Page 451 - Design and Operation of Heat Exchangers and their Networks

434 Design and operation of heat exchangers and their networks

is 171.428 10 4 $/kWh, and the cold utility cost (30–50°C) is
60.576 10 4 $/kWh. All heat exchangers including the heater and cooler
are of counterflow type. For the variation range of operation conditions
given in Table 9.1, determine the flexibility index and flexibility factor.
Solution
00
Let t i be the temperature of the ith fluid at the exit of the network before
000
entering a heater or cooler, and t i be that at the exit of the network after
leaving a heater or cooler. If there is no heater or cooler at the network
000 00
exit of the ith fluid, we have t i =t i .
For the case using a heater at the ith exit of the network, it requires
Table 9.1 Nominal operation conditions and deviations of Example 9.1.
_
Stream C (kW/K) t (°C) t (°C)
00
0
H1 1.4 0.4 310 10 50
H2 2.0 450 280
C1 3.0 40 120
C2 2.0 0.4 115 5 280

c
H2 2 1 2 H2
CU Ch1
Ch2 HU
c 2
E3
C2 4 4 C2
c 3
Ch3
Ch4
E2
H1 1 1 H1
c 4
Ch5
Ch6
E1
C1 3 3 C1
c 5
Fig. 9.1 Network configuration for the example.

Table 9.2 Overall heat transfer coefficients and areas of heat
exchangers in Example 9.1.
2
2
Heat exchanger Match k (kW/m K) A (m )
E1 H1-C1 0.8 103.83
E2 H1-C2 0.8 56.583
E3 H2-C2 0.8 23.315
HU HU-C2 0.8 23.770
CU H2-CU 0.8 13.013