# ChE455 Heat Exchanger Networks by 1G6xMwj5

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```									Heat Exchanger Networks
& Utility Minimization
NMSU Chemical Engineering
Ch E 455
Outline

• Heat Integration
• Design Procedure for MUMNE
–   Temperature interval diagram
–   Temperature-Enthalpy diagram
–   Minimum number of exchangers
–   Design above and below pinch
Heat Integration

• Heat exchange networks
• It saves money to match streams rather than pay to
heat one and pay to cool another
• You have already done this on ad hoc basis in design
projects
Heat Integration

• There is a rigorous methodology
• We will learn MUMNE (Minimum Utility, Minimum
Number of Exchangers) method
• Not necessarily (and unlikely to be) economic
optimum
Design Procedure

1. Complete energy balance on all streams to
determine all temperatures, mC p values, and heat
flows.
2. Choose minimum approach temperature. Typically,
this is between 5°C and 20°C, but any positive
number is valid.
3. Complete temperature interval diagram, Each
stream is drawn and labeled. The heat flow in each
interval is calculated.
Design Procedure

4. Complete the cascade diagram. The energy excess
or deficit is calculated for each interval on the
temperature interval diagram.
5. Find the minimum hot and cold utility requirements
and identify the pinch temperature.
6. Complete the composite temperature enthalpy
diagram. This is a T-Q diagram for the entire
process.
Design Procedure

7. Determine the minimum number of heat
exchangers required above and below the pinch.
8. Design the heat exchanger network.
Pinch Technology

– simple, does not require elaborate mathematics
– sets performance targets before actual design (minimum
required theoretical utility for entire process)
– analysis provides network design by matching hot and cold
streams for heat integration
– graphical representation (composite curve) used to
increase conceptual understanding of system
– method table used to predict minimum utility
requirements
Application of Pinch Technology

• Design an integrated heat exchanger system with a
minimum approach temperature of 10°F to
minimize utility for the following six streams:

hot streams         cold streams
str    A       B     C     D       E        F
m     4000 10000 6000      6000    9000     6000   lb/hr
Cp     0.65 1.00 0.50       0.70    0.95     0.55   btu/lb°F
To      410 370       270    260     310      340   °F
Tf     350 290       250    300     370      390   °F
mCp    2600 10000 3000      4200    8550     3300   btu/hr°F
Example

• The heat flow values of Q (or ΔH) are calculated from
the energy balance. The sign convention is positive
for heat available from a stream and negative for
heat needed by a stream.
• Choose the minimum approach temperature. For
this problem, it is 10°C.
Example

• Draw and label the temperature interval diagram.
Label the intervals beginning with “A” for the
highest temperature interval. The heat flow for each
interval is calculated from
Q=   åmC pDT
• where the sum is over all streams existing in that
interval.
Temperature Interval Diagram
Prepare the composite curves,                      A   B   C       F   E   D
distinguishing interval of temperature
400°F
where streams influent/effluent
temperatures begin/end
hot streams                  cold streams                1               1
str      A       B       C         D         E       F
m            hot streams
4000 10000 6000            6000      streams
cold 9000 6000 lb/hr 2                     2
350°F
Cpstr      A       B
0.65 1.00 0.50   C          D
0.70      E
0.95     F
0.55 btu/lb°F
Tom       4000 370
410 10000 270  6000        6000 310
260      9000 340 °F
6000 lb/hr
3
Cp
Tf                1.00 0.50
0.65 290
350 streams 250             0.70 370
300       0.95 390 °F 3
0.55 btu/lb°F
To        cold 370
410            270         260 8550 3300 btu/hr°F
310    340 °F                      4
mCp      2600 10000 3000
D 350 E290 F               4200       300°F
Tf                       250         300      370    390 °F
00 mCp     6000cold streams
9000 6000
2600 10000 3000 lb/hr 4200          8550 3300 btu/hr°F4                  5
C
50            D
0.70        E
0.95      F
0.55 btu/lb°F
000         6000 310  9000 340 °Flb/hr
6000                                5
70          260                                  250°F
0.50
50            0.70 370
300                0.55
0.95 390 °Fbtu/lb°F
270
00         4200         310     340 °F
260 8550 3300 btu/hr°F
250           300      370     390 °F
000         4200      8550 3300 btu/hr°F
Temperature Interval Diagram
Treating all Hot streams and all Cold              A      B     C           F   E   D

streams together, determine heat flow in   400°F
each interval                                          H1  104,000
1
H  m c p T
                                                                    1

2
H1  m1c p,1T1
                                  350°F
2

H1  4,000lb hr                                                    3
3

 0.65 Btu lb  hr  F         300°F
4

 370  410F                                                4               5

H1  104,000Btu hr                     250°F
5
Temperature Interval Diagram
Treating all Hot streams and all Cold                A     B       C           F     E        D
streams together, determine heat flow in   400°F
each interval                                          H1  104,000
1          H1  66,000
H  m c p T
                                                                              1
H2  252,000                    H2  355,500
2                         2
H2  m1c p,1  m2c p,2 T2
350°F
                                                H3  600,000

H2  4,0000.65
3
3                H3  256,500

 10,0001.0 
H4  0            4
300°F

H4  0              H5  168,000

 350  370
4                         5
H5  60,000
5
H2  252,000                         250°F
Composite                      420
410
hot
cold
Define pinch point as temperature approach

400                                       Pinch
Curves                         390
380
Point

370
360

temperature (°F)
350
only reject
340
heat below
330    the pinch
320      point                                                      Qhot
310
300
Never transfer heat       only add heat
290
across the pinch point.     above the
280       Qcold                                                 pinch point
270
260
250
0.E+00       2.E+05        4.E+05      6.E+05      8.E+05     1.E+06      1.E+06
H
Method Table

