Environmental Engineering Reference
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temperature will be {T*(obtained from graph)+(∆Tmin/2)} and the actual cold utility
temperature will be {T*(obtained from graph)-(∆Tmin/2)}
The point of “zero” heat flow in the GCC is the pinch point. The open “jaws” at the top
and the bottom represent QHmin and QCmin respectively.
The grand composite curve (GCC) provides a convenient tool for setting the targets for
the multiple utility levels of heating utilities as illustrated above.
The graphs below further illustrate such capability for both heating and cooling utilities.
Figure 16. Grand Composite Curve for Utility Selection-1
The above figure 16(a) shows a situation where HP steam is used for heating and
refrigeration is used for cooling the process. In order to reduce utilities cost, intermediate
utilities MP steam and cooling water (CW) can be introduced. The second graph (b) shows
the targets for all the utilities. The target for the MP steam is set via simply drawing a
horizontal line at the MP steam temperature level starting from the vertical axis until it
touches the GCC. The remaining heat duty required is then satisfied by the HP steam. This
maximizes the MP steam consumption prior to the remaining heating duty be fulfilled by the
HP steam and therefore minimizes the total utilities cost. Similar logic is followed below the
pinch to maximize the use of the cooling water prior the use of the refrigeration.
The points where the MP steam and CW levels touch the GCC are called utility pinches
since these are caused by utility levels. The graph, 17(C) below, shows a different possibility
of utility levels where furnace heating is used instead of HP steam. Considering that furnace
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