FB_HVACEnthalpy

Application

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 air pressure are required for the calculation of these parameters.
Enthalpy is a measure of the energy of a thermodynamic system.

VAR_INPUT

rTemperature           : REAL;         [°C ]
rRelativeHumidity      : REAL;         [ % ]       1..100
rBarometricPressure    : REAL;         [hPa] 

rTemperature: input for the temperature value, in degrees Celsius.

rRelativeHumidity: input for relative humidity, in percent. The value must equal or greater 1.

rBarometricPressure: input for air pressure, in hectopascal.

VAR_OUTPUT

rDewPoint          : REAL;        [°C ]    
rEnthalpy          : REAL;        [kJ/kg]
rAbsoluteHumidity  : REAL;        [kg/kg]
rRho               : REAL;        [kg/m³]
rYpsilon           : REAL;        [m³/kg]
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 humid air, showing the weight of the mass of the mixture in relation to volume. The unit is [ kg mixture / m³ ].

rYpsion: 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.

Fig. 1 h,x diagram [www.air2000.de]

Example 1:

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

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

Example 2:

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

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