Buildings.Fluids.MassExchangers

Information

Package Content

Name	Description
ConstantEffectiveness	Heat and moisture exchanger with constant effectiveness
Examples	Collection of models that illustrate model use and test models
HumidifierPrescribed	Ideal electric heater or cooler, no losses, no dynamics

Buildings.Fluids.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,

For a sensible heat exchanger, use Buildings.Fluids.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.

Parameters

Type	Name	Default	Description
replaceable package Medium_1		Modelica.Media.Interfaces.Pa...	Medium 1 in the component
replaceable package Medium_2		Modelica.Media.Interfaces.Pa...	Medium 2 in the component
Real	epsS	0.8	Sensible heat exchanger effectiveness
Real	epsL	0.8	Latent heat exchanger effectiveness
Nominal condition
MassFlowRate	m0_flow_1		Nominal mass flow rate [kg/s]
MassFlowRate	m0_flow_2	m0_flow_1	Nominal mass flow rate [kg/s]
Initialization
MassFlowRate	m_flow_1.start	0	Mass flow rate from port_a1 to port_b1 (m_flow_1 > 0 is design flow direction) [kg/s]
Pressure	dp_1.start	0	Pressure difference between port_a1 and port_b1 [Pa]
MassFlowRate	m_flow_2.start	0	Mass flow rate from port_a2 to port_b2 (m_flow_2 > 0 is design flow direction) [kg/s]
Pressure	dp_2.start	0	Pressure difference between port_a2 and port_b2 [Pa]
Assumptions
Boolean	allowFlowReversal_1	system.allowFlowReversal	= true to allow flow reversal in medium 1, false restricts to design direction (port_a -> port_b)
Boolean	allowFlowReversal_2	system.allowFlowReversal	= true to allow flow reversal in medium 2, false restricts to design direction (port_a -> port_b)
Advanced
MassFlowRate	m_flow_1_small	1E-4*m0_flow_1	Small mass flow rate for regularization of zero flow [kg/s]
MassFlowRate	m_flow_2_small	1E-4*m0_flow_2	Small mass flow rate for regularization of zero flow [kg/s]
Diagnostics
Boolean	show_V_flow	true	= true, if volume flow rate at inflowing port is computed
Initialization
AbsolutePressure	p_a1_start	system.p_start	Guess value for inlet pressure [Pa]
AbsolutePressure	p_b1_start	p_a1_start	Guess value for outlet pressure [Pa]
AbsolutePressure	p_a2_start	system.p_start	Guess value for inlet pressure [Pa]
AbsolutePressure	p_b2_start	p_a2_start	Guess value for outlet pressure [Pa]

Connectors

Type	Name	Description
FluidPort_a	port_a1	Fluid connector a1 (positive design flow direction is from port_a1 to port_b1)
FluidPort_b	port_b1	Fluid connector b1 (positive design flow direction is from port_a1 to port_b1)
FluidPort_a	port_a2	Fluid connector a2 (positive design flow direction is from port_a2 to port_b2)
FluidPort_b	port_b2	Fluid 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 Fluids.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";
  Medium_1.MassFraction XWat_in1 "Inlet water mass fraction of medium 1";
  Medium_2.MassFraction XWat_in2 "Inlet water mass fraction of medium 2";

  Modelica.SIunits.ThermalConductance C_flow_1 
    "Heat capacity flow rate medium 1";
  Modelica.SIunits.ThermalConductance C_flow_2 
    "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 m_flow_1 >= 0 then
     T_in1  = sta_a1.T;
     XWat_in1 = sta_a1.X[Medium_1.Water];
  else
     T_in1 = sta_b1.T;
     XWat_in1 = sta_b1.X[Medium_1.Water];
  end if;

  if m_flow_2 >= 0 then
     T_in2  = sta_a2.T;
     XWat_in2 = sta_a2.X[Medium_2.Water];
  else
     T_in2 = sta_b2.T;
     XWat_in2 = sta_b2.X[Medium_2.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.
  C_flow_1 = abs(m_flow_1)* Medium_1.specificHeatCapacityCp(sta_a1);
  C_flow_2 = abs(m_flow_2)* Medium_2.specificHeatCapacityCp(sta_a2);

  CMin_flow = min( C_flow_1, C_flow_2);
  QMax_flow = CMin_flow * (T_in2 - T_in1);

  // transferred heat
  Q_flow_1 = epsS * QMax_flow;
  0 = Q_flow_1 + Q_flow_2;

  // mass exchange
  mMax_flow = min(abs(m_flow_1), abs(m_flow_2)) * (XWat_in2 - XWat_in1);
  mWat_flow = epsL * mMax_flow;

  for i in 1:Medium_1.nXi loop
     mXi_flow_1[i] = if ( i == Medium_1.Water) then mWat_flow else 0;
  end for;

  for i in 1:Medium_2.nXi loop
     mXi_flow_2[i] = if ( i == Medium_2.Water) then -mWat_flow else 0;
  end for;

  // no pressure drop
  dp_1 = 0;
  dp_2 = 0;
end ConstantEffectiveness;

Buildings.Fluids.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 m0_flow. The input signal u and the nominal moisture flow rate added to the air stream m0_flow can be positive or negative. If the product u * m0_flow are 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 if the mass flow rate tends to zero, the moisture difference over this component tends to infinity for non-zero m_flow, so add proper control 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.

Parameters

Type	Name	Default	Description
replaceable package Medium		Modelica.Media.Interfaces.Pa...	Medium in the component
MassFlowRate	mWat0_flow		Water mass flow rate at u=1, positive for humidification [kg/s]
Temperature	T	293.15	Temperature of water that is added to the fluid stream (used only if T_in is unconnected) [K]
Nominal condition
MassFlowRate	m0_flow		Nominal mass flow rate [kg/s]
Initialization
MassFlowRate	m_flow.start	0	Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressure	dp.start	0	Pressure difference between port_a and port_b [Pa]
Assumptions
Boolean	allowFlowReversal	system.allowFlowReversal	= true to allow flow reversal, false restricts to design direction (port_a -> port_b)
Advanced
MassFlowRate	m_flow_small	1E-4*m0_flow	Small mass flow rate for regularization of zero flow [kg/s]
Diagnostics
Boolean	show_V_flow	true	= true, if volume flow rate at inflowing port is computed
Initialization
AbsolutePressure	p_a_start	system.p_start	Guess value for inlet pressure [Pa]
AbsolutePressure	p_b_start	p_a_start	Guess value for outlet pressure [Pa]

Connectors

Type	Name	Description
FluidPort_a	port_a	Fluid connector a (positive design flow direction is from port_a to port_b)
FluidPort_b	port_b	Fluid connector b (positive design flow direction is from port_a to port_b)
input RealInput	T_in	Temperature of water added to the fluid stream
input RealInput	u

Modelica definition

model HumidifierPrescribed 
  "Ideal electric heater or cooler, no losses, no dynamics"
  extends Fluids.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;
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;

  dp = 0;
  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;