•   draw hot/cold temperature scales offset by Tmin
•   plot stream temperatures on appropriate scales
•   determine temperature intervals
•   divide stream’s temperature change into intervals
based on supply & target temperatures for each
stream
Method Table
int   T(°   mcc -    Hi     cumulative
A   B   C   F   E   D
F)    mch                                                               400°F
400°F
1
1      10    -2.60    -26.0     -26.0
2      20    +0.70    +14.0     -12.0
2
3      10    +9.25    +92.5     +80.5                                       3   pinch
point
4      20    -0.75    -15.0     -15.0                                       4    350°F
350°F
5      30    -1.45    -43.5     -58.5
6      10    -10.00   -100.0   -158.5                                       5
7      20    -5.80    -116.0   -274.5
8      20    +4.20    +84.0    -190.5
6   300°F
9      20    -2.50    -50.0    -250.5       300°F                           7

heat surplus may be transferred                                             8
to the lower temperature interval
9   250°F
250°F

-26000
400

400                  -26000             390

14000
380   Qhot           -12000             370

-80500      92500
pinch
370                        0            360
point
-15000
350                  -15000             340

-43500
320                  -58500             310

-100000
310                 -158500             300

-116000
290                 -274500             280

84000
270                 -190500    Qcold    260

-60000       250500
250                        0            240
Minimum Energy Network Design
mcp          A     B    C     D      E     F
(x   10-3   Btu/hr°F)   2.6   10   3.0   4.2   8.55   3.3
400°C
400°C

350°C
350°C

300°C
300°C

250°C
250°C
Minimum Energy Network Design
mcp          A           B           C         D              E          F
(x   10-3   Btu/hr°F)   2.6         10          3.0       4.2           8.55        3.3
400°C
400°C

402.9°F
85.5                                                      85.5

HA,  mc p T
                                        350°C
350°C
85.5  103  2600T  370
T  402.9F
300°C
300°C                                                    HE,  mc p T


 855010
 85.5  103 Btu hr
250°C
250°C
Minimum Energy Network Design
mcp           A           B           C          D             E     F
(x   10-3   Btu/hr°F)    2.6         10          3.0        4.2          8.55   3.3
400°C
400°C

18.5                                                            18.5
402.9°F

350°C
350°C
HA,  mc p T


 2600410  402.9 
 18.5  103 Btu hr
300°C
300°C

250°C
250°C
Minimum Energy Network Design
mcp          A           B           C           D            E           F
(x   10-3   Btu/hr°F)   2.6         10          3.0         4.2         8.55         3.3
400°C
400°C
365.6°F
18.5
402.9°F

350°C
350°C                                 HF,  mc p T


18.5  103  3300T  360
 365.6F
300°C
300°C

250°C
250°C
Minimum Energy Network Design
mcp          A           B          C           D            E           F
(x   10-3   Btu/hr°F)   2.6         10         3.0         4.2         8.55         3.3
400°C
400°C
366.6°F
QH = 80.5
402.9°F

350°C
HF,  mc p T

350°C
 3300390  365.6 
 80.5  103 Btu hr
300°C
300°C

250°C
250°C
Minimum Energy Network Design
mcp          A             B              C            D             E            F
(x   10-3   Btu/hr°F)   2.6           10             3.0          4.2          8.55          3.3
400°C
400°C
366.6°F
QH = 80.5
402.9°F

350°C
350°C
52.0               If we do this, stream B will no longer           66.0
have heat available at a sufficiently              14.0
high temperature to supply stream E
427.5
300°C
300°C

800.0

168
250°C
250°C
50.0
Minimum Energy Network Design
mcp          A             B              C            D             E             F
(x   10-3   Btu/hr°F)   2.6           10             3.0          4.2          8.55           3.3
400°C
400°C
366.6°F
QH = 80.5
402.9°F

350°C
350°C
52.0               If we do this, stream B will no longer            66.0
have heat available at a sufficiently               14.0
high temperature to supply stream F
427.5
300°C
300°C

800.0

168
250°C
250°C
50.0
Minimum Energy Network Design
mcp          A             B                C      D           E             F
(x   10-3   Btu/hr°F)   2.6           10               3.0    4.2        8.55           3.3
400°C
400°C
366.6°F
QH = 80.5
402.9°F

350°C
350°C
360.3°F
52.0                                                           66.0
344.4°F                                     14.0

427.5
300°C
300°C

800.0
Split streams B and F into
168   two streams to prevent
violation of the second law     250°C
250°C
50.0
Minimum Energy Network Design
mcp          A             B              C      D      E            F
(x   10-3   Btu/hr°F)   2.6           10             3.0    4.2   8.55          3.3
400°C
400°C
366.6°F
QH = 80.5
402.9°F

350°C
350°C
360.3°F
52.0                                                   66.0
344.4°F                               14.0

427.5
300°C
300°C                               320.0°F
QC = 190.5
116 684

QC = 50.0            168     QCtotal = 240.5         250°C
250°C
50.0
Minimum Energy Network Design
A              B              C   D   E                  F
400°C
400°C
366.6°F
402.9°F

350°C
350°C
360.3°F

344.4°F

A – F, E, F        300°C
300°C                       320.0°F               B1 – F, U
B2 – E, D, U
C–U
D – B2
E – B2, A
F – B1/A, A, U     250°C
250°C
Minimum Energy Network Design

402.9
410

370

350
A

370
18.5                       85.5                           52.0

340
360
365

F

390

290
360
14

370
B

310 320
250
270

C
360
427.5

A – F, E, F                                                        E
B1 – F, U

260 300
B2 – E, D, U
C–U

300
D – B2                                                           168
E – B2, A
D
F – B1/A, A, U

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