Lecture 4 - Heat Exchanger Design

8/11/2019 Lecture 4 - Heat Exchanger Design

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Heat exchanger designHeat exchanger designHeat exchanger designHeat exchanger design

Copyright@Dominic Foo H82PLD - Plant Design HE Design - 2

TheTheTheThe Onion diagramOnion diagramOnion diagramOnion diagram

Reactor

Separation &

recycle

Heat exchangernetwork

Utilities (Linnhoffet al., 1982;Smith, 1995, 2005)

We are herenow


Lecture outlineLecture outlineLecture outlineLecture outline

Different types of heat

exchangers

Important equations for heat

exchangers design

Conceptual design for shell-and-tube

heat exchanger


IntroductionIntroductionIntroductionIntroduction This lecture file is a courtesy of Dr John Balwin at

Texas A&M University

The word exchanger refers to all types ofequipment in which heat is exchanged

Other specific terms for heat exchangers: Process fluid heated/cooled by plant service stream in a

heater/cooler

Process stream is vaporised in a vaporiser

Reboiler is used in a distillation column

Evaporator is used to concentrate a solution

Fired exchanger exchangers heated by combustiongases, e.g. boiler


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Standard flow patternsStandard flow patternsStandard flow patternsStandard flow patterns


AirAirAirAir----cooled heat exchangercooled heat exchangercooled heat exchangercooled heat exchanger


Cross sectional viewCross sectional viewCross sectional viewCross sectional view


2 different types2 different types2 different types2 different types

Air pushed by fans below tubes.Air pulled by fans

above tubes


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Bay arrangementsBay arrangementsBay arrangementsBay arrangements


AAAA----frame typeframe typeframe typeframe type airairairair----cooled exchangercooled exchangercooled exchangercooled exchanger


Engineering view of AEngineering view of AEngineering view of AEngineering view of A----frame airframe airframe airframe air----cooled exchangercooled exchangercooled exchangercooled exchanger


Other typesOther typesOther typesOther types

Combined air/water coolerSteam flow condenser


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Transporting exchangersTransporting exchangersTransporting exchangersTransporting exchangers sizesizesizesizematters!matters!matters!matters!


Plate heat exchangerPlate heat exchangerPlate heat exchangerPlate heat exchanger

Small footprint

Very high convection coefficients

Easy to assemble and disassemble

Self cleaning

Normally for relatively low pressures and relatively normal temp

Can easily be modified to vary the heat transfer area


Spiral plate heat exchangerSpiral plate heat exchangerSpiral plate heat exchangerSpiral plate heat exchanger

Provide true countercurrent flow

Hot fluid enters at the spiral centre & flows outward; whilecold fluid enters at the periphery & flows inward.

Competitive with the shell-and-tube exchanger for heating

& cooling of highly viscous, corrosive, fouling and scalingfluids at ambient to moderate pressures


Spiral plate heat exchangerSpiral plate heat exchangerSpiral plate heat exchangerSpiral plate heat exchanger

Reflux condenser Down-flow condenser


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Spiral tube heat exchangerSpiral tube heat exchangerSpiral tube heat exchangerSpiral tube heat exchanger

For high pressure operation

1 fluid flows through the tube coil, other fluid flowscounter-currently in the spiral gap.


Fired exchangerFired exchangerFired exchangerFired exchanger boiler interiorboiler interiorboiler interiorboiler interior


Brazed aluminum (core) heatBrazed aluminum (core) heatBrazed aluminum (core) heatBrazed aluminum (core) heatexchangerexchangerexchangerexchanger

Useful for low temperature applications

Primary application: the cold box of olefins plant.

Frequently handle 5 10 (or even more) streams


Scraped surface heat exchangerScraped surface heat exchangerScraped surface heat exchangerScraped surface heat exchanger

Crystallizers

Highly viscous services

Daily application: ice creamfreezer


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Plate coil heat exchangerPlate coil heat exchangerPlate coil heat exchangerPlate coil heat exchanger

Internal immersion External clamp-on

Useful for small duty applications,particularly for auxiliary heating.


