Buildings.Fluid.HeatExchangers

Package with heat exchanger models

Information

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

Package Content

Name	Description
ConstantEffectiveness	Heat exchanger with constant effectiveness
DryCoilCounterFlow	Counterflow coil with discretization along the flow paths and without humidity condensation
DryCoilDiscretized	Coil with discretization along the flow paths and no humidity condensation
DryEffectivenessNTU	Heat exchanger with effectiveness - NTU relation and no moisture condensation
EvaporatorCondenser	Evaporator or condenser with refrigerant experiencing constant temperature phase change
HeaterCooler_u	Heater or cooler with prescribed heat flow rate
Heater_T	Heater with prescribed outlet temperature
PrescribedOutlet	Ideal heater, cooler, humidifier or dehumidifier with prescribed outlet conditions
SensibleCooler_T	Sensible cooling device with prescribed outlet temperature
WetCoilCounterFlow	Counterflow coil with discretization along the flow paths and humidity condensation
WetCoilDiscretized	Coil with discretization along the flow paths and humidity condensation
ActiveBeams	Package with active beams
CoolingTowers	Package with cooling tower models
DXCoils	DX(Direct Expansion) cooling coil models
Ground	Package with ground-coupled heat exchanger models
RadiantSlabs	Package with radiant slab models
Radiators	Package with radiators models for hydronic space heating systems
Examples	Collection of models that illustrate model use and test models
Validation	Collection of models that validate the heat exchanger models
BaseClasses	Package with base classes for Buildings.Fluid.HeatExchangers

Buildings.Fluid.HeatExchangers.ConstantEffectiveness

Heat exchanger with constant effectiveness

Information

Model for a heat exchanger with constant effectiveness.

This model transfers heat in the amount of

Q = Q_max ε,

where ε is a constant effectiveness and Q_max is the maximum heat that can be transferred.

For a heat and moisture exchanger, use Buildings.Fluid.MassExchangers.ConstantEffectiveness instead of this model.

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

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatExchangers.DryCoilCounterFlow

Counterflow coil with discretization along the flow paths and without humidity condensation

Information

Model of a discretized coil without water vapor condensation. The coil consists of two flow paths which are, at the design flow direction, in opposite direction to model a counterflow heat exchanger. The flow paths are discretized into nEle elements. Each element is modeled by an instance of Buildings.Fluid.HeatExchangers.BaseClasses.HexElementSensible. Each element has a state variable for the metal.

The convective heat transfer coefficients can, for each fluid individually, be computed as a function of the flow rate and/or the temperature, or assigned to a constant. This computation is done using an instance of Buildings.Fluid.HeatExchangers.BaseClasses.HADryCoil.

To model humidity condensation, use the model Buildings.Fluid.HeatExchangers.WetCoilCounterFlow instead of this model, as this model computes only sensible heat transfer.

Implementation

At very small flow rates, which may be caused when the fan is off but there is wind pressure on the building that entrains outside air through the HVAC system, large temperature differences could occur if diffusion were neglected. This model therefore approximates a small diffusion between the elements to have more uniform medium temperatures if the flow is near zero. The approximation is done using the heat conductors heaCon1 and heaCon2. As this is a rough approximation, neighboring elements are connected through these heat conduction elements, ignoring the actual geometrical configuration. Also, radiation between the coil surfaces on the air side is not modelled explicitly, but rather may be considered as approximated by these heat conductors.

Extends from Buildings.Fluid.Interfaces.PartialFourPortInterface (Partial model transporting fluid between two ports without storing mass or energy), Buildings.Fluid.Interfaces.FourPortFlowResistanceParameters (Parameters for flow resistance for models with four ports).

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatExchangers.DryCoilDiscretized

Coil with discretization along the flow paths and no humidity condensation

Information

Model of a discretized coil with no water vapor condensation. The coil consists of nReg registers that are perpendicular to the air flow path. Each register consists of nPipPar parallel pipes, and each pipe can be divided into nPipSeg pipe segments along the pipe length. Thus, the smallest element of the coil consists of a pipe segment. Each pipe segment is modeled by an instance of Buildings.Fluid.HeatExchangers.BaseClasses.HexElementSensible. Each element has a state variable for the metal.

If the parameter energyDynamics is different from Modelica.Fluid.Types.Dynamics.SteadyState, then a mixing volume of length dl is added to the duct connection. This can help reducing the dimension of the nonlinear system of equations.

