartialCondensingGases "Medium model";

  Modelica.Blocks.Interfaces.RealInput Tdb(
    start=303,
    final quantity="ThermodynamicTemperature",
    final unit="K",
    min=0) "Dry bulb temperature";
  Modelica.Blocks.Interfaces.RealInput p(  final quantity="Pressure",
                                           final unit="Pa",
                                           min = 0) "Pressure";
  Modelica.Blocks.Interfaces.RealOutput Twb(
    start=293,
    final quantity="ThermodynamicTemperature",
    final unit="K",
    min=0) "Wet bulb temperature";
  Modelica.Blocks.Interfaces.RealInput Xi[Medium.nXi] 
    "Species concentration at dry bulb temperature";
protected 
  Medium.BaseProperties dryBul "Medium state at dry bulb temperature";
  Medium.BaseProperties wetBul(Xi(nominal=0.01*ones(Medium.nXi))) 
    "Medium state at wet bulb temperature";
 parameter Integer i_w(min=1, fixed=false) "Index for water substance";
initial algorithm 
  i_w :=1;
    for i in 1:Medium.nXi loop
      if Modelica.Utilities.Strings.isEqual(Medium.substanceNames[i], "Water") then
        i_w :=i;
      end if;
    end for;

equation 
  dryBul.p = p;
  dryBul.T = Tdb;
  dryBul.Xi = Xi;
  wetBul.phi = 1;
  wetBul.p = dryBul.p;
  wetBul.h = dryBul.h + (wetBul.X[i_w] - dryBul.X[i_w])
         * Medium.enthalpyOfLiquid(dryBul.T);
  Twb = wetBul.T;
end Twb_TdbXi;