Buildings.Fluid.HeatExchangers.ThermalWheels.Latent.BaseClasses
Package with base classes for enthalpy recovery devices
Information
This package contains base classes that are used to construct the models in Buildings.Fluid.HeatExchangers.ThermalWheels.Latent.
Extends from Modelica.Icons.BasesPackage (Icon for packages containing base classes).
Package Content
| Name | Description |
|---|---|
 Effectiveness
|
Model for calculating the heat exchange effectiveness |
 HeatExchangerWithInputEffectiveness
|
Heat and moisture exchanger with varying effectiveness |
 PartialWheel
|
Partial model for enthalpy recovery wheel |
 Validation
|
Collection of models that validate the module in the base classes |
Buildings.Fluid.HeatExchangers.ThermalWheels.Latent.BaseClasses.Effectiveness
Model for calculating the heat exchange effectiveness
Information
This block calculates the sensible and latent effectiveness of the heat exchanger under heating and cooling modes at different flow rates of the supply air and the exhaust air.
It calculates the ratio of the average operating flow rate to the nominal supply flow rate as
rat = max(0.5, min(1.3, (mSup_flow + mExh_flow)/(2*mSup_flow_nominal))),
where mSup_flow is the mass flow rate of the supply air,
mExh_flow is the mass flow rate of the exhaust air,
mSup_flow_nominal is the nominal mass flow rate of the supply air, and
rat is the flow ratio.
It then calculates the sensible and latent heat exchanger effectiveness by:
epsSen = (epsSenPL + (epsSen_nominal - epsSenPL) * (rat - 0.75)/0.25), epsLat = (epsLatPL + (epsLat_nominal - epsLatPL) * (rat - 0.75)/0.25),
where epsSen and epsLat are the effectiveness
for the sensible and latent heat transfer, respectively,
epsSen_nominal and epsSenPL are the effectiveness
for the sensible heat transfer when rat is 1 and 0.75, respectively,
epsLat_nominal and epsLatPL are the effectiveness
for the latent heat transfer when Rat is 1 and 0.75, respectively.
Note:
The value of the rat is suggested to be between 0.5 and 1.3 during normal operation
to ensure reasonable extrapolation.
Likewise, an unbalanced air flow ratio less than 2, i.e., VSup_flow/VExh_flow > 0.5
and VSup_flow/VExh_flow < 2, is recommended.
References
U.S. Department of Energy 2016. "EnergyPlus Engineering Reference".
Extends from Modelica.Blocks.Icons.Block (Basic graphical layout of input/output block).
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| Efficiency | epsSen_nominal | Nominal sensible heat exchanger effectiveness [1] | |
| Efficiency | epsLat_nominal | Nominal latent heat exchanger effectiveness [1] | |
| Efficiency | epsSenPL | Part load (75% of the nominal supply flow rate) sensible heat exchanger effectiveness [1] | |
| Efficiency | epsLatPL | Part load (75% of the nominal supply flow rate) latent heat exchanger effectiveness [1] | |
| MassFlowRate | mSup_flow_nominal | Nominal supply air mass flow rate [kg/s] |
Connectors
| Type | Name | Description |
|---|---|---|
| input RealInput | mSup_flow | Supply air mass flow rate [kg/s] |
| input RealInput | mExh_flow | Exhaust air mass flow rate [kg/s] |
| output RealOutput | epsSen | Sensible heat exchanger effectiveness [1] |
| output RealOutput | epsLat | Latent heat exchanger effectiveness [1] |
Modelica definition
model Effectiveness
"Model for calculating the heat exchange effectiveness"
extends Modelica.Blocks.Icons.Block;
parameter Modelica.Units.SI.Efficiency epsSen_nominal(final max=1)
"Nominal sensible heat exchanger effectiveness";
parameter Modelica.Units.SI.Efficiency epsLat_nominal(final max=1)
"Nominal latent heat exchanger effectiveness";
parameter Modelica.Units.SI.Efficiency epsSenPL(final max=1)
"Part load (75% of the nominal supply flow rate) sensible heat exchanger effectiveness";
parameter Modelica.Units.SI.Efficiency epsLatPL(final max=1)
"Part load (75% of the nominal supply flow rate) latent heat exchanger effectiveness";
parameter Modelica.Units.SI.MassFlowRate mSup_flow_nominal
"Nominal supply air mass flow rate";
Buildings.Controls.OBC.CDL.Interfaces.RealInput mSup_flow(final unit="kg/s")
"Supply air mass flow rate";
Buildings.Controls.OBC.CDL.Interfaces.RealInput mExh_flow(final unit="kg/s")
"Exhaust air mass flow rate";
Buildings.Controls.OBC.CDL.Interfaces.RealOutput epsSen(final unit="1")
"Sensible heat exchanger effectiveness";
Buildings.Controls.OBC.CDL.Interfaces.RealOutput epsLat(final unit="1")
"Latent heat exchanger effectiveness";
protected
Real rat
"Ratio of the average operating air flow rate to the nominal supply air flow rate";
Real ratRes
"Ratio of the average operating air flow rate to the nominal supply air flow rate, restricted to valid domain";
equation
// Calculate the average volumetric air flow and flow rate ratio.
