Threshold problems, heat exchanger

To design the heat exchanger network for a threshold problem, it is normal to start at the most constrained point. The problem can often be treated as one half of a problem exhibiting a pinch. 

At what value of the minimum approach temperature does the problem in Example 3.16 become a threshold problem Design a heat exchanger network for the resulting threshold problem. What insights does this give into the design proposed in Example 3.16 ... 

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