Area Target

Section StreamPopulation

jk

Cold Streams

Hot Streams

H

T

Qj

enthalpy interval

kq

kq

j jk k

k

jlmtot h

qT

A ||1

,

Townsend, D. W. and B. Linnhoff, “Surface Area Targets for Heat Exchanger Networks”, IChemE Annual Research Meeting, Bath, England (April, 1984)

H

T Steam

CW

CPC=14

CPH=7T=100

T=80

T=90

T=75

T=100

T=80

T=90

T=75

CP = 0.5CP = 2.5

CP = 1

CP = 1

CP = 6

Figure B.1 within each enthalpy interval it is possible to design a network in(S - 1) matches. (From Ahmad and Simth, IChemE, ChERD, 67: 481, 1989; reproduced by permission of the Institution of Chemical Engineers.)

250240230220210200190180170160150140130120110100 90 0 200 400 600

Table 8.1-1Approach temperature = 10

+

++

+

++

+ +

+ +

+

Enthalpy, 1000 Btu/hr = Hot + = Cold

Temperature,

F

FIGURE 8.3-1Tempreature-enthalpy diagram.

176

Hotutility

Coldutility

1

2

2

21

1 1

1

2Number of cold streamsin this interval

j

Hot composite Curve

T (F) H (BTU/hr)103

100 0 120 80 140 180 160 280 200 480 250 530

Cold composite Curve

T (F) H (103 BTU/hr)

90 60 130 180 150 360 190 600

MINIMUM HEAT TRANSFER AREA IN INTERVAL j Stream No. CP 103 qk,j 103 TS TT and Type (BTU/hr.F) (BTU/hr) (F) (F)

1. Hot 1 36 176 140 2. Hot 4 144 176 140 3. Cold 3 - 60 130 150 4. Cold 6 - 120 130 150

Stream No. CP 103 qk,j 103 TS TT and Type (BTU/hr.F) (BTU/hr) (F) (F)

1. Hot 1 36 176 140 2. Hot 2/3 24 176 140 31/3 120 176 140 3. Cold 1.8 - 36 130 150 1.2 - 24 130 150 4. Cold 6 - 120 130 150

1

2

3

MINIMUM HEAT TRANSFER AREA IN INTERVAL j

MIN

42,32,31,

1112011241136hhThhThhT

Ajlmjlmjlm

j

4321,

12060144361hhhhT jlm

MIN

jN

k k

jk

jlmj h

q

TA

1

,

,

1

where

Nj = total number of process streams in interval j

jN

k k

jkN

j jlmtot h

q

TA

1

,

1 ,

1MIN

Thot-cold

Tcold

PINCH

(a)

A Screening Procedure For Detection of “Bad” Matches

T

Tcold

(b)

A Screening Procedure For Detection of “Bad” Matches

A Good Match

T

Tcold

(c)

A Screening Procedure For Detection of “Bad” Matches

Two Bad Matches

1-1 Exchanger (a) Temperature

Length

countercurrent

Figure 7.7 1-1 shells approach pure countercurrent flow, whereas 1-2 shells exhibitpartial countercurrent and partial cocurrent flow.

Q = UATLM

TH1

TH2TC2

TC1

TH1

TC2

TH2

TC1

TC2 TH1

TH2TC1

Q =(UATLM) FT FT = f (R,P)

H

C

CC

HH

CPCP

TTTTR

12

21

11

12

CH

CC

TTTTP

1-2 Exchanger (b) Temperature

Length

TC1

TC2

TH1

TH2

partial cuntercurrentpartial cocurrent

R=1

Figure 7.8

Figure 7.8 Designs with a temperature approach or small temperature cross can be accommodated in a single 1-2 shell, whereas designs with a large temperature cross become infeasible. (From Ahmad, Linnhoff, and Smith, Trans. ASME, J. Heat Transfer, 110: 340, 1988; reproduced by permission of the American Society of Mechanical Engineers.)

temperature cross large

P large

(a) A single 1-2 shell is infeasible.

Figure 7.10 A large overall temperature cross requires shells in series to reduce the cross in individual exchangers.(From Ahmad, Linnhoff, and Smith, Trans. ASME, J. Heat Transfer, 110: 340, 1988; reproduced by permission of the American Society of Mechanical Engineers.)

A

B

A B

temperature crosses smaller

Figure 7.10 A large overall temperature cross requires shells in series to reduce the cross in individual exchangers.(From Ahmad, Linnhoff, and Smith, Trans. ASME, J. Heat Transfer, 110: 340, 1988; reproduced by permission of the American Society of Mechanical Engineers.)

(b) Putting shells in series reduces the temperature cross in individual exchangers.