Heat Exchanger DesignHeat Exchanger Design

Anand V P GurumoorthyAnand V P GurumoorthyAssociate ProfessorAssociate Professor

Chemical Engineering DivisionChemical Engineering DivisionSchool of Mechanical & Building SciencesSchool of Mechanical & Building Sciences

VIT UniversityVIT UniversityVellore, IndiaVellore, India

Heat Exchanger Heat Exchanger ClassificationClassification• Recuperative:Recuperative:

– Cold and hot fluid flow through the unit without Cold and hot fluid flow through the unit without mixing with each other. The transfer of heat mixing with each other. The transfer of heat occurs through the metal wall.occurs through the metal wall.

• Regenerative:Regenerative:– Same heating surface is alternately exposed to Same heating surface is alternately exposed to

hot and cold fluid. Heat from hot fluid is stored by hot and cold fluid. Heat from hot fluid is stored by packings or solids; this heat is passed over to the packings or solids; this heat is passed over to the cold fluid.cold fluid.

• Direct contact:Direct contact:– Hot and cold fluids are in direct contact and Hot and cold fluids are in direct contact and

mixing occurs among them; mass transfer and mixing occurs among them; mass transfer and heat transfer occur simultaneously.heat transfer occur simultaneously.

Heat Exchanger Standards and Heat Exchanger Standards and CodesCodes

• British Standard BS-3274British Standard BS-3274

• TEMA standards are universally used.TEMA standards are universally used.

• TEMA standards cover following classes of TEMA standards cover following classes of exchangers:exchangers:– Class R – designates severe requirements of Class R – designates severe requirements of

petroleum and other related processing applicationspetroleum and other related processing applications– Class C – moderate requirements of commercial and Class C – moderate requirements of commercial and

general process applicationsgeneral process applications– Class B – specifies design and fabrication for Class B – specifies design and fabrication for

chemical process service.chemical process service.

Shell and Tube Heat Shell and Tube Heat ExchangerExchanger• Most commonly used type of heat transfer Most commonly used type of heat transfer

equipment in the chemical and allied industries.equipment in the chemical and allied industries.• Advantages:Advantages:

– The configuration gives a large surface area in a The configuration gives a large surface area in a small volume.small volume.

– Good mechanical layout: a good shape for pressure Good mechanical layout: a good shape for pressure operation.operation.

– Uses well-established fabrication techniques.Uses well-established fabrication techniques.– Can be constructed from a wide range of materials.Can be constructed from a wide range of materials.– Easily cleaned.Easily cleaned.– Well established design procedures.Well established design procedures.

Types of Shell and Tube Heat Types of Shell and Tube Heat ExchangersExchangers

• Fixed tube designFixed tube design– Simplest and cheapest type.Simplest and cheapest type.– Tube bundle cannot be removed for cleaning.Tube bundle cannot be removed for cleaning.– No provision for differential expansion of shell and No provision for differential expansion of shell and

tubes.tubes.– Use of this type limited to temperature difference Use of this type limited to temperature difference

upto 80upto 8000C.C.

• Floating head designFloating head design– More versatile than fixed head exchangers.More versatile than fixed head exchangers.– Suitable for higher temperature differentials.Suitable for higher temperature differentials.– Bundles can be removed and cleaned (fouling liquids)Bundles can be removed and cleaned (fouling liquids)

Design of Shell and Tube Heat Design of Shell and Tube Heat ExchangersExchangers• Kern method:Kern method:

– Does not take into account bypass and leakage streams.Does not take into account bypass and leakage streams.– Simple to apply and accurate enough for preliminary design Simple to apply and accurate enough for preliminary design

calculations.calculations.– Restricted to a fixed baffle cut (25%).Restricted to a fixed baffle cut (25%).

• Bell-Delaware methodBell-Delaware method– Most widely used.Most widely used.– Takes into account:Takes into account:

• Leakage through the gaps between tubes and baffles and the Leakage through the gaps between tubes and baffles and the baffles and shell.baffles and shell.

• Bypassing of flow around the gap between tube bundle and shell.Bypassing of flow around the gap between tube bundle and shell.

• Stream Analysis method (by Tinker)Stream Analysis method (by Tinker)– More rigorous and generic.More rigorous and generic.– Best suited for computer calculations; basis for most Best suited for computer calculations; basis for most

commercial computer codes.commercial computer codes.

