Design Heat Exchanger

7/27/2019 Design Heat Exchanger

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Heat Exchanger DesignAnand V P Gurumoorthy

Associate Professor

Chemical Engineering Division

School of Mechanical & Building Sciences

VIT University

[EDITED BY AFRAZ ]

ICET PU[UNIVERSITY OF THE PUNJAB LAHORE]


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Heat Exchanger Classification

Recuperative:Cold and hot fluid flow through the unit without mixing

with each other. The transfer of heat occurs through themetal wall.

Regenerative:Same heating surface is alternately exposed to hot and

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

Direct contact:Hot and cold fluids are in direct contact and mixing occurs

among them; mass transfer and heat transfer occursimultaneously.


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Heat Exchanger Standards andCodes

British Standard BS-3274

TEMA standards are universally used.

TEMA standards cover following classes ofexchangers:Class Rdesignates severe requirements of petroleum

and other related processing applications

Class Cmoderate requirements of commercial and

general process applicationsClass Bspecifies design and fabrication for chemical

process service.


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Shell and Tube Heat Exchanger

Most commonly used type of heat transferequipment in the chemical and allied industries.

Advantages:

The configuration gives a large surface area in a smallvolume.

Good mechanical layout: a good shape for pressureoperation.

Uses well-established fabrication techniques.

Can be constructed from a wide range of materials.

Easily cleaned.

Well established design procedures.


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Types of Shell and Tube HeatExchangers

Fixed tube designSimplest and cheapest type.

Tube bundle cannot be removed for cleaning.

No provision for differential expansion of shell and tubes.

Use of this type limited to temperature difference upto800C.

Floating head design

More versatile than fixed head exchangers.

Suitable for higher temperature differentials.

Bundles can be removed and cleaned (fouling liquids)


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Design of Shell and Tube HeatExchangers

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

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

Bell-Delaware methodMost widely used.Takes into account:

Leakage through the gaps between tubes and baffles and the baffles andshell.

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

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

computer codes.


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Construction DetailsTubeDimensions

Tube diameters in the range 5/8 inch (16 mm) to 2inch (50 mm).

Smaller diameters (5/8 to 1 inch) preferred since

this gives compact and cheap heat exchangers. Larger tubes for heavily fouling fluids.

Steel tubesBS 3606; Other tubesBS 3274.

Preferred tube lengths are 6 ft, 8 ft, 12 ft, 16 ft, 20

ft and 24 ft; optimum tube length to shell diameterratio ~ 510.

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


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Construction DetailsTubeArrangements

Tubes usually arranged in equilateral triangular, square or rotatedsquare patterns.

Tube pitch, Pt, is 1.25 times OD.


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Construction Details - Shells

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

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


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Construction Details - Shell-Bundle Clearance


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Construction DetailsTubeCount Bundle diameter depends not only on number of tubes but also number of

tube passes.

Ntis the number of tubes

Dbis the bundle diameter (mm)

D0is tube outside diameter (mm)

n1and K1 are constants

1/1

1

0

n

t

b

K

NdD


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Construction Details - Baffles

Bafflesare used:To direct the fluid stream across the tubes

To increase the fluid velocity

To improve the rate of transfer

Most commonly used baffle is the single segmental baffle.

Optimal baffle cut ~ 20-25%


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Basic Design Procedure

General equation for heat transfer is:

where Q is the rate of heat transfer (duty),

U is the overall heat transfer coefficient,

A is the area for heat transfer

Tmis the mean temperature difference

We are not doing a mechanical design, only athermal design.

mTUAQ


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Overall Heat Transfer Coefficient

Overall coefficient given by:

h0 (hi) is outside (inside) film coefficient

hod(hid) is outside (inside) dirt coefficient

kwis the tube wall conductivitydo(di) is outside (inside) tube diameters

iiidiw

i

od hd

d

hd

d

k

d

dd

hhU

11

2

ln111 00

00

00


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Individual Film Coefficients

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

radiation, etc.)

Physical properties of fluids

Fluid flow rates

Physical layout of heat transfer surface

Physical layout cannot be determined until area is

known; hence design is a trial-and-error procedure.


