Cooling Tower

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Contents

General Description

The cooling Tower has two calculation modes:

  1. Simple -- This is a very basic water evaporation model. The models assumes the cooling effect comes from water evaporation only and does not take into account the heat exchanged with air flow or tower design.
  2. Merkel -- The implementation of the Merkel method allows the user to obtain some tower design characteristics and calculate the tower outlet temperature based on the Air Wet bulb temperature and Liquid to Gas mass flow ratio.

The cooling tower unit can pass back downstream liquid out flowrate Demand, based on the feed to cooling tower and total losses, it can calculate the water make up required through a demand feeder. For information on how to set up the cooling tower in demand mode, please see Hints and comments.


Note: The Cooling Tower Project and Evaporation Project, which are distributed with SysCAD in the Examples folder, demonstrate the use of this model in a SysCAD project.

Diagram

CoolingTower01.png

The diagram shows the default drawing of the cooling tower, with the required connecting streams. The unit will not operate unless all of the above streams are connected. There are two optional output connections for the loss streams.

The physical location of the connections is not important; the user may connect the streams to any position on the drawing.

Inputs and Outputs

Label Required
Optional
Input
Output
Number of Connections Description
Min Max.
Feed 1 Required In 1 20 The warm water feed.
Vapour 1 Required Out 1 1 The evaporation loss.
LiquorLoss Optional Out   1 Cooling tower water loss.
DriftLoss Optional Out   1 Cooling tower drift loss.
Liquor 1 Required Out 1 1 The cooled water outlet.


Behaviour when Model is OFF

If the user disables the unit, by un-ticking the On tick box, then the following actions occur:

  • No no evaporation or cooling will occur, all cooling water feed will report to the "Liquor" output stream.

Model Theory

Simple Method

The way this model works is to cool the water inlet by water evaporation. The user is required to specify the air wet bulb temperature, and the approach temperature to this wet bulb temperature. Enough water is then evaporated to achieve this.

Please NOTE: This method does NOT take into account the heat loss by contact with air. It is cooling by water evaporation only. For a better estimation of heat balance, please use the Merkel method.

Merkel Method

The warm water entering the tower is cooled by transferring Sensible and latent heat from water droplets to the surrounding air.

Merkel has developed a method to analyse this heat transfer base on the enthalpy potential difference as the driving force. Please refer to references for the full theory. However, the equations implemented by SysCAD will be briefly outlined below for quick reference. NB Cooling towers are generally analyzed on the basis of cooling per unit of tower internal ground area.

The integrated form of the Merkel equation is:

 \mathbf {\mathrm{\frac{KaV}{L} = \int\limits_{T2}^{T1}\frac{CdT}{h_w-h_a}}}

Where:

KaV/L = tower characteristic
K = mass transfer coefficient (lb water/h ft2)
a = contact area/tower volume (ft2/ft3)
V = active cooling volume/plan area (ft3/ft2)
L = water loading, mass flow of liquid per unit of ground area into the top of the tower (lb/h ft2)
C = specific heat of water (assumed to be a constant 1.0 BTU/lb°F)
T1 = hot water temperature (°F)
T2 = cold water temperature (°F)
T = bulk water temperature (°F)
hw = enthalpy of air-water vapour mixture at bulk water temperature (Btu/lb dry air)
ha = enthalpy of air-water vapour mixture at wet bulb temperature (Btu/lb dry air)


Thermodynamics also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air, thus:

 \mathbf{\mathrm{\frac{L}{G} = \frac{h_2-h_1}{C(T_1-T_2)}}}

where:

G = air loading, mass flow of dry air per unit of ground area into bottom of the tower (lb/h ft2)
L/G = liquid to gas mass flow ratio (lb/lb or kg/kg)
C = specific heat of water (assumed to be a constant 1.0 BTU/lb°F)
T1 = hot water temperature (°F)
T2 = cold water temperature (°F)
h2 = enthalpy of air-water vapour mixture at exhaust wet-bulb temperature (Btu/lb dry air)
h1 = enthalpy of air-water vapour mixture at inlet wet-bulb temperature (Btu/lb dry air)


Using the above equations, the user can either solve for:

KaV/L -- by providing the L/G ratio, ambient wet bulb temperature, and water outlet temperature required (or the approach temperature).
Water outlet temperature -- by providing the L/G ratio, ambient wet bulb temperature, and tower characteristic (KaV/L).
NOTE: Tower characteristic values can be obtained through vendors or by looking up nomographs, such as the one found in Perry's Chemical Engineer's Handbook 6th edition page 12-15. Typical numbers used for mechanical draft cooling towers are:
L/G ranging from 0.75 to 1.5, and
KaV/L ranging from 0.5 to 2.5.

Water Losses

A number of methods are available to calculate the water losses. Water losses include evaporation, drift (water entrained in discharge vapour), and blowdown (water released to discard solids). See Perry's Chemical Engineer's Handbook for more information.