Construction Details – Tube Construction Details – Tube DimensionsDimensions• Tube diameters in the range 5/8 inch (16 mm) Tube diameters in the range 5/8 inch (16 mm)

to 2 inch (50 mm).to 2 inch (50 mm).• Smaller diameters (5/8 to 1 inch) preferred Smaller diameters (5/8 to 1 inch) preferred

since this gives compact and cheap heat since this gives compact and cheap heat exchangers.exchangers.

• Larger tubes for heavily fouling fluids.Larger tubes for heavily fouling fluids.• Steel tubes – BS 3606; Other tubes – BS 3274.Steel tubes – BS 3606; Other tubes – BS 3274.• Preferred tube lengths are 6 ft, 8 ft, 12 ft, 16 Preferred tube lengths are 6 ft, 8 ft, 12 ft, 16

ft, 20 ft and 24 ft; optimum tube length to ft, 20 ft and 24 ft; optimum tube length to shell diameter ratio ~ 5 – 10.shell diameter ratio ~ 5 – 10.

• ¾ in (19 mm) is a good starting trial tube ¾ in (19 mm) is a good starting trial tube diameter.diameter.

Construction Details – Tube Construction Details – Tube Arrangements Arrangements

• Tubes usually arranged in equilateral Tubes usually arranged in equilateral triangular, square or rotated square triangular, square or rotated square patterns.patterns.

• Tube pitch, PTube pitch, Ptt, is 1.25 times OD., is 1.25 times OD.

Construction Details - ShellsConstruction Details - Shells

• Shell should be a close fit to the tube Shell should be a close fit to the tube bundle to reduce bypassing.bundle to reduce bypassing.

• Shell-bundle clearance will depend Shell-bundle clearance will depend on type of heat exchanger.on type of heat exchanger.

Construction Details - Shell-Construction Details - Shell-Bundle ClearanceBundle Clearance

Construction Details – Tube Construction Details – Tube CountCount• Bundle diameter depends not only on number of tubes but also number of Bundle diameter depends not only on number of tubes but also number of

tube passes.tube passes.

• NNtt is the number of tubes is the number of tubes• DDbb is the bundle diameter (mm) is the bundle diameter (mm)• DD00 is tube outside diameter (mm) is tube outside diameter (mm)

• nn11 and K and K1 1 are constantsare constants

1/1

10

n

tb K

NdD

Construction Details - Construction Details - BafflesBaffles• Baffles are used:Baffles are used:

– To direct the fluid stream across the tubesTo direct the fluid stream across the tubes– To increase the fluid velocityTo increase the fluid velocity– To improve the rate of transferTo improve the rate of transfer

• Most commonly used baffle is the single segmental Most commonly used baffle is the single segmental baffle.baffle.

• Optimal baffle cut ~ 20-25%Optimal baffle cut ~ 20-25%

Basic Design ProcedureBasic Design Procedure

• General equation for heat transfer is:General equation for heat transfer is:

where Q is the rate of heat transfer (duty),where Q is the rate of heat transfer (duty),U is the overall heat transfer coefficient,U is the overall heat transfer coefficient,A is the area for heat transferA is the area for heat transfer

ΔΔTTmm is the mean temperature difference is the mean temperature difference• We are not doing a mechanical design, only We are not doing a mechanical design, only

a thermal design.a thermal design.

mTUAQ

Overall Heat Transfer Overall Heat Transfer CoefficientCoefficient

• Overall coefficient given by:Overall coefficient given by:

hh0 0 (h(hii) is outside (inside) film coefficient) is outside (inside) film coefficient

hhodod (h (hidid) is outside (inside) dirt coefficient) is outside (inside) dirt coefficient

kkww is the tube wall conductivity is the tube wall conductivity

ddoo (d (dii) is outside (inside) tube diameters) is outside (inside) tube diameters

iiidiw

i

od hd

d

hd

d

k

d

dd

hhU

11

2

ln111 00

00

00

Individual Film CoefficientsIndividual Film Coefficients

• Magnitude of individual coefficients will Magnitude of individual coefficients will depend on:depend on:– Nature of transfer processes (conduction, Nature of transfer processes (conduction,

convection, radiation, etc.)convection, radiation, etc.)– Physical properties of fluidsPhysical properties of fluids– Fluid flow ratesFluid flow rates– Physical layout of heat transfer surfacePhysical layout of heat transfer surface

• Physical layout cannot be determined until Physical layout cannot be determined until area is known; hence design is a trial-and-area is known; hence design is a trial-and-error procedure.error procedure.

