Composite Curves

A design engineer can analyze the heat exchanges between several hot and cold streams in the same way as in the preceding two-stream, heat exchange example. A single composite of all hot and a single composite of all cold streams can be produced in the T/H diagram and handled in the same way as the two-stream problem.

The first step in constructing composite curves is to correctly identify the streams that undergo enthalpy changes as hot or cold. A hot stream is defined as one that requires cooling; a cold stream is defined as one that requires heating. The objective is to determine the minimum amount of residual heating or cooling necessary after the heat interchange between the process streams has been fully exploited.

The design engineer extracts stream data from the process flowsheet which contains heat and material balance information. The items of interest are mass flow rates, specific heat capacity (CP), and supply and target temperatures. This procedure is called data extraction.

Starting from the individual streams, the design engineer can construct one composite curve of all hot streams in the process and another of all cold streams by simply adding the heat contents over the temperature range.

In part a of Figure 3.11.2, three hot streams are plotted separately, with their supply and target temperatures defining a series of interval temperatures T1 to T5. Between T1 and T2, only stream B exists, so the heat available in this interval is given by CPB (T1 — T2). However, between T2 and T3, all three streams exist, so the heat available in this interval is (CPA + CPB + CPC) X (T2 — T3). A series

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