Heat is removed from the heat exchanger by forcing
cool outside air through the heat exchanger by an
external blower. There is no physical contact between
the internal and external air. In some applications, the
internal air is replaced by an inert gas such as nitrogen
to prevent oxidation.
AIR-TO-LIQUID COOLING
A more efficient heat transfer is possible by
replacing the air-to-air heat exchanger with an
air-to-liquid heat exchanger (fig. 1-5). In this method,
the internal air is also circulated past the heat producing
part and through a heat exchanger, but the heat is
removed from the heat exchanger by a liquid coolant
circulating through the heat exchanger.
Air-to-liquid cooling systems usually employ
built-in safety devices to shut down the equipment to
prevent overheating.The overheating could be caused
by low or no liquid flow, liquid too hot, an inoperative
circulating fan, or reduced heat exchanger efficiency
because of improper maintenance.
This type of cooling system is normally found on
large equipment instillations where a huge amount of
heat is developed.
Many radar transmitters, for
example, require cooling of this type. The other types
that we have discussed would not be able to dissipate
the heat that a high-powered radar transmitter develops.
A disadvantage of this type of cooling system is that they
are larger and more complex. However, for this reason
and because they are a part of the more complex
systems, we describe the typical liquid cooling system
used aboard ship. In this way, you will be given abetter
understanding of the function of individual components
within a system and the basic maintemnce required to
maintain the system to a high state of readiness.
LIQUID COOLING
Cooling systems are essential to the satisfactory
operation of all shipboard combat systems equipment.
In fact, some form of cooling is required for all
shipboard electronic equipment. As we have indicated,
liquid cooling is especially efficient for the transfer of
large amounts of heat. To maintain cooling systems,
you must have a broad understanding of the different
types of liquid cooling systems with which you are
involved.
A typical liquid cooling system is composed of two
basic coolant systems.
First, there is the primary
system. It provides the initial source of cooling water
that can be either seawater (SW) or chilled water (CW)
from the ship’s air-conditioning plant, or a combination
of both. Next, the secondary system transfers the heat
load from the electronic equipment to the primary
system. The coolant normally used in the secondary
system is distilled water (DW). This distilled water is
ultrapure and is maintained in that state by a
demineralize. In some secondary systems, ethylene
glycol is added to the water to prevent freezing when the
system is exposed to freezing weather.
TYPES OF LIQUID COOLING SYSTEMS
In the U.S. Navy, there are three basic
configurations of liquid cooling systems, and
conceivably you could be involved with all three. The
type or types with which you maybe involved, depends
upon the number and types of electronic equipment to
be cooled. The three types of systems areas follows:
Type I—Seawater/distilled water (SW/DW) heat
exchanger with SW/DW heat exchanger standby
Figure 1-5.—Air-to-liquid cooling.
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