Typical Overall CoefficientsTypical Overall Coefficients

Typical Overall CoefficientsTypical Overall Coefficients

Fouling Factors (Dirt Coeffs)Fouling Factors (Dirt Coeffs)

• Difficult to predict and usually based Difficult to predict and usually based on past experienceon past experience

Mean Temperature Difference Mean Temperature Difference (Temperature Driving Force)(Temperature Driving Force)

• To determine A, To determine A, ΔΔTTmm must be must be estimatedestimated

• True counter-current flow – True counter-current flow – “logarithmic temperature difference” “logarithmic temperature difference” (LMTD)(LMTD)

mTUAQ

LMTDLMTD

• LMTD is given by:LMTD is given by:

where Twhere T11 is the hot fluid temperature, inlet is the hot fluid temperature, inlet

TT22 is the hot fluid temperature, outlet is the hot fluid temperature, outlet

tt11 is the cold fluid temperature, inlet is the cold fluid temperature, inlet

tt22 is the cold fluid temperature, outlet is the cold fluid temperature, outlet

12

21

1221

ln

)()(

tT

tT

tTtTTlm

Counter-current Flow – Counter-current Flow – Temperature ProfliesTemperature Proflies

1:2 Heat Exchanger – 1:2 Heat Exchanger – Temperature ProfilesTemperature Profiles

True Temperature True Temperature DifferenceDifference• Obtained from LMTD using a correction Obtained from LMTD using a correction

factor:factor:

ΔΔTTmm is the true temperature difference is the true temperature difference

FFtt is the correction factor is the correction factor

• FFtt is related to two dimensionless ratios: is related to two dimensionless ratios:

lmtm TFT

)(

)(

12

21

tt

TTR

)(

)(

11

12

tT

ttS

Temp Correction Factor FTemp Correction Factor Ftt

Temperature correction factor, one shell pass, two or more even tube passes Temperature correction factor, one shell pass, two or more even tube passes

Fluid Allocation: Shell or Fluid Allocation: Shell or Tubes?Tubes?

• CorrosionCorrosion

• FoulingFouling

• Fluid temperaturesFluid temperatures

• Operating pressuresOperating pressures

• Pressure dropPressure drop

• ViscosityViscosity

• Stream flow ratesStream flow rates

Shell and Tube Fluid Shell and Tube Fluid VelocitiesVelocities• High velocities give high heat-transfer High velocities give high heat-transfer

coefficients but also high pressure drop.coefficients but also high pressure drop.• Velocity must be high enough to prevent settling Velocity must be high enough to prevent settling

of solids, but not so high as to cause erosion.of solids, but not so high as to cause erosion.• High velocities will reduce foulingHigh velocities will reduce fouling• For liquids, the velocities should be as follows:For liquids, the velocities should be as follows:

– Tube side: Process liquid 1-2m/sTube side: Process liquid 1-2m/sMaximum 4m/s if required to reduce foulingMaximum 4m/s if required to reduce fouling

Water 1.5 – 2.5 m/sWater 1.5 – 2.5 m/s– Shell side: 0.3 – 1 m/sShell side: 0.3 – 1 m/s

Pressure DropPressure Drop

• As the process fluids move through As the process fluids move through the heat exchanger there is the heat exchanger there is associated pressure drop.associated pressure drop.

• For liquids: viscosity < 1mNs/mFor liquids: viscosity < 1mNs/m22 35kN/m35kN/m22

Viscosity 1 – 10 mNs/mViscosity 1 – 10 mNs/m22 50- 50-70kN/m70kN/m22

Tube-side Heat Transfer Tube-side Heat Transfer CoefficientCoefficient• For turbulent flow inside conduits of uniform cross-section, For turbulent flow inside conduits of uniform cross-section,

Sieder-Tate equation is applicable:Sieder-Tate equation is applicable:

C=0.021 for gasesC=0.021 for gases =0.023 for low viscosity liquids=0.023 for low viscosity liquids =0.027 for viscous liquids=0.027 for viscous liquidsμμ= fluid viscosity at bulk fluid temperature= fluid viscosity at bulk fluid temperatureμμww=fluid viscosity at the wall=fluid viscosity at the wall

