Induced Draft Cooling Towers

8/2/2019 Induced Draft Cooling Towers

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IND ED DRAFT LIN T WER

(Fundamentals & Applications)

Power Management Institute,Noida

- -

DR. S. S. KACHHWAHAAssistant Professor

(Head, Training & Placement)

Delhi College of Engineering,Delhi-110 042


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Mechanism of Heat and Mass Transfer

Discussions Recent Developments

Conclusions


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Cooling Tower Zones

Heat and Mass Transfer Mechanism


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Classification

a ura ra

Induced Draft


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Cooling Tower Zones

Fill Zone


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Spray Zone

of water over the fill material

To develop spray pattern:e g o spray zone = . m nc


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Non-Uniformity in Spray Zone

Th li i n f w r n zzl

developing circular spray patterns results innon-uniform flow through the tower packing,thus limiting performance.

A method for determining nozzle depositionpro es resu ng n op mum per ormance ora specific packing configuration should be

Both thermal performance and uniformityshould be o timized.


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Non-uniformity in spray zone


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Fill Zone

Classification

(b) Trickle Fill

c m


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Fill Zone Fouling Al ae and bacteria (biolo ical rowth)

Colloidal material transported in the recirculation water Air borne dirt or particles

or suspen e so s n e ma e-up wa er

Scaling due to dissolved materials carried in solution

When selecting a particular fill for a cooling system, it isimportant not only to consider initial performance

performance and fouling characteristics.


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Rain Zone

Rain zone is required in a cooling tower to permituniform airflow into the fill.

Inefficient portion of the cooling tower (10 to 20% oftotal heat and mass interaction only in large sizetowers)

Droplets and jets are formed due to dripping of waterfrom the sheet of the fill.

Droplet radius in rain zone is large as compared tot at n spray zone.

For a 100 ton blow through tower:Rain zone = 0.90 m 36 inch ;

Fill zone = 0.90 m (36 inch);Spray zone = 0.45 m (18 inch).


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Terminologies

Approach

Cooling Load Zones of Cooling Tower


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Conclusions


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Modes

Convective heat transfer


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Merkel Method

)( mamaswfrfidma ii

Aahdi=

Water Temperature

a

dz

di

cm

m

dz

dt ma

pww

aw 1=

===wi

wo

t

t mamasw

wpw

w

fifid

w

fifrfid

Mii

dtc

G

Lah

m

LAahMe

)(


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Assumptions

The value of Lewis factor Le relatin heat and mass

transfer for air-water vapor system is equal to 1.

The air leaving the cooling tower is saturated withwater vapor and it is characterized only by itsenthal .

The reduction of water flow rate by evaporation is

neglected.


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Conclusions


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Design and Performance

Analysis

Objectives of Model DevelopmentDevelopment of a simple and efficient mathematical model

(a) for estimating heat and mass transfer between hot waterand air stream,

b to enable an accurate rediction of coolin towerperformance and fan power simultaneously with availableempirical relations for pressure drop.

Manufacturer (to design the cooling tower system)

User (to cross check the specifications)


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Induced Draft Counter Flow Cooling Tower with Geometry


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Problem Formulation

D i n An l i

Given Water mass flow rate

Inlet water temperature

Coolin ran e Air inlet temperature (WBT & DBT)

To calculate

Air mass flow rate Fill size


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Required Equations

,

Draft Equation (with Fan Characteristics)

Empirical Equations


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Energy Equation

The amount of heat transferred,q (J/s) to the air stream fromthe circulating water is expressed by energy equation as

q = mw . cpwm . (twi two) = ma (imas5 ima1)

imasw5 = enthalpy of saturated air-vapor at 5

ima1 = enthalpy of air-vapor at cooling tower inlet

The amount of water lost due to evaporation [mw(evap)] is givenb

mw(evap) = (mav5 mav1)


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Draft E uationThe Draft Equation obtained by matching fan performance curve

(Kilfi+ Krzfi+ Kfsfi+ Kfi+ Kspfi+ Kwdfi+ Kdefi+ Kctfi+ Kupfi) x(mav15/Afr)

2/(2 av15) (KFs(mav5/Ac)2/ (2 av6) = 0

where K = denotes the loss coefficient and

m = avera e air-va or mass flow rate between 1 and 5

Afr = frontal area of the fill

av15 = harmonic mean density of air-vapor = 2 /(1/av1+1/av5)= -av

Ac = area of the fan casing

av6 = density of air-vapor at 6.


