Cooling Tower

From SysCAD Documentation

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Contents 1 General Description 1.1 Diagram 2 Inputs and Outputs 3 Model Theory 3.1 Simple Method 3.2 Merkel Method 3.3 Water Make-up 3.4 Assumptions, Limitations and comments 3.5 References 4 Data Sections 4.1 Requirements 4.2 Characteristics 4.3 Water Loss / Makeup 4.4 Air-Water Mixture Estimates 5 Adding this Model to a Project

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.

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

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	Input / Output	No. of Connections		Description
		Min	Max
Feed	In	1	10	The warm water feed.
Vapour	Out	1	1	The evaporation loss.
LiquorLoss	Out		1	Cooling tower water loss.
DriftLoss	Out		1	Cooling tower drift loss.
Liquor	Out	1	1	The cooled water outlet.

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.

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 Reference 1 for the full theory. However, the equations implemented by SysCAD will be briefly outlined below for quick reference.

The integrated form of the Merkel equation is:

Where:

KaV/L = tower characteristic

K = mass transfer coefficient (lb water/h ft2)

a = contact area/tower volume (ft/ft)

V = active cooling volume/plan area (ft/ft)

L = water rate (lb/h ft²)

T₁ = hot water temperature (°F)

T₂ = cold water temperature (°F)

T = bulk water temperature (°F)

h_w = enthalpy of air-water vapour mixture at bulk water temperature (Btu/lb dry air)

h_a = 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:

where:

L/G = liquid to gas mass flow ratio (lb/lb or kg/kg)

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 characteristics.

NOTE: Tower characteristics 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 Make-up

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 is the ratio of solids in the circulating water to the solids in the make-up water

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 6^th or 7^th Edition, pp 12-12 to 12-17, McGraw-Hill 1984

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. The first section, which has the name CoolTwr, contains general information relating to the unit
2. The Info section, contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents. This is fully described in Common Data Sections.
3. Links tab, only visible in SysCAD 9.2, 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.

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

Tag / Symbol	Input / Calc	Description/Calculated Variables / Options
Common First Data Section
Requirements
Method	List Box	Simple -- The cooling is provided by water evaporation only.
		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.
ApproachT	Input/Calc	The approach temperature to 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.
Feed.T	Display	The feed water temperature.
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.
ApproachT	Input	The approach temperature to ambient air wet bulb temperature.
FeedQm	Display	The mass flowrate of the tower inlet is displayed.
Feed.T	Display	The feed water temperature.
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.
	Mass Frac	Specify the total water loss required as a mass fraction (does not include the evaporation loss).
	Mass Flow	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 Mass Frac Loss Method is selected. Total loss required specified as fraction of feed.
RqdLossQm	Input	Only available when Mass Flow Loss Method is selected. Total loss required specified as a flow rate.
FracOfLossToDrift	Input	Only available when Mass Flow or Mass Frac 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	Only available when the Drift&Blowdown method is selected. (>=2)
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.
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.

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

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Heat Transfer

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Cooling Tower

See Insert Unit for general information on inserting units.