The convective heat transfer coefficients can, for each fluid individually, be computed as a function of the flow rate and/or the temperature, or assigned to a constant. This computation is done using an instance of Buildings.Fluid.HeatExchangers.BaseClasses.HADryCoil.

In this model, the water (or liquid) flow path needs to be connected to port_a1 and port_b1, and the air flow path need to be connected to the other two ports.

To model humidity condensation, use the model Buildings.Fluid.HeatExchangers.WetCoilDiscretized instead of this model, as this model computes only sensible heat transfer.

Implementation

At very small flow rates, which may be caused when the fan is off but there is wind pressure on the building that entrains outside air through the HVAC system, large temperature differences could occur if diffusion were neglected. This model therefore approximates a small diffusion between the elements to have more uniform medium temperatures if the flow is near zero. The approximation is done using the heat conductors heaCon1 and heaCon2. As this is a rough approximation, neighboring elements are connected through these heat conduction elements, ignoring the actual geometrical configuration. Also, radiation between the coil surfaces on the air side is not modelled explicitly, but rather may be considered as approximated by these heat conductors.

Extends from Buildings.Fluid.Interfaces.PartialFourPortInterface (Partial model transporting fluid between two ports without storing mass or energy), Buildings.Fluid.Interfaces.FourPortFlowResistanceParameters (Parameters for flow resistance for models with four ports).

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatExchangers.DryEffectivenessNTU

Heat exchanger with effectiveness - NTU relation and no moisture condensation

Information

Model of a heat exchanger without humidity condensation. This model transfers heat in the amount of

Q = Q_max ε
ε = f(NTU, Z, flowRegime),

where Q_max is the maximum heat that can be transferred, ε is the heat transfer effectiveness, NTU is the Number of Transfer Units, Z is the ratio of minimum to maximum capacity flow rate and flowRegime is the heat exchanger flow regime. such as parallel flow, cross flow or counter flow.

The flow regimes depend on the heat exchanger configuration. All configurations defined in Buildings.Fluid.Types.HeatExchangerConfiguration are supported.

For a heat and moisture exchanger, use Buildings.Fluid.MassExchangers.ConstantEffectiveness instead of this model.

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

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatExchangers.EvaporatorCondenser

Evaporator or condenser with refrigerant experiencing constant temperature phase change

Information

Model for a constant temperature evaporator or condenser based on a ε-NTU heat exchanger model.

The heat exchanger effectiveness is calculated from the number of transfer units (NTU):

ε = 1 - exp(UA ⁄ (ṁ c_p))

Optionally, this model can have a flow resistance. If no flow resistance is requested, set dp_nominal=0.

Limitations

This model does not consider any superheating or supercooling on the refrigerant side. The refrigerant is considered to exchange heat at a constant temperature throughout the heat exchanger.

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

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatExchangers.HeaterCooler_u

Heater or cooler with prescribed heat flow rate

Information

Model for an ideal heater or cooler with prescribed heat flow rate to the medium.

This model adds heat in the amount of Q_flow = u Q_flow_nominal to the medium. The input signal u and the nominal heat flow rate Q_flow_nominal can be positive or negative. A positive value of Q_flow means heating, and negative means cooling.

The outlet conditions at port_a are not affected by this model, other than for a possible pressure difference due to flow friction.

Optionally, this model can have a flow resistance. Set dp_nominal = 0 to disable the flow friction calculation.

For a model that uses as an input the fluid temperature leaving at port_b, use Buildings.Fluid.HeatExchangers.PrescribedOutlet

Limitations

This model does not affect the humidity of the air. Therefore, if used to cool air below the dew point temperature, the water mass fraction will not change.

Validation

The model has been validated against the analytical solution in the example Buildings.Fluid.HeatExchangers.Validation.HeaterCooler_u.

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

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatExchangers.Heater_T

Heater with prescribed outlet temperature

Information

Model for an ideal heater that controls its outlet temperature to a prescribed outlet temperature.

This model forces the outlet temperature at port_b to be no lower than the temperature of the input signal TSet, subject to optional limits on the capacity. By default, the model has unlimited heating capacity.

The output signal Q_flow is the heat added to the medium if the mass flow rate is from port_a to port_b. If the flow is reversed, then Q_flow=0.

The outlet conditions at port_a are not affected by this model, other than for a possible pressure difference due to flow friction.