Compact diffusion bonded heatCompact diffusion bonded heatCompact diffusion bonded heatCompact diffusion bonded heatexchangerexchangerexchangerexchanger Compact size

Robust construction. Small fluid flow channels that are

easily blocked during fouling


DoubleDoubleDoubleDouble----pipe heat exchangerspipe heat exchangerspipe heat exchangerspipe heat exchangers

Simplest form of heat exchanger: 1 inner & 1 outer pipe

1 stream flows through the inner pipe; while 1 stream flowscountercurrently through the annual passage between theinner & outer pipe.

When more heat transfer area needed, return bends & heads

are used hairpin unit.

Double pipe Multi-tube; double pipe


DoubleDoubleDoubleDouble----pipe heat exchangerspipe heat exchangerspipe heat exchangerspipe heat exchangers Hairpin units are available up to 200 ft2 of heat transfer area

Competitive with shell-and-tube exchangers in the range of100 - 200 ft2 .

Not recommended for boiling & vaporisation purposes


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DoubleDoubleDoubleDouble----pipe heat exchangerspipe heat exchangerspipe heat exchangerspipe heat exchangers1-1 unit

Hairpin unit


ShellShellShellShell----andandandand----tube heat exchangertube heat exchangertube heat exchangertube heat exchanger


CrossCrossCrossCross----sectional viewsectional viewsectional viewsectional view


The important partsThe important partsThe important partsThe important parts


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ShellShellShellShell----andandandand----tube heat exchangertube heat exchangertube heat exchangertube heat exchanger


Shell flow patternShell flow patternShell flow patternShell flow patternTwo-shell-pass

Four-shell-pass


Different configurationsDifferent configurationsDifferent configurationsDifferent configurations

Most simplest type: 1-shell-pass; 1-tube-pass (denoted as 1-1)

Limitation for fixed heat type 1-1 exchanger:

Outside surface cannot be cleaned

When large Tbetween shell & tube sides, differential expansion mayexceed limits for bellows or expansion joins

Velocity of the tube side too low for good heat transfer coefficient Overcome by other configuration, e.g. floating head



12 fixed head

12 floating head


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12 U-tube

24 floating head


Baffle arrangementsBaffle arrangementsBaffle arrangementsBaffle arrangements


Other baffle arrangementsOther baffle arrangementsOther baffle arrangementsOther baffle arrangements


Tube arrangementTube arrangementTube arrangementTube arrangement Triangular & rotated

square: Higher heat transfer

Higher pressure drop

Square/rotatedsquare: Heavy fouling fluids

Ease for mechanicalcleaning at the outerlayer

Recommended tubepitch = 1.25 x tube OD

Square pitch Triangular pitch

Square pitch rotated Triangular pitch(with cleaning lane)

Pt

Pt

Pt P

t


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Kettle typeKettle typeKettle typeKettle type reboilerreboilerreboilerreboiler


Large shellLarge shellLarge shellLarge shell----andandandand----tube exchangerstube exchangerstube exchangerstube exchangers

On a 42-wheeler?


TubeTubeTubeTube----sheet for large shellsheet for large shellsheet for large shellsheet for large shell----andandandand----tubetubetubetubeexchangersexchangersexchangersexchangers

33 rowsof tubes!


TubeTubeTubeTube----sheet of a fouled exchangersheet of a fouled exchangersheet of a fouled exchangersheet of a fouled exchanger


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TubeTubeTubeTube----sheet of a fouled exchangersheet of a fouled exchangersheet of a fouled exchangersheet of a fouled exchanger


A dissembled tube bundleA dissembled tube bundleA dissembled tube bundleA dissembled tube bundle


A UA UA UA U----tube bundletube bundletube bundletube bundle


Tube bundle while cleaningTube bundle while cleaningTube bundle while cleaningTube bundle while cleaning


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Horizontal shellHorizontal shellHorizontal shellHorizontal shell----side condenserside condenserside condenserside condenser

