Buildings.Fluids.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.Fluids.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, use for example YorkCalc.mo.

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
TemperatureDifference	TApp	2	Approach temperature [K]
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
Pressure	dp_nominal		Pressure [Pa]
Initialization
MassFlowRate	m_flow.start	0	Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressure	dp.start	0	Pressure difference between port_a and port_b [Pa]
Temperature_degC	TWatIn_degC.start	35	Water inlet temperature [degC]
Temperature_degC	TWatOut_degC.start	28	Water outlet temperature [degC]
Temperature_degC	TAirIn_degC.start	25	Air dry-bulb inlet temperature [degC]
Assumptions
Boolean	allowFlowReversal	system.allowFlowReversal	= true to allow flow reversal, false restricts to design direction (port_a -> port_b)
Advanced
MassFlowRate	m_flow_small	1E-4*m_flow_nominal	Small mass flow rate for regularization of zero flow [kg/s]
Diagnostics
Boolean	show_V_flow	true	= true, if volume flow rate at inflowing port is computed
Initialization
AbsolutePressure	p_a_start	system.p_start	Guess value for inlet pressure [Pa]
AbsolutePressure	p_b_start	p_a_start	Guess value for outlet pressure [Pa]
Flow resistance
Boolean	from_dp	true	= 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

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.Fluids.HeatExchangers.CoolingTowers.BaseClasses.PartialStaticTwoPortCoolingTower;
  parameter Modelica.SIunits.TemperatureDifference TApp(min=0) = 2 
    "Approach temperature";
equation 
 Q_flow = m_flow * (Medium.specificEnthalpy(Medium.setState_pTX(port_b.p, TAir+TApp, inStream(port_b.Xi_outflow)))-inStream(port_a.h_outflow));
end FixedApproach;

Buildings.Fluids.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 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 that is implemented in the EnergyPlus building energy simulation program.

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)
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
Pressure	dp_nominal		Pressure [Pa]
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]
Initialization
MassFlowRate	m_flow.start	0	Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressure	dp.start	0	Pressure difference between port_a and port_b [Pa]
Temperature_degC	TWatIn_degC.start	35	Water inlet temperature [degC]
Temperature_degC	TWatOut_degC.start	28	Water outlet temperature [degC]
Temperature_degC	TAirIn_degC.start	25	Air dry-bulb inlet temperature [degC]
Assumptions
Boolean	allowFlowReversal	system.allowFlowReversal	= true to allow flow reversal, false restricts to design direction (port_a -> port_b)
Advanced
MassFlowRate	m_flow_small	1E-4*m_flow_nominal	Small mass flow rate for regularization of zero flow [kg/s]
Diagnostics
Boolean	show_V_flow	true	= true, if volume flow rate at inflowing port is computed
Initialization
AbsolutePressure	p_a_start	system.p_start	Guess value for inlet pressure [Pa]
AbsolutePressure	p_b_start	p_a_start	Guess value for outlet pressure [Pa]
Flow resistance
Boolean	from_dp	true	= 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

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.Fluids.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 "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.temperature(sta_a) - Medium.temperature(sta_b);
  // 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.Functions.spliceFunction(
                                                 pos=TAppCor, neg=TAppFreCon,
         x=y-yMin/2, deltax=yMin/2);
  TWatOut_degC = TApp + TAirIn_degC;
  PFan = y^3 * PFan0;
end YorkCalc;