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Loss Coefficient due to Inlet Louvers (Kilfi )

The specified loss coefficient due to inlet louvers (K ilfi ) referred

Kilfi = Kil (av15/av1){(Wi.Bi) / (2H3.Wi)} (mav1/mav15)2

where Kil denotes loss coefficient for inlet louvers and

av1 = density of air-vapor at 1

Wi = tower inlet widthBi = tower breadth or length

H3 = tower inlet height

m = air-va or mass flow rate u stream of fill


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rz

rzfi

conditions through the fill is given byKrzfi = Krz. (av15/av1). (mav1/mav15)2

where Krz = loss coefficient for the rain zone


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fsfi

(Kfsfi) referred to the mean conditions through the fill is given by

Kfsfi

= Kfs. (

av15/

av1). (m

av1/m

av15)2

fs =


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Suitable fans for mechanical draft cooling towers areselected based to a large extent, on the loss coefficient of

.

An inaccurate representation of the loss coefficients in the

form of empirical relations can have financial implications ift e coo ng tower oes not meet es gn spec cat ons.


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Fill Loss Coefficient (Kfdm

)

= bdl cdlm . . w a

w dl, dl, dl p

a given fill.


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Loss Coefficient Correlation For Fill (contd.)

Pressure drop is coupled with the loss coefficient

pfi = Kfi . v2/2Kfi = c1Gw

c2Gac3 + c4Gw

c4Gac6

(form drag) (viscous drag)

This e uation will enerall correlate measured ressure losscoefficients accurately for all types of fills under all types of practicaloperating conditions as it make provision for a spectrum of forces due

to shear and drag. Film fill empirical relations:

Kfdml =19.658921 Gw0.281255Ga

0.175177

Kfdml =3.897830 Gw0.777271Ga

0.215975 + 15.327472Gw0.215975Ga

0.079696


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Loss Coefficient Correlation For Fill contd.

Precautions in selecting the correlations

Range and applicability of Gw and Ga

correlation coefficient to compensate for any uncertainties.

Same water spray system must be employed in the fill testan e su sequen app ca on o e o e m na e eeffect of drop size and elimination on the loss coefficient.


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Actual Fill Loss Coefficient (Kfi

)

(Kfi ) applicable to cooling tower is given by

Kfi

= Kfdm

+ [(Gav5

2/av5

) - (Gav1

2/av1

)] / (Gav5

2/av15

)

where Gav1 = mass velocity of air-vapor at 1 [G = m / Afr]

Gav5 = mass velocity of air-vapor at 5


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Loss Coefficient throu h the S ra Zone K

(Ks fi) above the fill referred to the mean conditions through thefill is given by

Kspfi = Lsp[0.4(Gw/Ga) + 1].(av15/av5). (mav5/mav15)2

Lsp = height of the spray zone

Gw = mass velocity of water based on frontal area of filla = mass ve oc y o ry a r ase on ron a area o e


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w

(Kwdfi) referred to the mean conditions through the fill is given by

Kwdfi = Kwd (av15/av5). (mav5/mav15)2

where Kwd = loss coefficient for water distribution system


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Loss Coefficient for Drift Eliminator K

(Kdefi) based on the fill conditions is given by

Kdefi = ade. Rybde. (av15/av5). (mav5/mav15)2

Ry = characteristic flow parameter = m / (. Afr )

In the present case, commercially available type c drifteliminator has been selected for which

=

bde = 0.14247


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Inlet Loss Coefficient K

(Kct(norz) )for an induced draft, isotropically packed, rectangularcooling tower is given by

Kct(norz) = 0.2339 + (3.919 x10-3 Kfie2 6.84 x10-2 Kfie + 2.5267) xexp[Wi{0.5143 0.1803 exp(0.0163 Kfi)}/H3]

sinh-1[2.77 exp(0.958 Wi/H3)

exp{Kfie(2.4571.015 Wi/H3) x 10-2}(ri/Wi 0.013028)]

where the effective loss coefficient in the vicinity of the fill (Kfie) isgiven by

Kfie = Kfsfi + Kfi +Kspfi + Kwdfi + Kdefi


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Fan Upstream Loss Coefficient (Kupfi)

The specified fan upstream loss coefficient (Kupfi) referredto mean conditions through the fill is given by

Kupfi = Kup. (av15/av5). (mav5/mav15)2.(Afr/Ac)2

where Kup = fan upstream losses


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a

pa5 = pa1[1(0.009754(H3 +Lfi/2)/ta1]3.5(1+w1)(1-w1/(w1+0.622))

(Kilfi+Krzfi +Kfsfi +Kfi +Kspfi +Kwdfi +Kdefi +Kctfi) x

(mav15/Afr)2/ (2 av15)

Here, it is assumed that the air-vapor leaving the cooling tower issaturated.


