Fouling

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-transfer components through ingredients contained in cooling water or gases, and even the development of 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:

• 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;
• 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;
• Ship hulls – creates additional drag, increases fuel usage, reduces maximum speed;
• Turbines – reduces efficiency, increases probability of failure;
• Solar panels – decreases the electrical power generated;
• Reverse osmosis membranes – increases pressure drop, increases energy expenditure, reduces flux, membrane failure (in severe cases);
• Electrical heating elements – increases temperature of the element, increases corrosion, reduces lifespan;
• Firearm barrels – increases chamber pressure; hampers loading for muzzleloaders
• Nuclear fuel in pressurized water reactors – axial offset anomaly, may need to de-rate the power plant;
• Injection/spray nozzles (e.g., a nozzle spraying a fuel into a furnace) – incorrect amount injected, malformed jet, component inefficiency, component failure;
• Venturi tubes, orifice plates – inaccurate or incorrect measurement of flow rate;
• Pitot tubes in airplanes – inaccurate or incorrect indication of airplane speed;
• Spark plug electrodes in cars – engine misfiring;
• Production zone of petroleum reservoirs and oil wells – decreased petroleum production with time; plugging; in some cases complete stoppage of flow in a matter of days;
• Teeth – promotes tooth or gum disease, decreases aesthetics;
• 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 coarse matter of either biological or inorganic origin, for example industrially produced refuse. Such matter enters into the cooling water circuit through the cooling water pumps from sources like the open sea, rivers or lakes. In closed circuits, like cooling towers, 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:

• Manmade refuse;
• Detached internal parts of components;
• Tools and other "foreign objects" accidentally left after maintenance;
• Algae;
• Mussels;
• Leaves, parts of plants up to entire trunks.

Micro fouling

As to micro fouling, distinctions are made between:

• Scaling or precipitation fouling, as crystallization of solid salts, oxides, and hydroxides from water solutions (e.g., calcium carbonate or calcium sulfate)
• Particulate fouling, i.e., accumulation of particles, typically colloidal particles, on a surface
• Corrosion fouling, i.e., in-situ growth of corrosion deposits, for example, magnetite on carbon steel surfaces
• Chemical reaction fouling, for example, decomposition or polymerization of organic matter on heating surfaces
• Solidification fouling – when components of the flowing fluid with a high-melting point freeze onto a subcooled surface
• Biofouling, like settlements of bacteria and algae
• Composite fouling, whereby fouling involves more than one foulant or fouling mechanism

Precipitation fouling

Scaling or precipitation fouling involves crystallization of solid salts, oxides, and hydroxides 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 saturation, 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:

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