14.0

33.08.0 PrRe

w

CNu

f

eik

dhNu etduRe

f

p

kC

Pr

Tube-side Heat Transfer Tube-side Heat Transfer CoefficientCoefficient• Butterworth equation:Butterworth equation:

• For laminar flow (Re<2000):For laminar flow (Re<2000):

• If Nu given by above equation is less than 3.5, it should be taken as If Nu given by above equation is less than 3.5, it should be taken as 3.53.5

505.0205.0 PrRe ESt

pt

iCu

hNuSt PrRe

2Pr)(ln0225.0exp0225.0 E

14.033.033.0Pr)(Re86.1

w

e

L

dNu

Heat Transfer Factor, jHeat Transfer Factor, jhh

• ““j” factor similar to friction factor used for j” factor similar to friction factor used for pressure drop:pressure drop:

• This equation is valid for both laminar and This equation is valid for both laminar and turbulent flows.turbulent flows.

14.0

33.0PrRe

wh

f

ii jk

dh

Tube Side Heat Transfer Tube Side Heat Transfer FactorFactor

Heat Transfer Coefficients for Heat Transfer Coefficients for WaterWater

• Many equations for hMany equations for hii have developed have developed specifically for water. One such equation is:specifically for water. One such equation is:

where hwhere hii is the inside coefficient (W/m is the inside coefficient (W/m2 02 0C)C) t is the water temperature (t is the water temperature (00C)C)

uutt is water velocity (m/s) is water velocity (m/s)

ddt t is tube inside diameter (mm) is tube inside diameter (mm)

2.0

8.0)02.035.1(4200

i

ti d

uth

Tube-side Pressure DropTube-side Pressure Drop

where where ΔΔP is tube-side pressure drop (N/mP is tube-side pressure drop (N/m22)) NNpp is number of tube-side passes is number of tube-side passes uutt is tube-side velocity (m/s) is tube-side velocity (m/s) L is the length of one tubeL is the length of one tube m is 0.25 for laminar and 0.14 for turbulentm is 0.25 for laminar and 0.14 for turbulent jjff is dimensionless friction factor for heat is dimensionless friction factor for heat exchanger tubes exchanger tubes

25.28

2t

m

wifpt

u

d

LjNP

Tube Side Friction FactorTube Side Friction Factor

Shell-side Heat Transfer and Shell-side Heat Transfer and Pressure DropPressure Drop

• Kern’s methodKern’s method

• Bell’s methodBell’s method

Procedure for Kern’s MethodProcedure for Kern’s Method

• Calculate area for cross-flow ACalculate area for cross-flow Ass for the hypothetical row of for the hypothetical row of tubes in the shell equator.tubes in the shell equator.

pptt is the tube pitch is the tube pitchdd00 is the tube outside diameter is the tube outside diameterDDss is the shell inside diameter is the shell inside diameterllBB is the baffle spacing, m. is the baffle spacing, m.

• Calculate shell-side mass velocity GCalculate shell-side mass velocity Gss and linear velocity, u and linear velocity, uss..

where Wwhere Wss is the fluid mass flow rate in the shell in kg/s is the fluid mass flow rate in the shell in kg/s

t

bsts p

DdpA

)( 0

s

ss A

WG

s

s

Gu

Procedure for Kern’s MethodProcedure for Kern’s Method

• Calculate the shell side equivalent Calculate the shell side equivalent diameter (hydraulic diameter).diameter (hydraulic diameter).– For a square pitch arrangement:For a square pitch arrangement:

– For a triangular pitch arrangementFor a triangular pitch arrangement0

20

2

44

d

dp

d

t

e

2

42

187.0

24

0

20

d

dp

p

dt

t

e

Shell-side Reynolds NumberShell-side Reynolds Number

• The shell-side Reynolds number is given by:The shell-side Reynolds number is given by:

• The coefficient hThe coefficient hss is given by: is given by:

where jwhere jhh is given by the following chart is given by the following chart

eses dudG

Re

14.0

3/1PrRe

wh

f

es jk

dhNu

Shell Side Heat Transfer Shell Side Heat Transfer FactorFactor

Shell-side Pressure DropShell-side Pressure Drop

• The shell-side pressure drop is given The shell-side pressure drop is given by:by:

where jwhere jff is the friction factor given by is the friction factor given by following chart.following chart.

