modelPartialHexElement

Information

Element of a heat exchanger with dynamics of the fluids and the solid. The hA value for both fluids is an input. The driving force for the heat transfer is the temperature difference between the fluid volumes and the solid.

The heat capacity C of the metal is assigned as follows. Suppose the metal temperature is governed by

C dT ⁄ dt = (hA)₁ (T₁ - T) + (hA)₂ (T₂ - T)

where hA are the convective heat transfer coefficients times heat transfer area that also take into account heat conduction in the heat exchanger fins and T₁ and T₂ are the medium temperatures. Assuming (hA)₁=(hA)₂, this equation can be rewritten as

C dT ⁄ dt = 2 (UA)₀ ( (T₁ - T) + (T₂ - T) )

where (UA)₀ is the UA value at nominal conditions. Hence we set the heat capacity of the metal to

C = 2 (UA)₀ τ_m

where τ_m is the time constant that the metal of the heat exchanger has if the metal is approximated by a lumped thermal mass.

Note: This model is introduced to allow the instances Buildings.Fluid.HeatExchangers.BaseClasses.HexElementLatent and Buildings.Fluid.HeatExchangers.BaseClasses.HexElementSensible to redeclare the volume as final, thereby avoiding that a GUI displays the volume as a replaceable component.

Parameters

Connectors

Components

Revisions

• October 19, 2017, by Michael Wetter:
  Changed initialization of pressure from a constant to a parameter.
  This is for Buildings, issue 1013.
• July 17, 2015, by Michael Wetter:
  Added prescribedHeatFlowRate=false for both volumes. This is for issue 282 of the Annex 60 library.
• February 5, 2015, by Michael Wetter:
  Changed initalize_p from a parameter to a constant. This is only required in finite volume models of heat exchangers (to avoid consistent but redundant initial conditions) and hence it should be set as a constant.
• 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.
• July 2, 2014, by Michael Wetter:
  Conditionally removed the mass of the metall mas.
• June 26, 2014, by Michael Wetter:
  Removed parameters energyDynamics1 and energyDynamics2, and used instead of these two parameters energyDynamics. This was done as this complexity is not required.
• September 11, 2013, by Michael Wetter:
  Separated old model into one for dry and for wet heat exchangers. This was done to make the coil compatible with OpenModelica.
• May 1, 2013, by Michael Wetter:
  Changed the redeclaration of vol2 to be replaceable, as vol2 is replaced in some models.
• April 19, 2013, by Michael Wetter:
  Made instance MassExchange replaceable, rather than conditionally removing the model, to avoid a warning during translation because of unused connector variables.
• July 11, 2011, by Michael Wetter:
  Removed assignment of medium in vol1 and vol2, since this assignment is already done in the base class using the final modifier.
• August 12, 2008, by Michael Wetter:
  Introduced option to compute each medium using a steady state model or a dynamic model.
• March 25, 2008, by Michael Wetter:
  First implementation.