Buildings.Fluid.MassExchangers

Information

Extends from Modelica.Icons.VariantsPackage (Icon for package containing variants).

Package Content

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

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,

In the region mK_flow_small > abs(mK_flow) > mK_flow_small/2, for K = 1 or 2, the effectivness epsS and epsL are transitioned from their user-specified value to 0. This improves the numerical robustness near zero flow.

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 Buildings.Fluid.HeatExchangers.BaseClasses.PartialEffectiveness (Partial model to implement heat exchangers based on effectiveness model).

Parameters

Type	Name	Default	Description
replaceable package Medium1		PartialMedium	Medium 1 in the component
replaceable package Medium2		PartialMedium	Medium 2 in the component
HeatFlowRate	Q1_flow	epsS*QMax_flow	Heat transfered into the medium 1 [W]
MassFlowRate	mXi1_flow[Medium1.nXi]	{if (i == Medium1.Water) the...	Mass flow rates of independent substances added to the medium 1 [kg/s]
HeatFlowRate	Q2_flow	-Q1_flow	Heat transfered into the medium 2 [W]
MassFlowRate	mXi2_flow[Medium2.nXi]	{if (i == Medium2.Water) the...	Mass flow rates of independent substances added to the medium 2 [kg/s]
Boolean	sensibleOnly1	false	Set to true if sensible exchange only for medium 1
Boolean	sensibleOnly2	false	Set to true if sensible exchange only for medium 2
Real	epsS	0.8	Sensible heat exchanger effectiveness
Real	epsL	0.8	Latent heat exchanger effectiveness
Nominal condition
MassFlowRate	m1_flow_nominal		Nominal mass flow rate [kg/s]
MassFlowRate	m2_flow_nominal		Nominal mass flow rate [kg/s]
Pressure	dp1_nominal		Pressure [Pa]
Pressure	dp2_nominal		Pressure [Pa]
Initialization
MassFlowRate	m1_flow.start	0	Mass flow rate from port_a1 to port_b1 (m1_flow > 0 is design flow direction) [kg/s]
Pressure	dp1.start	0	Pressure difference between port_a1 and port_b1 [Pa]
MassFlowRate	m2_flow.start	0	Mass flow rate from port_a2 to port_b2 (m2_flow > 0 is design flow direction) [kg/s]
Pressure	dp2.start	0	Pressure difference between port_a2 and port_b2 [Pa]
Assumptions
Boolean	allowFlowReversal1	system.allowFlowReversal	= true to allow flow reversal in medium 1, false restricts to design direction (port_a -> port_b)
Boolean	allowFlowReversal2	system.allowFlowReversal	= true to allow flow reversal in medium 2, false restricts to design direction (port_a -> port_b)
Advanced
Initialization
SpecificEnthalpy	h_outflow_a1_start	Medium1.h_default	Start value for enthalpy flowing out of port a1 [J/kg]
SpecificEnthalpy	h_outflow_b1_start	Medium1.h_default	Start value for enthalpy flowing out of port b1 [J/kg]
SpecificEnthalpy	h_outflow_a2_start	Medium2.h_default	Start value for enthalpy flowing out of port a2 [J/kg]
SpecificEnthalpy	h_outflow_b2_start	Medium2.h_default	Start value for enthalpy flowing out of port b2 [J/kg]
MassFlowRate	m1_flow_small	1E-4*abs(m1_flow_nominal)	Small mass flow rate for regularization of zero flow [kg/s]
MassFlowRate	m2_flow_small	1E-4*abs(m2_flow_nominal)	Small mass flow rate for regularization of zero flow [kg/s]
Boolean	homotopyInitialization	true	= true, use homotopy method
Diagnostics
Boolean	show_V_flow	false	= true, if volume flow rate at inflowing port is computed
Boolean	show_T	false	= true, if actual temperature at port is computed (may lead to events)
Flow resistance
Medium 1
Boolean	from_dp1	false	= true, use m_flow = f(dp) else dp = f(m_flow)
Boolean	linearizeFlowResistance1	false	= true, use linear relation between m_flow and dp for any flow rate
Real	deltaM1	0.1	Fraction of nominal flow rate where flow transitions to laminar
Medium 2
Boolean	from_dp2	false	= true, use m_flow = f(dp) else dp = f(m_flow)
Boolean	linearizeFlowResistance2	false	= true, use linear relation between m_flow and dp for any flow rate
Real	deltaM2	0.1	Fraction of nominal flow rate where flow transitions to laminar

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 Buildings.Fluid.HeatExchangers.BaseClasses.PartialEffectiveness(
  sensibleOnly1=false,
  sensibleOnly2=false,
  Q1_flow = epsS * QMax_flow,
  Q2_flow = -Q1_flow,
  mXi1_flow = {if ( i == Medium1.Water) then mWat_flow else 0 for i in 1:Medium1.nXi},
  mXi2_flow = {if ( i == Medium2.Water) then -mWat_flow else 0 for i in 1:Medium2.nXi});

  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";

  Medium1.MassFraction X_w_in1 "Inlet water mass fraction of medium 1";
  Medium2.MassFraction X_w_in2 "Inlet water mass fraction of medium 2";

