Page 148 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

5.10 Design illustration 145

Table P5.1C Temperature interval heat balance and problem table cascade.
‘0’ kW +90 kW
HU HU
Surplus/
(∑CP C –∑CP H ) ∆H interval
Deficit
190°
20° 3 +60 Surplus +60 +60
170° 1 –60 +30
30° 1 +30 Surplus +30 +30
CP = 2
140° 2 90 0
T * 25° –3 –75 Deficit –75 –75
115° CP = 4 CP = 3 45° –0.4 –18 Deficit –18 –15 –18 +75

70° 3 CP = 2.6 –3.4 –102 Deficit –102 + 3 –102 +93

40° 4 30° +105 +195
10° –6 –60 Deficit –60 –60
30° +165 +255
kW kW CU
T pinch = 140°, T h,pinch = 140 + 20/2 = 150°C
T c,pinch = 140 – 20/2 = 130°C
Q HU = 90 kW, Q CU = 255 kW

After estimation of T pinch , the entire temperature interval is divided into the hot section above the
pinch temperature and the cold section below the pinch temperature. In the hot section, the hot streams
terminate at T h, pinch while the cold streams originate from T c,pinch and in the cold section, the cold
streams terminate at T c,pinch while the hot streams originate from T h,pinch .
The tick off heuristics (Section 5.4.4) is employed to generate the grid diagram for the hot and cold
section. Since feasibility criteria for stream matches at the pinch as per Table 5.5 i.e. (N out ¼ N in ) and
(CP out CP in ) is satisfied, stream splitting is not required. The CP table shows that criteria 3 of
Table 5.5 is satisfied as can be seen from Table P5.1D.

Table P5.1D CP table for hot/cold section.

CP h £CP c ; N h £N c ðhot sectionÞ
CP h ‡CP c ; N h ‡N c ðcold sectionÞ
Stream no. Hot Cold Stream no.
1 2.0
2 4.0
. . 3.0 1
. . 2.6 2
P P
Total CP h ¼ 6 CP c ¼ 5.6