Lecture 15 HT PNSheth

7/24/2019 Lecture 15 HT PNSheth

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Heat Exchangers

By

Dr Pratik N ShethChemical Engineering

Department


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Heat Transfer Equipment

Single-pass heat exchanger ui! "#sthr"ugh "nly "nce$

Parallel "r C"-current "# ui!s "# in

the same !irecti"n$ C"untercurrent "# "r C"unter"#- ui!s

"# in "pp"site !irecti"ns$

Cr"ss"# t#" ui!s "# at right anglest" "ne an"ther$


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D"u%le Pipe Heat Exchanger


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D"u%le Pipe Heat Exchanger

The %asic c"mp"nent is c"nstructe! fr"m t#" pipes& "ne insi!e the "ther$

'ne ui! ()lui! B* "#s insi!e the inner pipe

#hile the sec"n! ui! ()lui! +* "#s in the annular space

%et#een the t#" pipes$ 'ne ui! is h"tter than the "ther an! heat "#s

thr"ugh the #all "f the inner pipe fr"m the h"t ui!t" the c"l! ui!$

T" "%tain larger heat exchange area& se,eral pipes are arrange! si!e-%y-si!e an! ttings are

attache! t" all"# the ui!s t" c"ntact the pipes in seriess" the ",erall "# is the same as if "nly "ne ,ery l"ng pipe#ere use! rather than se,eral sh"rter "nes$


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Shell an! Tu%e HeatExchangers

.f ,ery large heat exchange areas are require!& the shell-and-tube heat exchanger !esign is use!$

+ %un!le "f small-!iameter tu%es #hich are arrange! parallel t" each "ther an! resi!e insi!e

a much larger-!iameter tu%e calle! the /shell0& The tu%es are all manif"l!e! t"gether at either en! s" that

the /tu%e ui!0 enters the left si!e an! is !istri%ute!equally am"ng all the tu%es$

+t the rightsi!e& the ui! exits fr"m each tu%e& is mixe!t"gether in a sec"n! manif"l!& then lea,es as a singlestream$

The sec"n! ui!& calle! the /shell ui!0& "#s in thespace in %et#een the "utsi!e "f tu%es$ Ba1e plates insi!e the shell f"rce the shell ui! t" "#

acr"ss the tu%es repeate!ly as the ui! m",es al"ng thelength "f the shell$


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2-2


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Tu%e Pitch

The %un!le "f tu%es in a shell-an!-tu%e heatexchanger can %e stacke! in "ne "f t#" !i3erent #ays

The use "f triangular pitch all"#s the tu%es t" %e m"retightly packe! -- m"re tu%es an! theref"re m"re areaper unit ,"lume "f shell$ This makes the shell cheaper$

'n the "ther han!& the square pitch has the a!,antagethat it is easier t" clean$ 4"u can get a l"ng %rush in%et#een t#" r"#s "r %et#een t#" c"lumns$ 'f c"urse&the shell has t" %e pulle! "3 "f the tu%e %un!le %ef"rey"u can get at it in either case$

5egar!less "f #hich ge"metry is use!& the center-t"center !istances %et#een a!6acent tu%es is calle! thetube pitch$


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Tu%e Pitch


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Ba1es

T" s"me extent& it is a!,antage"us t" re!ucethe cr"ss-secti"nal area a,aila%le f"r "#$

)"r the same ,"lumetric "# rate& this #illincrease the mass ux "r the mass ,el"city

"f the ui! "#& #hich #ill increase h"$ 'f c"urse& this als" increases the pressure

!r"p an! the pumping c"st$ + c"mm"n technique f"r thus increasing h"is

t" install %a1e plates& #hich partially %l"ckthe cr"ss-secti"nal area "n the shell si!e$ +%a1e plate is a !isk #h"se !iameter is theinsi!e !iameter "f the shell7


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Ba1es


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Parallel-fow heatexchanger


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Counter-fow heatexchanger


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Double pipe heat exchanger (A) andcrossfow heat exchanger (B)

A B


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Shell-an!-tu%e

+rrangement E.g. Tube-side fuid a!es two passes" shell-side

fuid a!es one pass.

#ood ixing o$ the shell-side fuid a!es one pass.


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8"g-9ean Temperature

Di3erence Temperature pr"les f"r single-pass !"u%le-

pipe heat exchanger


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C"unter"# analysis

Temperature ,s$ c"ntact area


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8"g-9ean Temperature

Di3erence (continued) )irst-la#-"f-therm"!ynamics

Energy transfer %et#een the t#" ui!s

. .

p c p H

c H

q mC T mC T = =

. .

p c c c p H H H

c H

dq mC dT C dT mC dT C dT = = = =

( )

( )

H C

H C H C

dq UdA T T

T T T d T dT dT

= = =


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8"g-9ean Temperature Di3erence(continued)


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!q : ;


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Example >2


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Example >2 (c"ntinue!*


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Shell-an!-Tu%e Heat Exchanger (2*


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Shell-an!-Tu%e Heat Exchanger (=*


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Shell-an!-Tu%e Heat Exchanger (?*


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Shell-an!-Tu%e Heat Exchanger (@*


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Cr"ss )l"# Heat Exchanger(2*


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Cr"ss )l"# HeatExchanger (=*


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Cr"ss )l"# HeatExchanger (?*


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Example > =

?A

?A

= ?A

= ?22$2

?A ?A

%" &" 'ater * -+ ,.

T" C" il ,/0-+ ,0*


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Heat Exchanger Design


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Heat Exchangers

Type "f Exchanger


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Design Stan!ar!s

Sc"pe "f stan!ar!

