15.3. Composite Temperature-Enthalpy Diagram

Physically, the pinch zone, previously mentioned, represents a point in the heat-exchanger network at which at least one heat exchanger, or two streams, will have the minimum approach temperature set in Step 1. This point is perhaps more clearly shown in a composite enthalpy-temperature diagram. This diagram is essentially the same as the combination of all the T-Q diagrams for all the exchangers in the network. Such a diagram for this example is shown in Figure 15.9 and is constructed by plotting the enthalpy of all the hot streams and all the cold streams as a function of temperature:

Figure 15.9. Composite Temperature-Enthalpy Diagram for Showing Effect of Changing Minimum Temperature Approach