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Fouling is the accumulation of solid material other than
scale in a way that hampers the operation of plant equipment or
contributes to its deterioration.
Examples of fouling are:
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- Dirt and silt
- Sand
- Corrosion Products
- Natural Organic
- Microbial Masses
- Aluminum Phosphates
- Iron Phosphates
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The most important factors that have influence over fouling are
water characteristics, temperature, flow, velocity, microbial
growths, corrosion, and contamination. Most waters contain the
dissolved and suspended materials that can cause a significant
fouling problem under certain conditions.
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As the temperature increases, the tendency to foul increases.
Since heat transfer surfaces are hotter than the cooling water, they
will accelerate the tendency to foul, and become the most likely
area of fouling.
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At low flow rates (1 foot per second or less), fouling occurs due
to natural settling of suspended material. At higher flow rates (3
feet per second), fouling can still occur, but usually is less
severe.
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Microorganisms can form deposits on any surface. In addition,
corrosive or iron-depositing bacteria will cause or utilize
corrosion products, which subsequently deposit as voluminous
foulants. All microbial colonies act as a collection site for silt
and dirt, causing a deposit of different foulants.
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Corrosion can form insoluble corrosion products that migrate and
mix with debris, process contamination, or microbial masses to
aggravate fouling.
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Material that leaks from the process side of heat exchange
equipment can cause serious fouling problems in several ways:
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- Depositing as insoluble products.
- Providing nutrients for microorganisms and causing severe microbial growths.
- Reacting with scale or corrosion inhibitors to form insoluble foulants
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Fouling can be controlled mechanically or by the use of a
chemical treatment program. The best method depends on the type of
fouling present. Control of fouling in a cooling water system
involves three steps:
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- Prevention – Whatever can be done to prevent foulants from entering the cooling system.
- Reduction – Steps taken to remove or reduce the volume of foulants that unavoidably enter the system. Examples would be side-stream filtering or periodic cleaning of the tower basin.
- Problem Control – Taking regular action to minimize deposition of the foulants in the system. This would include the use of specific treatments for control of fouling, air bumping or back-flushing of heat exchangers.
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