muhammad jawhar

1. 1
SalahaddinUniversity
College of Engineering
Mechanical and Mechatronics Department
Types Of Cooling Tower.A Review
Prepared by:
Ahmed Naseh latif
Ismail Farhad Kareem
Ibrahim Abdullah Ahmed
Muhammad Abdullah
Grade 4th
Academic year (2020-2021)
Supervised by
Dr. Ramzi Raphael IbraheemBarwari
(Assistant Professor)
Erbil-Kurdistan

2. 2
Types Of Cooling Tower.A Review
Abstract
Closed cooling systems use dry, wet or hybrid cooling towers. In order to compare these three
types of cooling towers together, the design of a cooling tower that can cool 100 MW is
performed for each type. The dimensioning takes into account the heat and mass transfer for the
thermal part and the pressure drop on the air and water sides for the hydraulic part. The two
elements of comparison used are electricity consumption and water loss. The airflows needed for
the required cooling for each type of cooling tower are calculated and the electric extraction
power of this air and that of the water pumping system is evaluated. The water loss is also
evaluated for only the wet type and the hybrid type. The application is made for several regions
in Algeria considering the most aggressive climates. The results presented in this work relate to
the highland region.
Key words: Cooling towers, Dry, Wet, Hybrid, Heat transfer, Electric power, Water loss,
Consumption.
Nomenclature:
Contain

3. 3
1-Introduction
Large power reactors [1] [2] use dry, wet or hybrid cooling towers when the cooling system is
closed. The choice of a type of cooling tower depends essentially on the characteristics of the site
where they will be installed and especially the costs generated. Two systems generate the
majority of the operating costs: the first system comprises the water pumping circuit and the
water spray circuit and the second system the air extraction circuit. The technology of the three
types of cooling towers differs from one model to another. In a dry cooling tower simple tubes
provide heat transfer between water inside the tubes and air in a cross-flow configuration ensures
the cooling of this water. In contrast to wet cooling towers where the contact is direct between
water and air within a packing for a water film flow. The performance of this cooling tower is
better because the heat transfer is increased by a mass transfer and the temperature of the humid
bulb of the air is the reference temperature for the calculations. For hybrid cooling towers, the
flow as in dry cooling towers is taken, to which is added a water spraying system as used in wet
cooling towers. The cooling is even better than for the other two types of cooling towers. The
study presented here deals with a comparison of these three types of cooling towers. The
equation of the thermal and hydraulic design of each cooling tower type is presented. The
original work involved a battery of cooling towers for the evacuation of a 1,000 MW. This
battery consists of ten cooling towers of 100 MW each one and an additional number to ensure a
given redundancy. So the load that will be used for this study is 100 MW for each cooling tower
type. When the design is finalized, the power consumption for each type will be evaluated and
the water loss generated for the completion of the cooling process will be evaluated.
2. Procedure for dimensioning the wet cooling tower
In figure (1), the design of a wet cooling tower is presented. For the mathematical model, the
equations governing the energy transferred by water and the one recovered by air are given
respectively by equations (1) and (2):
𝑄𝑐 = 𝑚𝑐𝜌𝑐𝐶𝑝𝑐Δ𝑡𝑐
𝑑𝑄𝑎 = 𝑚𝑎𝑑ℎ𝑎
(1) & (2)
Where Qc and Qa are the energiestransmittedbywater andair,c anda the waterand air densities,CPc
the specificheatof water,tc the watertemperature difference andthe airenthalpydifference.
Figure 1: Wet coolingtower

