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Utility selection and optimization
 Another masterpiece of Pinch Analysis is the residual heat
curve also known as the Grand Composite Curve. This graph has 2 clearly
distinct zones located above and below the Pinch point met where the abcissa has
a zero value. It is a graphical representation of the net amount of
heating (portion above the Pinch point) and cooling (below the Pinch) versus the temperature level. Since the
Pinch point represents the temperature
through which no heat transfer is allowed to minimize the utility requirements,
it is consequently found where the residual heat available is zero.
This tool is mostly used to optimize the utility
level and load selection
and to study the appropriate integration of heat pumps, cogen systems (CHP) or new
unit operations like evaporators. For
example, we can use it to maximize the use of low pressure steam and cooling water which are cheap utilities, and to maximize the production of low pressure
steam by energy recovery below the Pinch as shown on the graph.
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Design and establishment of a strategic plan of action in
energy efficiency
 Design rules are used to develop solutions that meet the
prescribed targets. The design rules rest on the 3 basic Pinch principles:
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No heat transfer across the
Pinch |
|
No heating utility below the
Pinch |
|
No cooling utility above the
Pinch |
This design phase ends with a final optimization
and is followed by a budget cost estimation. Figure 7 shows the design grid
representation used for this phase (not related to the hot water process
example). The savings and the
final profitability of each project is then calculated. The ultimate result is a set of
integrated projects obtained for, typically, 2 recommended scenarios. As desired
by the plant's management, each project can be implemented over the years.
The following table is
a short example of results provided to the Client (TMP pulp mill). Often, 2 scenarios are presented, one for short
payback projects (6-12 months), and a second one for 18 to 24 months payback
period solutions for a much larger level of process integration and energy recovery.
About the conventional approach...
In the conventional approach to energy efficiency, the
engineer will mostly refer to state-of-the-art knowledge for his process and
proceed by inspection and intuition. Often,
energy efficiency literature and reference books include extensive
checklists of
possibilities that can be many pages long (example),
often giving very general ideas. Selecting and studying some of them can be
a costly and time-consuming solution, and you never know what your
targets are, if you have reached them, and if any better solution exists. In
comparison, you can use
Pinch Analysis to find and select your energy and capital cost targets ahead of
design. The design rules will naturally and rapidly lead you
towards the one out of 100 projects of the literature listings that is the most appropriate to
your case.
Design for the hot water tank example
For the example presented here, the results of Step 1 through
7 gives the following design where the minimum steam
consumption target is met.
| Case
studies, publications and references |
Many case studies have been presented in the past in the
literature and in conferences. Pragmathic is also highly active in R&D
activities to keep improving its expertise in this field. We have developped
over the years very efficient and unique ways to get more precise targets for retrofit studies, and we use our own softwares for this
purpose.
Please consult the
Download section where we have many papers and congress presentations available
for download. A
textbook on Pinch Analysis is available here.
Here are
some good papers to consult.
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"Step
by step through the pinch", Smith, G., Patel, A., The Chemical
Engineer, November 1987, p. 26.
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"Pinch
Analysis - A state-of-the-art overview",
Linnhoff, B., Trans IChemE, vol. 71, Part A, September 1993, p. 503.
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