FB_HVACEnthalpy

This function block is used to calculate the dew point, the specific enthalpy and the absolute humidity. The temperature, the relative humidity and the barometric pressure are required for calculating these parameters.
The enthalpy is a measure for the energy of a thermodynamic system.

VAR_INPUT

rTemperature           : REAL;
rRelativeHumidity      : REAL;
rBarometricPressure    : REAL;

rTemperature: Input for the temperature value [°C].

rRelativeHumidity: Input for the relative humidity [%]. The value must equal or greater 1.

rBarometricPressure: Input for the air pressure [hPa].

VAR_OUTPUT

rDewPoint          : REAL;
rEnthalpy          : REAL;
rAbsoluteHumidity  : REAL;
rRho               : REAL;
rYpsilon           : REAL;
bInvalidParameter  : BOOL;

rDewPoint: Dew point [°C ].

rEnthalpy: Enthalpy [kJ / kg].

rAbsoluteHumidity: Absolute humidity [kg/kg]. The result must be multiplied by 1000 in order to obtain the absolute humidity in [g/kg].

rRho: Density of moist air, which specifies the mixture as a mass in relation to the volume. The unit is [kg/m³].

rYpsilon: Specific volume per 1 kg of dry air. The unit is [m³/kg] dry air.

bInvalidParameter: Set to TRUE if the input variable rRelativeHumidity or rBarometricPressure is smaller than or equal to zero.

H,x-diagram [www.air2000.de]:

Example 1:

rTemperature: = 20 °C
rRelativeHumidity:= 50%
rBarometricPressure:= 1013.15hPa

Results from the function block: rDewPoint:= 9.4°C
rEnthalpy:= 38.6kJ/kg
rAbsoluteHumidity:= 0.0072kg/kg, converted to g/kg ==> 7.2 g/kg

Example 2:

rTemperature: = 25 °C
rRelativeHumidity:= 60%
rBarometricPressure:= 1013.15hPa

Results from the function block: rDewPoint:= 17°C
rEnthalpy:= 55.6kJ/kg
rAbsoluteHumidity:= 0.0119kg/kg, converted to g/kg ==> 11.9 g/kg

Requirements