HEAT EXCHANGER APPLICATIONS
DOE-HDBK-1018/1-93
Heat Exchangers
HEAT EXCHANGER APPLICATIONS
This chapter describes some specific applications of heat exchangers.
EO 1.5
LIST at least three applications of heat exchangers.
EO 1.6
STATE the purpose of a condenser.
EO 1.7
DEFINE the following terms:
a.
Hotwell
b.
Condensate depression
EO 1.8
STATE why condensers in large steam cycles are
operated at a vacuum.
Introduction
Heat exchangers are found in most chemical or mechanical systems. They serve as the system's
means of gaining or rejecting heat. Some of the more common applications are found in
heating, ventilation and air conditioning (HVAC) systems, radiators on internal combustion
engines, boilers, condensers, and as preheaters or coolers in fluid systems. This chapter will
review some specific heat exchanger applications. The intent is to provide several specific
examples of how each heat exchanger functions in the system, not to cover every possible
applicaton.
Preheater
In large steam systems, or in any process requiring high temperatures, the input fluid is usually
preheated in stages, instead of trying to heat it in one step from ambient to the final temperature.
Preheating in stages increases the plant's efficiency and minimizes thermal shock stress to
components, as compared to injecting ambient temperature liquid into a boiler or other device
that operates at high temperatures. In the case of a steam system, a portion of the process steam
is tapped off and used as a heat source to reheat the feedwater in preheater stages. Figure 8 is
an example of the construction and internals of a U-tube feedwater heat exchanger found in a
large power generation facility in a preheater stage. As the steam enters the heat exchanger and
flows over and around the tubes, it transfers its thermal energy and is condensed. Note that the
steam enters from the top into the shell side of the heat exchanger, where it not only transfers
sensible heat (temperature change) but also gives up its latent heat of vaporization (condenses
steam into water). The condensed steam then exits as a liquid at the bottom of the heat
exchanger. The feedwater enters the heat exchanger on the bottom right end and flows into the
tubes. Note that most of these tubes will be below the fluid level on the shell side.
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