rat=(mSup_flow+mExh_flow)/2/mSup_flow_nominal;
// Check if the air flows are too unbalanced, unless rat < 0.05, in which case
// the system is likely off or transitioning to on or off.
assert(rat<0.05 or (mSup_flow-2*mExh_flow<1e-5 and mExh_flow-2*mSup_flow<1e-5),
"In " + getInstanceName() + ": The ratio of the supply flow rate to the exhaust flow rate should be in the range of [0.5, 2] when flow rates are non-zero.",
level=AssertionLevel.warning);
// Calculate effectiveness
ratRes=Buildings.Utilities.Math.Functions.smoothLimit(x=rat, l=0.5, u=1.3, deltaX=0.01);
epsSen=(epsSenPL+(epsSen_nominal-epsSenPL)*(ratRes-0.75)/0.25);
epsLat=(epsLatPL+(epsLat_nominal-epsLatPL)*(ratRes-0.75)/0.25);
assert(epsSen>=0 and epsSen<1,
"In " + getInstanceName() + ": The sensible heat exchange effectiveness epsSen = " + String(epsSen) + ". It should be in the range of [0, 1].
Check if the part load (75% of the nominal supply flow rate) or nominal sensible heat exchanger effectiveness is too high or too low.",
level=AssertionLevel.error);
assert(epsLat>=0 and epsLat<1,
"In " + getInstanceName() + ": The latent heat exchange effectiveness epsLat = " + String(epsLat) + ". It should be in the range of [0, 1],
Check if the part load (75% of the nominal supply flow rate) or nominal latent heat exchanger effectiveness is too high or too low.",
level=AssertionLevel.error);
end Effectiveness;
Buildings.Fluid.HeatExchangers.ThermalWheels.Latent.BaseClasses.HeatExchangerWithInputEffectiveness
Heat and moisture exchanger with varying effectiveness
Information
This block is identical to Buildings.Fluid.MassExchangers.ConstantEffectiveness, except that the sensible and latent heat exchanger effectiveness are inputs rather than parameters.
This model transfers heat and moisture in the amount of
QSen = epsSen * Q_max, m = epsLat * mWat_max,
where epsSen and epsLat are the input effectiveness
for the sensible and latent heat transfer, respectively;
Q_max is the maximum sensible heat that can be transferred,
m is the moisture that is transferred, and
mWat_max is the maximum moisture that can be transferred.
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 | epsSen*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 |
| 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) |
| input RealInput | epsSen | Sensible heat exchanger effectiveness [1] |
| input RealInput | epsLat | Latent heat exchanger effectiveness [1] |
Modelica definition
model HeatExchangerWithInputEffectiveness
"Heat and moisture exchanger with varying 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 = epsSen * QMax_flow + QLat_flow,
Q2_flow = -Q1_flow,
mWat1_flow = +mWat_flow,
mWat2_flow = -mWat_flow);
Buildings.Controls.OBC.CDL.Interfaces.RealInput epsSen(unit="1")
"Sensible heat exchanger effectiveness";
Buildings.Controls.OBC.CDL.Interfaces.RealInput epsLat(unit="1")
"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 = epsLat * mMax_flow;
// As enthalpyOfCondensingGas is dominated by the latent heat of phase change,
// we use Medium1.enthalpyOfVaporization for the
// latent heat that is exchanged among the fluid streams.
// This is 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 HeatExchangerWithInputEffectiveness;
Buildings.Fluid.HeatExchangers.ThermalWheels.Latent.BaseClasses.PartialWheel
Partial model for enthalpy recovery wheel
Information
Partial model of an enthalpy recovery wheel.
Extends from Modelica.Blocks.Icons.Block (Basic graphical layout of input/output block).
