Buildings.Controls.OBC.CDL.Psychrometrics

Package with psychrometric blocks

Information

This package contains blocks for psychrometric calculations.

Package Content

Name	Description
DewPoint_TDryBulPhi	Block to compute the dew point temperature based on relative humidity
SpecificEnthalpy_TDryBulPhi	Block to compute the specific enthalpy based on relative humidity
WetBulb_TDryBulPhi	Block to compute the wet bulb temperature based on relative humidity
Validation	Collection of models that validate the psychrometrics blocks of the CDL

Buildings.Controls.OBC.CDL.Psychrometrics.DewPoint_TDryBulPhi

Block to compute the dew point temperature based on relative humidity

Buildings.Controls.OBC.CDL.Psychrometrics.DewPoint_TDryBulPhi

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°C and 93°C. It is the correlation from 2009 ASHRAE Handbook Fundamentals, p. 1.9, equation 39.

Connectors

Type	Name	Description
input RealInput	TDryBul	Dry bulb temperature [K]
input RealInput	phi	Relative air humidity [1]
output RealOutput	TDewPoi	Dew point temperature [K]

Modelica definition

block DewPoint_TDryBulPhi "Block to compute the dew point temperature based on relative humidity" Buildings.Controls.OBC.CDL.Interfaces.RealInput TDryBul( final quantity="ThermodynamicTemperature", final unit="K", final min=100) "Dry bulb temperature"; Buildings.Controls.OBC.CDL.Interfaces.RealInput phi( final min=0, final max=1, unit="1") "Relative air humidity"; Buildings.Controls.OBC.CDL.Interfaces.RealOutput TDewPoi( final quantity="ThermodynamicTemperature", final unit="K", final min=100) "Dew point temperature"; protected Real p_w( final quantity="Pressure", final unit="Pa", displayUnit="Pa") "Water vapor pressure"; constant Real C14=6.54 "Constant used in the equation"; constant Real C15=14.526 "Constant used in the equation"; constant Real C16=0.7389 "Constant used in the equation"; constant Real C17=0.09486 "Constant used in the equation"; constant Real C18=0.4569 "Constant used in the equation"; Real alpha "Variable used in the equation"; equation p_w=phi*Buildings.Utilities.Psychrometrics.Functions.saturationPressure(TDryBul); alpha=Modelica.Math.log( p_w/1000.0); TDewPoi=(C14+C15*alpha+C16*alpha^2+C17*alpha^3+C18*(p_w/1000.0)^0.1984)+273.15; end DewPoint_TDryBulPhi;

Buildings.Controls.OBC.CDL.Psychrometrics.SpecificEnthalpy_TDryBulPhi

Block to compute the specific enthalpy based on relative humidity

Buildings.Controls.OBC.CDL.Psychrometrics.SpecificEnthalpy_TDryBulPhi

Information

This block computes the specific enthalpy for a given dry bulb temperature, relative air humidity and atmospheric pressure. The specific enthalpy is zero if the temperature is 0°C and if there is no moisture.

The correlation used in this model is from the 2009 ASHRAE Handbook Fundamentals, p. 1.9, equation 32.

Parameters

Type	Name	Default	Description
Real	pAtm	101325	Atmospheric pressure [Pa]

Connectors

Type	Name	Description
input RealInput	TDryBul	Dry bulb temperature [K]
input RealInput	phi	Relative air humidity
output RealOutput	h	Specific enthalpy [J/kg]

Modelica definition

block SpecificEnthalpy_TDryBulPhi "Block to compute the specific enthalpy based on relative humidity" parameter Real pAtm( final quantity="Pressure", final unit="Pa")=101325 "Atmospheric pressure"; Buildings.Controls.OBC.CDL.Interfaces.RealInput TDryBul( final quantity="ThermodynamicTemperature", final unit="K", final min=100) "Dry bulb temperature"; Buildings.Controls.OBC.CDL.Interfaces.RealInput phi( final min=0, final max=1) "Relative air humidity"; Buildings.Controls.OBC.CDL.Interfaces.RealOutput h( final quantity="SpecificEnergy", final unit="J/kg") "Specific enthalpy"; protected Real TDryBul_degC( final unit="degC", displayUnit="degC") "Dry bulb temperature in degree Celsius"; Real p_w( final quantity="Pressure", final unit="Pa", displayUnit="Pa") "Water vapor pressure"; Real w( final unit="1", nominal=0.01) "Water vapor mass fraction in kg per kg dry air"; equation TDryBul_degC=TDryBul-273.15; p_w=phi*Buildings.Utilities.Psychrometrics.Functions.saturationPressure(TDryBul); w=0.6219647130774989*p_w/(pAtm-p_w); h=1006*TDryBul_degC+w*(2501014.5+1860*TDryBul_degC); end SpecificEnthalpy_TDryBulPhi;

Buildings.Controls.OBC.CDL.Psychrometrics.WetBulb_TDryBulPhi

Block to compute the wet bulb temperature based on relative humidity

Buildings.Controls.OBC.CDL.Psychrometrics.WetBulb_TDryBulPhi

Information

This block computes the wet bulb temperature for a given dry bulb temperature, relative air humidity and atmospheric pressure.

The block uses the approximation of Stull (2011) to compute the wet bulb temperature without requiring a nonlinear equation. The approximation by Stull is valid for a relative humidity of 5% to 99%, a temperature range from -20°C to 50°C and standard sea level pressure. For this range of data, the approximation error is -1 Kelvin to +0.65 Kelvin, with a mean error of less than 0.3 Kelvin.

The model is validated in Buildings.Controls.OBC.CDL.Psychrometrics.Validation.WetBulb_TDryBulPhi.

References

Stull, Roland. Wet-Bulb Temperature from Relative Humidity and Air Temperature Roland Stull. Journal of Applied Meteorology and Climatology. Volume 50, Issue 11, pp. 2267-2269. November 2011 DOI: 10.1175/JAMC-D-11-0143.1

Connectors

Type	Name	Description
input RealInput	TDryBul	Dry bulb temperature [K]
input RealInput	phi	Relative air humidity
output RealOutput	TWetBul	Wet bulb temperature [K]

Modelica definition

block WetBulb_TDryBulPhi "Block to compute the wet bulb temperature based on relative humidity" Buildings.Controls.OBC.CDL.Interfaces.RealInput TDryBul( final quantity="ThermodynamicTemperature", final unit="K", final min=100) "Dry bulb temperature"; Buildings.Controls.OBC.CDL.Interfaces.RealInput phi( final min=0, final max=1) "Relative air humidity"; Buildings.Controls.OBC.CDL.Interfaces.RealOutput TWetBul( final quantity="ThermodynamicTemperature", final unit="K", final min=100) "Wet bulb temperature"; protected constant Real uniCon(final unit="1/rad") = 1 "Unit conversion to satisfy unit check"; Real TDryBul_degC( final unit="degC", displayUnit="degC") "Dry bulb temperature in degree Celsius"; Real rh_per( min=0) "Relative humidity in percentage"; equation TDryBul_degC=TDryBul-273.15; rh_per=100*phi; TWetBul=273.15 + TDryBul_degC * uniCon * Modelica.Math.atan(0.151977*sqrt(rh_per+8.313659)) + uniCon * ( Modelica.Math.atan(TDryBul_degC+rh_per) - Modelica.Math.atan(rh_per-1.676331) + 0.00391838*rh_per^(1.5)*Modelica.Math.atan(0.023101*rh_per)) -4.686035; end WetBulb_TDryBulPhi;