Coker A.K. Fortran Programs for Chemical Process Design, Analysis, and Simulation

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Table 8-14

Factors Which Influence the Selection Process

Fluids in a Shell and Tube Heat Exchanger

Tube Side Shell Side

CORROSION: When special alloy

materials are necessary to minimize

corrosion. It is more economical to

place the corrosive fluids in the

tubes. Only the tubes, tube sheets

and channel need to be alloy. In

comparison, if the shell side was

selected, the shell side and baffles

would have to be alloy in addition to

the tubes and tube sheets.

CONDENSING SERVICE: The

large free area on the shell side

permits minimum pressure drop

arrangements as well as higher

condensate loadings.

EXCESSIVE FOULING: If

mechanical cleaning is desired, the

fouling medium should be placed in

the tube side. This enables the tubes

to be cleaned without removing the

bundle from the shell.

BOILING SERVICE: Vapor disen-

gaging space may be provided on

the shell side which eliminates the

need for a separate dry drum.

HIGH TEMPERATURE: If high

temperature service requires special

alloy construction, the cost of an

alloy shell and bonnet will be saved

by using the tube side.

CRITICAL PRESSURE DROP: By

varying the baffle, spacing, very low

pressure drops may be attained.

HIGH PRESSURE: Tubes, tube

sheets, and channel need be

designed only for high pressure if

the tube side is utilized. Also,

integral tube sheet and channel

design may be used, thus eliminating

one gasketed joint.

VISCOUS FLUIDS: If mechanical

cleaning is not required, higher heat

transfer rates may be obtained by

placing the viscous fluid on the shell

side. Due to the flow pattern across

the tube bank, turbulent flow may be

maintained on the shell side at mass

velocities which would yield laminar

flow on the tube side.

Source: Morton [7]

627

628

Fortran Programs for Chemical Process Design

(text continued from page 623)

a s = flow area, ft 2

B = baffle spacing, inch

C = clearance between adjacent tube, inch

D~ = equivalent diameter, ft

F c = temperature difference correction factor, At = (Fc)(LMTD)

F T = tube side fouling factor, hr ft2~

F s = shell side fouling factor, hr ft2~

f~ = tube side friction factor (ft2/in 2)

fs = shell side friction factor (ft2/in 2)

G t = tube side mass velocity, (lb/ft2hr) or (lb/ft2sec)

G~ = shell side mass velocity, (lb/ft2hr)

g = acceleration due to gravity, 32.17 ft/sec 2 (4.18 x l0 s ft/hr 2)

h i, h o = heat transfer coefficient for inside and outside fluids,

respectively, Btu/hrft2~

h~o = value of hi when referred to the tube outside diameter,

ID = inside diameter, inch (ft)

JH = factor for heat transfer, dimensionless.

k = thermal conductivity, Btu/hrft~

L = tube length, ft

LMTD = log mean temperature difference, ~

N = number of shell side baffles

N~ = number of shells per unit

N t =

number of tubes

n = number of tube passes

OD = outside diameter, inch

P = temperature group

(t 2 -- tl)/(T 1 -

t~)

P~ = prandtl number, dimensionless

Pt =

tube pitch, inch

AP T, AP t = total, tube side pressure drop, psi.

AP~, AP. = return and nozzle side pressure drops, psi

AP~ = shell side pressure drop, psi

Q = heat flow, Btu/hr

R = temperature group (T~-

Tz)/(t 2 -tl),

dimensionless

Re = reynolds number, dimensionless.

R d = combined dirt factor, hr ftZ~

Tj = hot fluid inlet temperature, ~

Y 2 =

hot fluid outlet temperature, ~

t~ = cold fluid inlet temperature, ~

t 2 =

cold fluid outlet temperature, ~

At = true temperature difference, ~

Heat Transfer 629

U = overall heat transfer coefficient, Btu/hrft2~

V n = nozzle side fluid velocity, ft/s

V~ = shell side fluid velocity, ft/s

V, = tube side fluid velocity, ft/s

W = mass flow rate of hot fluid, lb/hr

w = mass flow rate of cold fluid, lb/hr

~) = viscosity ratio, (~)0 ~4

~t = fluid viscosity, cP x 2.42 = lb/ft, hr.

