Documentation

Enthalpy (T29)

T29_ENTHALPY

ST call

`PROGRAM TEST_T29	VAR 		REAL1,REAL2,REAL3,REAL4,REAL5,REAL6: REAL;	END_VAR	REAL1:= LIB.HVAC.V1_0.T29_ENTHALPY(P:= REAL2, TEMP:= REAL3, RH:= REAL4, PWATER=>REAL5,R=> REAL6);	END_PROGRAM`

LIB\HVAC

V1.0

Description

The function calculates enthalpy, or energy of humid air, partial water vapour pressure, and absolute humidity from air pressure, temperature and humidity.

Inputs

 Input Type Description Default value P REAL Pressure (Pa), standard value is 101325 Pa 0 TEMP REAL Air temperature (°C) 22 RH REAL Relative air humidity (%rH) 70

Outputs

Output Type Description
=> REAL Humid air enthalpy (kJ/kg)
PWATER REAL Partial water vapours pressure
R REAL Absolute humidity (g/kg in dry air)

Function

One of the most important air-conditioning calculations is to learn the amount of energy necessary to bring the room air to required temperature and humidity. The outside air is processed by mixing, heating, cooling, humidification and dehumidification. Most of the processes also brings along changes in thermal energy of the processed air. In thermodynamics, the amount of heat in 1 kg of the material is called nominal enthalpy h [kJ/kg]. Enthalpy is a relative value: absolutely dry air of 0 °C and water contents of X = 0 g/kg has, according to definition, enthalpy H = 0 kJ/kg; this state is the zero point at the enthalpy scale. Enthalpy values < 0 kJ/kg are called negative (with the minus sign).

Application example

Absolute humidity controller (g/kg); it is used in air handling units supplying production technologies, where a constant contents of water in the air must be kept, even if the technology influences the air temperature by its rest heat. It usually is enhanced by an independent temperature controller to compensate room temperature changes. It also can be used in heat recovery units where enthalpy shows the heat recovery efficiency and energies in the respective sections of the air handling unit.