14.02

28

w

s

Be

sfs

uL

d

DjP

Shell Side Friction FactorShell Side Friction Factor

)(

)(

12

21

tt

TTR

)(

)(

11

12

tT

ttS

(Figure 8 in notes)

mTUAQ

1/1

10

n

tb K

NdD

(Figure 4 in notes)

(Figure 2)

2.0

8.0)02.035.1(4200

i

ti d

uth

(Figure 9 in notes)

t

bsts p

DdpA

)( 0

2

42

187.0

24

0

20

d

dp

p

dt

t

e

iiidiw

i

od hd

d

hd

d

k

d

dd

hhU

11

2

ln111 00

00

00

(Figure 10 in notes)

(Table 3 in notes)

25.28

2t

m

wifpt

u

d

LjNP

14.02

28

w

s

Be

sfs

uL

d

DjP

(Figure 12 in notes)

Bell’s MethodBell’s Method

• In Bell’s method, the heat transfer In Bell’s method, the heat transfer coefficient and pressure drop are coefficient and pressure drop are estimated from correlations for flow estimated from correlations for flow over ideal tube banks. over ideal tube banks.

• The effects of leakage, by-passing, The effects of leakage, by-passing, and flow in the window zone are and flow in the window zone are allowed for by applying correction allowed for by applying correction factors.factors.

Bell’s Method – Shell-side Heat Bell’s Method – Shell-side Heat Transfer CoefficientTransfer Coefficient

where where hhococ is heat transfer coeff for cross flow is heat transfer coeff for cross flow over ideal tube banksover ideal tube banks

FFnn is correction factor to allow for no. is correction factor to allow for no. of vertical tube rowsof vertical tube rows

FFww is window effect correction factor is window effect correction factor

FFbb is bypass stream correction factor is bypass stream correction factor

FFLL is leakage correction factor is leakage correction factor

Lbwnocs FFFFhh

Bell’s Method – Ideal Cross Bell’s Method – Ideal Cross Flow CoefficientFlow Coefficient

• The Re for cross-flow through the The Re for cross-flow through the tube bank is given by:tube bank is given by:

GGss is the mass flow rate per unit area is the mass flow rate per unit area

dd00 is tube OD is tube OD

• Heat transfer coefficient is given by:Heat transfer coefficient is given by:

00RedudG ss

14.0

3/10 PrRe

wh

f

oc jk

dh

Bell’s Method – Tube Row Bell’s Method – Tube Row Correction FactorCorrection Factor

• For Re>2100, FFor Re>2100, Fnn is obtained as a function of is obtained as a function of NNcvcv (no. of tubes between baffle tips) from the (no. of tubes between baffle tips) from the chart below:chart below:

• For Re 100<Re<2100, FFor Re 100<Re<2100, Fnn=1.0=1.0• For Re<100, For Re<100,

18.0' )( cn NF

Bell’s Method – Window Bell’s Method – Window Correction FactorCorrection Factor• FFww, the window correction factor is obtained from the , the window correction factor is obtained from the

following chart:following chart:

where Rwhere Rww is the ratio of bundle cross-sectional area in the is the ratio of bundle cross-sectional area in the window zone to the tube bundle cross-sectional area window zone to the tube bundle cross-sectional area (obtained from simple formulae).(obtained from simple formulae).

Bell’s Method – Bypass Bell’s Method – Bypass Correction FactorCorrection Factor

• Clearance area between the bundle Clearance area between the bundle and the shelland the shell

• For the case of no sealing strips, FFor the case of no sealing strips, Fbb as a function of Aas a function of Abb/A/Ass can be obtained can be obtained from the following chartfrom the following chart

)( bsBb DDA

Bell’s Method – Bypass Bell’s Method – Bypass Correction FactorCorrection Factor

• For sealing strips, for NFor sealing strips, for Nss<N<Ncvcv/2 (N/2 (Nss is the number is the number of baffle strips)of baffle strips)

where where αα=1.5 for Re<100 and =1.5 for Re<100 and αα=1.35 for Re>100.=1.35 for Re>100.