4. 4
In orderto evaluate the airflowforthe requiredcooling,the Merkelmethod [3] isused.Thismethod
takesintoaccount the heatand mass transferperformancesonone handandthe performancesof the
packingon the otherhand[4], it isgivenbyequation(3)
∫
ℎ𝑒𝑐
ℎ𝑠𝑐
𝑑ℎ𝑐
(ℎ𝑠 − ℎ𝑎)
=
𝛽𝜌𝐺 𝑎𝐻
𝑚𝑐
(3)
Where β isthe masstransfercoefficient,Gthe air density,the surface volume of the packing,the height
of the packing,mc the massflowrate of water,hecand hsc are respectivelythe enthalpyof wateratthe
entryand at the exitof the coolingtower.
The data requiredforthe calculationsare:the massflow rate of water,the inletandoutlet
temperaturesof wateraswell asthe temperature of dryand humidairbulb.Thisdata isrepresented
graphicallyonFigure (2):
To evaluate (hs–ha) as a functionof the enthalpyof water,several flowratiosare considered.Foreach
flowratio,the enthalpyof the airat the outletiscalculatedusingequation(4) toplotthe variationinthe
operatingline.
ℎ𝑓,𝑠 =
𝑚𝑐
𝑚𝑎
∗ 𝐶𝑝𝑐 ∗ (ℎ𝑐,𝑒 − ℎ𝑐,𝑠 ) (4)
We thenevaluate (hs- ha) foreachflowratiobytakingthe average overfourpointsof the evolutionline
of the enthalpyof wateraccordingtoBritishStandardBS4485. Once the valuesof (hs–ha) are evaluated
for all flowratios,we canthencalculate the termon the leftof equation(3) by:
𝐼𝑀 =
ℎ𝑐,𝑒 − ℎ𝑐,𝑆
ℎ𝑠 − ℎ𝑎
(5)
The evolutioncurve of IMisplottedasa functionof the flow ratiosfor the firstpart of equation(5).The
secondpart issolvedbythe same way,ie for several flow ratios,isevaluatedfromitsequivalent
equation[3]:
𝐼𝑝 =
𝛽𝜌𝐺𝑎𝐻
𝑚𝑐
= 𝜆 ∗ 𝐻 ∗ (
𝑚𝑐
𝑚𝑎
)
−𝑛
(6)

5. 5
Where 𝜆and n are coefficientsreferringtothe packingused,determinedexperimentallyintestcellsor
foundfromtwooperatingpointsof 𝐼𝑝&𝐼𝑚 the coolingtower.The solutionpointSisthe intersection
pointof and , the soughtflowratioand the air flow neededforthe requiredcoolingisthen:
𝑚𝑎 =
𝑚𝑐
𝑠
(7)
Once the air flowisknown,one can thendefine the dimensionsof the coolingtower.Afterwards,a
pressure losscalculationisdone toknow the characteristicsof the airextractionsystem.The pumping
systemcan alsobe calculated.Toevaluate the waterlossthe systemof twoequations(8) and(9) is
solvedaccordingto the difference of enthalpyof airbetweenentryandexitof the coolingtowerand
thusto knowexactlythe amountof water lostbymass transfer.
3. Procedure for dimensioning a dry cooling tower
In a dry coolingtower,figure (3),the contactisnot directbetweenthe twofluids(waterandair).The
water(hotfluid) flowsinsidefinnedtubesandthe air(coldfluid) flowsoutside these tubesasina cross-
flowheatexchanger.Anairextractionsystemisinstalleddownstreamof the airoutletof thisheat
exchanger.The dimensioningof suchatoweramountsto the dimensioningof awater-airheat
exchanger.The procedure isknownandgivesgoodresults.

6. 6
Where,mc andma are the massflowrates of the twofluidsCpcthe specificheatof the hot fluid,Cpa
the specificheatof the coldfluid,Δtc andΔta are the temperature difference betweenthe inletandthe
outletof the two fluids,tcmandtam are the average temperaturesof the twofluids,Uref the reference
heatexchange coefficientandSexchange isthe exchangesurface.
The convectionheattransfercoefficientinsidethe tubesαi iscalculatedaccordingtoGnielinski [5]:
𝛼𝑖 =
𝑁𝑢𝑐∗ 𝐾𝐶
𝑑𝑖
(13)
The numberof Nusseltisgivenbythe followingexpression
The expressiongivingthe Reynoldsnumberforasimple cross-currentexchangerisgivenbythe
followingformula:
𝑅𝑒𝑐 = 4 ∗
𝑚𝑐
𝜋 ∗ 𝑑𝑖 ∗ 𝑁𝑡𝑜𝑡 ∗ 𝜇𝑐
(15)
Where Rec isthe Reynoldsnumberforthe hotside,di the inside diameterof tubes,Ntotthe total
numberof tubesand the dynamicviscosity.Forasmoothturbulentflow,the Darcycoefficient(Ω) in
equation(14) isdependantof the value of the Reynoldsnumber.
The convective heatexchange coefficientoutside the tubesiscalculatedaccordingtothe arrangement
of the tubesinthe beam.For a staggeredbeamPFR[6],it is givenby:
𝛼𝑒 = 0.29 ∗
𝑘𝑓
𝑑𝑒
∗ 𝑅𝑒𝑓
0.633
∗ 𝑃𝑟
𝑓
1
3
∗ (
𝑆𝑎
𝑆𝑡𝑜𝑡
)
−0.17
(18)
Where kf is the transfercoefficientof the coldfluid(air), de the outside diameter,Sathe finssurface
and Stotthe total exchange surface.
The Reynoldsnumberof the coldfluidRef isgivenby:

