Cooling Tower

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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.

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. Perry's Chemical Engineer's Handbook, 6^th or 7^th Edition, pp 12-12 to 12-17 has very good explanation on this topic. Please refer to it for full theory; however, the equations implemented by SysCAD will be briefly outlined below for quick reference.

The integrated form of the Merkel equation is:

File:Models-Cooling-Tower-image002.gif

[math]\displaystyle{ \mathbf {\mathrm{\frac{KaV}{L} = \int\limits_{T2}^{T1}\frac{dT}{h_w-h_a}}} }[/math]

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:

File:Models-Cooling-Tower-image004.gif

[math]\displaystyle{ \mathbf{\mathrm{\frac{L}{G} = \frac{h_2-h_1}{T_1-T_2}}} }[/math]

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 venders 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.

Data Sections

Tag / Symbol	Input / Calc	Description
Common First Data Section
Requirements:
Method	List Box	Simple -- The cooling is provided by water evaporation.
		Merkel -- See model theory.
Characteristics -- NOTE the availability of some of the following fields will be method dependent.
CalcType	List Box	Only available when Merkel method is selected.
		KaV/L -- Tower Characteristics is calculated from required water outlet T.
		Outlet T -- Water outlet temperature is calculated from the KaV/L.
AirWetBulbT	Input	The ambient air wet bulb temperature.
ApproachT	Input/Calc	The approach temperature to ambient air wet bulb temperature
LG_Ratio	Input	L/G - The liquid to Gas mass flow ratio.
KaVL	Input/Calc	KaV/L - The tower characteristic -- see model theory.
FeedQm	Feedback	The mass flowrate of the tower inlet is displayed.
TempFeed	Calc	The feed water temperature.
Tdrop	Calc	The cooling range.
FinalT	Calc	The water outlet temperature.
HeatTransfer	Calc	The amount of energy transfered to heat up the air stream. NOTE: Only applicable to Merkel method.
FinalP	Calc	The pressure of the cooling tower. NOTE: the cooling tower works at atmospheric pressure.
Water Loss/Makeup:
LossMethod	List Box	None -- No water loss.
		Mass Frac -- specify total water loss required as a mass fraction (does not include the evaporation loss).
		Mass Flow -- specify total water loss required as a mass mass flow (does not include the evaporation loss).
		Drift&Blowdown -- alternative method to specify required drift loss and blowdown loss.
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:
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 psychrometric 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.
AirWaterMix Qm	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.
AirWaterMix CpEst	Calc	This is an estimate of the air-water vapour mixture Cp. Using a simple mass weighted mean calculation.
AirWaterMix TEst	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.

Audit, fully described in Audit Section. See Model Examples for enthalpy calculation Examples.