Buildings.Fluid.MassExchangers

Package with mass exchanger models

Information


This package contains models for mass exchangers.
For heat exchanger models with humidity transfer, see the package

Buildings.Fluid.HeatExchangers.

Package Content

NameDescription
Buildings.Fluid.MassExchangers.ConstantEffectiveness ConstantEffectiveness Heat and moisture exchanger with constant effectiveness
Buildings.Fluid.MassExchangers.Examples Examples Collection of models that illustrate model use and test models
Buildings.Fluid.MassExchangers.HumidifierPrescribed HumidifierPrescribed Ideal humidifier or dehumidifier with prescribed water mass flow rate addition or subtraction


Buildings.Fluid.MassExchangers.ConstantEffectiveness Buildings.Fluid.MassExchangers.ConstantEffectiveness

Heat and moisture exchanger with constant effectiveness

Buildings.Fluid.MassExchangers.ConstantEffectiveness

Information


Model for a heat and moisture exchanger with constant effectiveness.

This model transfers heat and moisture in the amount of

  Q = epsS * Q_max,
  m = epsL * mWat_max,
where epsS and epsL are constant effectiveness for the sensible and latent heat transfer, Q_max is the maximum heat that can be transferred and mWat_max is the maximum moisture that can be transferred.

For a sensible heat exchanger, use Buildings.Fluid.HeatExchangers.ConstantEffectiveness instead of this model.

This model can only be used with medium models that define the integer constant Water which needs to be equal to the index of the water mass fraction in the species vector.


Extends from Fluid.Interfaces.PartialStaticFourPortHeatMassTransfer (Partial model transporting two fluid streams between four ports without storing mass or energy), Buildings.BaseClasses.BaseIcon (Base icon).

Parameters

TypeNameDefaultDescription
replaceable package Medium1PartialMediumMedium 1 in the component
replaceable package Medium2PartialMediumMedium 2 in the component
RealepsS0.8Sensible heat exchanger effectiveness
RealepsL0.8Latent heat exchanger effectiveness
Nominal condition
MassFlowRatem1_flow_nominal Nominal mass flow rate [kg/s]
MassFlowRatem2_flow_nominalm1_flow_nominalNominal mass flow rate [kg/s]
Pressuredp1_nominal Pressure [Pa]
Pressuredp2_nominal Pressure [Pa]
Initialization
MassFlowRatem1_flow.start0Mass flow rate from port_a1 to port_b1 (m1_flow > 0 is design flow direction) [kg/s]
Pressuredp1.start0Pressure difference between port_a1 and port_b1 [Pa]
MassFlowRatem2_flow.start0Mass flow rate from port_a2 to port_b2 (m2_flow > 0 is design flow direction) [kg/s]
Pressuredp2.start0Pressure difference between port_a2 and port_b2 [Pa]
Assumptions
BooleanallowFlowReversal1system.allowFlowReversal= true to allow flow reversal in medium 1, false restricts to design direction (port_a -> port_b)
BooleanallowFlowReversal2system.allowFlowReversal= true to allow flow reversal in medium 2, false restricts to design direction (port_a -> port_b)
Advanced
MassFlowRatem1_flow_small1E-4*m1_flow_nominalSmall mass flow rate for regularization of zero flow [kg/s]
MassFlowRatem2_flow_small1E-4*m2_flow_nominalSmall mass flow rate for regularization of zero flow [kg/s]
Diagnostics
Booleanshow_V_flowtrue= true, if volume flow rate at inflowing port is computed
Initialization
AbsolutePressurep_a1_startsystem.p_startGuess value for inlet pressure [Pa]
AbsolutePressurep_b1_startp_a1_startGuess value for outlet pressure [Pa]
AbsolutePressurep_a2_startsystem.p_startGuess value for inlet pressure [Pa]
AbsolutePressurep_b2_startp_a2_startGuess value for outlet pressure [Pa]
Flow resistance
Medium 1
Booleanfrom_dp1true= true, use m_flow = f(dp) else dp = f(m_flow)
BooleanlinearizeFlowResistance1false= true, use linear relation between m_flow and dp for any flow rate
RealdeltaM10.1Fraction of nominal flow rate where flow transitions to laminar
Medium 2
Booleanfrom_dp2true= true, use m_flow = f(dp) else dp = f(m_flow)
BooleanlinearizeFlowResistance2false= true, use linear relation between m_flow and dp for any flow rate
RealdeltaM20.1Fraction of nominal flow rate where flow transitions to laminar

