Page 136 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

5.4 Pinch design analysis 133

Table 5.5 Feasibility criteria for stream matches at the pinch.
Criteria Reasoning
1) N out N in For minimum utility design, since coolers are not allowed above the pinch,
this implies each hot stream in the hot section (above the pinch) must be brought to the
pinch temperature by heat exchange with a cold process stream.
N h N c ðhot sectionÞ
The same logic applies to the cold section (below the pinch) problem.
N h N c ðcold sectionÞ
2) CP out CP in Since ðDT ¼ DT min Þ at the pinch, violation results in ðDT < DT min Þ at the
other end of the exchanger.
0CP h CP c ðhot sectionÞ
a
0CP h CP c ðcold sectionÞ
3) For any exchanger operating at For one exchanger with ðDCP > DCP ov Þ, there has to be another
exchanger with negative DCP that violates criteria (2).
the pinch ðDCP DCP ov Þ
where
DCP ¼ CP c CP h
X X
CP h
DCP ov ¼ CP c
The summation is taken over
only those streams that inter-
sect at the pinch
a
It is important to remember that the CP inequality applies only when a match is made between two streams both at the pinch. This
ensures exchangers operating near the pinch have a temperature difference equal to ðDT min Þ at least on one side. It is not essential
to obey the CP inequality for matches not adjacent to the pinch.

A CP table is used for identifying feasible pinch matches for the hot and the cold section design.
This table lists the hot streams followed by the cold streams. The sum ( P CP)is
entered below each column. Each table includes only those streams that intersect the
pinch. The feasibility criteria are mentioned on the top as a reminder. Table 5.6
CP table
presents the typical CP table format.

Table 5.6 A typical CP table format for hot/cold section design.

CP h £CP c ðhot sectionÞ N h £N c ðhot endÞ
CP h ‡CP c ðcold sectionÞ N h ‡N c ðcold endÞ
Stream no. Hot Cold Stream no.

H1 C1
H2 C2
. . . .
. . . .
P P
Total CP h ¼ CP c ¼