Composite curves show the scope for energy recovery and the hot and cold utility targets. Generally, several utilities at different temperature levels and of different costs are available to a design engineer. Another pinch technology tool, the grand composite curve, helps the design engineer to select the best individual utility or utility mix.
The grand composite curve presents the profile of the horizontal (enthalpy) separation between the composite curves with a built-in allowance for ATmin. As shown in Figure 3.11.5, its construction involves bringing the composite curves together vertically (to allow for ATmin) and then plotting the horizontal separation (a in Figure 3.11.5).
Figure 3.11.6 shows how the grand composite curve reveals where heat is transferred between utilities and process and where the process can satisfy its own heat demand (Linnhoff, Polley, and Sahdev 1988).
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