Excess vapor or

noncondensablesvent

Gravity-controlled

flow

Shear-controlled

flow

Vapor in

Baffle,vertical cut

Condensate outCoolant in

Coolant out


Fluid allocationFluid allocationFluid allocationFluid allocation Material of construction tube side if expensive

material is required for corrosive or high tempfluid

Fouling tube side if fluid that has tendency tofoul (higher velocity in tube reduces fouling)

Operating pressure tube side for higher pressurestream (tube needs thinner wall due to smaller

diameter, compared to shell) Pressure drop tube side for lower P (higher

heat transfer coefficient in tube)


Fluid allocation (continue)Fluid allocation (continue)Fluid allocation (continue)Fluid allocation (continue) Viscosity shell side for more viscous material,

provided it is turbulent flow (higher heat transfercoefficient)

Stream flowrate lower flowrate to the shell side(higher heat transfer coefficient)

Fluid temperature hotter fluid in tubes (reduceheat loss & safety reasons)


Design ofDesign ofDesign ofDesign of shellshellshellshell----andandandand----tube HEtube HEtube HEtube HE 1-1 HE (1 shell 1 tube

pass) Lowest surface area

requirement

Normal practise: hot

fluid flows verticallydown

hot liquid becomesdenser as it is cooled

1-2 HE Normal practise: cold

stream flowsupwards

Less dense when

heated Liquid may vaporise


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Important equations for heatImportant equations for heatImportant equations for heatImportant equations for heatexchanger designexchanger designexchanger designexchanger design Heat transfer (Q) across a surface:

Q = UATLmwhere,

U= overall heat transfer coefficient;

A = heat transfer area;

TLm = log mean temp difference

Change of an individual stream: Q = mCpT

where,m = mass flowrate;

Cp = heat capacity;

T= temp difference across heat exchanger


5 resistances to heat transfer5 resistances to heat transfer5 resistances to heat transfer5 resistances to heat transfer Overall heat transfer coefficient (U):

Ti

o

TFi

o

i

oo

SFS hd

d

hd

d

d

d

k

d

hhU

11ln

2

111++

++=

(hS)

(hSF

)(h

TF)

(hT)

(k)


Film transfer coefficient (Film transfer coefficient (Film transfer coefficient (Film transfer coefficient (hhhhSSSS,,,, hhhhTTTT))))


Fouling coefficient (Fouling coefficient (Fouling coefficient (Fouling coefficient (hhhhSFSFSFSF,,,, hhhhTFTFTFTF))))


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Tube wall coefficient (k)Tube wall coefficient (k)Tube wall coefficient (k)Tube wall coefficient (k)


TTTTLmLmLmLm calculationcalculationcalculationcalculation

TH, in = 260C

TH, out = 220C

TC, in = 150C

TC, out = 200C

TH =60

TC =70

Q

T

C________

lnlnC

H

CH

inC,outH,

outC,inH,

inC,outH,outC,inH,

Lm =

=

=

T

T

TT

TT

TT

TTTTT


Limitation ofLimitation ofLimitation ofLimitation of TTTTLmLmLmLm Only applicable in the following

situation: Stream flows are at steady-state

Co-current & counter-current flow

Sensible heat transfer, with constantspecific heat

Overall heat transfer coefficient isconstant

No heat losses

In most situation, flow in heatexchanger is a mixture of co-current, counter-current & cross-

flow, e.g. 1-2 exchanger.

Counter-current

Co-current


1111----2 design HE2 design HE2 design HE2 design HE Many practical advantages:

Allowance for thermalexpansion

Easy mechanical cleaning

Good heat transfer coefficienton tube side (high velocity)

Flow arrangement: mixture ofco-current & counter-current larger area than 1-1 design

Correction factor (Ft) needed:

Q = UAFtTLm

where 0 < Ft < 1.


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Correction factor (FCorrection factor (FCorrection factor (FCorrection factor (Ftttt)))) Correlated with 2 dimensionless ratio:

Ration of heat capacity flowrate

Thermal effectiveness of HE

Hence, Ft

depends only on Tin

& Tout

of streams.

