Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses

Package with base classes for Buildings.Fluid.HeatExchangers.CoolingTowers

Information

This package contains base classes that are used to construct the models in Buildings.Fluid.HeatExchangers.CoolingTowers.

Extends from Modelica.Icons.BasesPackage (Icon for packages containing base classes).

Package Content

Name	Description
CoolingTower	Base class for cooling towers
Merkel	Model for thermal performance of Merkel cooling tower
Characteristics	Functions for fan characteristics
Functions

Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses.CoolingTower

Base class for cooling towers

Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses.CoolingTower

Information

Base class for a steady-state cooling tower. The variable TAirHT is used to compute the heat transfer with the water side of the cooling tower. For a dry cooling tower, this is equal to the dry-bulb temperature. For a wet cooling tower, this is equal to the wet-bulb temperature.

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
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
PressureDifference	dp_nominal		Pressure difference [Pa]
Assumptions
Boolean	allowFlowReversal	true	= false to simplify equations, assuming, but not enforcing, no flow reversal
Advanced
MassFlowRate	m_flow_small	1E-4*abs(m_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
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...	Type of energy balance: dynamic (3 initialization options) or steady state
Dynamics	massDynamics	energyDynamics	Type of mass balance: dynamic (3 initialization options) or steady state
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)
output RealOutput	TLvg	Leaving water temperature [K]

Modelica definition

partial model CoolingTower "Base class for cooling towers" extends Buildings.Fluid.Interfaces.TwoPortHeatMassExchanger( redeclare final Buildings.Fluid.MixingVolumes.MixingVolume vol); Modelica.Blocks.Interfaces.RealOutput TLvg( final unit="K", displayUnit="degC") "Leaving water temperature"; Modelica.SIunits.HeatFlowRate Q_flow = preHea.Q_flow "Heat input into water circuit"; protected Modelica.Thermal.HeatTransfer.Sources.PrescribedHeatFlow preHea "Prescribed heat flow"; Modelica.Thermal.HeatTransfer.Sensors.TemperatureSensor TVol "Water temperature in volume, leaving at port_b"; equation connect(preHea.port, vol.heatPort); connect(TVol.port, vol.heatPort); connect(TVol.T, TLvg); end CoolingTower;

Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses.Merkel

Model for thermal performance of Merkel cooling tower

Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses.Merkel

Information

Model for the thermal peformance of the Merkel cooling tower.

For the documentation, see Buildings.Fluid.HeatExchangers.CoolingTowers.Merkel.

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...	Medium in the component
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate of water [kg/s]
Real	ratWatAir_nominal		Water-to-air mass flow rate ratio at design condition [1]
Heat transfer
Temperature	TAirInWB_nominal		Nominal outdoor (air inlet) wetbulb temperature [K]
Temperature	TWatIn_nominal		Nominal water inlet temperature [K]
Temperature	TWatOut_nominal		Nominal water outlet temperature [K]
Real	fraFreCon		Fraction of tower capacity in free convection regime [1]
UAMerkel	UACor	redeclare parameter Building...	Coefficients for UA correction
Fan
Real	yMin		Minimum control signal until fan is switched off (used for smoothing between forced and free convection regime) [1]

Connectors

Type	Name	Description
replaceable package Medium		Medium in the component
input RealInput	y	Fan control signal [1]
input RealInput	m_flow	Water mass flow rate [kg/s]
input RealInput	TWatIn	Inlet water temperature [K]
input RealInput	TAir	Entering air wet bulb temperature [K]
output RealOutput	Q_flow	Heat removed from water