7. 7
Withand respectivelythe transversepitchrelative tothe outertube diameterandthe longitudinal pitch
relative tothe outertube diameter.The characteristiclengthof the flow Γisgivenby.
Once the twoconvectioncoefficientscalculated,one canthenevaluate the global exchangecoefficient
usingequation(22):
𝑈 = [(
1
𝛼𝑖
+ ℜ𝑖)
𝑆𝑒
𝑆𝑖
+
𝑆𝑒
2 ∗ 𝜋 ∗ 𝑘𝑡∗𝑙
Ln (
𝑑𝑒
𝑑𝑖
) +
1
𝜂𝑔𝛼𝑒
+ ℜ𝑒]
−1
(22)
Where and are respectivelythe foulingfilmresistance forthe innerside andthe outerside,the fin
efficiencygivenby,ξ the anisothermycoefficientgivenby,and the finnedsurface global efficiency
givenby,
The fincorrectedlenght[5] isgivenby(23):
𝑙𝑎𝑐 = 0.5𝑑𝑒(𝐷𝑎
∗ − 1)[1 + 0.35 Ln(𝐷𝑎
∗ )] (23)
With(
𝑑𝑎
𝑑𝑒
)the diameterof the finrelatedtothe hydraulicdiameterand
𝑆𝑒
2∗𝜋∗𝑘𝑡∗𝑙
∗−1+0.5 ∗2−1
Where w,ail isthe thicknessof the finandk,ail the finheatexchange coefficient,The exchange surface
of finsalone isgivenby:
𝑆𝑎𝑠 = 𝑆𝑎 − 𝜋𝑑𝑒(1 − 𝑤𝑎𝑖𝑙 ∗ 𝑁𝑎𝑖𝑙) (24)
The efficiencyof the drycoolingtowerisgivenbythe equation(25) deducedbythe NUTmethod:
𝜀 =
1 − exp (−
𝑈+𝑆𝑡𝑜𝑡
𝑚𝑎
∗ 𝐶𝑝𝑎
(1 +
𝑚𝑎∗𝑐𝑝𝑎
𝑚𝑐∗𝐶𝑝𝑐
))
1 +
𝑚𝑎∗𝐶𝑝𝑎
𝑚𝑐∗𝐶𝑝𝑐
∗ 100 (25)

8. 8
Once the dimensionsof the drycoolingtowerfixed,the pressure dropcanbe evaluatedforthe air
extractionsystemandforthe waterpumpingsystem.
4. Procedure for dimensioning an hybrid cooling tower
A hybridcoolingtowerhasa water-to-airheatexchangerasina dry coolingtoweranda waterspraying
systemasin a wetcoolingtower,asshowninfigure (4).Hot waterflowsinside finnedtubesandair
flowsfrombottomto topto cool these tubes.The waterspraywaterflowsfromtopto bottomand at
the same time coolsdry air andfinnedtubes.A double use of thishybridtoweristhenpossible eitherin
a completelydrysystemorina wetsystembutwithindirectcontactbetweenthe twocoolants.
In orderto evaluate watertemperature inthe heatexchanger,acorrelationsmodel isused.Thismodel
was firstintroducedby[7].Inthismodel,the whole surface of the tubesissupposedtobe wet;this
assumptionisvalidfora uniformsprayingof the water.The variationof the temperature of the water
spray can be evaluated;however,the temperatureof the watersprayisconsideredequal tothe
temperature of the water/airfilminterface.The variationof the temperature of waterinsidetubescan
be calculatedthanksto the equilibriumequationbetweenwaterandair.
Variationsof watertemperature andairenthalpyare givenrespectivelybyequations(26) and(27):
𝑑𝑇𝑐
𝑑𝐴
=
𝑈
𝑚
˙
𝑐𝐶𝑝𝑐
(𝑇𝑐 − 𝑇𝑠𝑝)
𝑑ℎ𝑎
𝑑𝐴
=
𝛽
𝑚
˙
𝑓
(ℎ𝑖 − ℎ𝑎)
(26)&(27)
Where 𝑇𝑠𝑝 isthe water spraytemperature and ℎ𝑖 the enthalpyof the water/airfilminterfaceThe
evolutionof waterspraytemperatureiscalculatedbyneglectingthe evaporationof water.

