Hot composite stream

Figure 0.2 Constructing a hot composite stream using superposition (the dashed line represents the composite line).

Next, both composite streams are plotted on the same diagram (Fig. 9.4). On this diagram, thermodynamic feasibility of heat exchange is guaranteed if at any heat-exchange level (which corresponds to a horizontal line), the temperature of the hot composite stream is located to the right of the cold composite stmam. Therefore, the cold composite stream can be slid down until it touches the hot composite stream. The ptoint where the two composite streams touch is called... 

Solution Figures 9.6-9.8 illustrate the hot composite stream, the cold composite stream and the pinch diagram, respectively. As can be seen from Fig. 9.8, the two composite streams touch at 310 K on the hot scale (300 K on the cold scale). The minimum heating and cooling utilities are 2,620 and 50 kW, respectively, leading to an annual operating cost of... 

Composite curves were developed for heat recovery targeting (Linnhoff et al., 1982). The word composite reveals the basic concept behind the composite curves method A system view of the overall heat recovery system. One hot composite stream represents all the hot process streams, while one cold composite stream represents all the cold process streams. In this manner, the problem of assessing a complex heat recovery system involving multiple hot and cold streams is simplified as a problem of two composite streams. In essence, the hot composite stream represents a single process heat source, while the cold composite stream represents a single process heat sink. 

What characteristics should a hot composite stream possess to represent the three hot streams The composite stream should have two features (i) It should go through the exact same temperature ehanges as the three streams do, and (ii) it should have the same total heat load as the summation of the heat loads of three streams. 

Specifying the hot utility or cold utility or AT m fixes the relative position of the two curves. As with the simple problem in Fig. 6.2, the relative position of the two curves is a degree of freedom at our disposal. Again, the relative position of the two curves can be changed by moving them horizontally relative to each other. Clearly, to consider heat recovery from hot streams into cold, the hot composite must be in a position such that everywhere it is above the cold composite for feasible heat transfer. Thereafter, the relative position of the curves can be chosen. Figure 6.56 shows the curves set to ATn,in = 20°C. The hot and cold utility targets are now increased to 11.5 and 14 MW, respectively. 

In a similar manner to constructing the hot-composite line, a cold composite stream is plotted (see Fig. 9.3 for a two-cold-stream example). 

Comparing the composite curve, Figure 3.22, with Figure 3.237 shows that the heat introduced to the cascade is the minimum hot utility requirement and the heat removed at the bottom is the minimum cold utility required. The pinch occurs in Figure 3.23b where the heat flow in the cascade is zero. This is as would be expected from the rule that for minimum utility requirements no heat flows across the pinch. In Figure 3.23b the pinch temperatures are 80 and 90°C, as was found using the composite stream curves. 

Within each temperature range, the streams are combined to produce a composite hot stream. This composite hot stream has a CP in any temperature range that is the sum of the individual streams. Also, in any temperature range, the enthalpy change of the composite stream is the sum of the enthalpy changes of the individual streams. Figure 16.3b... 

Figure 16.3 The hot streams can be combined to obtain a composite stream.

A new design situation would start from the grand composite curves of each of the processes on the site and would combine them together to obtain a picture of the overall site utility system12. This is illustrated in Figure 23.27, where two processes have their heat sink and heat source profiles from their grand composite curves combined to obtain a site hot composite curve and a site cold composite curve, using the procedure developed for composite curves in Chapter 16. Wherever there is an overlap in temperature between streams, the heat loads... 

On the cold composite curve, each stream that is to be heated must enter or leave an exchanger at the pinch point. On the hot composite curve, each stream that is to be cooled must enter or leave an exchanger at the pinch point. 

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