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Heat
Exchanger - Heat Transfer
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[ Heat
Transfer Table of Contents]
The transfer of thermal energy between fluids
is one of the most important and frequently used processes
in engineering. The transfer of heat is usually accomplished
by means of a device known
as a heat exchanger. Common applications of heat exchangers
in the nuclear field include boilers,
fan coolers, cooling water heat exchangers, and condensers.
The basic design of a heat exchanger normally
has two fluids of different temperatures separated by
some conducting medium. The most common design has one fluid
flowing through metal tubes
and the other fluid flowing around the tubes. On either side
of the tube, heat is transferred by
convection. Heat is transferred through the tube wall by
conduction. Heat
exchangers may be divided into several categories or
classifications. In the most commonly used
type of heat exchanger, two fluids of different temperature
flow in spaces separated by a tube
wall. They transfer heat by convection and by conduction
through the wall. This type is referred
to as an "ordinary heat exchanger," as compared to
the other two types classified as "regenerators"
and "cooling towers." An
ordinary heat exchanger is single-phase or two-phase. In a
single-phase heat exchanger, both of
the fluids (cooled and heated) remain in their initial
gaseous or liquid states. In two-phase exchangers,
either of the fluids may change its phase during the heat
exchange process. The steam generator and main condenser of
nuclear facilities are of the two-phase, ordinary heat
exchanger classification.
Single-phase heat exchangers are usually of
the tube-and-shell type; that is, the exchanger consists
of a set of tubes in a container called a shell (Figure 8).
At the ends of the heat exchanger,
the tube-side fluid is separated from the shell-side fluid by
a tube sheet. The design of
two-phase exchangers is essentially the same as that of
single-phase exchangers.
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