3/12

1expcv

s

s

bb N

N

A

AF

Bell’s Method – Leakage Bell’s Method – Leakage Correction FactorCorrection Factor• Tube-baffle clearance area ATube-baffle clearance area Atbtb is given by: is given by:

• Shell-baffle clearance area AShell-baffle clearance area Asbsb is given by: is given by:

where Cwhere Css is baffle to shell clearance and is baffle to shell clearance and θθbb is the angle subtended by baffle chord is the angle subtended by baffle chord

• AALL=A=Atbtb+A+Asbsb

where where ββLL is a factor obtained from following chart is a factor obtained from following chart

)(2

8.0 0wttb NN

dA

)2(2 bss

sb

DCA

L

sbtbLL A

AAF

)2(1

Coefficient for FCoefficient for FLL, Heat , Heat TransferTransfer

Shell-side Pressure DropShell-side Pressure Drop

• Involves three components:Involves three components:– Pressure drop in cross-flow zonePressure drop in cross-flow zone– Pressure drop in window zonePressure drop in window zone– Pressure drop in end zonePressure drop in end zone

Pressure Drop in Cross Flow Pressure Drop in Cross Flow ZoneZone

where where ΔΔPPii pressure drop calculated for an equivalent ideal tube pressure drop calculated for an equivalent ideal tube bankbankFFbb’ is bypass correction factor’ is bypass correction factorFFLL’ is leakage correction factor’ is leakage correction factor

where jwhere jff is given by the following chart is given by the following chart NNcvcv is number of tube rows crossed is number of tube rows crossed uuss is shell-side velocity is shell-side velocity

''Lbic FFPP

14.02

28

w

scvfi

uNjP

Friction Factor for Cross Flow Friction Factor for Cross Flow BanksBanks

Bell’s Method – Bypass Bell’s Method – Bypass Correction Factor for Pressure Correction Factor for Pressure DropDrop

αα is 5.0 for laminar flow, Re<100 is 5.0 for laminar flow, Re<1004.0 for transitional and turbulent flow, Re>1004.0 for transitional and turbulent flow, Re>100

AAbb is the clearance area between the bundle and shell is the clearance area between the bundle and shell

NNss is the number of sealing strips encountered by is the number of sealing strips encountered by bypass bypass streamstream

NNcvcv is the number of tube rows encountered in the is the number of tube rows encountered in the cross-cross-flow sectionflow section

3/1

' 21exp

cv

s

s

bb N

N

A

AF

Bell’s Method – Leakage Factor Bell’s Method – Leakage Factor for Pressure Dropfor Pressure Drop

where Awhere Atbtb is the tube to baffle clearance area is the tube to baffle clearance area

AAsbsb is the shell to baffle clearance area is the shell to baffle clearance area

AALL is total leakage area = A is total leakage area = Atbtb+A+Asbsb

ββLL’ is factor obtained from following ’ is factor obtained from following chartchart

L

sbtbLL A

AAF

)2(1 ''

Coefficient for FCoefficient for FLL’’

Pressure Drop in Window Pressure Drop in Window ZonesZones

where uwhere uss is the geometric mean velocity is the geometric mean velocity

uuww is the velocity in the window zone is the velocity in the window zone

WWss is the shell-side fluid mass flow is the shell-side fluid mass flow

NNwvwv is number of restrictions for cross-flow in window zone, is number of restrictions for cross-flow in window zone, approximately equal to the number of tube rows.approximately equal to the number of tube rows.

2)6.00.2(

2' z

wvLw

uNFP

swz uuu

ws

w A

Wu

Pressure Drop in End ZonesPressure Drop in End Zones

• NNcvcv is the number of tube rows is the number of tube rows encountered in the cross-flow sectionencountered in the cross-flow section

• NNwvwv is number of restrictions for cross- is number of restrictions for cross-flow in window zone, approximately flow in window zone, approximately equal to the number of tube rows.equal to the number of tube rows.