Various plots of Ft are available.

inC,outC,

outH,inH,

H,H

C,C

TT

TT

Cm

CmR

p

p

==

inC,inH,

inC,outC,

TT

TTS

=


Correction factor for 1Correction factor for 1Correction factor for 1Correction factor for 1----2222






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Graphical approachGraphical approachGraphical approachGraphical approach For design, it is desire to have F

t> 0.85

Ft

< 0.75 is not acceptable, as below this value, curves turn

sharply downward small errors in R & S result in Ftmuch lower than anticipated.

When Ft is unsatisfactory, a multiple-shell-pass HE is used.

More shell passes higher Ft value.


Solution:

TLm = _________

R = _________ ; S = _________

Determine Ft : Exchanger 1-2: temp crossover

Exchanger 2-4: < 0.5

Exchanger 3-6: < 0.7 (risky)

Exchanger 4-8: 0.85 (satisfactory)

FtTLm = _________

Example 1Example 1Example 1Example 1 A hot stream is being cooled from 200F to 140F by

a cold stream that enters the exchanger at 100F &exists at 190F. Determine the true meantemperature driving force for multiple tube-pass

shell-and-tube exchanger.

200F

140F

100F

190F

Thot=10

Tcold=40

Q

T


Graphical approachGraphical approachGraphical approachGraphical approach For design, it is desire to have Ft > 0.85

Ft < 0.75 is not acceptable

When Ft is unsatisfactory, a multiple-shell-pass

heat exchanger is used

iterative procedure tofind no of shell (e.g. try 1-2 2-4 3-6, etc.)


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An insight on FAn insight on FAn insight on FAn insight on Fttttplotplotplotplot

0.75Temperature

approach

Q

T

Temperature

cross

Q

T

Large

temperature

cross

Q

T

(Smith, 2005)Length

T


An insight on FAn insight on FAn insight on FAn insight on Fttttplotplotplotplot

To be confident in design,steep part of the F

t

chartshould be avoided,irrespective of F

t> 0.75.

For any value of R, there isa maximum asymtropicvalue value for S, i.e. Smax,where Ft tends to .

Practical design will be limited to some fraction of Smax, i.e.

S = XPSmax ; where 0 < XP < 1

For conceptual design, a value of XP = 0.9 is reasonable.

This approach restricted to 1-2 heat exchanger.

0.75

XP

= 0.9

(Smith, 2005)


Algebraic approachAlgebraic approachAlgebraic approachAlgebraic approach

( )

( ) ( )

( )

+++

++

+

=

112

112ln1

1

1ln1

:1For

2

2

2

RRS

RRSR

RS

SR

F

R

t

Lm

TUAFQt

=( )

( )

+

=

=

222

222ln

1

2

:1For

S

S

S

S

F

R

tinC,inH,

inC,outC,

inC,outC,

outH,inH,;where

TT

TTS

TT

TTR

=

=

tinQ

T

Tin

tout

Tout


Algebraic approachAlgebraic approachAlgebraic approachAlgebraic approach Situation often encountered where design is infeasible in a single 1-2

shellmultiple shell is considered.

By using two 1-2 shells in series, temp cross is reduced.

(Smith, 2005)


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Basic principlesBasic principlesBasic principlesBasic principles In principle, profile achieved by a 2-4 shell is achievable by two 1-2 shell.

For Nshells in series, some heuristics to remember : Ft of each shell = Ft across all Nshells shell passes

R of each shell = R across all Nshells shell passes

Values of S of each shell pass (S1-2) are equal, but not equal to S across allNshells shell (SN-2N).

Equation to determine Nshell :

Notes: Nshell is round up to the next largest number to obtain no of shell

S in this equation is applicable to both S1-2 and SN-2N Xp is chosen to satisfy the min allowable Ft, e.g. for Ft, min = 0.75; XP = 0.9.