If the parameter energyDynamics is different from Modelica.Fluid.Types.Dynamics.SteadyState, the component models the dynamic response using a first order differential equation. The time constant of the component is equal to the parameter tau. This time constant is adjusted based on the mass flow rate using

τ_eff = τ |ṁ| ⁄ ṁ_nom

where τ_eff is the effective time constant for the given mass flow rate ṁ and τ is the time constant at the nominal mass flow rate ṁ_nom. This type of dynamics is equal to the dynamics that a completely mixed control volume would have.

Optionally, this model can have a flow resistance. Set dp_nominal = 0 to disable the flow friction calculation.

For a similar model that is a sensible cooling device, use Buildings.Fluid.HeatExchangers.SensibleCooler_T. For a model that uses a control signal u ∈ [0, 1] and multiplies this with the nominal heating or cooling power, use Buildings.Fluid.HeatExchangers.HeaterCooler_u

Limitations

If the flow is from port_b to port_a, then the enthalpy of the medium is not affected by this model.

Validation

The model has been validated against the analytical solution in the examples Buildings.Fluid.HeatExchangers.Validation.PrescribedOutlet and Buildings.Fluid.HeatExchangers.Validation.PrescribedOutlet_dynamic.

Extends from Buildings.Fluid.HeatExchangers.BaseClasses.PartialPrescribedOutlet (Ideal heater, cooler, humidifier or dehumidifier with prescribed outlet conditions).

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatExchangers.PrescribedOutlet

Ideal heater, cooler, humidifier or dehumidifier with prescribed outlet conditions

Information

Model that allows specifying the temperature and mass fraction of the fluid that leaves the model from port_b.

This model forces the outlet temperature at port_b to be equal to the temperature of the input signal TSet, subject to optional limits on the heating or cooling capacity QMax_flow ≥ 0 and QMin_flow ≤ 0. Similarly than for the temperature, this model also forces the outlet water mass fraction at port_b to be no lower than the input signal X_wSet, subject to optional limits on the maximum water vapor mass flow rate that is added, as described by the parameter mWatMax_flow. By default, the model has unlimited capacity, but control of temperature and humidity can be subject to capacity limits, or be disabled.

The output signal Q_flow is the heat added (for heating) or subtracted (for cooling) to the medium if the flow rate is from port_a to port_b. If the flow is reversed, then Q_flow=0.

The outlet conditions at port_a are not affected by this model.

If the parameter energyDynamics is not equal to Modelica.Fluid.Types.Dynamics.SteadyState, the component models the dynamic response using a first order differential equation. The time constant of the component is equal to the parameter tau. This time constant is adjusted based on the mass flow rate using

τ_eff = τ |ṁ| ⁄ ṁ_nom

where τ_eff is the effective time constant for the given mass flow rate ṁ and τ is the time constant at the nominal mass flow rate ṁ_nom. This type of dynamics is equal to the dynamics that a completely mixed control volume would have.

Optionally, this model can have a flow resistance. If no flow resistance is requested, set dp_nominal=0.

For a model that uses a control signal u ∈ [0, 1] and multiplies this with the nominal heating or cooling power, use Buildings.Fluid.HeatExchangers.HeaterCooler_u

Limitations

This model only adds or removes heat or water vapor for the flow from port_a to port_b. The enthalpy of the reverse flow is not affected by this model.

If this model is used to cool air below the dew point temperature, the water mass fraction will not change.

Note that for use_TSet = false, the enthalpy of the leaving fluid will not be changed, even if moisture is added. The enthalpy added (or removed) by the change in humidity is neglected. To properly account for change in enthalpy due to humidification, use instead Buildings.Fluid.Humidifiers.SprayAirWasher_X.

Validation

The model has been validated against the analytical solution in the examples Buildings.Fluid.HeatExchangers.Validation.PrescribedOutlet and Buildings.Fluid.HeatExchangers.Validation.PrescribedOutlet_dynamic.

Extends from Buildings.Fluid.HeatExchangers.BaseClasses.PartialPrescribedOutlet (Ideal heater, cooler, humidifier or dehumidifier with prescribed outlet conditions).

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatExchangers.SensibleCooler_T

Sensible cooling device with prescribed outlet temperature

Information

Model for an ideal sensible-only cooler that controls its outlet temperature to a prescribed outlet temperature.