')(b

cv

cvwvie F

N

NNPP

Bell’s Method – Total Shell-side Bell’s Method – Total Shell-side Pressure DropPressure Drop

zoneswindowN

zonescrossflowNzonesendP

b

bs

)1(2

wbcbes PNPNPP )1(2

Effect of FoulingEffect of Fouling

• Above calculation assumes clean tubesAbove calculation assumes clean tubes

• Effect of fouling on pressure drop is given by table aboveEffect of fouling on pressure drop is given by table above

CondensersCondensers

• Construction of a condenser is similar to other shell and Construction of a condenser is similar to other shell and tube heat exchangers, but with a wider baffle spacingtube heat exchangers, but with a wider baffle spacing

• Four condenser configurations:Four condenser configurations:– Horizontal, with condensation in the shellHorizontal, with condensation in the shell– Horizontal, with condensation in the tubesHorizontal, with condensation in the tubes– Vertical, with condensation in the shellVertical, with condensation in the shell– Vertical, with condensation in the tubesVertical, with condensation in the tubes

• Horizontal shell-side and vertical tube-side are the Horizontal shell-side and vertical tube-side are the most commonly used types of condenser.most commonly used types of condenser.

sB Dl

Heat Transfer MechanismsHeat Transfer Mechanisms

• Filmwise condensationFilmwise condensation– Normal mechanism for heat transfer in commercial condensersNormal mechanism for heat transfer in commercial condensers

• Dropwise condensationDropwise condensation– Will give higher heat transfer coefficients but is unpredictableWill give higher heat transfer coefficients but is unpredictable– Not yet considered a practical proposition for the design of Not yet considered a practical proposition for the design of

condenserscondensers• In the Nusselt model of condensation laminar flow is In the Nusselt model of condensation laminar flow is

assumed in the film, and heat transfer is assumed to assumed in the film, and heat transfer is assumed to take place entirely by conduction through the film.take place entirely by conduction through the film.

• Nusselt model strictly applied only at low liquid and Nusselt model strictly applied only at low liquid and vapor rates when the film is undisturbed.vapor rates when the film is undisturbed.

• At higher rates, turbulence is induced in the liquid film At higher rates, turbulence is induced in the liquid film increasing the rate of heat transfer over that predicted increasing the rate of heat transfer over that predicted by Nusselt model.by Nusselt model.

Condensation Outside Condensation Outside Horizontal TubesHorizontal Tubes

where (hwhere (hcc))11 is the mean condensation film coefficient, for a single tube is the mean condensation film coefficient, for a single tubekkLL is the condensate thermal conductivity is the condensate thermal conductivityρρLL is the condensate density is the condensate densityρρvv is the vapour density is the vapour densityμμLL is the condensate viscosity is the condensate viscosityg is the gravitational accelerationg is the gravitational accelerationΓΓ is the tube loading, the condensate flow per unit length of is the tube loading, the condensate flow per unit length of

tube.tube.• If there are NIf there are Nrr tubes in a vertical row and the condensate is assumed tubes in a vertical row and the condensate is assumed

to flow smoothly from row to row, and if the flow is laminar, the top to flow smoothly from row to row, and if the flow is laminar, the top tube film coefficient is given by:tube film coefficient is given by:

3/1

1

)(95.0)(

L

vLLLc

gkh

4/11)()( rcNc Nhh

r

•

Condensation Outside Condensation Outside Horizontal TubesHorizontal Tubes• In practice, condensate will not flow smoothly from tube In practice, condensate will not flow smoothly from tube

to tube.to tube.• Kern’s estimate of mean coefficient for a tube bundle is Kern’s estimate of mean coefficient for a tube bundle is

given by:given by:

L is the tube lengthL is the tube lengthWWcc is the total condensate flow is the total condensate flowNNtt is the total number of tubes in the bundle is the total number of tubes in the bundleNNrr is the average number of tubes in a vertical tube row is the average number of tubes in a vertical tube row

• For low-viscosity condensates the correction for the For low-viscosity condensates the correction for the number of tube rows is generally ignored.number of tube rows is generally ignored.

6/1

3/1)(

95.0)(

rhL

vLLLbc N

gkh

t

ch LN

W

Condensation Inside and Condensation Inside and Outside Vertical TubesOutside Vertical Tubes• For condensation inside and outside vertical tubes For condensation inside and outside vertical tubes

the Nusselt model gives:the Nusselt model gives:

where (hwhere (hcc))vv is the mean condensation coefficient is the mean condensation coefficient

ΓΓvv is the vertical tube loading, condensate per unit is the vertical tube loading, condensate per unit tube perimetertube perimeter

• Above equation applicable for Re<30Above equation applicable for Re<30• For higher Re the above equation gives a For higher Re the above equation gives a

conservative (safe) estimate.conservative (safe) estimate.• For Re>2000, turbulent flow; situation analyzed For Re>2000, turbulent flow; situation analyzed

by Colburn and results in following chart.by Colburn and results in following chart.

