.Buildings.Fluid.HeatExchangers.DryCoilCounterFlow

Information

Model of a discretized coil without water vapor condensation. The coil consists of two flow paths which are, at the design flow direction, in opposite direction to model a counterflow heat exchanger. The flow paths are discretized into nEle elements. Each element is modeled by an instance of Buildings.Fluid.HeatExchangers.BaseClasses.HexElementSensible. Each element has a state variable for the metal.

The convective heat transfer coefficients can, for each fluid individually, be computed as a function of the flow rate and/or the temperature, or assigned to a constant. This computation is done using an instance of Buildings.Fluid.HeatExchangers.BaseClasses.HADryCoil.

To model humidity condensation, use the model Buildings.Fluid.HeatExchangers.WetCoilCounterFlow instead of this model, as this model computes only sensible heat transfer.

Implementation

At very small flow rates, which may be caused when the fan is off but there is wind pressure on the building that entrains outside air through the HVAC system, large temperature differences could occur if diffusion were neglected. This model therefore approximates a small diffusion between the elements to have more uniform medium temperatures if the flow is near zero. The approximation is done using the heat conductors heaCon1 and heaCon2. As this is a rough approximation, neighboring elements are connected through these heat conduction elements, ignoring the actual geometrical configuration. Also, radiation between the coil surfaces on the air side is not modelled explicitly, but rather may be considered as approximated by these heat conductors.

Revisions

• October 19, 2018, by Kino:
  Changed model to use a replaceable model as this allows translation in OpenModelica.
  This is for #1258.
• November 12, 2017, by Michael wetter:
  Changed time constant to more reasonable values, which also makes closed loop control tuning easier.
• November 4, 2017, by Michael wetter:
  Added approximation of diffusion.
  This is for Buildings, #1038.
• September 8, 2017, by Michael Wetter:
  Changed computation of temperature used for hA calculation to avoid a state variable with small time constant for some model parameterizations.
  This is for Buildings, #678.
• September 12, 2014, by Michael Wetter:
  Changed assignment of T_m to avoid using the conditionally enabled model ele[:].mas.T, which is only valid in a connect statement. Moved assignments of Q1_flow, Q2_flow, T1, T2 and T_m outside of equation section to avoid mixing graphical and textual modeling within the same model.
• July 3, 2014, by Michael Wetter:
  Added parameters initialize_p1 and initialize_p2. This is required to enable the coil models to initialize the pressure in the first volume, but not in the downstream volumes. Otherwise, the initial equations will be overdetermined, but consistent. This change was done to avoid a long information message that appears when translating models.
• June 26, 2014, by Michael Wetter:
  Removed parameters energyDynamics1 and energyDynamics2, and used instead of these two parameters the new parameter energyDynamics. This was done as this complexity is not required.
• February 2, 2012, by Michael Wetter:
  Corrected error in assignment of dp2_nominal. The previous assignment caused a pressure drop in all except one element, instead of the opposite. This caused too high a flow resistance of the heat exchanger.
• October 8, 2011, by Michael Wetter:
  Set show_T=false to avoid state events near zero flow.
• May 27, 2010, by Michael Wetter:
  First implementation.