Page 270 - Design and Operation of Heat Exchangers and their Networks

258 Design and operation of heat exchangers and their networks

45°C 125°C 125°C 175°C
H1 H1
(10 kW/K) (10 kW/K)
65°C 125°C
H2 (40 kW/K) H2
20°C 120°C 120°C 155°C
C1 C1
(20 kW/K) (20 kW/K)
40°C 112°C
C2
(15 kW/K)
Cold end part Pinch Hot end part
(below the pinch) (above the pinch)
Fig. 6.4 Pinch decomposition of Example 6.4 (H2C2_175R), Δt m ¼5K.

_
_
temperature intervals. Since C H1 < C C1 , according to Eq. (6.77), a splitting
in C1 with a new match H2C1 and a corresponding splitting in H2 is
necessary. Furthermore, as has been analyzed in the aforementioned
problem table, the minimum cooling duty for hot stream H1 is 120kW,
and the minimum heating duty for cold stream C1 is 200kW; therefore,
a cooler is added to hot stream H1, and a heater is added to cold stream
C1. Thus, we obtain a network configuration shown in Fig. 6.5.
To determine the thermal capacity rates of the splits, we assume at first
that the mixing of the splits is isothermal. According to the mass and energy
balance constraints, we have
_

_
_
_
_
= t
C h,E4 ¼ C c,2 t 00 c,2 t 0 c,2 0 h,2 t 00 h,2 ¼ 18kW=K, C h,E3 ¼ C h,2 C h,E4
¼ 22 kW=K
_
_

0
= t c,pinch t
C c,E3 ¼ C h,E,3 t h,2 t 00 h,2 0 c,1 ¼ 13:2kW=K,
_
_
_
C c,E2 ¼ C c,1 C c,E3 ¼ 6:8kW=K
E1 t E2
175 pinch 45
H1
(10)
E3
123 65
H2
(40)
155 20
(20) C1
112 40 C2
– Dt (15)
t pinch m E4
Fig. 6.5 Network configuration of Example 6.4 (H2C2_175R).