3/1)(

926.0)(

vL

vLLLvc

gkh

Colburn’s ResultsColburn’s Results

Boyko-Kruzhilin CorrelationBoyko-Kruzhilin Correlation

• A correlation for shear-controlled condensation in tubes; simple to use.A correlation for shear-controlled condensation in tubes; simple to use.• The correlation gives mean coefficient between two points at which vapor quality, x, (mass fraction of vapour) is The correlation gives mean coefficient between two points at which vapor quality, x, (mass fraction of vapour) is

known.known.

1,2 refer to inlet and outlet conditions respectively1,2 refer to inlet and outlet conditions respectively

• In a condenser, the inlet stream will normally be saturated vapour and vapour will be totally condensed. For these In a condenser, the inlet stream will normally be saturated vapour and vapour will be totally condensed. For these conditions:conditions:

• For design of condensers with condensation inside the tubes and downward vapor flow, coefficient should be For design of condensers with condensation inside the tubes and downward vapor flow, coefficient should be evaluated using Colburn’s method and Boyko-Kruzhilin correlation and the evaluated using Colburn’s method and Boyko-Kruzhilin correlation and the higherhigher value selected. value selected.

xJwhereJJ

hhv

vLiBKc

12

)(2/1

22/1

1

43.08.0 PrRe021.0

i

Li d

kh

2

1)( v

L

iBKc hh

Flooding in Vertical TubesFlooding in Vertical Tubes

• When the vapor flows up the tube, tubes When the vapor flows up the tube, tubes should not flood.should not flood.

• Flooding should not occur if the following Flooding should not occur if the following condition is satisfied:condition is satisfied:

where uwhere uvv and u and uLL are velocities of vapor and are velocities of vapor and liquid and dliquid and dii is in metres. is in metres.

• The critical condition will occur at the bottom The critical condition will occur at the bottom of the tube, so vapor and liquid velocities of the tube, so vapor and liquid velocities should be evaluated at this point.should be evaluated at this point.

4/14/12/14/12/1 )(6.0 vLiLLvv gduu

Condensation Inside Horizontal Condensation Inside Horizontal TubesTubes• When condensation occurs, the heat transfer coefficient at any When condensation occurs, the heat transfer coefficient at any

point along the tube will depend on the flow pattern at that point.point along the tube will depend on the flow pattern at that point.

• No general satisfactory method exists that will give accurate No general satisfactory method exists that will give accurate predictions over a wide flow range.predictions over a wide flow range.

Two Flow ModelsTwo Flow Models

• Two flow models:Two flow models:– Stratified flowStratified flow

• Limiting condition at low condensate and vapor ratesLimiting condition at low condensate and vapor rates– Annular flowAnnular flow

• Limiting condition at high vapor and low condensate ratesLimiting condition at high vapor and low condensate rates

– For stratified flow, the condensate film coefficient can be estimated as:For stratified flow, the condensate film coefficient can be estimated as:

– For annular flow, the Boyko-Kruzhilin equation can be usedFor annular flow, the Boyko-Kruzhilin equation can be used– For condenser design, both annular and stratified flow should be considered and the higher value of mean For condenser design, both annular and stratified flow should be considered and the higher value of mean

coefficient should be selected.coefficient should be selected.

3/1)(

76.0)(

hL

vLLLsc

gkh

• Condensation of steamCondensation of steam– For air-free steam a coefficient of 8000 W/mFor air-free steam a coefficient of 8000 W/m22--00C should be C should be

used.used.

• Mean Temperature DifferenceMean Temperature Difference– A pure, saturated, vapor will condense at a constant A pure, saturated, vapor will condense at a constant

temperature, at constant pressure.temperature, at constant pressure.– For an isothermal process such as this, the LMTD is given For an isothermal process such as this, the LMTD is given

by:by:

where Twhere Tsatsat is saturation temperature of vapor is saturation temperature of vapor

tt11 (t (t22) is the inlet (outlet) coolant temperature) is the inlet (outlet) coolant temperature

– No correction factor for multiple passes is needed.No correction factor for multiple passes is needed.

2

1

12

ln

)(

tT

tT

ttlm

sat

satT