Page 321 - Corrosion Engineering Principles and Practice
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(b)
FIGURE 8.14 (continued)
one end to the other, or with U-tube in which the hot and cooled fluid
enter and exit from the same end of the exchanger.
The selection of materials for these applications is often a
compromise between the requirements of the process flow and
the type of water. Associated with such heat exchangers are
pumps, pipes, and valves to distribute the water and return it to
source. The various metals commonly used in heat exchangers
have quite different thermal conductivities (Table 8.9). However,
the thermal conductivity of the metal wall is only one component
of the resistance to heat transfer in a heat exchanger tube. In a
condenser (i.e., where steam is condensing on cold tubes), for
example, the resistance to heat transfer through a tube wall is
made up of five main components as illustrated in Fig. 8.15 [8].
The tube wall resistance is comparatively small so that changes in
thermal conductivity from the use of different metals in not
necessarily very significant.
Most metals used in heat exchangers perform well in clean water,
that is, free of sediment, debris, fouling organisms, and pollutants.