H

(b) (d) FIG. 3.11.1 Two-stream heat exchange in the T/H diagram.

of values of AH for each interval can be obtained in this way, and the result can be replotted against the interval temperatures as shown in part b of Figure 3.11.2. The resulting T/H plot is a single curve representing all hot streams. A similar procedures gives a composite of all cold streams in the problem.

Figure 3.11.3 shows a typical pair of composite curves. Shifting the curves leads to behavior similar to that shown in the two-stream problem. However, the kinked nature of the composites means that ATmin can occur anywhere in the interchange region and not just at one end. For a given value of ATmin, the utility quantities predicted are the minimum required to solve the heat recovery problem. Although many streams are in the problem, in general, ATmin occurs at only one point termed the pinch. Therefore, a network can be designed which uses the minimum utility requirement, where only the heat exchangers at the pinch must operate at AT values down to ATmin.

Figure 3.11.4 shows that the pinch divides the overall system into two thermodynamically separate systems, each of which is in enthalpy balance with its utility target. This example shows that utility targets can only be achieved if no heat transfers across the pinch. To guarantee minimum energy consumption, the design engineer must ensure that heat is not transferred across the pinch in developing a structure. The following design rules must be followed:

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