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Mitigate Heat
Exchanger Fouling
Fouling is one of the major causes of unreliable heat exchanger
operation. It can lead to increased energy costs, unit downtime and
maintenance expenses. Heat exchangers are usually designed for
specific plant operations and service conditions. They can be affected
by changes in feed rates and composition. Heat exchangers can also be
influenced by performance changes in other related units or by
seasonal variations in plant operation conditions.
Fouling can result from many sources. In petroleum operations this
can readily occur from the formation of hard carbon deposits (coking)
or deposition of undesirable polymers (polymerization). Other sources
of fouling can be scaling, deposition of solids in the system, and
formation of corrosion products. However, certain procedures can be
utilized to minimize such problems. These methods may simply involve
controlling tube wall temperatures within recommended limits to
prevent coking or polymerization or by prevention of formation of
excessive corrosion products or inorganic deposits that can promote
fouling.
Coking is usually prevalent at high temperatures (>400 F) and can
result in 50 to 80 percent loss in heat transfer capabilities with
greatest tendencies in hydrocarbon environments which have precursors
such as asphaltenic materials. Corrosion related fouling is usually
most severe in heavy hydrocarbon streams where sulfidic attack of
steel can be excessive. The use of higher Cr alloys can usually
eliminate this latter form of fouling. Other technologies that can
being utilized in these system also includes control of conditions and
use of twisted tube technology.
Fouling problems are commonly most severe when dealing with
recirculated cooling tower water. Here, excessive tube wall
temperatures are experienced since it is relatively easy to promote
inorganic scale formation in this location. This reduces cooling water
flow rates which usually further magnifies the problem and may also
add a the complication of deposition of silt or fine solids. Cooling
water fouling is caused by the precipitation of salts resulting in
scales on the inside pipe wall. Under-deposit corrosion can also be a
common side effect. To minimize this problem particular attention
should be paid to maintaining velocity, low tube wall temperature and
the selection of a proper cooling water treatment program.
Adapted from Mitigate Fouling to Improve Heat Exchanger
Reliability, H.M. Joshi, Hydrocarbon Processing (January 1999),
pp93-95
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