P = fluid density, lb/ft 3

Subscripts

s = shell

t = tube

DOUBLE PIPE HEAT EXCHANGER

A double pipe exchanger consists of one or more pipes or tubes

inside a pipe shell. Basically two straight pipe lengths are connected at

one end to form a U or "hair-pin." Longitudinal fins may be used on the

outside of the inner tube. The flow is true countercurrent, which can be

beneficial when very close temperature approaches or very long tem-

perature ranges are required. Such an exchanger is used in service when

the heat duty is moderate (that is, UA < 100,00 Btu/hr~ or when one

stream is a viscous liquid, or when flow rates are small. This type of

exchanger is suitable for high pressure applications because of their

smaller diameters. Also, several hairpin sections provide for flexibility

in matching heat exchanger requirements with changing process condi-

tions. A double pipe exchanger is suited for "dirty" service because it is

easy to dismantle and clean. In addition, a pipe exchanger can be con-

sidered in the following situations:

1. When the shell side coefficient is less than half that of the tube side;

the annular side coefficient can be made comparable to the tube side.

2. When a high pressure can be catered for more economically in the

annulus than in a larger diameter shell.

3. At duties requiring 100 - 200 ft 2 of the surface, which make it

more economical.

4. When a true countercurrent flow can be obtained, thus eliminat-

ing temperature crosses that require multishell shell and tube units.

The equations required for the rating of a double pipe exchanger are

as follows.

630

Fortran Programs for Chemical Process Design

The Equations

Process Conditions Required

Hot fluid: T~,

T2, W, C,

ILk k, AR

R D, s

or 9

Cold fluid:

t~,

t 2, W, C, ~

k, AR

R D, s or

The diameter of the pipe must be given or assumed. The fluids' veloci-

ties must be in the range of 3-10 ft/sec. The following assumes that the

cold fluid is in the inner pipe.

(1) The heat duty Q, Btu/hr, is"

- WC(T l - T2) - wc(t 2 - tl)

(8-53)

(2) The log mean temperature difference (LMTD).

LMTD - (T, - t 2 )

- (T 2 -

t, ) (8-5)

ln{T~

- t 2

Z 2 -il}

Inner Pipe

(3) The flow area, ap:

a z

71;D 2

ft 2 (8-54)

(4) The mass velocity, Gp

w lb

Gp - ~, (8-55)

ap ft2hr

(5) The Reynolds number, Re t

D. Gp

Re~ = (8-56)

(6) The Prandtl number, Pr t

Pr~ = c.g (8-57)

(7) The heat transfer coefficient, h~, Btu/hrft2~

Heat Transfer 631

hi'D =C "

k lLt

)o.,4 w

(8-58)

h i can be expressed by

k) 08 r'~ 033

h i -C. -~ Re t t'r t

(8-59)

where Wgw- 1

C - 0.021 for gases

C - 0.023 for non-viscous fluids

C - 0.027 for viscous fluids

converting

h i to h

io,

the heat transfer coefficient referred to the pipe

outside diameter, Btu/hrft2~

inside dia. of inner pipe /

hio

outside dia. of inner pipe

ID (8-60)

= hi.oD

Annulus

(8) The flow area, a a"

a a =

~(D2o

D 2

f12 (8-61)

(9) The equivalent diameter, D e

4 x flow area

wetted perimeter

- D 2

__ i

ft (8-62)

(10) The mass velocity, G,

w lb

G a = -- (8-63)

a a ' ft:hr

632

Fortran Programs for Chemical Process Design

(11) The Reynolds number, Rea

Re a --

De ~ Ga

(8-64)

(12) The Prandtl number, Pr a

Pr a -

c. la

(13) The heat transfer coefficient, h o

If Re a < 2100

(8-65)

ho _ 1.8 6 k

o33 pr:33( / ~

ea" -

(8-66)

If Re a > 2100

ho - C. k RepS

t.ra.r~

(14) The overall coefficient, U, Btu/hr.

ft. 2

1 1 1

= -I- +RD+R w

U hio

h o

(15) The heat transfer area, A,

ft 2

U D . LMTD

(8-67)

(8-68)

(8-69)

Vapor Service

(16) The heat transfer coefficient, h~, Btu/hrft 2~ F

0.0144CpG ~

D~ .2

(8-70)

and

hif

+ Fol (h i is corrected for fouling) (8-71)

Heat Transfer 633

Shell Side (Finned Tube)

Figure 8-13 shows examples of extended surfaces on one or both

sides of heat exchangers.