  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";

protected 
  Real gai1(min=0, max=1) "Auxiliary variable for smoothing at zero flow";
  Real gai2(min=0, max=1) "Auxiliary variable for smoothing at zero flow";
equation 
  // Definitions for effectiveness model
  X_w_in1 = Modelica.Fluid.Utilities.regStep(m1_flow,
                  state_a1_inflow.X[Medium1.Water],
                  state_b1_inflow.X[Medium1.Water], m1_flow_small);
  X_w_in2 = Modelica.Fluid.Utilities.regStep(m2_flow,
                  state_a2_inflow.X[Medium2.Water],
                  state_b2_inflow.X[Medium2.Water], m2_flow_small);

  // mass exchange
  // Compute a gain that goes to zero near zero flow rate.
  // This is required to smoothen the heat transfer at very small flow rates.
  // Note that gaiK = 1 for abs(mK_flow) > mK_flow_small
  gai1 = Modelica.Fluid.Utilities.regStep(abs(m1_flow) - 0.75*m1_flow_small,
              1, 0, 0.25*m1_flow_small);
  gai2 = Modelica.Fluid.Utilities.regStep(abs(m2_flow) - 0.75*m2_flow_small,
              1, 0, 0.25*m2_flow_small);

  mMax_flow = smooth(1, min(smooth(1, gai1 * abs(m1_flow)),
                            smooth(1, gai2 * abs(m2_flow)))) * (X_w_in2 - X_w_in1);
  mWat_flow = epsL * mMax_flow;

end ConstantEffectiveness;

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

ṁ_wat = u ṁ_wat,nom,

where u is the control input signal and ṁ_wat,nom is equal to the parameter mWat_flow_nominal. The parameter mWat_flow_nominal can be positive or negative. If ṁ_wat 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 the temperature of the water that is added to the air stream.

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 Buildings.Fluid.Interfaces.TwoPortHeatMassExchanger (Partial model transporting one fluid stream with storing mass or energy).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Boolean	use_T_in	false	Get the temperature from the input connector
Temperature	T	293.15	Temperature of water that is added to the fluid stream (used if use_T_in=false) [K]
MassFlowRate	mWat_flow_nominal		Water mass flow rate at u=1, positive for humidification [kg/s]
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
Pressure	dp_nominal		Pressure [Pa]
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*abs(m_flow_nominal)	Small mass flow rate for regularization of zero flow [kg/s]
Boolean	homotopyInitialization	true	= true, use homotopy method
Diagnostics
Boolean	show_V_flow	false	= true, if volume flow rate at inflowing port is computed
Boolean	show_T	false	= true, if actual temperature at port is computed (may lead to events)
Flow resistance
Boolean	from_dp	false	= true, use m_flow = f(dp) else dp = f(m_flow)
Boolean	linearizeFlowResistance	false	= true, use linear relation between m_flow and dp for any flow rate
Real	deltaM	0.1	Fraction of nominal flow rate where flow transitions to laminar
Dynamics
Nominal condition
Time	tau	30	Time constant at nominal flow (if energyDynamics <> SteadyState) [s]
Equations
Dynamics	energyDynamics	Modelica.Fluid.Types.Dynamic...	Formulation of energy balance
Dynamics	massDynamics	energyDynamics	Formulation of mass balance
Initialization
AbsolutePressure	p_start	Medium.p_default	Start value of pressure [Pa]
Temperature	T_start	Medium.T_default	Start value of temperature [K]
MassFraction	X_start[Medium.nX]	Medium.X_default	Start value of mass fractions m_i/m [kg/kg]
ExtraProperty	C_start[Medium.nC]	fill(0, Medium.nC)	Start value of trace substances

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	Control input

Modelica definition

model HumidifierPrescribed 
  "Ideal humidifier or dehumidifier with prescribed water mass flow rate addition or subtraction"
  extends Buildings.Fluid.Interfaces.TwoPortHeatMassExchanger(
    redeclare final Buildings.Fluid.MixingVolumes.MixingVolumeMoistAir vol);

  parameter Boolean use_T_in= false 
    "Get the temperature from the input connector";

  parameter Modelica.SIunits.Temperature T = 293.15 
    "Temperature of water that is added to the fluid stream (used if use_T_in=false)";

  parameter Modelica.SIunits.MassFlowRate mWat_flow_nominal 
    "Water mass flow rate at u=1, positive for humidification";

  Modelica.Blocks.Interfaces.RealInput T_in if use_T_in 
    "Temperature of water added to the fluid stream";
  Modelica.Blocks.Interfaces.RealInput u "Control input";
protected 
  Modelica.Blocks.Interfaces.RealInput T_in_internal 
    "Needed to connect to conditional connector";
  Modelica.Blocks.Math.Gain gai(k=mWat_flow_nominal) "Gain";
  Modelica.Thermal.HeatTransfer.Sources.PrescribedHeatFlow preHea 
    "Prescribed heat flow";
  Modelica.Blocks.Sources.RealExpression realExpression(y=
        Medium.enthalpyOfLiquid(T_in_internal));
  Modelica.Blocks.Math.Product pro 
    "Product to compute latent heat added to volume";
  Modelica.Blocks.Sources.RealExpression realExpression1(y=T_in_internal);
equation 
  // Conditional connect statement
  connect(T_in, T_in_internal);
  if not use_T_in then
    T_in_internal = T;
  end if;

  connect(u, gai.u);
  connect(gai.y, vol.mWat_flow);
  connect(realExpression.y, pro.u1);
  connect(gai.y, pro.u2);
  connect(pro.y, preHea.Q_flow);
  connect(preHea.port, vol.heatPort);
  connect(vol.TWat, realExpression1.y);
end HumidifierPrescribed;