9. 9
𝑑𝑇𝑠𝑝
𝑑𝐴
=
𝑚
˙
𝑎
𝑚
˙
𝑠𝑝𝐶𝑝𝑠𝑝
𝑑ℎ𝑎
𝑑𝐴
−
𝑚
˙
𝑐𝐶𝑝𝑐
𝑚
˙
𝑠𝑝𝐶𝑝𝑠𝑝
𝑑𝑇𝑐
𝑑𝐴
(28)
The evolutionof airmoisture rate canbe calculatedby:
𝑑𝑤𝑎
𝑑𝐴
=
𝛽
𝑚
˙
𝑎
(𝑤𝑖 − 𝑤𝑎) (29)
Where 𝑤𝑖isthe moisture rate at the water/airfilminterface.The global heatexchange coefficientis
givenby:
1
𝑈
=
1
𝛼𝑐
𝑑𝑒
𝑑𝑖
+
𝑑𝑒
2𝑘𝑡
ln (
𝑑𝑒
𝑑𝑖
) +
1
𝛼𝑠𝑝
(30)
Where 𝛼𝑠𝑝 isthe water filmconvectiveheatexchangecoefficientgivenby:
For a staggeredtube configurationandaturbulentflow,the Nusseltnumberisgivenby:
𝑁𝑢𝑒𝑎𝑢 =
(
𝑓𝐷
8
)(𝑅𝑒𝑐 − 1000) Pr𝑐 (1 + (
𝑑𝑖
𝐿
)
0.67
)
1 + 12.7(
𝑓𝐷
8
)
0.5
(𝑃𝑟𝐶
0.67
− 1)
(32)
For an internal flowinasmoothtube,the coefficientof friction 𝑓𝐷 isgivenby:
𝑓𝐷 = (1.82log10Reaue − 1.64)^ − 2
For a staggeredtube configurationwithapitchof 2Dext, the masstransfercoefficientβisgivenby:
𝛽 = 5.027 ∗ 10−8(𝑅𝑒𝑎)0.9(𝑅𝑒𝑠𝑝)
0.15
(𝐷𝑒)−2.6 (33)
The Reynoldsnumber 𝑅𝑒𝑠𝑝 forwatersprayis evaluatedby:
𝑅𝑒𝑠𝑝 =
𝑚
˙
𝑠𝑝
𝑁𝑡𝑢𝑏𝑒𝐿𝜇𝑠𝑝
(34)
Water lossesare evaluatedinthe same wayasfor wet coolingtowers.
Once the dimensional characteristicsare known,the pressure dropcanbe evaluated.We couldthus
size the air extractionsystem, the hotwaterpumpingsystemandfinallythe coldwatersprayingsystem.

10. 10
5. Energy consumption
In an industrial coolingsystem,mainenergyconsumersare pumpsandfans.Pumpsensure the
circulationof waterand fansensure the circulationof air.
The electrical pumppowerusedinthissystemiscalculatedbythe followingformula:
𝑃ele =
Δ𝑝𝑐
𝜂𝑡𝑝
𝜌𝑔𝑚𝑐 (35)
Where𝑃ele is the electrical power, 𝜌 the densityof the fluid, 𝑚𝑐the waterflow, Δ𝑝𝑐 the total pressure
inthe waterside, 𝑔the gravityaccelerationand 𝜂𝑡𝑝the pumpefficiency.Forthe fanpower,the same
equationisused.The airdensityistakenequal to1 kg/m3 :
𝑃ele =
Δ𝑝𝑓
𝜂𝑡𝑓
𝑔𝑚𝑓 (36)
Where 𝑚𝑓isthe air flow, Δ𝑝𝑓 the total pressure inthe airside and 𝜂𝑡𝑓 the fanefficiency.
6. Loss of water by evaporation
This isthe amountof waterevaporatedtoensure cooling.Itismainlyafunctionof the transferredheat.
As a firstapproximation,itisequal to1% of the flow of watercirculatinginthe coolingtower.Thiswater
lossisevaluated,assaid,byresolvingequations(8) and(9) or itis givenby[3][8]:
𝜔 =
𝜑𝑎𝑠 − 𝜑𝑎𝑒
𝑚 𝑐
𝑚𝑎
(37)
With 𝜑𝑎𝑠 and 𝜑𝑎𝑠 respectivelythe rate of waterinair respectivelyatthe exitandat the entrance of the
coolingtower
7. Applications and obtained results:
By applying the already detailed sizing methods, the obtained results for each type of cooling tower are
presentedhere after:
- Dry cooling towers: the study is done on a cooling system consisting of 155 tubes per row and 36 rows,
these tubes have an inside/outside diameter of 300/320 mm and a length of 40 m, the tubes
arrangement is staggered at an angle of 60°, the transverse pitch and the longitudinal pitch are
calculated from the equilateral tubes arrangement and depending on the dimensions of the tubes and
the geometry of the bundle of tubes, the material used is copper, these tubes carry aluminum fins in an
integral construction. They have the following dimensions: height of 100 mm for 1mm of thickness, their
number is 300 fins per unit of length, the necessary power to ensure the water circulation is 286.2
kWatt. It is assumed that the efficiency of the pump is 75%, the electric power is therefore 408.8 kWatt.
The power of the air extraction system is 5.047 MWatt. In the same way, it is assumed that the selected
fans have a yield of 80% so the electrical power will be 7.21Mwatt. - Wet cooling towers: The study is
made for four types of packing, of dimensions 5 m long, 8 m wide and 2 m high, for a heat exchange of
about 100 MW. The spacing between the plates of the packing is 10 mm. These dimensions were

