15-1
Chapter 15
15.1 Costs affected by changing the min temp approach for a HEN are
15.2 By decreasing the Tmin in a HEN, the process-process heat exchangers at the pinch will
15.3 (a) For streams requiring an MOC cheaper than CS, the film heat transfer coefficient, h,
for the stream should be increased. This means that smaller heat exchange areas will be
15.4 As Tmin increases the utility requirements increase (yearly operating costs) but the
process-process exchangers become smaller and less expensive. However, the costs of the
utility exchangers will increase. The costs for utilities and fixed capital investment
15.5 At the pinch, for streams above the pinch always match streams such that , ,
p
hot p cold
mc mc
Using this criterion avoids violating the Tmin criterion set at the start of the problem.
15.6 Composite temperature-enthalpy (T-Q) diagram
15.7 A cascade diagram illustrates the amounts of available energy, in excess of that required
by process streams, at each temperature level in the temperature interval diagram. The
15.9 For streams that change phase, two approaches are used
(i) if the stream is pure and the phase change occurs at a single temperature then we
may assume some arbitrary, small T of say 1 C and use an equivalent mcp for
15.10 MUMNE = minimum utility, minimum number of exchangers heat exchanger network. It
QH
hot streams
15-3
15.11 (a)
FhrBTU
Cm P4 4 3 5
Stream
Number 1 2 3 4
Temperature
(ºF)
Temperature
(ºF) hrBTU
Q
600 580
400 A 300 100
(b) A
100
B
-120
HU
20
100
20
Pinch at 470-450 ºF
15-4
(c)
Above
3 exchangers
1
520
HU
20
130
20
390
15-5
(d) Above PCPH CmCm
Stream Number 1 2 3 4
FhrBTUCm P 4 4 3 5
Or split stream 1
Stream Number 1 2 3 4
FhrBTUCm P 4 4 3 5
580
31
15-6
(e) Below pinch PCPH CmCm
Stream Number 1 2 3 4
FhrBTUCm P 4 4 3 5
OR
1314
Q4 = 330 BTU/hr
470 450
14
470
450
Q4 = 600 BTU/hr
44
15-7
15.12 (a)
Stream
Number 1 2 3 4
FhrBTU
Cm P2 4 3 2
Temperature
(ºF)
Temperature
(ºF) hrBTU
Q
400 390
160 A 160
(b)
A
160
160
15-8
(c)
Above
4 Exchangers
1
160
2
480
HU
60
3
420
4
280
160 260 220 60
(d) Above pinch PCPH CmCm
Stream Number 1 2 3 4
FhrBTUCm P 2 4 3 2
(e) Below pinch PCPH CmCm
Stream Number 1 2 3 4
FhrBTUCm P 2 4 3 2
31
15-10
15.13 (a)
Stream
Number 1 2 3 4
CkW/
P
Cm 3 5 3 2
Temperature
(ºC)
Temperature
(ºC) kW
Q
(b)
A
20
20
B
-80
HU
60
60
15-11
(c)
Above
3 exchangers
(d) Above pinch PCPH CmCm
Stream Number 1 2 3 4
CkW/Cm P 3 5 3 2
HU
60
1
180
60 60 120
170
3
HU Q3 = 60 kW
15-12
(e) Below pinch PCPH CmCm
Only one utility stream
Stream Number 1 2 3 4
C)kW/(Cm P 3 5 3 2
For two utility streams
Stream Number 1 2 3 4
CkW/Cm P 3 5 3 2
15-13
15.14 (a)
Stream
Number 1 2 3 4
FhrBTU
Cm P4 4 3 4
Temperature
(ºF)
Temperature
(ºF) hrBTU
Q
500 490
200 A 150 50
(b)
A
50
50
15-14
(c)
Above pinch
3 exchangers
1
280
HU
10
3
210
4
80
210 70 10
15-15
(d) Above pinch PCPH CmCm
Stream Number 1 2 3 4
FhrBTUCm P 4 4 3 4
(e) Below pinch PCPH CmCm
Stream Number 1 2 3 4
FhrBTUCm P 4 4 3 4
490
Exact match solution
Stream Number 1 2 3 4
FhrBTUCm P 4 4 3 4
15-17
15.15 (a)
Stream
Number 1 2 3 4
C)(kW/
Cm P3 5 4 3
Temperature
(ºC)
Temperature
(ºC) kW
Q
250 240
120 A 120
(b) A
120
120
15-18
(c)
Above pinch
4 exchangers
HU
120
1
150
2
500
3
440
4
330
120 320 150 180
15-19
(d) Above pinch PCPH CmCm
Stream Number 1 2 3 4
CkWCm P 3 5 4 3
Or
Stream Number 1 2 3 4
CkWCm P 3 5 4 3
3.2 1.8
2.9 2.1
15-20
(e) Below pinch PCPH CmCm
Stream Number 4
CkWCm P 3