Fouling is the accumulation of unwanted material on solid surfaces. The fouling materials can consist of either living organisms (biofouling, organic) or a non-living substance (inorganic). Fouling is usually distinguished from other surface-growth phenomena in that it occurs on a surface of a component, system, or plant performing a defined and useful function and that the fouling process impedes or interferes with this function.
Other terms used in the literature to describe fouling include deposit formation, encrustation, crudding, deposition, scaling, scale formation, slagging, and sludge formation. The last six terms have a more narrow meaning than fouling within the scope of the fouling science and technology, and they also have meanings outside of this scope; therefore, they should be used with caution.
Fouling phenomena are common and diverse, ranging from fouling of ship hulls, natural surfaces in the marine environment (marine fouling), fouling of heat exchanger components through ingredients contained in cooling water or gases, and even the development of dental plaque or calculus on teeth or deposits on solar panels on Mars, among other examples.
This article is primarily devoted to the fouling of industrial heat exchangers, although the same theory is generally applicable to other varieties of fouling. In cooling technology and other technical fields, a distinction is made between macro fouling and micro fouling. Of the two, micro fouling is the one that is usually more difficult to prevent and therefore more important.
Components subject to fouling
Examples of components that may be subject to fouling and the corresponding effects of fouling:
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Heat exchanger surfaces – reduces thermal efficiency, decreases heat flux, increases temperature on the hot side, decreases temperature on the cold side, induces under-deposit corrosion, increases use of cooling water;
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Piping, flow channels – reduces flow, increases pressure drop, increases upstream pressure, increases energy expenditure, may cause flow oscillations, slugging in two-phase flow, cavitation; may increase flow velocity elsewhere, may induce vibrations, may cause flow blockage;
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Ship hulls – creates additional drag, increases fuel usage, reduces maximum speed;
["Marine fouling and its prevention"; prepared for Bureau of Ships, Navy Dept, Woods Hole Oceanographic Institution, United States, Navy Dept. Bureau of Ship, 1952. (pdf)]
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– reduces efficiency, increases probability of failure;
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Solar panels – decreases the electrical power generated;
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Membrane fouling – increases pressure drop, increases energy expenditure, reduces flux, membrane failure (in severe cases);
[Siobhán Francesca E. Boerlage, Scaling and Particulate Fouling in Membrane Filtration Systems, Taylor & Francis; 2001, (Google books)]
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Electrical – increases temperature of the element, increases corrosion, reduces lifespan;
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Firearm barrels – increases chamber pressure; hampers loading for
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Nuclear fuel in pressurized water reactors – axial offset anomaly,
[Joshua M. Hawkes, "The Simulation and Study of Conditions Leading to Axial Offset Anomaly in Pressurized Water Reactors", Georgia Institute of Technology Master of Science Thesis, December 2004. (pdf) ] may need to de-rate the power plant;
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Injection/spray nozzles (e.g., a nozzle spraying a fuel into a furnace) – incorrect amount injected, malformed jet, component inefficiency, component failure;
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, – inaccurate or incorrect measurement of flow rate;
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in airplanes – inaccurate or incorrect indication of airplane speed;
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Spark plug electrodes in cars – engine misfiring;
["Spark Plug Faces", brochure "Bosch Spark Plugs 0307", Part 1 (pdf) ]
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Production zone of petroleum reservoirs and – decreased petroleum production with time; plugging; in some cases complete stoppage of flow in a matter of days;
[G.A. Mansoori "Physicochemical Basis of Arterial Blockage / Fouling. Prediction and Prevention." Department of Chemical Engineering, University of Illinois at Chicago, on-line publication, September 2001 (pdf) ]
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Teeth – promotes tooth or gum disease, decreases aesthetics;
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Living organisms – deposition of excess minerals (e.g., calcium, iron, copper) in tissues is (sometimes controversially) linked to aging/senescence.
Macro fouling
Macro fouling is caused by
debris matter of either biological or inorganic origin, for example industrially produced
refuse. Such matter enters into the cooling water circuit through the cooling
from sources like the open
sea,
or
. In closed circuits, like
, the ingress of macro fouling into the cooling tower basin is possible through open canals or by the wind. Sometimes, parts of the cooling tower internals detach themselves and are carried into the cooling water circuit. Such substances can foul the surfaces of heat exchangers and may cause deterioration of the relevant heat transfer coefficient. They may also create flow blockages, redistribute the flow inside the components, or cause
fretting damage.
- Examples
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Manmade refuse;
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Detached internal parts of components;
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Tools and other "foreign objects" accidentally left after maintenance;
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Algae;
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;
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Leaf, parts of plants up to entire trunks.
Micro fouling
As to micro fouling, distinctions are made between:
[T.R. Bott, "Fouling of Heat Exchangers (Chemical Engineering Monographs)", Elsevier Science, 1995.]
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Scaling or precipitation fouling, as crystallization of solid salts, , and from water solutions (e.g., calcium carbonate or calcium sulfate)
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Particulate fouling, i.e., accumulation of particles, typically colloidal particles, on a surface
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Corrosion fouling, i.e., in-situ growth of corrosion deposits, for example, magnetite on carbon steel surfaces
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Chemical reaction fouling, for example, decomposition or polymerization of organic matter on heating surfaces
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Solidification fouling – when components of the flowing fluid with a high-melting point freeze onto a subcooled surface
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Biofouling, like settlements of bacteria and algae
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Composite fouling, whereby fouling involves more than one foulant or fouling mechanism
Precipitation fouling
Scaling or precipitation fouling involves
crystallization of solid salts,
, and
from solutions. These are most often water solutions, but non-aqueous precipitation fouling is also known. Precipitation fouling is a very common problem in boilers and heat exchangers operating with
hard water and often results in
limescale.
Through changes in temperature, or solvent evaporation or degasification, the concentration of salts may exceed the Supersaturation, leading to a precipitation of solids (usually crystals).
As an example, the equilibrium between the readily soluble calcium bicarbonate - always prevailing in natural water - and the poorly soluble calcium carbonate, the following chemical equation may be written:
- \mathsf