11. 11
applied to the four types of packing. The water losses are 19.18, 23.93, 17.03 and 22.32 liters/s for types
(i), (e), (d) and (c) described in [9] respectively. The power consumed by the ventilation system with a
fan efficiency of 80% is 5.4 MW. Considering that, the calculation of the pumping systemis mainly at the
level of the heat exchangers, it will be neglected for the wet system. - Hybrid cooling towers: the hybrid
cooling system consists of 12 rows containing 25 tubes each. The tubes are made of stainless steel, 320
mm outside diameter and 300 mm inside diameter. The electrical power required for the operation of
the cooling tower is 1.58 MW for the water pumping system, 0.539 MW for the spray system and 1.4
MW for the air extractionsystemThe obtainedresultsforthese applicationsare presentedhere after:
Figure (1) shows the evolution of electric power as a function of air flow. One notes that electrical power
increases when air flow increases. The electric power depends on the type of the cooling tower used.
For hybrid and dry cooling towers, electrical power evolves in the same way with a fixed value that
separates them, because these two cooling towers have the same geometry. However, it is found that
the power in the case of hybrid cooling tower is higher than the one of dry cooling tower because of
losses due to air-waterfilm contact. For the case of wet cooling tower, the power is much higher than
the othertypes;thismeansthat the air pressure dropinthiskindof coolingtoweristhe larger.

12. 12
8. Conclusions
Each of the three cooling tower types offers advantages and disadvantages. For dry cooling towers,
energy consumption is the largest among the cooling towers studied, but they offer the advantage of
zero water loss. For wet cooling towers, the heat and mass transfer offers the advantage of having a
more compact geometry than the dry cooling tower which requires a large exchange surface; the energy
consumption is lower than the dry cooling towers but larger than that of hybrid cooling towers. Closed-
circuit hybrid cooling towers require less space than a dry cooling tower but more space than a wet
cooling tower. The main advantage of this type of cooling tower is their minimal energy consumption
amongthe three coolingtowersstudied,theycanbe usedinbothhumidordry environment.

13. 13
9. References
[1] L.E. Echávarri,L’énergie nucléaire aujourd’hui,Agence pourl’énergieNucléaire,(2003).
[2] ***, Nuclear Power Technology Development Section Division of Nuclear Power , Department of
Nuclear Energy, , Status of Small and Medium Sized Reactor Designs, International Atomic Energy
Agency,(2011).
[3] K. Sidi-Ali, Dimensioning of wet Cooling towers for nuclear power plants, original title in french:
dimensionnement des tours de Refroidissement humides des centrales électronucléaires, SPRUA, Ain-
Oussera(1998).
[4] K. Sidi-Ali, Effects of packing depth on the main parameters of a wet cooling tower, original title in
french: Effets de la profondeur du garnissage sur les principaux paramètres d’une tour de
refroidissementhumide,SIPE,Bechar(2002).
[5] V. Gnielinski, New equation for heat and mass transfer in turbulent pipe and channel flow, Int.
Chem.Eng.,vol.16, (1976).
[6] PFR Engineering Systems, Heat transfer and pressure drop characteristics of dry towers extended
surfaces. Part II: DATA analysis and correlation”, Calif. Marina del, Rey: PFR, Rapport d'étude n°BNWL-
BFR-7-102. (1976)
[7] Mizushina, R.I.T, Miyashita, H, Characteristics And Methods Of Thermal Design Of Evaporative
Coolers,Int.Chemical Engineering8(3).(1968)
[8] Stoecker, WF and Jones, JW, Cooling towers and evaporative condensers, Refrigerating and air
conditionning,McGraw-hill ISE,(1982).
[9] H.J. Lowe, D.G. Christie, Heat transfer and pressure drop data on cooling tower packings and model
studiesof the resistance of natural draughttowers,IHTCColorado,(1961).