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Typical Overall Coefficients


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Typical Overall Coefficients


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Fouling Factors (Dirt Coeffs)

Difficult to predict and usually based on past experience


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Mean Temperature Difference(Temperature Driving Force)

To determine A, Tmmust be estimated

True counter-current flowlogarithmic temperature difference(LMTD)

mTUAQ


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LMTD

LMTD is given by:

where T1is the hot fluid temperature, inlet

T2is the hot fluid temperature, outlet

t1is the cold fluid temperature, inlett2is the cold fluid temperature, outlet

12

21

1221

ln

)()(

tTtT

tTtTTlm


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Counter-current FlowTemperature Proflies


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1:2 Heat ExchangerTemperature Profiles


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True Temperature Difference

Obtained from LMTD using a correction factor:

Tmis the true temperature difference

Ftis the correction factor

Ftis related to two dimensionless ratios:

lmtm

TFT

)(

)(

12

21

tt

TTR

)(

)(

11

12

tT

ttS


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Temp Correction Factor Ft

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


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Fluid Allocation: Shell or Tubes?

Corrosion

Fouling

Fluid temperatures

Operating pressures Pressure drop

Viscosity

Stream flow rates


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Shell and Tube Fluid Velocities

High velocities give high heat-transfer coefficientsbut also high pressure drop.

Velocity must be high enough to prevent settling of

solids, but not so high as to cause erosion. High velocities will reduce fouling

For liquids, the velocities should be as follows:Tube side: Process liquid 1-2m/s

Maximum 4m/s if required to reduce fouling

Water 1.52.5 m/s

Shell side: 0.31 m/s


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Pressure Drop

As the process fluids move through the heat exchanger there isassociated pressure drop.

For liquids: viscosity < 1mNs/m235kN/m2

Viscosity 110 mNs/m250-70kN/m2


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Tube-side Heat TransferCoefficient For turbulent flow inside conduits of uniform cross-section, Sieder-Tateequation is applicable:

C=0.021 for gases

=0.023 for low viscosity liquids

=0.027 for viscous liquids

= fluid viscosity at bulk fluid temperature

w=fluid viscosity at the wall

14.0

33.08.0PrRe

w

CNu

f

ei

kdh

Nu

etduRef

p

k

C Pr


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Tube-side Heat TransferCoefficient

Butterworth equation:

For laminar flow (Re


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Heat Transfer Factor, jh

j factor similar to friction factor used for pressuredrop:

This equation is valid for both laminar and turbulentflows.

14.0

33.0PrRe

w

h

f

ii jk

dh


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Tube Side Heat Transfer Factor


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Heat Transfer Coefficients forWater

Many equations for hihave developed specificallyfor water. One such equation is:

where hiis the inside coefficient (W/m2 0C)

t is the water temperature (0C)utis water velocity (m/s)

dt is tube inside diameter (mm)

2.0

8.0

)02.035.1(4200

i

ti

duth


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Tube-side Pressure Drop

where P is tube-side pressure drop (N/m2)

Npis number of tube-side passes

utis tube-side velocity (m/s)

Lis the length of one tube

mis 0.25 for laminar and 0.14 for turbulentjfis dimensionless friction factor for heat

exchanger tubes

25.28

2

t

m

wi

fpt

u

d

LjNP


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Tube Side Friction Factor


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Shell-side Heat Transfer andPressure Drop

Kerns method

Bells method


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Procedure for Kerns Method

Calculate area for cross-flow Asfor the hypothetical row oftubes in the shell equator.

ptis the tube pitchd0is the tube outside diameter

Dsis the shell inside diameter

lBis the baffle spacing, m.

Calculate shell-side mass velocity Gsand linear velocity, u

s.

where Wsis the fluid mass flow rate in the shell in kg/s

t

bsts

p

DdpA

)(0

s

ss

A

WG

ss

Gu


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Procedure for Kerns Method

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

For a triangular pitch arrangement

0

2

0

2

44

d

dp

d

t

e

2

42

187.0

24

0

2

0

d

dp

p

d

tt

e


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Shell-side Reynolds Number

The shell-side Reynolds number is given by:

The coefficient hsis given by:

wherejhis given by the following chart

eses dudG Re

14.0

3/1PrRe

w

h

f

es jk

dhNu


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Shell Side Heat Transfer Factor


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Shell-side Pressure Drop

The shell-side pressure drop is given by:

where jfis the friction factor given by following chart.