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Fan Power EquationsThe actual air volume flow rate (VF, m

3/s) through the fan is given by

VF = mav5/av5

As actual air density and rotational speed of the fan are not thesame as the reference conditions for which fan performancecharacteristics were specified, the relevant fan laws are employed.Accordingly, air volume flow rate (VF/dif, m

3/s) is given by

VF/dif = VF.(NFr/ NF) . (dFr/dF)3

where NFr = reference fan rotational speed (r/min)

F

dFr = test fan diameter (m) and dF = fan diameter (m)


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Fan Power Equations (contd.)

The reference fan static pressure difference (pF/dif, N/m2) is giveny

pF/dif = 320.85 6.9604 VF/dif + 0.31373 VF/dif2 0.021393 VF/dif3 The actual fan static pressure difference (pFs, N/m2) is given by

pFs = pF/dif.(NF/ NFr)2 . (dF/dFr)2.(av6/r)

The fan shaft power at reference conditions (PF/dif, W) is given by

PF/dif = 4245.1 64.134 VF/dif + 17.586 VF/dif2 0.71079 VF/dif3 The actual fan shaft power (PF,W) is given by

PF= PF/dif.(NF/ NFr)3 . (dF/dFr)

5.(av6/r)

The static ressure rise coefficient of the fan K is

KF/difs= 2.pFs.av6/ [mav5/Ac]2


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otal Transfer Coefficienttwi dtcLahLAah

+++

=

===

iiii

pwpwpwpwwowi

t mamasw

wpw

ww

r

M

wo

iiGmMe

)4()3()2()1(

4)(

( )

4

ttc wowipwm =

+++

iiii)4()3()2()1(

1111

tw(1) = two + 0.1 (twi two)

= w(2) wo . wi wo

tw(3) = two + 0.6 (twi two)

t = t + 0.9 t t


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Transfer coefficient in Rain Zone (Merz) of the cooling tower from isgiven by

0.33rz . a v. a. w . a,in. d . rz d .

x ln[(ws+ 0.622)/(w + 0.622)] /(ws w)

x {5.01334.b1.

a

192121.7. b2

.a

2.57724 + 23.61842

x . 3.va,in. + . x . 4. rz

- . + .

x [43.0696 (b4. dd)0.7947 + 0.52]


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Formulations for three zones contd.

Transfer coefficient in fill zone (Mefi) of cooling tower for anyfill is given by

Mefi = ad. Lfi.Gwbd Ga cd

The coefficients a b and c are taken from the fill data

Transfer coefficient in spray zone (Mesp) of the cooling toweris given by

0.5sp . sp . a w

Total transfer characteristic of cooling tower (MeT ) is givenby

MeT = Merz + Mefi + Mesp


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Exer etic E uationsLimitations of conventional studies

Based on law of conservation of energy. Ener anal sis alone rovides no

information of energy transfer from the best

possible way (only a quantity of energytransfer).

It is insufficient to indicate some aspects of

energy utilization and may be misleading.


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Exergetic Equations

Law of Degradable of Energy (Exergy Analysis)

Powerful concept of exergy to fulfill of incompleteness

Exergy is a measure of the usefulness, quantity or potential of

energy to cause change, and it appears to be an effective measureof the potential of system to impact the environment

Importance

tower in various inlet air conditions performing thermodynamicallyvaluable.


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Exer of Water

The exergy (W) of water is given by

Xw = mw[(hfw hfwr) tr.(sfw sfwr) Rv. tr .ln (r)]

where r

= pa.w/(0.622 + w).p

vsand h and s represent enthalpy and entropy of waterrespectively.


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Exer of Air- a or

Exergy of air-vapor is sum of exergy of dry air and exergy.

