Cooling Tower

Meshal Khaled Al-SaeedMeshal Khaled Al-Saeed

423105653423105653  

Dr. Malik Al-AhmadDr. Malik Al-Ahmad

King Saud UniversityKing Saud UniversityCollege of EngineeringCollege of EngineeringChemical Engineering Dept.Chemical Engineering Dept.

Chemical Engineering LaboratoryChemical Engineering LaboratoryChE 403ChE 403

IntroductionIntroduction The cooling tower is one of the most important device in The cooling tower is one of the most important device in

chemical industries for example when the hot water come chemical industries for example when the hot water come from heat exchanger we use the cooling tower to cool it.from heat exchanger we use the cooling tower to cool it.

The purpose of cooling tower is to cool relatively warm The purpose of cooling tower is to cool relatively warm water by contacting with unsaturated air. The evaporation of water by contacting with unsaturated air. The evaporation of water mainly provides cooling.water mainly provides cooling.

In a typical water cooling water tower, warm water flows In a typical water cooling water tower, warm water flows countercurrent to an air stream. Typically, the warm water countercurrent to an air stream. Typically, the warm water enters the top of packed tower and cascades down through enters the top of packed tower and cascades down through the packing, leaving at the bottom.the packing, leaving at the bottom.

Air enters at the bottom of the tower and flows upward Air enters at the bottom of the tower and flows upward through the descending water. The tower packing often through the descending water. The tower packing often consists of slats of plastic or of packed bed. The water is consists of slats of plastic or of packed bed. The water is distributed by troughs and overflows to cascade over slat distributed by troughs and overflows to cascade over slat gratings or packing that provides large interfacial areas of gratings or packing that provides large interfacial areas of contact between the water and air in the form of droplets contact between the water and air in the form of droplets and films of water. The flow of air upward through the and films of water. The flow of air upward through the tower can be induced by the buoyancy of the warm air in tower can be induced by the buoyancy of the warm air in the tower (natural draft) or by the action of a fan.the tower (natural draft) or by the action of a fan.

The water cannot be cooled below the wet bulb The water cannot be cooled below the wet bulb temperature. The driving force for the evaporation of the temperature. The driving force for the evaporation of the water is approximately the vapor pressure of the water less water is approximately the vapor pressure of the water less the vapor pressure it would have at the wet bulb the vapor pressure it would have at the wet bulb temperature.temperature.

Theory Theory Overall Mass Balance:Overall Mass Balance:

In Put = Out PutIn Put = Out Put

LL22 + G + G11 = L = L11 + G + G22 → (1) → (1)

Water Mass Balance:Water Mass Balance:

LL22 - L - L11 = G = G22 * H * H22 - G - G11 * H * H11 → (2) → (2)

LL22 - L - L11 = G * (H = G * (H22 - H - H11) → (3)) → (3)

Energy Balance:Energy Balance:

Q = G * (HQ = G * (HG2G2 - H - HG1G1) → (4)) → (4)

GG11 = = * * airair → (5) → (5)

GG22 = G = G11 * H * H22 → (6) → (6)

HHGG = Cs * (T - T = Cs * (T - Too) + ) + oo * H → (7) * H → (7)

Cs = Cp Cs = Cp LiquidLiquid + Cp + Cp VaporVapor * H * H → (8)→ (8)

To determine the number of transfer unit (NTU):To determine the number of transfer unit (NTU):

To calculate height of transfer unit (HTU):To calculate height of transfer unit (HTU):

To calculate the mean driving force: To calculate the mean driving force:

Simpson’s rule:Simpson’s rule:

G2

G1

H

H GGi

G

HH

H dNTU

G1

G2

G1G2LM

HH

ln

HHΔH

)]f(x)4f(x)[f(x3

hf(x)dx 21o

x

x

2

o

.Pa.MK

GHTU

BG

(NTU) (HTU) Z ZdZ

0

Where:Where: LL22: : Flow rate of water in [kg/s.mFlow rate of water in [kg/s.m22].]. LL11: : Flow rate of water out [kg/s.mFlow rate of water out [kg/s.m22].]. GG11: : Air in [kg/s.mAir in [kg/s.m22].]. GG22: : Air out [kg/s.mAir out [kg/s.m22].]. : : Volumetric flow rate of air.Volumetric flow rate of air. H: H: Humidity of air [kg water/kg air].Humidity of air [kg water/kg air]. HHGG: : Enthalpy of the air [J/kg air].Enthalpy of the air [J/kg air]. KKGGa: a: Mass transfer coefficient of air [kg mol/s.mMass transfer coefficient of air [kg mol/s.m33.Pa]..Pa]. NTU:NTU: Number of transfer unit [dimensionless]. Number of transfer unit [dimensionless]. HTU:HTU: Height of transfer unit [m]. Height of transfer unit [m].

Schematic Apparatus Schematic Apparatus

Figure 1: Photo of cooling tower.Figure 1: Photo of cooling tower.

Figure 2: Schematic apparatus for cooling tower.Figure 2: Schematic apparatus for cooling tower.