Modelica definition

block Merkel "Model for thermal performance of Merkel cooling tower" extends Modelica.Blocks.Icons.Block; replaceable package Medium = Modelica.Media.Interfaces.PartialMedium "Medium in the component"; parameter Modelica.SIunits.MassFlowRate m_flow_nominal "Nominal mass flow rate of water"; final parameter Modelica.SIunits.MassFlowRate mAir_flow_nominal= m_flow_nominal/ratWatAir_nominal "Nominal mass flow rate of air"; parameter Real ratWatAir_nominal(min=0, unit="1") "Water-to-air mass flow rate ratio at design condition"; parameter Modelica.SIunits.Temperature TAirInWB_nominal "Nominal outdoor (air inlet) wetbulb temperature"; parameter Modelica.SIunits.Temperature TWatIn_nominal "Nominal water inlet temperature"; parameter Modelica.SIunits.Temperature TWatOut_nominal "Nominal water outlet temperature"; parameter Real fraFreCon(min=0, max=1, final unit="1") "Fraction of tower capacity in free convection regime"; replaceable parameter Buildings.Fluid.HeatExchangers.CoolingTowers.Data.UAMerkel UACor constrainedby Buildings.Fluid.HeatExchangers.CoolingTowers.Data.UAMerkel "Coefficients for UA correction"; parameter Real yMin(min=0.01, max=1, final unit="1") "Minimum control signal until fan is switched off (used for smoothing between forced and free convection regime)"; final parameter Modelica.SIunits.HeatFlowRate Q_flow_nominal(max=0)= -m_flow_nominal*cpWat_nominal*(TWatIn_nominal-TWatOut_nominal) "Nominal heat transfer, (negative)"; final parameter Modelica.SIunits.ThermalConductance UA_nominal= NTU_nominal*CMin_flow_nominal "Thermal conductance at nominal flow, used to compute heat capacity"; final parameter Real eps_nominal= Q_flow_nominal/((TAirInWB_nominal - TWatIn_nominal) * CMin_flow_nominal) "Nominal heat transfer effectiveness"; final parameter Real NTU_nominal(min=0)= Buildings.Fluid.HeatExchangers.BaseClasses.ntu_epsilonZ( eps=min(1, max(0, eps_nominal)), Z=Z_nominal, flowRegime=Integer(Buildings.Fluid.Types.HeatExchangerConfiguration.CounterFlow)) "Nominal number of transfer units"; Modelica.Blocks.Interfaces.RealInput y(unit="1") "Fan control signal"; Modelica.Blocks.Interfaces.RealInput m_flow(final unit="kg/s") "Water mass flow rate"; Modelica.Blocks.Interfaces.RealInput TWatIn( final min=0, final unit="K", displayUnit="degC") "Inlet water temperature"; Modelica.Blocks.Interfaces.RealInput TAir( final min=0, final unit="K", displayUnit="degC") "Entering air wet bulb temperature"; Modelica.SIunits.Temperature TAirOut( displayUnit="degC") "Outlet air temperature"; Modelica.SIunits.MassFlowRate mAir_flow "Air mass flow rate"; Modelica.SIunits.MassFraction FRWat "Ratio actual over design water mass flow ratio"; Modelica.SIunits.MassFraction FRAir "Ratio actual over design air mass flow ratio"; Real eps(min=0, max=1, unit="1") "Heat exchanger effectiveness"; Modelica.SIunits.SpecificHeatCapacity cpWat "Heat capacity of water"; Modelica.SIunits.ThermalConductance CAir_flow "Heat capacity flow rate of air"; Modelica.SIunits.ThermalConductance CWat_flow "Heat capacity flow rate of water"; Modelica.SIunits.ThermalConductance CMin_flow(min=0) "Minimum heat capacity flow rate"; Modelica.SIunits.HeatFlowRate QMax_flow "Maximum heat flow rate into air"; Modelica.SIunits.ThermalConductance UAe(min=0) "Thermal conductance for equivalent fluid"; Modelica.SIunits.ThermalConductance UA "Thermal conductance"; Modelica.Blocks.Interfaces.RealOutput Q_flow "Heat removed from water"; protected final package Air = Buildings.Media.Air "Package of medium air"; final parameter Air.ThermodynamicState staAir_default=Air.setState_pTX( T=TAirInWB_nominal, p=Air.p_default, X=Air.X_default[1:Air.nXi]) "Default state for air"; final parameter Medium.ThermodynamicState staWat_default=Medium.setState_pTX( T=TWatIn_nominal, p=Medium.p_default, X=Medium.X_default[1:Medium.nXi]) "Default state for water"; parameter Real delta=1E-3 "Parameter used for smoothing"; parameter Modelica.SIunits.SpecificHeatCapacity cpe_nominal= Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses.Functions.equivalentHeatCapacity( TIn = TAirInWB_nominal, TOut = TAirOutWB_nominal) "Specific heat capacity of the equivalent medium on medium 1 side"; parameter Modelica.SIunits.SpecificHeatCapacity cpAir_nominal= Air.specificHeatCapacityCp(staAir_default) "Specific heat capacity of air at nominal condition"; parameter Modelica.SIunits.SpecificHeatCapacity