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...