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| replaceable package Medium | Modelica.Media.Interfaces.Pa... | Air | |
| Generic | per | per(final have_latHEX=true) | Record with performance data |
| 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 |
| 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 |
| 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 |
Connectors
| Type | Name | Description |
|---|---|---|
| replaceable package Medium | Air | |
| output RealOutput | P | Electric power consumption [W] |
| output RealOutput | epsSen | Sensible heat exchanger effectiveness [1] |
| output RealOutput | epsLat | Latent heat exchanger effectiveness [1] |
| FluidPort_a | port_a1 | Fluid connector a1 of the supply air (positive design flow direction is from port_a1 to port_b1) |
| FluidPort_b | port_b2 | Fluid connector b2 of the exhaust air (positive design flow direction is from port_a2 to port_b2) |
| FluidPort_b | port_b1 | Fluid connector b1 of the supply air (positive design flow direction is from port_a1 to port_b1) |
| FluidPort_a | port_a2 | Fluid connector a2 of the exhaust air (positive design flow direction is from port_a2 to port_b2) |
Modelica definition
partial model PartialWheel
"Partial model for enthalpy recovery wheel"
extends Modelica.Blocks.Icons.Block;
replaceable package Medium =
Modelica.Media.Interfaces.PartialCondensingGases
"Air";
parameter Buildings.Fluid.HeatExchangers.ThermalWheels.Data.Generic per(
final have_latHEX=true)
"Record with performance data";
parameter Boolean allowFlowReversal1 = true
"= false to simplify equations, assuming, but not enforcing, no flow reversal for medium 1";
parameter Boolean allowFlowReversal2 = true
"= false to simplify equations, assuming, but not enforcing, no flow reversal for medium 2";
parameter Boolean from_dp1 = false
"= true, use m_flow = f(dp) else dp = f(m_flow)";
parameter Boolean linearizeFlowResistance1 = false
"= true, use linear relation between m_flow and dp for any flow rate";
parameter Boolean from_dp2 = false
"= true, use m_flow = f(dp) else dp = f(m_flow)";
parameter Boolean linearizeFlowResistance2 = false
"= true, use linear relation between m_flow and dp for any flow rate";
Buildings.Controls.OBC.CDL.Interfaces.RealOutput P(
final unit="W")
"Electric power consumption";
Buildings.Controls.OBC.CDL.Interfaces.RealOutput epsSen(final unit="1")
"Sensible heat exchanger effectiveness";
Buildings.Controls.OBC.CDL.Interfaces.RealOutput epsLat(final unit="1")
"Latent heat exchanger effectiveness";
Buildings.Fluid.Sensors.MassFlowRate senSupMasFlo(
redeclare package Medium = Medium)
"Supply air mass flow rate";
Buildings.Fluid.Sensors.MassFlowRate senExhMasFlo(
redeclare package Medium = Medium)
"Exhaust air mass flow rate";
Modelica.Fluid.Interfaces.FluidPort_a port_a1(
redeclare final package Medium = Medium)
"Fluid connector a1 of the supply air (positive design flow direction is from port_a1 to port_b1)";
Modelica.Fluid.Interfaces.FluidPort_b port_b2(
redeclare final package Medium = Medium)
"Fluid connector b2 of the exhaust air (positive design flow direction is from port_a2 to port_b2)";
Modelica.Fluid.Interfaces.FluidPort_b port_b1(
redeclare final package Medium = Medium)
"Fluid connector b1 of the supply air (positive design flow direction is from port_a1 to port_b1)";
Modelica.Fluid.Interfaces.FluidPort_a port_a2(
redeclare final package Medium = Medium)
"Fluid connector a2 of the exhaust air (positive design flow direction is from port_a2 to port_b2)";
protected
Buildings.Fluid.HeatExchangers.ThermalWheels.Latent.BaseClasses.Effectiveness
effCal(
final epsSen_nominal=per.epsSen_nominal,
final epsLat_nominal=per.epsLat_nominal,
final epsSenPL=per.epsSenPL,
final epsLatPL=per.epsLatPL,
final mSup_flow_nominal=per.mSup_flow_nominal)
"Calculate the effectiveness of heat exchanger";
Buildings.Fluid.HeatExchangers.ThermalWheels.Latent.BaseClasses.HeatExchangerWithInputEffectiveness
hex(
redeclare package Medium1 = Medium,
redeclare package Medium2 = Medium,
final m1_flow_nominal=per.mSup_flow_nominal,
final m2_flow_nominal=per.mExh_flow_nominal,
final allowFlowReversal1=allowFlowReversal1,
final allowFlowReversal2=allowFlowReversal2,
final from_dp1=from_dp1,
final from_dp2=from_dp2,
final linearizeFlowResistance1=linearizeFlowResistance1,
final linearizeFlowResistance2=linearizeFlowResistance2)
"Heat exchanger";
equation
connect(senSupMasFlo.m_flow, effCal.mSup_flow);
connect(senExhMasFlo.m_flow, effCal.mExh_flow);
connect(hex.port_b1, senSupMasFlo.port_a);
connect(senSupMasFlo.port_b, port_b1);
connect(senExhMasFlo.port_a, hex.port_b2);
connect(senExhMasFlo.port_b, port_b2);
end PartialWheel;