exchanger, the heat is transferred directly from
one fluid to another as the two fluids mix.
Practically all heat exchangers used aboard Navy
ships are of the indirect or surface type.
MAINTENANCE OF
HEAT EXCHANGERS
Foreign matter lodged on or in the tubes of
a heat exchanger interferes with and reduces the
rate of heat transfer from one fluid to the other.
Figure 6-34.--Counterflow in heat exchanger.
Heat exchangers requiring the most attention are
those where the cooling medium is seawater.
Marine growth can form on the tube surfaces and,
in extreme cases, completely block the flow.
The seawater side of the tubes should be
cleaned as often as necessary. The intervals will
depend on the rate that slime, marine growth,
scale, mud, oil, grease, and so forth, are deposited
on the tube walls. The amount of such deposits
depends on the operating environment and
existing conditions. Operation in shallow water,
for example, may cause rapid fouling of the tubes.
The resistance of heat exchanger tubes to
corrosion depends on a thin, adherent, continuous
film of corrosion products on the surface exposed
Figure 6-35.--Crossflow in heat exchanger.
to seawater; therefore, you must be extremely
careful when cleaning the tubes not to use abrasive
tools capable of marring or scratching the surface
and crossflow heat exchangers are more com-
of the tubes. You should never use wire brushes
monly used than the parallel flow aboard ship.
or rubber plugs having metal parts inside con-
In many heat exchangers, the types of flow are
combined in various ways so it is not always easy
denser tubes. Any scratch or perforation of the
corrosion-resistant film will form a pit that will
to determine whether the flow is basically parallel,
widen and deepen. Eventually this can result in
counter, or cross.
tube failure (through corrosion pitting), in the
same manner as occurs when foreign matter
Number of Passes
lodges on the tube surface.
For ordinary cleaning, push an air lance
Surface heat exchangers may be classified as
SINGLE-PASS units if one fluid passes another
through each tube, wash the tube sheets clean, and
remove all foreign matter from the water chests.
only once, or as MULTIPASS units if one fluid
In installations with severe fouling, push a water
passes another more than once. Multipass flow
lance through each tube to remove foreign matter.
is obtained by the arrangement of the tubes and
For extreme fouling, you should run a rotating
the fluid inlets and outlets or by the use of baf-
bristle brush through each tube-or drive soft
fles to guide a fluid so it passes the other fluid
rubber plugs, available at shipyards, through the
more than once before it leaves the heat
tubes by using air or water pressure. Afterwards,
exchanger.
ensure this procedure is followed by water-
lancing.
Path of Heat Flow
ZINCS
When classified according to the path of heat
flow, heat exchangers are of two basic types. In
Where zinc anodes are required to protect the
the INDIRECT or SURFACE heat exchanger, the
heat exchanger materials from electrolysis, good
heat flows from one fluid to the other through
metallic contact must exist between the anodes
a tube, plate, or other surface that separates the
and the metal of the condenser.
two fluids; in the DIRECT-CONTACT heat
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