Specific exergy of dry air (J/kg) is given bya = [xa.(cpa/Ma).{ t tr tr. ln(t/tr) } + (R/Ma).tr.(p/pr) +a . r. xa. n xa xar

Specific exergy of vapor is given by

v = x . c /M . t t t . ln t/t + R/M .t . / +(R/Mv).tr. xv. ln (xv/xvr)]

Usin above e uations, exer of air-va or mixture

becomesXav = ma [a + v]


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Exergy Balance

Total exergy entering = Total exergy leaving + destroyedexer

Total exergy entering = (Xwi + Xavi + Xwimakeup)

Total exergy leaving = (Xwo + Xavo)

xergy estruct on d s g ven y

Xd = (Xwi + Xavi + Xwimakeup) (Xwo + Xavo)


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Second Law Efficiency ( )

II = 1 [(Xd / (Xwi + Xavi + Xwimakeup)]

Th l Effi i ( )


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Thermal Efficiency (th)

efficiency of evaporative cooling is given by

th = wi wo wi wb


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Discussions

Recent Developments

Conclusions


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DISCUSSIONS

Input Parameters

Air/water conditions

Atmospheric pressure at ground level 1(Pa),pa1 101325.000

Water inlet temperature (K),twi 314.65

Water outlet tem erature K t 303.47

Inlet water mass flow rate(kg/s),mw 412.0000

Inlet air dr bulb tem erature K t 306.65

Inlet air wet bulb temperature(K),twb1 298.1500

Inp t P r meters (contd )


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Input Parameters (contd.)

Geometric parameters

Tower height,H9 (m) 12.5

Fan height,H6 (m) 9.5

Tower inlet height,H3 (m) 4.0

Tower inlet width,Wi (m) 12.0

Tower breadth or length, Bi

(m) 12.0

Fill height (m),Lfi 1.878

Height of the spray zone(m),Lsp 0.5

Inlet rounding (m),ri 0.025 Wi

Plenum chamber height (m),Hpl 2.4


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In ut Parameters contd.

an parame ers

Fan diameter(m),dF 8.0

Fan rotational speed (r/min),NF 120

es an ame er m , Fr .

Reference rotational speed (r/min),NFr 750

Reference air density (kg/m3), r 1.2


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In ut Parameters contd.Other specifications

Mean droplet diameter in rain zone, dd (m) 0.0035

, il .

Loss coefficient for fill support ,K 0.5

Loss coefficient for water distribution system, Kwd 0.5

Fan upstream losses, Kup 0.52


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In ut Parameters contd.

Guess Values

-cooling tower, mav15 (kg/s)

w

Pressure at 5, pa5, (N/m2) pa1

S.No. Calculated values (Output)


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1. Average mass flow rate of air-vapor(kg/s),mav15 441.7592

2. Pressure of air at 5 upstream of fan(Pa),pa5 101170.321

3. Air dry/wet bulb temperature at 5(K),ta5 306.76

4. Transfer coefficient for the rain zone, Merz 0.264781

5. Transfer coefficient for the fill zone, Mefi 0.886219

6. Transfer coefficient for the spray zone, Mes 0.102264

7. Total transfer coefficient / Merkel number for thecooling tower, MeT

1.253264

8. Merkel number by Chebyshevs formula, MeC 1.27580

9. Actual fan shaft power (W),PF 69242.37

10. Water lost due to evaporation (kg/s),mwevap 7.4834

. av1 w .

12. Evaporation loss of water (kg/s), mwevap 1.8164

13. Exergy destruction (W), Xd 2260169.503

14. Second law efficiency, II 0.9204

15. Thermal efficiency of the cooling tower, th 0.6777

Effect of variation in wet bulb


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Effect of variation in wet bulb

temperature of inlet air

twb1(K)

ta5(K) mav1/mw

mwevap( % )

PF(W) th

Xd(W) II

Run 1

. . . . . . .