ResultsResultsTable1: Temperature at Q = 1.0 kWTable1: Temperature at Q = 1.0 kW

Flow rate of water = 40 g/secFlow rate of water = 40 g/sec Initial pressure = 31 mm HInitial pressure = 31 mm H22OO Final pressure = 38 mm HFinal pressure = 38 mm H22OO Pressure drop = 7 mm HPressure drop = 7 mm H22OO Volume of evaporation water at 20 min = 1027 mlVolume of evaporation water at 20 min = 1027 ml

TimeTime(min)(min)

Air inAir in Air outAir out WaterWater

TT11

((ºCºC)) TT22

((ºCºC)) TT33

((ºCºC)) TT44

((ºCºC)) TT55

((ºCºC)) TT66

((ºCºC))

55212119192222222231312323

1010212119192020212129292222

1515212119192020212129292121

2020212119192020212129292121

Table2: Temperature at Q = 1.5 kWTable2: Temperature at Q = 1.5 kW

Flow rate of water = 40 g/secFlow rate of water = 40 g/sec Initial pressure = 31 mm HInitial pressure = 31 mm H22OO Final pressure = 38 mm HFinal pressure = 38 mm H22OO Pressure drop = 7 mm HPressure drop = 7 mm H22OO Volume of evaporation water at 20 min = 1025 mlVolume of evaporation water at 20 min = 1025 ml

TimeTime(min)(min)

Air inAir in Air outAir out WaterWater

TT11

((ºCºC)) TT22

((ºCºC)) TT33

((ºCºC)) TT44

((ºCºC)) TT55

((ºCºC)) TT66

((ºCºC))

00222219192222222231312121

55212119192323242432322222

1010212119192424242432322121

1515212119192424252532322222

2020212119192424252533332222

Where:Where: TT11:: Dry bulb temperature in. Dry bulb temperature in.

TT22:: Wet bulb temperature in. Wet bulb temperature in.

TT33:: Dry bulb temperature out. Dry bulb temperature out.

TT44:: Wet bulb temperature out. Wet bulb temperature out.

TT55:: Water temperature input. Water temperature input.

TT66:: Water temperature output. Water temperature output.

ConclusionConclusion

From conclusion point view:From conclusion point view:Cooling tower is used to cool relatively hot water.Cooling tower is used to cool relatively hot water.As the humidity of the inlet air decreased, the performance As the humidity of the inlet air decreased, the performance

of the cooling tower will be better. This leads to the better of the cooling tower will be better. This leads to the better mass transfer between water and gas phase. mass transfer between water and gas phase.

As the temperature of the inlet air decreased, the As the temperature of the inlet air decreased, the performance of the cooling tower will be better.performance of the cooling tower will be better.

As the temperature increased overall mass transfer As the temperature increased overall mass transfer coefficient Kcoefficient KGGa increased.a increased.

If the air flow rate is increased, the height of the cooling If the air flow rate is increased, the height of the cooling tower decrease.tower decrease.

RecommendationsRecommendations

It is better to open the windows or doors in the It is better to open the windows or doors in the lap to refresh the air and to make a good lap to refresh the air and to make a good deference in the driving force.deference in the driving force.

SummarySummary The main objective of this experiment is to perform mass and The main objective of this experiment is to perform mass and

energy balances over a cooling tower and to determine the mean energy balances over a cooling tower and to determine the mean driving force, the number of transfer units and the overall mass driving force, the number of transfer units and the overall mass transfer coefficient.transfer coefficient.

In this experiment can be calculated:In this experiment can be calculated: The outlets water for a typical cooling tower and compare The outlets water for a typical cooling tower and compare

it with the measured value.it with the measured value. The rate of heat transfer.The rate of heat transfer. The mean driving force.The mean driving force.

Also can be get HAlso can be get HGiGi from the Temperature Enthalpy diagram and from the Temperature Enthalpy diagram and the operating line then get the number of transfer unit (NTU) by the operating line then get the number of transfer unit (NTU) by determine the area under the curve then find the overall mass determine the area under the curve then find the overall mass transfer coefficient.transfer coefficient.

Results:Results: At Q = 1.0 kWAt Q = 1.0 kW LL11 = 0.04 kg/s = 0.04 kg/s Q = 0.524 kWQ = 0.524 kW NTU = 0.285NTU = 0.285 HTU = 1.68 mHTU = 1.68 m ∆ ∆HHGG Lm Lm = 49.23 kJ/kg dry air = 49.23 kJ/kg dry air KKGGa = 2.9 e-6 kg mol/s.ma = 2.9 e-6 kg mol/s.m22

At Q = 1.5 kWAt Q = 1.5 kW LL11 = 0.0401 kg/s = 0.0401 kg/s Q = 1.25 kWQ = 1.25 kW NTU = 0.0879NTU = 0.0879 HTU = 5.523 mHTU = 5.523 m ∆ ∆HHGG Lm Lm = 60.39 kJ/kg = 60.39 kJ/kg KKGGa = 9.73 e-7 kg mol/s.ma = 9.73 e-7 kg mol/s.m22

ReferencesReferences1. Chirstie J. Geankoplis, "Transfer Process and Unit 1. Chirstie J. Geankoplis, "Transfer Process and Unit

Operation", Operation", 33rdrd edition. edition.2. Dep. of Chemical Eng “Chemical Engineering 2. Dep. of Chemical Eng “Chemical Engineering

Laboratory II Manual”.Laboratory II Manual”.3. Perry’s, "Chemical Engineers Handbook", 3. Perry’s, "Chemical Engineers Handbook", 55thth edition. edition.4. From web side: 4. From web side: “ http://www.armfield.co.uk/ ”“ http://www.armfield.co.uk/ ”