Buildings.HeatExchangers.CoolingTowers

Package Content

Name	Description
BaseClasses	Package with base classes for heat exchanger models
Correlations	Package with correlations for cooling tower performance
Examples	Collection of models that illustrate model use and test models
FixedApproach	Cooling tower with constant approach temperature
YorkCalc	Cooling tower with variable speed using the York calculation for the approach temperature

Buildings.HeatExchangers.CoolingTowers.FixedApproach

Information

Model for a steady state cooling tower with constant approach temperature.

By connecting a signal that contains either the dry bulb or the wet bulb temperature, this model can be used to estimate the water return temperature from a cooling tower. For a more detailed model see for example YorkCalc.mo.

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Temperature	TApp	2	Approach temperature [K]
Initialization
MassFlowRate	m_flow		Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressure	dp		Pressure difference between port_a and port_b [Pa]
CelsiusTemperature	TWatIn_degC		Water inlet temperature [degC]
CelsiusTemperature	TWatOut_degC		Water outlet temperature [degC]
CelsiusTemperature	TAirIn_degC		Air dry-bulb inlet temperature [degC]
Advanced
Temp	flowDirection	Modelica_Fluid.Types.FlowDir...	Unidirectional (port_a -> port_b) or bidirectional flow component

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	TAir	Entering air dry or wet bulb temperature

Modelica definition

model FixedApproach 
  "Cooling tower with constant approach temperature" 
  extends Buildings.HeatExchangers.CoolingTowers.BaseClasses.PartialStaticTwoPortCoolingTower;
  parameter Modelica.SIunits.Temperature TApp = 2 "Approach temperature";
equation 
  TWatOut_degC = TApp + TAirIn_degC;
end FixedApproach;

Buildings.HeatExchangers.CoolingTowers.YorkCalc

Information

Model for a steady state cooling tower with variable speed fan using the York calculation for the aproach temperature.

This model uses a performance curve for a York cooling tower to compute the approach temperature. If the fan control signal is zero, then the cooling tower operates in a free convection mode. In the current implementation the fan power consumption is proportional to the control signal raised to the third power. Not yet implemented are the basin heater power consumption, the water usage and the option to provide a fan efficiency curve to compute the fan power consumption. Otherwise, the model is similar to the one in EnergyPlus.

References

EnergyPlus 2.0.0 Engineering Reference, April 9, 2007.

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Real	fraFreCon	0.125	Fraction of tower capacity in free convection regime
Real	yMin	0.3	Minimum control signal until fan is switched off (used for smoothing)
Initialization
MassFlowRate	m_flow		Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressure	dp		Pressure difference between port_a and port_b [Pa]
CelsiusTemperature	TWatIn_degC		Water inlet temperature [degC]
CelsiusTemperature	TWatOut_degC		Water outlet temperature [degC]
CelsiusTemperature	TAirIn_degC		Air dry-bulb inlet temperature [degC]
Nominal condition
Temperature	TAirInWB0	273.15 + 25.55	Design inlet air wet bulb temperature [K]
Temperature	TApp0	3.89	Design apprach temperature [K]
Temperature	TRan0	5.56	Design range temperature (water in - water out) [K]
MassFlowRate	mWat0_flow	0.15	Design water flow rate [kg/s]
Power	PFan0	275	Fan power [W]
Advanced
Temp	flowDirection	Modelica_Fluid.Types.FlowDir...	Unidirectional (port_a -> port_b) or bidirectional flow component

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	TAir	Entering air dry or wet bulb temperature
input RealInput	y	Fan control signal

Modelica definition

model YorkCalc 
  "Cooling tower with variable speed using the York calculation for the approach temperature" 
  extends Buildings.HeatExchangers.CoolingTowers.BaseClasses.PartialStaticTwoPortCoolingTower;
  parameter Modelica.SIunits.Temperature TAirInWB0 = 273.15+25.55 
    "Design inlet air wet bulb temperature";
  parameter Modelica.SIunits.Temperature TApp0 = 3.89 
    "Design apprach temperature";
  parameter Modelica.SIunits.Temperature TRan0 = 5.56 
    "Design range temperature (water in - water out)";
  parameter Modelica.SIunits.MassFlowRate mWat0_flow = 0.15 
    "Design water flow rate";
  parameter Modelica.SIunits.Power PFan0 = 275 "Fan power";
  parameter Real fraFreCon(min=0, max=1) = 0.125 
    "Fraction of tower capacity in free convection regime";
  parameter Real yMin(min=0.01, max=1) = 0.3 
    "Minimum control signal until fan is switched off (used for smoothing)";
  
  Modelica.SIunits.Temperature TApp(min=0, nominal=1) "Approach temperature";
  Modelica.SIunits.Temperature TAppCor(min=0, nominal=1) 
    "Approach temperature based on manufacturer correlation";
  Modelica.SIunits.Temperature TAppFreCon(min=0, nominal=1) 
    "Approach temperature for free convection";
  
  Modelica.SIunits.Temperature TRan(nominal=1) "Range temperature";
  Modelica.SIunits.MassFraction FRWat 
    "Ratio actual over design water mass flow ratio";
  Modelica.SIunits.MassFraction FRAir 
    "Ratio actual over design air mass flow ratio";
  Modelica.SIunits.Power PFan "Fan power";
protected 
  parameter Modelica.SIunits.MassFraction FRWat0(min=0, start=1, fixed=false) 
    "Ratio actual over design water mass flow ratio at nominal condition";
  parameter Modelica.SIunits.Temperature TWatIn0(fixed=false) 
    "Water inlet temperature at nominal condition";
  parameter Modelica.SIunits.Temperature TWatOut0(fixed=false) 
    "Water outlet temperature at nominal condition";
  parameter Modelica.SIunits.MassFlowRate mWatRef_flow(min=0, start=mWat0_flow, fixed=false) 
    "Reference water flow rate";
  
  Modelica.SIunits.Temperature dTMax(nominal=1) 
    "Maximum possible temperature difference";
  
public 
  Correlations.BoundsYorkCalc bou "Bounds for correlation";
  Modelica.Blocks.Interfaces.RealInput y(redeclare type SignalType = Real (min=
            0)) "Fan control signal";
initial equation 
  TWatOut0 = TAirInWB0 + TApp0;
  TRan0 = TWatIn0 - TWatOut0; // by definition of the range temp.
  TApp0 = Correlations.yorkCalc(TRan=TRan0, TWB=TAirInWB0,
                                FRWat=FRWat0, FRAir=1); // this will be solved for FRWat0
  mWatRef_flow = mWat0_flow/FRWat0;
equation 
  // range temperature
  TRan = medium_a.T - medium_b.T;
  // fractional mass flow rates
  FRWat = m_flow/mWatRef_flow;
  FRAir = y;
  
  TAppCor = Correlations.yorkCalc(TRan=TRan, TWB=TAir,
                                  FRWat=FRWat, FRAir=max(FRWat/bou.liqGasRat_max, FRAir));
  dTMax = TWatIn_degC - TAirIn_degC;
  TAppFreCon = (1-fraFreCon) * ( TWatIn_degC-TAirIn_degC)  + fraFreCon *
               Correlations.yorkCalc(TRan=TRan, TWB=TAir, FRWat=FRWat, FRAir=1);
  
  TApp = Buildings.Utilities.Math.spliceFunction(pos=TAppCor, neg=TAppFreCon,
         x=y-yMin/2, deltax=yMin/2);
  TWatOut_degC = TApp + TAirIn_degC;
  PFan = y^3 * PFan0;
end YorkCalc;