P

P

XRR

RXRRW

211

211where

2

2

+++

+++=

W

S

RS

NRln

1

1ln

:1For shells

=

2

21

1:1For shells

P

P

X

XS

S

NR

+

==


Calculation procedureCalculation procedureCalculation procedureCalculation procedure From R & SN-2N, determine Nshell .

Next, S1-2 is calculated for each shell:

Finally, substitute to obtain Ft:

Area can be calculated:

RZ

ZS

R

N

N

=

shell

shell

1

1

21

1

:1For

NN

NN

S

RSZ

2

2

1

1where

=

LmTUF

QA

t=

(Smith, 2005)

( )

( ) ( )( )

+++

++

+

=

112

112ln1

1

1ln1

:1For

2

2

2

RRS

RRSR

RS

SR

F

R

t

( )( )

+

=

=

222

222ln

1

2

:1For

S

S

S

S

F

R

t

shellsshells22

221

:1For

NNSS

SS

R

NNNN

NN

+=

=


Example 2Example 2Example 2Example 2A hot stream is to be cooled from 300 to 100C byexchange with a cold stream being heated from 60 to200C in a single unit. 1-2 shell-and-tube heatexchangers are to be used subject to XP = 0.9. The duty

for the exchanger is 3.5 MW & the overall heat transfercoefficient (U) is estimated to be 100 W.m-2.K-1.Calculate:

i. Number of shells required

ii. S1-2 for each shell

iii. Ft for the shells in series

iv. Heat transfer area

300C

100C

60CQ

T

200C


Solution for Example 2Solution for Example 2Solution for Example 2Solution for Example 2i. Number of shells required:

ii. S1-2 for each shell:

iii.Ft for the shells in series:

iv. Heat transfer area:

5833.060300

60200;4286.1

60200

1003002 =

==

=

NNSR

( )( )

)passesshell(32.33

6749.0372.2601.1;

ln5833.01

5833.04286.11ln

=

==

= WW

Ns

3805.04286.14.0

14.04.0

4167.0

1667.0

31

31

21 =

===

SZ

86.03805.0;4286.1 21 === tFSR

26

m96165.48x0.86x100

10x5.3==

=

TUF

QA

t

inC,outC,

outH,inH,

TT

TTR

=

inC,inH,

inC,outC,

TT

TTS

=


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Overall designOverall designOverall designOverall designprocedureprocedureprocedureprocedure

We have only learnt

until Step 5 (the rest

of the steps will

remain as your

coursework!)

(Sinnott, 2005) Copyright@Dominic Foo H82PLD - Plant Design HE Design - 74

BellBellBellBells methods methods methods method Heat transfer coefficient and pressure drop are

estimated from correlations for flow:

Over idea tube-banks

Effects of leakage

Bypassing

Flow in window zone

More satisfactory prediction than Kerns method


Basic conceptsBasic conceptsBasic conceptsBasic concepts Sometimes called the stream

analysis method

Based on the notion that the streamflow through a shell side of anexchanger consist of multiple flows.

The streams are as follows:

A. Leaking through the clearance between tubes and baffles

B. Main cross-flow stream flowing through one window across the cross-flowsection and out through the opposite window

C. Bundle bypass stream, flowing around the tube bundle between theoutermost tubes in the bundle and the inside of the shell

D. No stream with this ID

E. Shell to baffle leakage stream flowing through the clearance between thebaffles and the inside diameter of the shell

F. Flowing through any channels within the tube bundle caused by theprovision of pass dividers in the exchanger (only in multiple tube-passconfigurations)


ShellShellShellShell----side heat transferside heat transferside heat transferside heat transfercoefficientcoefficientcoefficientcoefficient

0.9.to0.6fromvarywillcorrectiontotalThe

factor.correctionLeakageF

factor,correctionstreamBypassF

factor,correctioneffectWindowF

rows,tubeverticalnumberforfactorCorrectionF

bank,tubeidealanovertcoefficientransferHeathwhere

bycalculatedistcoefficientransferheatside-shellThe

b

w

n

oc

=

=

=

=

=

=

l

Lbwnocs FFFFhh


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Lecture 4 - Heat Exchanger Design

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