modelCoolingTower
Information
This model represents a cooling tower with variable speed fans. If closedCircuit is false, the model
represents a cooling tower filled with metallic packing, where the incoming water is cooled by direct contact with humid air,
by means of evaporation. Dynamic models with Mp > 0 also represent heat storage in the packing and heat
transfer between the packing and the evaporating water.
If closedCircuit is true and Mp = 0, then the model represents a closed-circuit tower cooling an
external circuit, which is thermally connected through the 1D distributed heat port wallPort, representing the tube external surface.
The heat transfer between the packing or the tube surface and the evaporating water is computed by a specific
heat transfer coefficient, whose nominal value is gamma_wp_nom, and which varies with the nu_l-th power
of the liquid flow.
The mass and heat transfer from the hot water to humid ambient air is modelled according to Merkel's equation: the driving force for heat and mass transfer is the difference between the specific enthalpy of saturated humid air at the water temperature per unit dry air mass, and the specific enthalpy of saturated humid air at the wet bulb temperature Twb per unit dry air mass. The dry and wet bulb temperatures of incoming air are given by the settings of the system object.
The 1D counter-current heat and mass transfer equations are discretized by the finite volume method, with N-1 volumes; average quantities between volume inlet and volume outlet are used to compute the driving force of the mass and energy transfer.
Humid air is modelled as an ideal mixture of dry air and steam, using the IF97 water-steam model.
The hold-up of water in the packaging is modelled assuming a simple linear relationship between the hold-up in each volume and the corresponding outgoing flow, which is calibrated by the Mnom and M0 parameters. The energy storage in the water hold-up and in the packaging is accounted for.
The behaviour of the fan is modelled by kinematic similarity; the air flow is proportional to the fan rpm, while the consumption is proportional to the cube of the fan rpm.
Is it possible to neglect all dynamic behaviour and get a static model by setting staticModel=true.
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| Boolean | closedCircuit | false | true for closed-circuit tower |
| Boolean | staticModel | false | = true for a static model |
| Integer | Nt | 1 | Number of towers in parallel |
| Integer | N | 10 | Number of nodes |
| Integer | Nw | N - 1 | Number of volumes on the pipe wall |
| SI.MassFlowRate | wlnom | Nominal water mass flow rate (single tower) | |
| SI.VolumeFlowRate | qanom | Nominal air volume flow rate (single tower) | |
| SI.Density | rhoanom | Nominal air density | |
| SI.SpecificHeatCapacity | cp | 0 | Specific heat of packing |
| SI.Area | S | Surface of air/water mass and heat transfer (single tower) | |
| SI.CoefficientOfHeatTransfer | gamma_wp_nom | 0 | Nominal heat transfer coefficient beween water and packing or tubes |
| Real | k_wa_nom | Nominal total mass & heat transfer coefficient per unit surface (single tower) | |
| SI.PerUnit | nu_a | Exponent of air flow rate in mass & heat transfer coefficients | |
| SI.PerUnit | nu_l | Exponent of liquid flow rate in mass & heat transfer coefficients | |
| Real | rpm_nom | Nominal fan rotational speed [rpm] | |
| SI.Power | Wnom | Nominal power consumption (single tower) | |
| SI.Pressure | patm | 101325 | Atmospheric pressure |
| SI.MassFlowRate | wanom | qanom*rhoanom | Nominal air mass flow rate |
| Real | MMv | 0.029 | |
| Real | MMa | 0.018 | |
| Initialisation | |||
