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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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