832 Steady State Heat Exchangers

Section 8.4 treats heat exchangers in some detail. However, several important criteria for selection are listed below.

1. Flow Arrangements. These are characterized as:

Parallel flow Crossflow Counterflow Mixed flow

The flow arrangement helps to determine the overall effectiveness, the cost, and the highest achievable temperature in the heated stream. The



Figure 8.14 Heat wheel.

latter effect most often dictates the choice of flow arrangement. Figure 8.15 indicates the temperature profiles for the heating and heated streams, respectively. If the waste-heat stream is to be cooled below the cold stream exit, a counterflow heat exchanger must be used.

2. Character of the Exchange Fluids. It is necessary to specify the heated and cooled fluids as to:

Chemical composition

Physical phase (i.e., gaseous, liquid, solid, or multiphase) Change of phase, if any, such as evaporating or condensing These specifications may affect the optimum flow arrangement and/or the materials of construction.

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