1) LossMethod: Drift and Blowdown

Evaporation Factor = 0.00085
Evaporation Loss = Evaporation Factor * water flowrate * (T1-T2) [T1 and T2 in °F]
Drift losses = typically 0.1 to 0.2% of water supply
Blowdown Loss = Evaporation Loss/(Cycles-1)
where Cycles = the ratio of total dissolved solids (TDS) in the circulating water to the TDS in the make-up water. Normal number of Cycles is between 3 and 7.
Total Losses = Evaporation Losses + Drift Losses + Blowdown Losses

2) LossMethod: None

There are no drift or blowdown losses.

3) LossMethod: Mass Fraction and Mass Flow

Can specify the required loss directly as fraction or flow. In addition, using FracOfLossToDrift, the amount of this loss that reports to drift (remainder goes to blowdown ) can be set.


Optional stream connections for losses

The output streams LiqLoss and DriftLoss are optional connections. The drift and blowdown losses report as follows depending on if these streams are connected:

  1. No LiqLoss and No DriftLoss : All losses exit with Liquor stream
  2. LiqLoss present and No DriftLoss : All losses exit with LiqLoss stream
  3. LiqLoss present and DriftLoss present : Blowdown reports to LiqLoss stream and drift reports to DriftLoss stream
  4. No LiqLoss and DriftLoss present : Blowdown reports to Liquor stream and drift reports to DriftLoss stream

Assumptions, Limitations and comments

  1. The Simple method only accounts for the cooling effect of water evaporation in the cooling tower. Cooling by airflow is not accounted for neither is the tower design.
  2. The feed stream must contain water.

For Merkel method:

  1. The merkel method uses the air enthalpy difference to calculate the water outlet temperature; SysCAD does this internally using hardwired air enthalpy equations. Therefore, the user does not need to put in an air stream to the cooling tower. The required air flowrate is calculated from the required L/G ratio.
  2. The maximum valid air temperature (for air enthalpy calculation) is 70dC or 158dF.
  3. The ambient wet bulb temperature is required as an input. The cooling tower model in SysCAD does not handle relative humidity and so on.
  4. It is important to check for the validity of L/G and KaV/L values from nomographs otherwise you may have conditions where a solution cannot be found.
  5. L/G ratio is the actual ratio; design L/G and tower efficiency have not been accounted for.

For Air Water Mixture Estimates:

  1. The airflow to the cooling tower is not an actual connection on the flowsheet, but rather it is an estimate of what it should be based on user specified L/G ratio. The air-water mixture outlet properties are also estimated using user specified air feed conditions.
  2. If the psychrometric charts are handy, user should refer to it for wet bulb temperature and air humidity information.


References

  1. Perry et al Perry's Chemical Engineers' Handbook 6th or 7th Edition, pp 12-12 to 12-17, McGraw-Hill 1984
  2. DQ Kern Process Heat Transfer, Chapt 17, McGraw-Hill 1983 (this book gives an excellent development of the equations used)
  3. Merkel, F., Forschungsarb.. 275, 1-48 (1925)

Data Sections

The default sections and variable names are described in detail in the following table. The default Cooling Tower access window consists of 3 sections:

  1. CoolTwr tab - This first tab contains general information relating to the unit
  2. Info tab - contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  3. Links tab - contains a summary table for all the input and output streams.
  4. Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.


Cooling Tower Page

Class: CoolTwr - The first tab page in the access window will have this name.

Symbol / Tag

Input / Calc

Description/Calculated Variables / Options

Common First Data Section

Requirements

On Tick Box If this option is deselected, the cooling tower will not be operational and thus all material will report to the liquor stream.
Method Simple The cooling is provided by water evaporation only and does NOT take into account the heat exchange with air.
Merkel See model theory.

Characteristics

Merkel Method
CalcType KaV/L The water outlet temperature is calculated from the KaV/L.
OutletT The tower characteristics are calculated from required water outlet temperature.
AirWetBulbT Input The ambient air wet bulb temperature.
ApproachTemp / ApproachT Input/Calc The difference between the water outlet temperature and the ambient air wet bulb temperature. This is an Input when using the KaV/L method, and it is calculated when using the OutletT method.
LG_Ratio Input L/G - The liquid to Gas mass flow ratio.
KaVL Input/Calc KaV/L - The tower characteristic (see model theory). This is an calculated when using the KaV/L method, and it is an input when using the OutletT method.
FeedQm Display The mass flowrate of the tower inlet is displayed.
TempFeed / Feed.T Display The feed water temperature.
TempDrop / TDrop Calc The temperature change between the Feed and the Outlet temperatures.
FinalT Calc The water outlet temperature.
HeatTransfer Calc The amount of energy transferred to heat up the air stream.
FinalP Calc The pressure of the cooling tower. NOTE: the cooling tower works at atmospheric pressure.
Simple Method
AirWetBulbT Input The ambient air wet bulb temperature.
ApproachTemp / ApproachT Input The difference between the water outlet temperature and the ambient air wet bulb temperature.
FeedQm Display The mass flowrate of the tower inlet is displayed.
TempFeed / Feed.T Display The feed water temperature.
TempDrop / TDrop Calc The temperature change between the Feed and the Outlet temperatures.
FinalT Calc The water outlet temperature.
FinalP Calc The pressure of the cooling tower. NOTE: the cooling tower works at atmospheric pressure.

