11A - Chapter 11, Sec 11.1 - 11.3 Black

7/25/2019 11A - Chapter 11, Sec 11.1 - 11.3 Black

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Heat Exchangers:

Design Considerations

Chapter 11

Sections 11.1 through 11.3



Types

Heat Exchanger Types

Heat exchangers are ubiquitous to energy conversion and utiliation. They involve

heat exchange bet!een t!o "luids separated by a solid and enco#pass a !ide

range o" "lo! con"igurations.

$ Concentric%Tube Heat Exchangers

&arallel 'lo! Counter"lo!

Si#plest con"iguration.

Superior per"or#ance associated !ith counter "lo!.



Types (cont.)

$ Cross%"lo! Heat Exchangers

'inned%*oth 'luids

+n#ixed

+n"inned%,ne 'luid -ixed

the ,ther +n#ixed

'or cross%"lo! over the tubes "luid #otion and hence #ixing in the transverse

direction ( y) is prevented "or the "inned tubes but occurs "or the un"inned condition.

Heat exchanger per"or#ance is in"luenced by #ixing.



Types (cont.)

$ Shell%and%Tube Heat Exchangers

One Shell Pass and One Tube Pass

*a""les are used to establish a cross%"lo! and to induce turbulent #ixing o" the shell%side "luid both o" !hich enhance convection.

The nu#ber o" tube and shell passes #ay be varied e.g.:

,ne Shell &ass

T!o Tube &asses

T!o Shell &asses

'our Tube &asses



Types (cont.)

$ Co#pact Heat Exchangers

/idely used to achieve large heat rates per unit volu#e particularly !hen

one or both "luids is a gas.

Characteried by large heat trans"er sur"ace areas per unit volu#e s#all"lo! passages and la#inar "lo!.

(a) 'in%tube ("lat tubes continuous plate "ins)

(b) 'in%tube (circular tubes continuous plate "ins)

(c) 'in%tube (circular tubes circular "ins)

(d) &late%"in (single pass)

(e) &late%"in (#ultipass)



,verall Coe""icient

,verall Heat Trans"er Coe""icient

$ 0n essential require#ent "or heat exchanger design or per"or#ance calculations.

$ Contributing "actors include convection and conduction associated !ith the

t!o "luids and the inter#ediate solid as !ell as the potential use o" "ins on both

sides and the e""ects o" ti#e%dependent sur"ace "ouling.

$ /ith subscripts c and h used to designate the hot and cold "luids respectively

the #ost general expression "or the overall coe""icient is:

( ) ( )

( ) ( ) ( ) ( )

1 1 1

1 1

c h

f c f h

w

o o o oc c h h

UA UA UA

R R R

hA A A hAη η η η

= =

′′ ′′= + + + +



,verall Coe""icient

( )o

,verall sur"ace e""iciency o" "in array (Section 3..2)

1 1

o

f

c or h f

c or h

A

A

η

η η

→

= − − ÷

total sur"ace area ("ins and exposed base) sur"ace area o" "ins onlyt

f A A A = →→

0ssu#ing an adiabatic tip the "in e""iciency is

( )

tanh f c or h

c or h

mL

mL

η

= ÷

( ) 4c or h p w c or h

m U k t =

partial overall coe1

""icient p c or h

f c or h

hU hR

= → ÷ ÷′′+

"or a unit sur" 'ouling "act ace area (# /)or 54 f R′′ → ×

Table 11.1→

conduction resistan/all (54/ce )w R →



6-TD -ethod

0 -ethodology "or Heat Exchanger

Design Calculations% The 6og -ean Te#perature Di""erence (6-TD) -ethod %

$ 0 "or# o" 7e!ton8s 6a! o" Cooling #ay be applied to heat exchangers by using a log%#ean value o" the te#perature di""erence bet!een the t!o "luids:

1mq U A T ∆=

( )1

1

1 1n 4m

T T T

T T

∆ ∆∆

∆ ∆

−=

Evaluation o" depends on the heat exchanger type.1 andT T ∆ ∆

$ Counter%'lo! Heat Exchanger :

1 1 1

h c

h i c o

T T T

T T

∆ ≡ −

= −

h c

h o c i

T T T

T T

∆ ≡ −= −



6-TD -ethod (cont.)

$ &arallel%'lo! Heat Exchanger :

1 1 1

h c

h i c i

T T T

T T

∆ ≡ −

= −

h c

h o c o

T T T T T

∆ ≡ −= −

7ote that T c,o can not exceed T h,o "or a &' H9 but can do so "or a C' H9.

'or equivalent values o" UA and inlet te#peratures

1 1 m CF m PF T T ∆ ∆>

$ Shell%and%Tube and Cross%'lo! Heat Exchangers:

1 1 m m CF T F T ∆ ∆=

'igures 11.1 % 11.13 F →



Energy *alance

,verall Energy *alance

$ 0ssu#e negligible heat trans"er bet!een the exchanger and its surroundings and negligible potential and ;inetic energy changes "or each "luid.

( ) h i h ohq m i i×= −

( ) c c o c iq m i i×

= −

"luid enthalpyi →

$ 0ssu#ing no l/v phase change and constant speci"ic heats

( ) p h h i h ohq m c T T ×= − ( ) h h i h oC T T = −

( ) c p c c o c iq m c T T ×

= − ( ) c c o c iC T T = −

Heat capacity r sateh cC C →

$ 0pplication to the hot (h) and cold (c) "luids:



Special Conditions

Special ,perating Conditions

Case (a): C h>>C c or h is a condensing vapor ( ) .hC → ∞

– 7egligible or no change in ( ) .h h o h iT T T =

Case (b): C c>>C h or c is an evaporating liquid ( ) .cC → ∞

– 7egligible or no change in ( ) .c c o c iT T T =

Case (c): C h=C c

1 1mT T T ∆ ∆ ∆= = –



&roble#: ,verall Heat Trans"er Coe""icient

&roble# 11.2: Deter#ination o" heat trans"er per unit length "or heat recovery

device involving hot "lue gases and !ater.





(cont.)

( ) ( ) ( )! oc c h14 +0 14 h0 < 14 h0

η = + +

( ) ( )

( )

i i1 =!

ln D 4 D ln 3:4 =< >.1: 1: 5 4 /.

;6 2: / 4 # 5 l#π π

−= = = ×

⋅





(cont.)





(cont.)




&roble#: ,verall Heat Trans"er Coe""icient(cont.)

&roble#: ,cean Ther#al Energ Con ersion



&roble#: ,cean Ther#al Energy Conversion

&roble# 11.=>: Design o" a t!o%pass shell%and%tube heat exchanger to supply

vapor "or the turbine o" an ocean ther#al energy conversion

syste# based on a standard (<an;ine) po!er cycle. The po!er

cycle is to generate -/e at an e""iciency o" 3?. ,cean

!ater enters the tubes o" the exchanger at 35 and its desired

outlet te#perature is @5. The !or;ing "luid o" the po!er

cycle is evaporated in the tubes o" the exchanger at its

phase change te#perature o" @5 and the overall heat trans"er

coe""icient is ;no!n.

&roble#: ,cean Ther#al



&roble#: ,cean Ther#al Energy Conversion (cont)





Energy Conversion (cont)

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11A - Chapter 11, Sec 11.1 - 11.3 Black