Part a in Figure 3.11.1 shows two streams plotted in the temperature-enthalphy (T/H) diagram, one hot (i.e., requiring cooling) and one cool. The hot stream is represented by the line with an arrow pointing to the left, and the cold stream by the line with an arrow pointing to the right.

For feasible heat exchange between the two streams, the hot stream must be hotter than the cool stream at all points. However, because of the relative temperatures of the two streams, the construction of the heating and cooling curves shown in part a of Figure 3.11.1 represents a limiting case illustrated by the flow diagram shown in part b of the figure. The heat exchange between the hot stream countercurrent to the cold stream cannot be increased because the temperature difference between the hot and cold streams at the cold end of the exchanger is zero. This difference means that the heat available in the hot stream below 100°C must be rejected to the cooling water, and the balance of the heat required by the cold stream must be made up from steam heating.

In part c of Figure 3.11.1, the cold stream is shifted on the H-axis relative to the hot stream so that the minimum temperature difference, ATmin, is no longer zero but positive and finite. The effect of this shift increases utility heating and cooling by equal amounts and reduces the load on the heat exchanger by the same amount. The arrangement, which is now practical because ATmin is nonzero, is shown in the flow diagram in part d of Figure 3.11.1. Clearly, further shifting implies larger ATmin values and larger utility consumption.

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