.AixLib.Fluid.FMI.Adaptors.HVAC

Information

Adaptor that can be used to connect an HVAC system (with acausal ports) to input/output signals, which then can be exposed in an FMI interface.

The adaptor has a vector of fluid ports called ports. The supply and return air ducts need to be connected to these ports. Also, if a thermal zone has interzonal air exchange or air infiltration, these flow paths also need be connected to ports.

This model outputs at the port fluPor the mass flow rate for each flow that is connected to ports, together with its temperature, water vapor mass fraction per total mass of the air (not per kg dry air), and trace substances. These quantities are always as if the flow enters the room, even if the flow is zero or negative. If a medium has no moisture, e.g., if Medium.nXi=0, or if it has no trace substances, e.g., if Medium.nC=0, then the output signal for these properties are removed. These quantities are always as if the flow enters the room, even if the flow is zero or negative. Thus, a thermal zone model that uses these signals to compute the heat added by the HVAC system need to implement an equation such as

Q_sen = max(0, ṁ_sup)   c_p   (T_sup - T_air,zon),

where Q_sen is the sensible heat flow rate added to the thermal zone, ṁ_sup is the supply air mass flow rate from the port fluPor (which is negative if it is an exhaust), c_p is the specific heat capacity at constant pressure, T_sup is the supply air temperature and T_air,zon is the zone air temperature. Note that without the max(·, ·) function, the energy balance would be wrong.

The output signals of this model are the zone air temperature, the water vapor mass fraction per total mass of the air (unless Medium.nXi=0) and trace substances (unless Medium.nC=0). These output connectors can be used to connect to a controller. These values are obtained from the fluid stream(s) that flow into this component at the port fluPor, e.g., from the connector fluPor.backward. Note that there are nPorts of these signals. For a completely mixed room, they will all have the same value, but for a room with non-uniform temperatures, they can have different values.

Assumption and limitations

The mass flow rates at ports sum to zero, hence this model conserves mass.

This model does not impose any pressure, other than setting the pressure of all fluid connections to ports to be equal. The reason is that setting a pressure can lead to non-physical system models, for example if a mass flow rate is imposed and the HVAC system is connected to a model that sets a pressure boundary condition such as AixLib.Fluid.Sources.Outside. Also, setting a pressure would make it impossible to use multiple instances of this model (one for each thermal zone) and build in Modelica an airflow network model with pressure driven mass flow rates.

The model has no pressure drop. Hence, the pressure drop of an air diffuser or of an exhaust grill need to be modelled in models that are connected to ports.

Typical use and important parameters

See AixLib.Fluid.FMI.ExportContainers.HVACZone for a model that uses this model.

Contents

Name	Description
Medium	Medium in the component

Revisions

• January 18, 2019, by Jianjun Hu:
  Limited the media choice to moist air only. See #1050.
• September 13, 2017, by Michael Wetter:
  Removed erroneous each.
• October 4, 2016, by Michael Wetter:
  Corrected assignment of quantity in CZon.
• June 29, 2016, by Michael Wetter:
  Revised implementation.
• April 14, 2016, by Michael Wetter:
  First implementation.