The analysis of the composite curves guided by the socalled
Plus / Minus Principle is a crucial and innovative phase of the study where we identify judicious process modifications that could improve significantly the
energy saving potential at a better overall cost. Among others, the current
operation's factor that creates the Heat Recovery Pinch (the bottleneck to energy
recovery) can be pinpointed and challenged with the client. At this stage, very
innovative and surprising ways to change the process can often be discovered
even if we usually have to restrict the findings to more conventional ideas in order to
limit the inherent risk behind any "firstofitskind" solution.
Changes of flowrates by some recycling, temperatures and pressures increases or
reductions, and others, are among the options that are examined to achieve
additional energy efficiency improvements at a minimal cost. Thus, Pinch
Analysis is MUCH MORE THAN JUST OPTIMUM HEAT EXCHANGER NETWORK DESIGN !!
For
the hot water tank example, the Pinch point is created by the economizer outlet
water temperature.
We can observe on the composite curves of figure 5 that we can reduce the
heating load if we increase the slope of the cold composite curve located above
the pinch. This slope increase corresponds to a reduction in flowrates. This can
be done if we decrease the economizer water flowrate sent in the hot water tank
until its overflow in the warm water side equals zero.
Another
option would be to reduce the economizer outlet water temperature.
If this temperature is lowered, it will be possible to move the cold composite
curve further on the left for the same DT_{min }value,
thus leading to a smaller minimum heat load target. This option corresponds to reduce the
hot water overflow and to heat the warm water side only with the economizer.

Capital
cost targeting and
DT_{min
}selection for a global optimal solution
In addition to its energy targeting
capabilities, Pinch Analysis uses a set of rules to establish targets for
the minimum heat exchangers' area and the minimum number of installations or
projects for a given DT_{min }value. In
brief, these informations come from the composite curves and the streams
population met in the part of the graph where heat is exchanged from the upper
hot composite curve to the lower cold composite curve.
The temperature differences between
the curves and the individual streams' heat transfer coefficicients allow the
estimation of the total minimum heat exchangers area. This value is a minimum area
target because it is calculated assuming a perfectly vertical heat transfer
between the hot and the cold composite curves. Vertical heat transfer is synonym
of global pure countercurrent heat transfer. This is usually the most efficient
possible way to transfer heat while minimizing the exchangers' size.
The streams
population met in both composite curves serves to target the minimum number of
unit (or heat exchangers or projects) Umin. Following the Euler rules for nodes,
Umin equals the streams count minus 1. This calculation is done for both regions
above and below the Pinch, since both regions are considered independant. Since
the Pinch location moves with DTmin, the streams
count in each zone will change accordingly. Hence, Umin is also a function of DTmin.
These
area and number of unit targets (and other informations specific to each stream
like piping cost for example) are the primary information used to estimate the capital cost and the
profitability associated to a chosen DTmin. Costing
equations for different types of heat exchangers, metallurgy, pressure,
etc, are developped and used with these figures for capital cost
estimations.
Theses capital cost targets can be used for
prediction purposes ahead of any design because appropriate and
consistent design rules support them. Once the design is
completed, the final solutions obtained will be closed to the targeted values. In other
words, for any DTmin value, the predicted
energy savings and heat exchangers' minimum area and number are compatible
targets.
It is then possible to explore the economic space of the
problem to find the global optimum DTmin value
that
meets the management profitability criteria, thereafter to proceed to the final
design of projects. Thus, each project will be a piece of a global strategic set
of well integrated actions to be taken to achieve the management objectives in
energy efficiency.
For our hot
water tank example, the path presented in figure 6 is followed to find
the optimum DTmin = 7 deg. C.