Deniti"n


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Design Stan!ar!s

Stan!ar! C"rr"si"n +ll"#ances

)"r Car%"n steel Parts Pressure Parts

5 type 2 /(?$= mm*

CB type 22F0(2$F mm*

.nternal )l"ating Hea! c",ers

+ll #ette! part

Tu%e sheets

'n each si!e "f it N"n Pressure Parts

Tie r"!s& spacers& %a1es an! supp"rt plates are n"trequire! t" ha,e c"rr"si"n all"#ances


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Design Stan!ar!s

Tu%es

Tu%e length

GF0& 2=0& 2@@0& 2G=0 an! =@0

Tu%e !iameters an! auges


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Design Stan!ar!s

; Tu%es

9inimum tu%e #all thickness in the %entp"rti"n


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Design Stan!ar!s

Stan!ar! tu%e Pattern

Tu%e Pitch 5 type 9in$ centre t" centre !istance : 2$=A 'D

(mechanical cleaning must* 9in cleaning lane I0

C type 9in$ centre t" centre !istance : 2$=A 'D if tu%e !iaJ A0 & min c t" c !istance : 2$= 'D

B type 9in$ centre t" centre !istance : 2$=A 'D (mechanical cleaning must* 9in cleaning lane ?2F0 (shell Dia J2=0* (mechanical cleaning must* 9in cleaning lane I0 (shell Dia K2=0*


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Design Stan!ar!s

Shells an! its c",ers

9inimum shell thickness (5 type*


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Design Stan!ar!s

9inimum Shell Thickness (CB type*


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Tu%e Sheet 8ay"ut


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N"$ "f Tu%es


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Design Stan!ar!s

Ba1es an! Supp"rt Plates

Typical Ba1e cuts


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Design Stan!ar!s

Trans,erse Ba1e an! Supp"rtClearance


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Design Stan!ar!s

Trans,erse Ba1e an! Supp"rt9inimum thickness (5 type*


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Design Stan!ar!s

Trans,erse Ba1e an! Supp"rt9inimum thickness (CB type*


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Design Stan!ar!s

8"ngitu!inal Ba1es 5 type

9inimum metal thickness : I/

CB type CS minimum metal thickness : I0

+ll"y - minimum metal thickness : 20

Spacing "f Ba1es an! supp"rt plates Segmental %a1es sh"ul! n"t %e cl"ser

than 2A "f shell .D "r = incheshiche,er is greater


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Design Stan!ar!s

9aximum spacing


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Design Stan!ar!s

.mpingement Ba1e

5equire!

.f LM= K 2A (f"r n"n a%rasi,e single phase

"#* .f LM= K A (f"r all "ther liqui!& saturate!

liqui!* L l%ft?

M fts

Shell %un!le Entrance .n n" case LM= K @


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Design Stan!ar!s

Tier"!s an! spacers

5 type

CB type

eneral Design


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eneral DesignC"nsi!erati"ns

)lui! +ll"cati"n

C"rr"si"n

The m"re c"rr"si,e ui! sh"ul! %e all"cate!

t" the tu%e-si!eThis #ill re!uce the c"st "f expensi,e all"y

"r cla! c"mp"nents

)"uling

The ui! that has the greatest ten!ency t"f"ul the heat-transfer surfaces sh"ul! %eplace! in the tu%es

eneral Design


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This #ill gi,e %etter c"ntr"l ",er the !esignui! ,el"city& an! the higher all"#a%le ,el"cityin the tu%es #ill re!uce f"uling$ +ls"& the tu%es#ill %e easier t" clean

)lui! temperatures .f the temperatures are high en"ugh t" require

the use "f special all"ys placing the highertemperature ui! in the tu%es #ill re!uce the",erall c"st$

+t m"!erate temperatures& placing the h"tterui! in the tu%es #ill re!uce the shell surfacetemperatures& an! hence the nee! f"r laggingt" re!uce heat l"ss& "r f"r safety reas"ns&


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eneral Design


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Misc"sity enerally& a higher heat-transfer c"ecient

#ill %e "%taine! %y all"cating the m"re,isc"us material t" the shell-si!e& pr",i!ing

the "# is tur%ulentThe critical 5eyn"l!s num%er f"r tur%ulent

"# in the shell is in the regi"n "f =

.f tur%ulent "# cann"t %e achie,e! in the

shell it is %etter t" place the ui! in the tu%es&as the tu%e-si!e heat-transfer c"ecient can%e pre!icte! #ith m"re certainty

Shell an! Tu%e ui!


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Shell an! Tu%e ui!Mel"cities

High ,el"cities #ill gi,e high heat-transfer c"ecients %ut als" a high-pressure !r"p

The ,el"city must %e high en"ugh t"pre,ent any suspen!e! s"li!ssettling& %ut n"t s" high as t" cause

er"si"n$

Shell an! Tu%e ui!


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Shell an! Tu%e ui!Mel"cities


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Pressure Dr"p

.n many applicati"ns the pressure!r"p a,aila%le t" !ri,e the ui!sthr"ugh the exchanger #ill %e set %y

the pr"cess c"n!iti"ns& an! the a,aila%le pressure !r"p #ill

,ary fr"m a fe# milli%ars in ,acuum

ser,ice t" se,eral %ars in pressuresystems


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Pressure Dr"p

Ohen the !esigner is free t" selectthe pressure !r"p an ec"n"micanalysis can %e ma!e t" !etermine

the exchanger !esign #hich gi,esthe l"#est "perating c"sts& takingint" c"nsi!erati"n %"th capital an!

pumping c"sts$


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Shell si!e calculati"n


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i l


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Design Example


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Lecture 15 HT PNSheth