(a) (b) (e)

i'i LI

(d)

Ga~

flow

(g)

Liquid flow

Figure 8-13. Examples of extended surfaces on one or both sides. (a) Radial fins.

(b) Serrated radial fins. (c) Studded surface. (d) Joint between tubesheet and low fin

tube with three times bare surface. (e) External axial fins. (f) Internal axial fins. (g) Finned

surface with internal spiral to promote turbulence. (h) Plate fins on both sides. (i) Tubes

and plate fins. Source: S. M. Walas,

Chemical Process Equipment: Selection and

Design,

Butterworths Series in Chemical Engineering, 1988.

634

Fortran Programs for Chemical Process Design

(17) The equivalent diameter,

De,

4NFA

D e = (8-72)

rc(D,,~ + D~,o) - NO + 2HN

(18) The net free cross-sectional area, NFA, shell side,

ft 2

NFA- CSA- NH0

(19) The cross-sectional area without fins, CSA,

ft 2

(8-73)

CSA -

rc (D 2 _

D2 ) (8-74)

s,i t,o

(20) Finned surface area, ft 2

A t. - 2HN (8-75)

(21) Outside heat transfer area of tube,

ft2/ft

(for finned tubes in-

cludes fin area)

A o - rcD~, o + Af (8-76)

(22) Parameter for fin efficiency, X

)0.5

X - H hif

6Kt0

(8-77)

(23) The fin efficiency, e

e- tanhX = 11exp(X)-exp(-X) 1 (8-78)

X X exp(X) + exp(-X)

(24) The effective surface efficiency for fins, eel f,

eel f - e + 1- Ao

(8-79)

2 o

(25) The corrected film heat transfer coefficient,

hifd,

Btu/ft hr F

h~f d

- (hif)(eeff)

(8-80)

(26) The overall heat transfer coefficient, U, Btu/ft2hr~

Heat Transfer 635

1/ w /

U -- -~t tube wall

/a/

hi ~ tube

( h ira )shell

where

Ai _

~'cDt, i

reD + 2HN

t,O

Pressure Drop, AP

(27) The pressure drop in tubes is:

4fG2L

AF - (ft of liquid)

2gp2D

where

f - for Re < 2,300

Using, Wilson

et al.

[15] correlation for commercial pipe

f - 0.0035 +

0.264

Re 0.42

for Re < 2,300

APp-

(AF'P/'144

psi

Annulus

For an annulus, D~ is:

D e -

4~(D 2 o - D~)

4 rt(Do + D~ )

=Do-D i

Reynolds number, Re a

Re a -

D;Ga

(8-81)

(8-82)

(8-83)

(8-84)

(8-85)

(8-86)

(8-87)

(8-88)

636

Fortran Programs for Chemical Process Design

Pressure drop, AF., ft

_ f 2

AF. - 4 G L ft (8-89)

2gpZD~ '

The pressure drop due to the reversal of flow in the annulus for each

hairpin is:

AF r = ~, fl/hair

pin (8-90)

The total annulus pressure drop is"

AP-I (AF~ +AFr)9]

psi (8-91)

Nomenclature

A = cross-sectional area, f12

Ai = inside heat transfer area-tube,

ft2/ft

A o = outside heat transfer area-tube, ft2/ft (for finned tubes includes

fin area)

Ar-

finned transfer area, f12

ap- flow area, ft 2

C = specific heat of hot fluid, Btu/lb~

c = specific heat of cold fluid, Btu/lb~

CSA = cross-sectional area, shell side without fins, ft 2

D = inside diameter, fl

D~, D o = for annuli, D~ is the outside diameter of inner pipe, D o is the

inside diameter of the outer pipe, inch

Dt, ~ = tube inside diameter, inch

Dt, o =

tube outside diameter, inch

D~,~ = shell inside diameter, inch

D e = equivalent diameter for heat-transfer, ft

D e = equivalent diameter for pressure drop, ft.

e = fin efficiency

e" = effective surface efficiency for fins

FOL = fouling factor, hrfl2~

f- friction factor, dimensionless

AF = pressure drop, ft