The composite curves built with the streams' data set are a
synthetized representation of the overall relationship
between the amount of heat available in the process (upper hot composite curve)
and needed
(lower cold composite curve) against the temperature. **Figure
4** applies to our hot water process example.

A given representation of the composite curves shows the
amount of energy that can be exchanged in the process from the hot composite curve to
supply part of the requirements of the cold composite curve. This corresponds to
energy recovery by heat transfer. It also gives the required heating and
cooling loads to be supplied by the utilities.

This graphical representation shown in figure
4 is characterized by a point
where the temperature difference between both curves is minimum (DTmin_{
}).

Once DTmin is set by
the engineer, its selection determines the relative position of both curves to
each other which, in turn, gives values of utility requirements and energy
recovery load. The DTmin value can be
increased or reduced as desired to evaluate its conceptual impact on the energy recovery
potential and the utility loads. In conclusion, for any DTmin
value, the composite curves give targets for the thermal energy
requirements of your process from a rigorous thermodynamic representation.
These loads are the minimum possible values, and they represent ** MINIMUM** utility
requirement targets.

For
our example, a DTmin value of 15 gives an
energy target of 3560 kW, which is 8% smaller than the actual steam usage. The
graph "Energy targeting - Hot water process" gives the hot utility
load vs DTmin and summarizes the
energy targeting results for this case. We can see a minimum steam
consumption of 1025 kW is obtained for zero DTmin
. This represents a 75% energy saving potential.

The location where the temperature driving force is the
smallest is called the Pinch point and it has a major significance. This is the place
where is met the bottleneck to energy recovery, and the design of energy
recovery in the process must be done taking this into consideration, otherwise
it will be impossible to obtain solutions that will use the minimum utility
loads obtained from the composite curves.

Let's observe that you can take some heat
from a
part of the hot composite curve located **above** the Pinch point,
like point
1, to heat a
point of the cold composite curve located **below** it, like point 2. The temperature difference is
appropriate for that, and is in fact much larger than DTmin. But the opposite is not
true. You cannot exchange heat from a portion of the hot composite curve located **below** the pinch point
(pt 3) to heat the cold composite curve situated **above** it (pt 4). The temperature differences
seen there **
are always smaller than DTmin** and can
easily be 0 or negative. In such cases, no heat transfer is even
possible. Thus, the amount of heat that can be supplied from point 3 to 4 (with
0 < DT < DTmin) is
in proportion very small and far from being sufficient to compensate for 1 or
more 1-2 type of match. So a 1-2 match represents a waste of precious DTs
in a location where this resource is very rare, and it will bring, as a
consequence, an increase in the hot utilities to fulfill the point 4's heating load.
This design choice will naturally lead to an overall **increase** in utility
requirements. This is why existing installations use more energy than their
minimum values. Having ignored the Pinch location during the design, many
existing equivalent 1-2 matches transfer heat across the Pinch, causing an energy
penalty as a consequence of a wrong use of the overall DTs
available within the process.

We can also note that a feasible
3-4 match with 0 < DT < DTmin
would lead to larger and more expensive heat exchangers than a 3-2 match for
which DT > DTmin, without
any
gain in energy efficiency. This would give a non-optimal design, capital
cost-wise.

In conclusion, the
** Pinch point divides the process in 2
separate zones**, and the ** design must be done independently** in both zones allowing
no heat transfer across the Pinch. Design rules exist to guide the engineer in
the selection of the right matches between the process streams to avoid this
event. They guide the engineer into the identification of appropriate hot and
cold streams for a match, and in the establishment of the right heat transfer
load and appropriate temperature levels for both streams.

**Wrap up**

The previous material is correct
when the DTmin value is small because the
thermodynamic constraint created by the limited availability of the temperature
driving force in the process to recover energy is the dominant factor. For high
values of DTmin (> 60 deg. C), this is no more the
case, but large values of DTmin are of no interest
when we want to design highly energy efficient processes. With the Pinch
principles, the engineer can identify where is located, among all the process
and utility streams, the bottleneck to heat transfer: the Pinch point. Once
done, the designer makes sure no heat transfer is done across the Pinch in order
to avoid matches (like the above 1-2 match) that waste the precious available DTs
where this driving force is the most limited, with no real possibility of
compensation with matches like the above 3-4 combination. This design rule - no
heat transfer across the Pinch - is a rigid rule to be followed for the first
stage of the design in order to correctly manage the use of DTs
and obtain solutions using the minimum energy requirement, also called MER
(Maximum Energy Recovery) solutions. In subsequent stages, this rigid rule is
relaxed for the final optimization of the designs obtained. Some heat transfer
through the Pinch is then assessed mostly to simplify the design and reduce the
total number of heat exchangers. Experience has shown that no major evolution is
achieved there and that MER solutions are excellent starting points that are
very close to the final global optimum design. This is a normal consequence of a
design that minimizes its energy usage, because the energy cost term is by far
the major component of the total cost equation.

We hope that all these explanations
have helped you understanding the heart of Pinch technology...