14.02

28

w

s

Be

s

fs

uL

d

DjP

Shell Side Friction Factor


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Shell Side Friction Factor


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)(

)(

12

21

tt

TTR

)(

)(

11

12

tT

ttS

(Figure 8 in notes)


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mTUAQ

1/1

1

0

n

tb

K

NdD

(Figure 4 in notes)

(Figure 2)


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2.0

8.0)02.035.1(4200

i

ti

d

uth


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(Figure 9 in notes)


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t

bst

s p

Ddp

A

)( 0

2

42

187.0

24

0

2

0

d

dp

p

d

tt

e


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iiidiw

i

od hd

d

hd

d

k

d

dd

hhU

11

2

ln111 00

0

0

00

(Figure 10 in notes)

(Table 3 in notes)


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25.28

2

t

m

wi

fpt

u

d

LjNP

14.02

28

w

s

Be

sfs

uL

d

DjP

(Figure 12 in notes)


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Bells Method

In Bells method, the heat transfer coefficient and pressure drop areestimated from correlations for flow over ideal tube banks.

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


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Bells Method Shell-side HeatTransfer Coefficient

where hocis heat transfer coeff for cross flow

over ideal tube banks

Fnis correction factor to allow for no.of vertical tube rows

Fwis window effect correction factorFbis bypass stream correction factor

FLis leakage correction factor

Lbwnocs FFFFhh


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Bells Method Ideal Cross Flow


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Bell s Method Ideal Cross FlowCoefficient

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

Gsis the mass flow rate per unit area

d0is tube OD

Heat transfer coefficient is given by:

00Re dudG

ss

14.0

3/10 PrRe

w

h

f

oc jk

dh


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Bells Method Tube RowCorrection Factor

For Re>2100, Fnis obtained as a function of Ncv(no. of tubes betweenbaffle tips) from the chart below:

For Re 100


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Bells Method WindowCorrection Factor

Fw, the window correction factor is obtained from thefollowing chart:

whereRwis the ratio of bundle cross-sectional area in thewindow zone to the tube bundle cross-sectional area(obtained from simple formulae).


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Bells Method BypassCorrection Factor

Clearance area[Ab]between the bundle and the shell

For the case of no sealing strips, Fbas a function of A

b/A

scan be

obtained from the following chart

)( bsBb DDA


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Bells Method BypassCorrection Factor

For sealing strips, for Ns


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Bells Method LeakageCorrection Factor Tube-baffle clearance area Atbis given by:

Shell-baffle clearance area Asb

is given by:

where Csis baffle to shell clearance and bis the angle subtended by baffle chord

AL=Atb+Asb

where Lis a factor obtained from following chart

)(2

8.0 0wttb NN

dA

)2(2

bss

sb

DCA

L

sbtbLL

A

AAF )2(1


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Coefficient for FL, Heat Transfer


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Shell-side Pressure Drop

Involves three components:

Pressure drop in cross-flow zone

Pressure drop in window zone

Pressure drop in end zone

C


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Pressure Drop in Cross FlowZone

where Pipressure drop calculated for an equivalent ideal tubebank

Fb is bypass correction factor

FL is leakage correction factor

wherejfis given by the following chart

Ncvis number of tube rows crossed

usis shell-side velocity

''

Lbic FFPP

14.02

28

w

scvfi

uNjP

F i ti F t f C Fl


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Friction Factor for Cross FlowBanks

Bells Method Bypass


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Bell s Method BypassCorrection Factor for Pressure

Drop

is 5.0 for laminar flow, Re100

Abis the clearance area between the bundle and shell

Nsis the number of sealing strips encountered by bypass

streamNcvis the number of tube rows encountered in the cross- flow section

3/1

' 21exp

cv

s

s

bb

N

N

A

AF

B ll M th d L k F t


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Bells Method Leakage Factorfor Pressure Drop

where Atbis the tube to baffle clearance area

Asbis the shell to baffle clearance area

ALis total leakage area = Atb+Asb

L is factor obtained from following chart

L

sbtbLL

A

AAF

)2(1

''


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Coefficient for FL


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Pressure Drop in Window Zones

whereusis the geometric mean velocity

uwis the velocity in the window zone

Wsis the shell-side fluid mass flow

Nwvis number of restrictions for cross-flow in windowzone, approximately equal to the number of tube rows.