Run 2

294.15 303.9592 1.0752 1.9399 70082.76 0.5455 3498385 0.877

296.15 305.3476 1.0693 1.8785 69668.17 0.6044 2877996 0.8987

Run 4 298.15 306.7647 1.0631 1.8164 69242.37 0.6777 2260170 0.9204

300.15 308.2106 1.0568 1.7535 68804.93 0.7712 1646612 0.9419

Run 6 302.15 309.6848 1.0503 1.6898 68355.37 0.8946 1039240 0.9633

Run 7 303.4677 310.6714 1.0459 1.6476 68052.33 1 643454.6 0.9773

Air outlet temperature v/s wet bulb temperature of inlet air


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Air outlet temperature v/s wet bulb temperature of inlet air

310

312

304

306

308

ttemperature(K)

300

302

Airoutle

292.15 294.15 296.15 298.15 300.15 302.15 303.468

Wet bulb temprature of inlet air(K)

Inlet mass flow rate ratio v/s wet bulb


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Inlet mass flow rate ratio v/s wet bulb


1.08

1.09

1.05

1.06

1.07

s

flowrateratio

1.03

1.04

Inletma

.

292.15 294.15 296.15 298.15 300.15 302.15 303.468


Thermal efficiency v/s wet bulb


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Thermal efficiency v/s wet bulb


1

1.2

0.6

0.8

alefficiency

0.2

0.4Ther

292.15 294.15 296.15 298.15 300.15 302.15 303.468


Exergy destruction v/s wet bulb


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Exergy destruction v/s wet bulb


3500000

4000000

4500000

1500000

2000000

2500000

3000000

y

destruction(W

0

500000

1000000

5 5 5 5 5 5 68

Exer

292.

294.

296.

298.

300.

302.

303.


Second law efficiency v/s wet bulb


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Second law efficiency v/s wet bulb


0.96

0.98

1

0.88

0.9

0.92

0.94

d

lawefficiency

0.8

0.82

0.84

0.86

Seco

.

292.15 294.15 296.15 298.15 300.15 302.15 303.468


The variation of air conditions


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The variation of air conditions

Second law efficiency and exergy destruction


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y gy

to the variation of inlet dry bulb temperature.

Dry air flow rate required to the variation


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y q

of inlet dry bulb temperature



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to the variation of inlet relative humidity.

Exergy change of water and air to the


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gy g

variation of inlet relative humidity.

Dry air flow rate re uired to the variation


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Dry air flow rate re uired to the variation

of inlet relative humidity


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Discussions

Recent Developments

Conclusions

Recent Develo ments


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towers.

[SCT].

Limitations of Conventional Cooling Towers


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Lower water temperature drop and performance degradation withtime due to foulin .

Higher power consumption and noise of motor.

Fills are easy to get blocked due to salt deposition and.

Electric fans are easy to be damaged.

Unstable cooling effect.

Difficulty for the fills to be replaced and cleaned.

Tend to age, change, embrittle, crack and jam, so the technicale ui ment and the i in are ammed with fra ment debris,affecting the distribution of air and water greatly.

Shower Cooling Tower (SCT)


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Breakthrough

Fill are eliminated completely and tiny water

droplets replace the fill as the mode of heatan mass rans er.

Better heat and mass transfer promotion.

Performance Characteristics of SCT


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In a SCT, efficient low pressure atomization devices replacethe conventional fill so the resistance of the coolin mediumin the tower decreases considerably.

The duration of heat transfer between the water and the air inthe counter flow SCT is lon er and hence the effect on thetemperature drop is better.

Tiny droplets causes large contact surface area with the cool

,increases greatly.

The synchronous reliability of SCT components is better, so,

and the operation life span can extend to 15 years.

Principle of Shower Cooling Towers(SCT)


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Cooling effect of SCT depends on the following factors:

The ratio of the mass flow rate of dry air to that of water

(same working conditions this ratio increase 15-20% for SCT).

the tower.

The retention time of hot water droplet inside the tower.


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Discussions

Recent Developments

Conclusions

Conclusion


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For a given cooling tower load (mass flow rate of water andcooling range,the model successfully predicts the air outletconditions, fan power requirements, make up waterrequirements and various evaluation parameters such asmass flow rate ratio, thermal efficiency of cooling tower,

exergy destruction and second law efficiency.

overall performance of the induced draft cooling tower. From parametric study, it may be concluded that increase in

wet bulb temperature of inlet air causes increase in air outletempera ure, erma e c ency an secon aw e c ency andecrease in inlet mass flow rate ratio,evaporation loss, fanpower requirements and exergy destruction.

Dro let diameter in the rain zone has no si nificant role in theperformance of cooling tower.

The present model can be successfully applied for airconditioning and power plant applications for wide range of

.


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Documents

Induced Draft Cooling Towers