Water Loss / Makeup

LossMethod None No water loss.
MassFrac Specify the total water loss required as a mass fraction (does not include the evaporation loss).
MassFlow Specify total water loss required as mass flow (does not include the evaporation loss).
Drift&Blowdown Alternative method to specify separate drift and blowdown losses.
RqdLossFrac Input Only available when MassFrac Loss Method is selected. Total loss required specified as fraction of feed.
RqdLossQm Input Only available when MassFlow Loss Method is selected. Total loss required specified as a flow rate.
FracOfLossToDrift Input Only available when MassFlow or MassFrac Loss Method is selected. Portion of total loss that reports to drift loss, the remainder reports to blowdown (or liquid) loss.
DriftLoss Input Only available when the Drift&Blowdown method is selected. (0.1 - 0.2)
Cycles Input This is the ratio of total dissolved solids (TDS) in the circulating water to the TDS in the make-up water. Normal number of Cycles is between 3 and 7, but must be greater than 2. This is only available when the Drift and Blowdown method is selected.
MaxEvapFrac Input This is for the simple method only, where the user can limit the % of water evaporation. The minimum amount the user can set is 1%. NOTE: if this is used, the outlet temperature may not meet specifications.
EvapFactor Input This is for the merkel method only. The default is 0.00085 (see model theory).
DriftLossQm Calc Loss due to Drift in mass flow.
BlowdownLossQm Calc The blowdown loss in mass flow.
LossQm Calc Total loss mass flow (sum of drift and blowdown loss).
EvapLossQm Calc The mass flow of water evaporated.
TotalLossQm Calc The mass of total water loss including evaporation, thus, can be used as water makeup requirements.
WaterVapQm / EvapQm Calc The mass flow of the total vapour.
WaterVapFrac Calc The percent feed water loss through evaporation.

Air-Water Mixture Estimates

(Only visible with the Merkel method.)

AirEnthOut Calc The enthalpy of exhaust air-water vapour mixture, which can used to look up the Air-Water mixture outlet wet bulb T from the psychometric charts.
HeatAvailable Calc The amount of heat available to heat up the air stream.
AirQm Calc The air mass flowrate is calculated based on liquid feed flowrate and user specified L/G ratio.
AirCp Input The air Cp is used to estimate the Air outlet Temp.
AirInDryBulbT Input The air inlet dry bulb temperature is used to estimate the Air outlet temp.
AirTRise Calc This is the temperature rise assuming HeatAvailable is used to heat up the dry air only.
AiroutT Calc This is the temperature out assuming HeatAvailable is used to heat up the dry air only.
AirWaterMixQm Calc This is the estimate of the total air-water vapour mixture flow rate. NOTE that the airflow is not an actual stream in SysCAD.
AirWaterMixCpEst Calc This is an estimate of the air-water vapour mixture Cp. Using a simple mass weighted mean calculation.
AirWaterMixTEst Calc This is an estimate of the air-water vapour mixture Temperature. Using a simple mass weighted mean calculation. NOTE: This would serve as an estimate for the air-water outlet wet bulb T, if the psychrometric charts were not at hand.

General Demand

(Only visible if Liquid Outlet pipe (or downstream pipe) has demand specified)

Liquor.DemandQm Calc Displays the Liquor Outlet flowrate required (as specified in the outlet pipe Qm_Demand.Rqd.
Liquor.Qm Calc Displays the Liquor Outlet actual flowrate
Feed.DemandQm Calc Displays the Makeup stream flowrate into the cooling tower. (This flow is from the feeder with Demand.On selected.)
Feed.FixedQm Calc Displays the remaining feed rates into the cooling tower (less the make up stream).

Hints and Comments

Setting the Cooling Tower Water Makeup on demand mode:

CoolingTower02.png

  1. Set up one feeder with Demand.On, any remaining feeder must have feed rates defined or cross-page connected.
  2. Set up the Cooling Tower Liquor Out pipe with a demand flowrate in the Qm_Demand.Rqd field. (This can be on a downstream pipe if preferred, the demand will pass back to this pipe.)
  3. SysCAD will then calculate how much makeup water is required.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

 

DLL:

HeatXch1.dll

Units/Links

Heat Transfer: Cooling Tower

or

Group:

Energy Transfer

Units/Links

Heat Transfer: Cooling Tower

See Project Configuration for more information on adding models to the configuration file.


Insert into Project

 

Insert Unit

Heat Transfer

Cooling Tower

See Insert Unit for general information on inserting units.

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