2)6.00.2(

2

' z

wvLw

u

NFP

swz

uuu

w

swA

Wu


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Pressure Drop in End Zones

Ncv

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

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

')(b

cv

cvwvie F

N

NNPP

B ll M th d T t l Sh ll id


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Bells Method Total Shell-sidePressure Drop

zoneswindowN

zonescrossflowNzonesendP

b

bs

)1(2

wbcbes PNPNPP )1(2

Eff t f F li


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Effect of Fouling

Above calculation assumes clean tubes

Effect of fouling on pressure drop is given by table above


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Condensers

Construction of a condenser is similar to other shell and tubeheat exchangers, but with a wider baffle spacing

Four condenser configurations:Horizontal, with condensation in the shell

Horizontal, with condensation in the tubes

Vertical, with condensation in the shell

Vertical, with condensation in the tubes

Horizontal shell-side and vertical tube-side are the mostcommonly used types of condenser.

sB Dl


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Heat Transfer Mechanisms

Filmwise condensationNormal mechanism for heat transfer in commercial condensers

Dropwise condensationWill give higher heat transfer coefficients but is unpredictableNot yet considered a practical proposition for the design of

condensers In the Nusselt model of condensation laminar flow is

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

Nusselt model strictly applied only at low liquid and vaporrates when the film is undisturbed.

At higher rates, turbulence is induced in the liquid filmincreasing the rate of heat transfer over that predicted byNusselt model.

Condensation Outside Horizontal


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Condensation Outside HorizontalTubes

where (hc)1is the mean condensation film coefficient, for a single tubekLis the condensate thermal conductivity

Lis the condensate density

vis the vapour density

Lis the condensate viscosity

g is the gravitational acceleration

is the tube loading, the condensate flow per unit length of tube.

If there are Nrtubes in a vertical row and the condensate is assumed to flowsmoothly from row to row, and if the flow is laminar, the top tube film coefficientis given by:

3/1

1)(95.0)(

L

vLLLc

gkh

4/1

1)()( rcNc Nhh r


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Condensation Outside Horizontal


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Condensation Outside HorizontalTubes

In practice, condensate will not flow smoothly from tube totube.

Kerns estimate of mean coefficient for a tube bundle isgiven by:

Lis the tube length

Wcis the total condensate flow

Ntis the total number of tubes in the bundle

Nris the average number of tubes in a vertical tube row For low-viscosity condensates the correction for the number

of tube rows is generally ignored.

6/1

3/1

)(95.0)(

rhL

vLLLbc Ng

kh

t

ch

LNW

Condensation Inside and Outside


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Condensation Inside and OutsideVertical Tubes

For condensation inside and outside vertical tubes the Nusseltmodelgives:

where (hc)vis the mean condensation coefficient

vis the vertical tube loading, condensate per unit tube perimeter

Above equation applicable for Re2000, turbulent flow; situation analyzed by Colburn andresults in following chart.

3/1

)(926.0)(

vL

vLLLvc

gkh


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Colburns Results

Boyko-Kruzhilin Correlation


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Boyko-Kruzhilin Correlation 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 ofvapour) is known.

1,2 refer to inlet and outlet conditions respectively

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

For design of condensers with condensation inside the tubes and downward vapor flow, coefficientshould be evaluated using Colburns method and Boyko-Kruzhilin correlation and the highervalueselected.

xJwhereJJ

hhv

vLiBKc

1

2)(

2/12

2/11

43.08.0

PrRe021.0

i

L

id

k

h

2

1

)( v

L

iBKc hh


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Flooding in Vertical Tubes

When the vapor flows up the tube, tubes should notflood.

Flooding should not occur if the following condition is

satisfied:

where uvand uLare velocities of vapor and liquid anddiis in metres.

The critical condition will occur at the bottom of thetube, so vapor and liquid velocities should beevaluated at this point.

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

Condensation Inside Horizontal


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Condensation Inside HorizontalTubes When condensation occurs, the heat transfer coefficient at any point along the

tube will depend on the flow pattern at that point.

No general satisfactory method exists that will give accurate predictions over awide flow range.


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Two Flow Models Two flow models:

Stratified flow

Limiting condition at low condensate and vapor rates

Annular flow

Limiting condition at high vapor and low condensate rates

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

For annular flow, the Boyko-Kruzhilin equation can be used

For condenser design, both annular and stratified flow should be considered andthe higher value of mean coefficient should be selected.

3/1

)(76.0)(

hL

vLLLsc

gkh

Condensation of steam


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For air-free steam a coefficient of 8000 W/m2-0C should be used.

Mean Temperature DifferenceA pure, saturated, vapor will condense at a constant temperature, at

constant pressure.

For an isothermal process such as this, the LMTD is given by:

where Tsatis saturation temperature of vapor

t1(t2) is the inlet (outlet) coolant temperature

No correction factor for multiple passes is needed.

2

1

12

ln

)(

tT

tT

tt

lm

sat

sat

T

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