Types Of Fouling In Heat Exchangers

What is fouling in Heat Exchangers? 

           Fouling is the accumulation of unwanted material on the tube surfaces of the heat exchanger. After a period of operation the heat transfer surface of heat exchanger may become coated with various deposits presents in flow system. This coating represents an additional resistance to heat flow and thus decreased in performance. Fouling factors are best determined from experience with similar units in the same or similar service. When such information is not available, recourse may be had to publish data. The most comprehensive tabulation of fouling factors is the one developed by TEMA, which is available in Refs. [1, 2]. Fouling can occur by a number of mechanisms operating either alone or in combination. These include:

• Corrosion:

          Corrosion products such as rust can gradually build up on tube walls, resulting in reduced heat transmission and eventual tube failure. This type of fouling can be minimized or eliminated by the proper choice of corrosion-resistant materials of construction in the design process.

• Crystallization:

      Crystallization typically occurs with cooling water streams containing dissolved sulfates and carbonates. Since the solubility of these salts decreases with increasing temperature, they tend to precipitate on heat-transfer surfaces when the water is heated, forming scale. This type of fouling can be minimized by restricting the outlet water temperature to a maximum of 110–125◦F.

• Decomposition:

         Some organic compounds may decompose when they are heated or come in contact with a hot surface, forming carbonaceous deposits such as coke and tar. In cracking furnaces, partial decomposition of the hydrocarbon feedstock is the objective and coke formation is an undesired but unavoidable result.

• Polymerization:

      Polymerization reactions can be initiated when certain unsaturated organic compounds are heated or come in contact with a hot metal tube wall. The resulting reaction products can form a very tough plastic-like layer that can be extremely difficult to remove from heat-transfer surfaces. 

• Sedimentation:

    Sedimentation fouling results from the deposition of suspended solids entrained in many process streams such as cooling water and flue gases. High fluid velocities tend to minimize the accumulation of deposits on heat-transfer surfaces.

• Biological activity:

      Biological fouling is most commonly caused by micro-organisms, although macroscopic marine organisms can sometimes cause problems as well. Cooling water and some other process streams may contain algae or bacteria that can attach and grow on heat-transfer surfaces, forming slimes that are very poor heat conductors. Metabolic products of these organisms can also cause corrosion of metal surfaces. Biocides and copper-nickel alloy tubing can be used to inhibit the growth of micro-organisms and mitigate this type of fouling.

References:

1. Kakac, S. and H. Lui, Heat Exchangers: Selection, Rating and Thermal Design, CRC Press Boca Raton FL, 1997.
2. Standards of the Tubular Exchanger Manufacturers Association, 8th edn, Tubular Exchanger Manufacturers Assiociation, Inc, Tarrytown, NY, 1999.