Connectors

TypeNameDescription
FluidPort_aport_a1Fluid connector a1 (positive design flow direction is from port_a1 to port_b1)
FluidPort_bport_b1Fluid connector b1 (positive design flow direction is from port_a1 to port_b1)
FluidPort_aport_a2Fluid connector a2 (positive design flow direction is from port_a2 to port_b2)
FluidPort_bport_b2Fluid connector b2 (positive design flow direction is from port_a2 to port_b2)

Modelica definition

model ConstantEffectiveness 
  "Heat and moisture exchanger with constant effectiveness"
  extends Fluid.Interfaces.PartialStaticFourPortHeatMassTransfer;
  extends Buildings.BaseClasses.BaseIcon;
  parameter Real epsS(min=0, max=1) = 0.8 
    "Sensible heat exchanger effectiveness";
  parameter Real epsL(min=0, max=1) = 0.8 "Latent heat exchanger effectiveness";

  Modelica.SIunits.Temperature T_in1 "Inlet temperature of medium 1";
  Modelica.SIunits.Temperature T_in2 "Inlet temperature of medium 2";
  Medium1.MassFraction XWat_in1 "Inlet water mass fraction of medium 1";
  Medium2.MassFraction XWat_in2 "Inlet water mass fraction of medium 2";

  Modelica.SIunits.ThermalConductance C1_flow 
    "Heat capacity flow rate medium 1";
  Modelica.SIunits.ThermalConductance C2_flow 
    "Heat capacity flow rate medium 2";
  Modelica.SIunits.ThermalConductance CMin_flow(min=0) 
    "Minimum heat capacity flow rate";
  Modelica.SIunits.HeatFlowRate QMax_flow "Maximum heat flow rate";

  Modelica.SIunits.MassFlowRate mWat_flow 
    "Water flow rate from medium 2 to medium 1";
  Modelica.SIunits.MassFlowRate mMax_flow 
    "Maximum water flow rate from medium 2 to medium 1";
equation 

  // Definitions for effectiveness model
  if m1_flow >= 0 then
     T_in1  = Medium1.temperature(sta_a1);
     XWat_in1 = sta_a1.X[Medium1.Water];
  else
     T_in1 = Medium1.temperature(sta_b1);
     XWat_in1 = sta_b1.X[Medium1.Water];
  end if;

  if m2_flow >= 0 then
     T_in2  = Medium2.temperature(sta_a2);
     XWat_in2 = sta_a2.X[Medium2.Water];
  else
     T_in2 = Medium2.temperature(sta_b2);
     XWat_in2 = sta_b2.X[Medium2.Water];
  end if;

  // The specific heat capacity is computed using the state of the
  // medium at port_a. For forward flow, this is correct, for reverse flow,
  // this is an approximation.
  C1_flow = abs(m1_flow)* Medium1.specificHeatCapacityCp(sta_a1);
  C2_flow = abs(m2_flow)* Medium2.specificHeatCapacityCp(sta_a2);

  CMin_flow = min( C1_flow, C2_flow);
  QMax_flow = CMin_flow * (T_in2 - T_in1);

  // transferred heat
  Q1_flow = epsS * QMax_flow;
  0 = Q1_flow + Q2_flow;

  // mass exchange
  mMax_flow = min(abs(m1_flow), abs(m2_flow)) * (XWat_in2 - XWat_in1);
  mWat_flow = epsL * mMax_flow;

  for i in 1:Medium1.nXi loop
     mXi1_flow[i] = if ( i == Medium1.Water) then mWat_flow else 0;
  end for;

  for i in 1:Medium2.nXi loop
     mXi2_flow[i] = if ( i == Medium2.Water) then -mWat_flow else 0;
  end for;
end ConstantEffectiveness;

Buildings.Fluid.MassExchangers.HumidifierPrescribed Buildings.Fluid.MassExchangers.HumidifierPrescribed

Ideal humidifier or dehumidifier with prescribed water mass flow rate addition or subtraction

Buildings.Fluid.MassExchangers.HumidifierPrescribed

Information


Model for an air humidifier or dehumidifier.

