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 ?

Data from example 3.16

Determine the pinch temperatures and the minimum utility requirements for the streams set out in the table below, for a  minimum temperature difference between the streams of 20°C. Devise a heat exchanger network to achieve the maximum energy recovery.

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Stream Heat capacity flow number Туре rate kW/°C hot 40.0 hot 30.0 cold 60.0 cold 20.0 |1 2 34 4 Source temp. °C 180 150 30 80 Target temp. °C 40 60 180 160 Heat load KW 5600 2700 9000 1600