This model forces the outlet temperature at port_b to be no higher than the temperature of the input signal TSet, subject to optional limits on the capacity. By default, the model has unlimited cooling capacity.

The output signal Q_flow ≤ 0 is the heat added to the medium if the mass flow rate is from port_a to port_b. If the flow is reversed, then Q_flow=0.

The outlet conditions at port_a are not affected by this model, other than for a possible pressure difference due to flow friction.

If the parameter energyDynamics is different from Modelica.Fluid.Types.Dynamics.SteadyState, the component models the dynamic response using a first order differential equation. The time constant of the component is equal to the parameter tau. This time constant is adjusted based on the mass flow rate using

τ_eff = τ |ṁ| ⁄ ṁ_nom

where τ_eff is the effective time constant for the given mass flow rate ṁ and τ is the time constant at the nominal mass flow rate ṁ_nom. This type of dynamics is equal to the dynamics that a completely mixed control volume would have.

Optionally, this model can have a flow resistance. Set dp_nominal = 0 to disable the flow friction calculation.

For a similar model that is a heater, use Buildings.Fluid.HeatExchangers.Heater_T. For a model that uses a control signal u ∈ [0, 1] and multiplies this with the nominal heating or cooling power, use Buildings.Fluid.HeatExchangers.HeaterCooler_u.

Limitations

If the flow is from port_b to port_a, then the enthalpy of the medium is not affected by this model.

This model does not affect the humidity of the air. Therefore, if used to cool air below the dew point temperature, the water mass fraction will not change.

Validation

The model has been validated against the analytical solution in the examples Buildings.Fluid.HeatExchangers.Validation.PrescribedOutlet and Buildings.Fluid.HeatExchangers.Validation.PrescribedOutlet_dynamic.

Extends from Buildings.Fluid.HeatExchangers.BaseClasses.PartialPrescribedOutlet (Ideal heater, cooler, humidifier or dehumidifier with prescribed outlet conditions).

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatExchangers.WetCoilCounterFlow

Counterflow coil with discretization along the flow paths and humidity condensation

Information

Model of a discretized coil with water vapor condensation. The coil consists of two flow paths which are, at the design flow direction, in opposite direction to model a counterflow heat exchanger. The flow paths are discretized into nEle elements. Each element is modeled by an instance of Buildings.Fluid.HeatExchangers.BaseClasses.HexElementLatent. Each element has a state variable for the metal.

The convective heat transfer coefficients can, for each fluid individually, be computed as a function of the flow rate and/or the temperature, or assigned to a constant. This computation is done using an instance of Buildings.Fluid.HeatExchangers.BaseClasses.HADryCoil.

In this model, the water (or liquid) flow path needs to be connected to port_a1 and port_b1, and the air flow path needs to be connected to the other two ports.

The mass transfer from the fluid 2 to the metal is computed using a similarity law between heat and mass transfer, as implemented by the model Buildings.Fluid.HeatExchangers.BaseClasses.MassExchange.

This model can only be used with medium models that implement the function enthalpyOfLiquid and that contain an integer variable Water whose value is the element number where the water vapor is stored in the species concentration vector. Examples for such media are Buildings.Media.Air and Modelica.Media.Air.MoistAir.

To model this coil for conditions without humidity condensation, use the model Buildings.Fluid.HeatExchangers.DryCoilCounterFlow instead of this model.

Extends from Buildings.Fluid.HeatExchangers.DryCoilCounterFlow (Counterflow coil with discretization along the flow paths and without humidity condensation).

Parameters

Connectors

Modelica definition

Buildings.Fluid.HeatExchangers.WetCoilDiscretized

Coil with discretization along the flow paths and humidity condensation

Information

Model of a discretized coil with humidity condensation. This model is identical to Buildings.Fluid.HeatExchangers.DryCoilDiscretized but in addition, the mass transfer from fluid 2 to the metal is computed. The mass transfer is computed using a similarity law between heat and mass transfer, as implemented by the model Buildings.Fluid.HeatExchangers.BaseClasses.MassExchange. See this model for details.

This model can only be used with medium models that implement the function enthalpyOfLiquid and that contain an integer variable Water whose value is the element number where the water vapor is stored in the species concentration vector. Examples for such media are Buildings.Media.Air and Modelica.Media.Air.MoistAir.

Extends from DryCoilDiscretized (Coil with discretization along the flow paths and no humidity condensation).

Parameters

Connectors

Modelica definition