Buildings.Fluid.MassExchangers

Information

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
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)
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]
Advanced
MassFlowRate	m1_flow_small	1E-4*m1_flow_nominal	Small mass flow rate for regularization of zero flow [kg/s]
MassFlowRate	m2_flow_small	1E-4*m2_flow_nominal	Small mass flow rate for regularization of zero flow [kg/s]
Diagnostics
Boolean	show_V_flow	false	= true, if volume flow rate at inflowing port is computed
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);
  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";

equation 
  // transfered heat
  Q1_flow = epsS * QMax_flow;
  // no heat loss to ambient
  0 = Q1_flow + Q2_flow;
  // 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
  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;

  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

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).

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*m_flow_nominal	Small mass flow rate for regularization of zero flow [kg/s]
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

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 Fluid.Interfaces.PartialStaticTwoPortHeatMassTransfer(
     sensibleOnly = false);

  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 
  constant Modelica.SIunits.MassFraction[Medium.nXi] Xi_w = {1} 
    "Mass fraction of water";
  Modelica.SIunits.MassFlowRate mWat_flow "Water flow rate";
  Modelica.Blocks.Interfaces.RealInput T_in_internal 
    "Needed to connect to conditional connector";
equation 
  connect(T_in, T_in_internal);
  if not use_T_in then
    T_in_internal = T;
  end if;

  mWat_flow = u * mWat_flow_nominal;
  Q_flow = Medium.enthalpyOfLiquid(T_in_internal) * 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;