cpWat_nominal= Medium.specificHeatCapacityCp(staWat_default) "Specific heat capacity of water at nominal condition"; parameter Modelica.SIunits.ThermalConductance CAir_flow_nominal= mAir_flow_nominal*cpe_nominal "Nominal capacity flow rate of air"; parameter Modelica.SIunits.ThermalConductance CWat_flow_nominal= m_flow_nominal*cpWat_nominal "Nominal capacity flow rate of water"; parameter Modelica.SIunits.ThermalConductance CMin_flow_nominal= min(CAir_flow_nominal, CWat_flow_nominal) "Minimal capacity flow rate at nominal condition"; parameter Modelica.SIunits.ThermalConductance CMax_flow_nominal= max(CAir_flow_nominal, CWat_flow_nominal) "Maximum capacity flow rate at nominal condition"; parameter Real Z_nominal( min=0, max=1) = CMin_flow_nominal/CMax_flow_nominal "Ratio of capacity flow rate at nominal condition"; parameter Modelica.SIunits.Temperature TAirOutWB_nominal(fixed=false) "Nominal leaving air wetbulb temperature"; Real UA_FAir "UA correction factor as function of air flow ratio"; Real UA_FWat "UA correction factor as function of water flow ratio"; Real UA_DifWB "UA correction factor as function of differential wetbulb temperature"; Real corFac_FAir "Smooth factor as function of air flow ratio"; Real corFac_FWat "Smooth factor as function of water flow ratio"; Modelica.SIunits.SpecificHeatCapacity cpEqu "Specific heat capacity of the equivalent fluid"; initial equation // Heat transferred from air to water at nominal condition Q_flow_nominal = mAir_flow_nominal*cpe_nominal*(TAirInWB_nominal - TAirOutWB_nominal); assert(eps_nominal > 0 and eps_nominal < 1, "eps_nominal out of bounds, eps_nominal = " + String(eps_nominal) + "\n To achieve the required heat transfer rate at epsilon=0.8, set |TAirInWB_nominal-TWatIn_nominal| = " + String(abs(Q_flow_nominal/0.8*CMin_flow_nominal)) + "\n or increase flow rates. The current parameters result in " + "\n CMin_flow_nominal = " + String(CMin_flow_nominal) + "\n CMax_flow_nominal = " + String(CMax_flow_nominal)); equation // For cp water, we use the inlet temperatures because the effect is small // for actual water temperature differences, and in case of Buildings.Media.Water, // cp is constant. cpWat = Medium.specificHeatCapacityCp(Medium.setState_pTX( p=Medium.p_default, T=TWatIn, X=Medium.X_default)); // Determine the airflow based on fan speed signal mAir_flow = Buildings.Utilities.Math.Functions.spliceFunction( pos=y*mAir_flow_nominal, neg=fraFreCon*mAir_flow_nominal, x=y - yMin + yMin/20, deltax=yMin/20); FRAir = mAir_flow/mAir_flow_nominal; FRWat = m_flow/m_flow_nominal; // UA for equivalent fluid // Adjust UA UA_FAir =Buildings.Fluid.Utilities.extendedPolynomial( x=FRAir, c=UACor.cAirFra, xMin=UACor.FRAirMin, xMax=UACor.FRAirMax) "UA correction factor as function of air flow fraction"; UA_FWat =Buildings.Fluid.Utilities.extendedPolynomial( x=FRWat, c=UACor.cWatFra, xMin=UACor.FRWatMin, xMax=UACor.FRWatMax) "UA correction factor as function of water flow fraction"; UA_DifWB =Buildings.Fluid.Utilities.extendedPolynomial( x=TAirInWB_nominal - TAir, c=UACor.cDifWB, xMin=UACor.TDiffWBMin, xMax=UACor.TDiffWBMax) "UA correction factor as function of differential wet bulb temperature"; corFac_FAir =Buildings.Utilities.Math.Functions.smoothHeaviside( x=FRAir - UACor.FRAirMin/4, delta=UACor.FRAirMin/4); corFac_FWat =Buildings.Utilities.Math.Functions.smoothHeaviside( x=FRWat - UACor.FRWatMin/4, delta=UACor.FRWatMin/4); UA = UA_nominal*UA_FAir*UA_FWat*UA_DifWB*corFac_FAir*corFac_FWat; UAe = UA*cpEqu/Buildings.Utilities.Psychrometrics.Constants.cpAir; // Capacity for air and water CAir_flow =abs(mAir_flow)*cpEqu; CWat_flow =abs(m_flow)*cpWat; CMin_flow =Buildings.Utilities.Math.Functions.smoothMin( CAir_flow, CWat_flow, delta*CMin_flow_nominal); // Calculate epsilon eps = Buildings.Fluid.HeatExchangers.BaseClasses.epsilon_C( UA=UAe, C1_flow=CAir_flow, C2_flow=CWat_flow, flowRegime=Integer(Buildings.Fluid.Types.HeatExchangerConfiguration.CounterFlow), CMin_flow_nominal=CMin_flow_nominal, CMax_flow_nominal=CMax_flow_nominal, delta=delta); // QMax_flow is maximum heat transfer into medium air: positive means heating QMax_flow = CMin_flow*(TWatIn - TAir); eps*QMax_flow =CAir_flow*(TAirOut - TAir); cpEqu = Buildings.Fluid.HeatExchangers.CoolingTowers.BaseClasses.Functions.equivalentHeatCapacity( TIn=TAir, TOut=TAirOut); Q_flow = -eps * QMax_flow; end Merkel;