This model adds (or removes) moisture from the air stream. The amount of exchanged moisture is equal to m_flow = u * m_flow_nominal. The input signal u and the nominal moisture flow rate added to the air stream m_flow_nominal can be positive or negative. If the product u * m_flow_nominal is positive, then moisture is added to the air stream, otherwise it is removed.

If the connector T_in is left unconnected, the value set by the parameter T is used for temperature of the water that is added to the air stream.

Note that for non-zero m_flow, if the mass flow rate tends to zero, then the moisture difference over this component tends to infinity. Hence, using a proper control for u is essential when using this component.

This model can only be used with medium models that define the integer constant Water which needs to be equal to the index of the water mass fraction in the species vector.


Extends from Fluid.Interfaces.PartialStaticTwoPortHeatMassTransfer (Partial model transporting fluid between two ports without storing mass or energy), Buildings.BaseClasses.BaseIcon (Base icon).

Parameters

TypeNameDefaultDescription
replaceable package MediumPartialMediumMedium in the component
MassFlowRatemWat0_flow Water mass flow rate at u=1, positive for humidification [kg/s]
TemperatureT293.15Temperature of water that is added to the fluid stream (used only if T_in is unconnected) [K]
Nominal condition
MassFlowRatem_flow_nominal Nominal mass flow rate [kg/s]
Pressuredp_nominal Pressure [Pa]
Initialization
MassFlowRatem_flow.start0Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressuredp.start0Pressure difference between port_a and port_b [Pa]
Assumptions
BooleanallowFlowReversalsystem.allowFlowReversal= true to allow flow reversal, false restricts to design direction (port_a -> port_b)
Advanced
MassFlowRatem_flow_small1E-4*m_flow_nominalSmall mass flow rate for regularization of zero flow [kg/s]
Diagnostics
Booleanshow_V_flowtrue= true, if volume flow rate at inflowing port is computed
Initialization
AbsolutePressurep_a_startsystem.p_startGuess value for inlet pressure [Pa]
AbsolutePressurep_b_startp_a_startGuess value for outlet pressure [Pa]
Flow resistance
Booleanfrom_dptrue= true, use m_flow = f(dp) else dp = f(m_flow)
BooleanlinearizeFlowResistancefalse= true, use linear relation between m_flow and dp for any flow rate
RealdeltaM0.1Fraction of nominal flow rate where flow transitions to laminar

Connectors

TypeNameDescription
FluidPort_aport_aFluid connector a (positive design flow direction is from port_a to port_b)
FluidPort_bport_bFluid connector b (positive design flow direction is from port_a to port_b)
input RealInputT_inTemperature of water added to the fluid stream
input RealInputuControl input

Modelica definition

model HumidifierPrescribed 
  "Ideal humidifier or dehumidifier with prescribed water mass flow rate addition or subtraction"
  extends Fluid.Interfaces.PartialStaticTwoPortHeatMassTransfer;
  extends Buildings.BaseClasses.BaseIcon;

  parameter Modelica.SIunits.MassFlowRate mWat0_flow 
    "Water mass flow rate at u=1, positive for humidification";
  parameter Modelica.SIunits.Temperature T = 293.15 
    "Temperature of water that is added to the fluid stream (used only if T_in is unconnected)";
  Modelica.Blocks.Interfaces.RealInput T_in 
    "Temperature of water added to the fluid stream";
  Modelica.Blocks.Interfaces.RealInput u "Control input";
protected 
  constant Modelica.SIunits.MassFraction[Medium.nXi] XiWat = {1} 
    "Mass fraction of water";
  Modelica.SIunits.MassFlowRate mWat_flow "Water flow rate";
equation 
  if cardinality(T_in)==0 then
    T_in = T;
  end if;
  mWat_flow = u * mWat0_flow;
  Q_flow = Medium.enthalpyOfLiquid(T_in) * mWat_flow;
  for i in 1:Medium.nXi loop
     mXi_flow[i] = if ( i == Medium.Water) then  mWat_flow else 0;
  end for;
end HumidifierPrescribed;

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