| ThermoPower.Choices.Init.Options | initOpt | system.initOpt | Initialization option |
| SI.Mass[N - 1] | Mstart | ones(N - 1)*Mnom/(N - 1) | Start value of water holdup in each volume |
| SI.SpecificEnthalpy[N] | hlstart | fill(120e3, N) | Start values of liquid enthalpy at volume boundaries |
| SI.Temperature[N - 1] | Tpstart | ones(N - 1)*(30 + 273.15) | Start value of packing temperature in each volume |
| SI.Temperature | Twbstart | system.T_wb | Start value of average wet bulb temperature |
| Dynamic model only | |||
| SI.Mass | M0 | 0 | Water hold-up at zero water flow rate (single column) |
| SI.Mass | Mnom | 0 | Water hold-up at nominal water flow rate (single tower) |
| SI.Mass | Mp | 0 | Mass of packing (single tower) |
Connectors
| Type | Name | Default | Description |
|---|---|---|---|
| ThermoPower.Water.FlangeA | waterInlet | ||
| ThermoPower.Water.FlangeB | waterOutlet | ||
| Modelica.Blocks.Interfaces.RealInput | fanRpm | Fan rotational speed in rpm | |
| Modelica.Blocks.Interfaces.RealOutput | powerConsumption | Total fan power consumption | |
| Thermal.DHTVolumes | tubeWalls | Interface to tube walls @ wall temperature |
Components
| Type | Name | Default | Description |
|---|---|---|---|
| ThermoPower.System | system | System object | |
| SI.MassFlowRate[N] | wl | Water flow rate | |
| SI.MassFlowRate | wa | Dry air flow rate | |
| SI.MassFlowRate[N - 1] | wev | Evaporation flow rate | |
| SI.VolumeFlowRate | qfan | Air volume flow rate through fan | |
| Water.SpecificEnthalpy[N] | hl | Water specific enthalpy | |
| DryAir.SpecificEnthalpy[N] | ha | Specific enthalpy of saturated humid air @ Twb, per unit dry mass | |
| SI.SpecificEnthalpy[N] | hw | Specific enthalpy of saturated humid air @ water temperature, per unit dry mass | |
| Water.SpecificEnthalpy[N] | hvw | Specific enthalpy of saturated steam at water temperature | |
| Water.SpecificEnthalpy[N] | hva | Specific enthalpy of saturated steam at interface (i.e. Twb) | |
| DryAir.SpecificEnthalpy[N] | haw | Dry air specific enthalpy @ liquid temperature | |
| DryAir.SpecificEnthalpy[N] | haa | Dry air specific enthalpy @ interface (i.e. Twb) | |
| Water.AbsolutePressure[N] | pvw | Saturation pressure at water temperature | |
| Water.AbsolutePressure[N] | pva | Saturation pressure at interface (i.e. Twb) | |
| SI.MassFraction[N] | Xvw | Absolute humidity (Mv/Ma_dry) of saturated air at the liquid temperature | |
| SI.MassFraction[N] | Xva | Absolute humidity (Mv/Ma_dry) of air at the interface | |
| Water.Temperature[N] | Tl | Water temperature at the nodes | |
| Water.Temperature[N - 1] | Tla | Average water temperature for each volume | |
| Water.Temperature[N] | Twb | Wet bulb temperature of air at interface | |
| SI.Mass[N - 1] | M | Water hold-up | |
| Water.SpecificEnthalpy[N - 1] | hltilde | Specific enthalpy state variable | |
| SI.Temperature[N - 1] | Tp | Temperature of packing or tube surface | |
| SI.Power[N - 1] | Qwp | Thermal power transfer between water and packing or tubes | |
| SI.Power[N - 1] | Q | Total thermal power transfer water -> air interface (@Twb) | |
| SI.Power | W | Power consumption of the fan (single column) | |
| SI.Power | Wtot | Power consumption of the fans (all columns) | |
| SI.CoefficientOfHeatTransfer[N] | gamma_wp | Heat transfer coefficient between water and packing or tubes | |
| Real | k_wa | Total evaporation heat transfer coefficient per unit surface | |
| SI.Temperature | Tlin | Inlet water temperature | |
| SI.Temperature | Tlout | Outlet water temperature |
Contents
| Name | Description |
|---|---|
 Water | |
 DryAir |
Revisions
- 28 Jul 2017 Francesco Casella:
First release.