Buildings.Fluid.MassExchangers
Package with mass exchanger models
Information
This package contains models for mass exchangers. For heat exchanger models without humidity transfer, see the package Buildings.Fluid.HeatExchangers.
Extends from Modelica.Icons.VariantsPackage (Icon for package containing variants).
Package Content
| Name | Description |
|---|---|
 ConstantEffectiveness
|
Heat and moisture exchanger with constant effectiveness |
 Examples
|
Collection of models that illustrate model use and test models |
| Validation
|
Validation examples |
Buildings.Fluid.MassExchangers.ConstantEffectiveness
Heat and moisture exchanger with constant effectiveness
Information
Model for a heat and moisture exchanger with constant effectiveness.
This model transfers heat and moisture in the amount of
QSen = 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 sensible 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 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 + QLat_flow | Heat transferred into the medium 1 [W] |
| MassFlowRate | mWat1_flow | +mWat_flow | Moisture mass flow rate added to the medium 1 [kg/s] |
| HeatFlowRate | Q2_flow | -Q1_flow | Heat transferred into the medium 2 [W] |
| MassFlowRate | mWat2_flow | -mWat_flow | Moisture mass flow rate 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 |
| Efficiency | epsS | 0.8 | Sensible heat exchanger effectiveness [1] |
| Efficiency | epsL | 0.8 | Latent heat exchanger effectiveness [1] |
| Nominal condition | |||
| MassFlowRate | m1_flow_nominal | Nominal mass flow rate [kg/s] | |
| MassFlowRate | m2_flow_nominal | Nominal mass flow rate [kg/s] | |
| PressureDifference | dp1_nominal | Pressure difference [Pa] | |
| PressureDifference | dp2_nominal | Pressure difference [Pa] | |
| Assumptions | |||
| Boolean | allowFlowReversal1 | true | = false to simplify equations, assuming, but not enforcing, no flow reversal for medium 1 |
| Boolean | allowFlowReversal2 | true | = false to simplify equations, assuming, but not enforcing, no flow reversal for medium 2 |
| Advanced | |||
| 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] |
| Diagnostics | |||
| Boolean | show_T | false | = true, if actual temperature at 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 |
|---|---|---|
| replaceable package Medium1 | Medium 1 in the component | |
| replaceable package Medium2 | Medium 2 in the component | |
| 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(
redeclare replaceable package Medium1 =
Modelica.Media.Interfaces.PartialCondensingGases,
redeclare replaceable package Medium2 =
Modelica.Media.Interfaces.PartialCondensingGases,
sensibleOnly1=false,
sensibleOnly2=false,
final prescribedHeatFlowRate1=true,
final prescribedHeatFlowRate2=true,
Q1_flow = epsS * QMax_flow + QLat_flow,
Q2_flow = -Q1_flow,
mWat1_flow = +mWat_flow,
mWat2_flow = -mWat_flow);
parameter Modelica.Units.SI.Efficiency epsS(max=1) = 0.8
"Sensible heat exchanger effectiveness";
parameter Modelica.Units.SI.Efficiency epsL(max=1) = 0.8
"Latent heat exchanger effectiveness";
Modelica.Units.SI.HeatFlowRate QLat_flow
"Latent heat exchange from medium 2 to medium 1";
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.Units.SI.MassFlowRate mWat_flow
"Water flow rate from medium 2 to medium 1";
Modelica.Units.SI.MassFlowRate mMax_flow
"Maximum water flow rate from medium 2 to medium 1";
protected
parameter Integer i1_w(min=1, fixed=false) "Index for water substance";
parameter Integer i2_w(min=1, fixed=false) "Index for water substance";
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";
initial algorithm
i1_w:= -1;
i2_w:= -1;
for i in 1:Medium1.nXi loop
if Modelica.Utilities.Strings.isEqual(string1=Medium1.substanceNames[i],
string2="Water",
caseSensitive=false) then
i1_w := i;
end if;
end for;
for i in 1:Medium2.nXi loop
if Modelica.Utilities.Strings.isEqual(string1=Medium2.substanceNames[i],
string2="Water",
caseSensitive=false) then
i2_w := i;
end if;
end for;
assert(i1_w > 0, "Substance 'water' is not present in Medium1 '"
+ Medium1.mediumName + "'.\n"
+ "Check medium model.");
assert(i2_w > 0, "Substance 'water' is not present in Medium2 '"
+ Medium2.mediumName + "'.\n"
+ "Check medium model.");
equation
// Definitions for effectiveness model
X_w_in1 = Modelica.Fluid.Utilities.regStep(m1_flow,
state_a1_inflow.X[i1_w],
state_b1_inflow.X[i1_w], m1_flow_small);
X_w_in2 = Modelica.Fluid.Utilities.regStep(m2_flow,
state_a2_inflow.X[i2_w],
state_b2_inflow.X[i2_w], 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;
// As enthalpyOfCondensingGas is dominated by the latent heat of phase change,
// we simplify and use Medium1.enthalpyOfVaporization for the
// latent heat that is exchanged among the fluid streams.
// This is simply added to QSen_flow, while mass is conserved because
// of the assignment of mWat1_flow and mWat2_flow.
QLat_flow = mWat_flow * Medium1.enthalpyOfVaporization(Medium1.T_default);
end ConstantEffectiveness;