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Heat Exchanger DesignShell-and-Tube Exchanger

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Shell-and-tube Exchangers

Provide large amounts of heat transfer area--upto 5000 ft2--in relatively small space

Available in standard sizes--specified by TEMA--of shell diameters and tube counts

Number of tubes per shell depends on shell

diameter, tube diameter, and tube pitch Tube lengths available in multiples of 4 ft, e.g.,

8, 12, 16, 20 ft

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Shell-and-Tube Exchanger

3

im sm P

Ds

Cross sectional area of tubes:

Outside heat transfer area: LDNA

D

N

N

S

otubeso

i

passes

tubes

i

4

2

P = baffle pitch

Ds = shell inside diameter

Di = tube inside diameter

Do

= tube outside diameter

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Selection of number of tube passes

Inside film coefficient changes with the 0.8power of the number of tube passes

Pressure drop changes with the 2.8 power of thenumber of tube passes

Example:

4

1 100 0.2

2 174 1.4

4 303 9.7

6 419 30.2

8 528 67.6

Npasses hi p, psi

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Selection of baffle pitch

Shell fluid velocity is inversely proportional tothe baffle pitch (distance between baffles)

ho increases with velocity to the 0.6 power p increases with velocity to the 2.8 power

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6 500 100.0

8 421 44.7

10 368 23.9

12 330 14.4

18 259 4.6

P, inches ho p, psi

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Temperature Patterns

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Correction to LMTD

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flowrentcountercurassumingLMTD

L

LG

T

TFT

For one shell pass, 2, 4, 6, tube passes:

caha

cacbH

cacb

hbha

H

H

H

H

G

TT

TT

TT

TTZ

ZZ

ZZ

ZZ

Z

F

11/2

11/2ln

1

1ln

1

1

2

2

2

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Correction to LMTD

8

flowrentcountercurassumingLMTD

L

LG

T

TFT

For two shell passes, 4, 8, 12, tube passes:

caha

cacbH

cacb

hbha

HHHH

HHHH

H

H

G

TT

TT

TT

TTZ

ZZZ

ZZZ

ZZ

Z

F

111/21/2

111/21/2

ln

1

1ln

12

1

2

2

2

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LMTD Correction Factor FG When either fluid temperature is constant,FG

= 1

It does not matter which fluid is in the shelland which is in the tubes

It does not matter in which direction the shellfluid flows

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Rating an existing exchanger

Exchanger exists and we know:

Number of tubes and tube passes,Nt,Npasses

Tube diameter, length, and rating,Do,Di,L,xw

Shell diameter, baffle pitch, tube pitch,Ds,P,p

Compute area of heat transfer:

Compute design coefficient:

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LDNA oto

TA

qU

o

oD

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Exchanger Rating (continued)

Compute film coefficients and clean over-allcoefficient:

Compute available fouling resistance:

If fouling resistance is large enough, theexchanger is suitable for the service

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ii

o

mL

wo

ooc hD

D

kD

xD

hU

11

ocoD

f

UU

r11

hi from Sieder-Tate

ho from Donohue

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Selecting a Standard Exchanger

Estimate the heat transfer coefficient U(see McCabe, Smith, and Harriott, 7th ed., Table 11.2, page 343)

Calculate an estimate of the area and number oftubes

Select an appropriate shell diameter from TEMA

tube count tables Rate the selected exchanger.

12

LD

AN

TU

qA

o

oto

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Summary

Shell-and-tube heat exchangers provide largeheat transfer area in a compact space

Fluid velocities are controlled by the number oftube passes and baffle spacing

LMTD must be corrected for theparallel/countercurrent patterns

Select and rate exchangers by determining theavailable fouling resistance

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