Fouling
Fouling refers to the accumulation and deposition of living organisms (biofouling) and certain non-living material on hard surfaces, most often in an aquatic environment. This can be the fouling of ships, pilings, and natural surfaces in the marine environment (marine fouling), fouling of heat-transferring components through ingredients contained in the cooling water or gases, and even the development of plaque on teeth or deposits on solar panels on Mars, among other examples. This article is mostly devoted to the fouling of heat exchanger systems, although many of the points made are applicable to other varieties of fouling. In the cooling technology and other technical fields, a distinction is made between macro fouling and micro fouling. Of the two, micro fouling is the one which is usually more difficult to prevent and therefore more important.

# 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.
- Manmade refuse
- Detached internal parts of components
- 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, for example
Calcium carbonate (aragonite or calcite)
Calcium sulfate (anhydrite, hemihydrate, gypsum)
Silicates (serpentine, analcite, acmite, gyrolite, noselite, gehlenite, amorphous silica, cristoballite)
Magnesium hydroxide (brucite)
Aluminum oxide (boehmite, gibbsite, corundum)
Copper (metalic copper, cuprite)
Barium sulfate
- Calcium carbonate (aragonite or calcite)
- Calcium sulfate (anhydrite, hemihydrate, gypsum)
- Silicates (serpentine, analcite, acmite, gyrolite, noselite, gehlenite, amorphous silica, cristoballite)
- Magnesium hydroxide (brucite)
- Aluminum oxide (boehmite, gibbsite, corundum)
- Copper (metalic copper, cuprite)
- Barium sulfate
- Particulate fouling, i.e., accumulation of particles, typically  colloidal particles, on a surface
Iron oxides and oxyhydroxides (magnetite, hematite, lepidocrocite, maghemite, goethite)
- Iron oxides and oxyhydroxides (magnetite, hematite, lepidocrocite, maghemite, goethite)
- Sedimentation fouling by silt and other relatively coarse suspended matter
- 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
- Biofouling, like settlements of bacteria and algae
- Composite fouling, whereby fouling involves more than one foulant or fouling mechanism.

## Precipitation fouling
Through changes in temperature, or solvent evaporation or degasification, the concentration of salts may exceed the saturation, leading to a precipitation of salt crystals. Precipitation fouling is a very common problem in boilers and heat exchangers operating with hard water and often results in limescale.
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:
The calcium carbonate that has formed through this reaction precipitates. Due to the temperature dependence of the reaction, the scaling is higher at the hotter outlet of the heat exchanger than at the cooler inlet. In general, the dependence of the salt solubility on temperature or presence of evaporation will often be the driving force for precipitation fouling. The important distinction is between salts with "normal" or "retrograde" dependence of solubility on temperature. The salts with the "normal" solubility decrease their solubility with temperature and thus will foul the cooling sufaces. The salts with "inverse" or "retrograte" solubility will foul the heating surfaces.

## Particulate fouling
Fouling by particles suspended in water ("crud") or in gas progresses by a mechanism different than precipitation fouling. This process is usually most important for colloidal particles, i.e., particles smaller than about 1 m in at least one dimension  (but which are much larger than atomic dimensions). Particles are transported to the surface by a number of mechanisms and there they can attach themselves, e.g., by flocculation or coagulation. Note that the attachment of colloidal particles typically involves electrical forces and thus the particle behaviour defies the experience from the macroscopic world. The probability of attachment is sometimes referred to as "sticking probability", which for colloidal particles is a function of both the surface chemistry and the local thermohydraulic conditions. With time, the resulting surface deposit may harden through processes collectively known as "deposit consolidation" or, colloquially, "aging".

## Chemical reaction fouling
Chemical reactions may occur on contact of the components of the process fluid with heat transfer surfaces. In such cases, the metallic surface sometimes acts as a catalyst. For example, corrosion and polymerization occurs in cooling water for the chemical industry which has a minor content of hydrocarbons. Systems in petroleum processing are prone to polymerization of olefins or deposition of heavy fractions (asphaltenes, waxes, etc). High tube wall temperatures may lead to carbonizing of organic matter.  Food industry, for example milk processing, also experiences fouling problems by chemical reactions.

## Corrosion fouling
Corrosion deposits are created in-situ by the corrosion of the substrate. They are distinguished from fouling deposits, which form from material originating ex-situ. Corrosion deposits should not be confused with fouling deposits formed by ex-situ generated corrosion products. Corrosion deposits will normally have composition related to the composition of the substrate. An example of corrosion fouling can be formation of an iron oxide or oxyhydroxide deposit from corrosion of the carbon steel underneath.

## Biofouling
Biofouling or biological fouling is the undesirable accumulation of micro-organisms, plants, algae, and animals on structures, for example ships' hulls. Water piping systems carrying untreated water are also often subject of biofouling.

## Composite fouling
Composite fouling is common. This type of fouling involves more than one foulant or more than one fouling mechanism working simultaneously. The multiple foulants or mechanisms may interact with each other resulting in a synergistic fouling which is not a simple arithmetic sum of the individual components.

## Fouling on Mars
NASA Mars Exploration Rovers (Spirit and Opportunity) experienced fouling of solar panels by dust particles from the  Martian atmosphere. Some of the deposits subsequently spontaneously cleaned off. This illustrates the universal nature of the fouling phenomena.

# Fouling modelling
Fouling of a system can be modelled as consisting of several steps:
- Generation or ingress of the species that causes fouling ("foulant sourcing")
- Foulant transport with the stream of the process fluid
- Foulant transport from the bulk of the process fluid to the fouling surface
- Induction period, i.e., a near-nil fouling rate at the initial period of fouling (observed only for some fouling mechanisms)
- Foulant crystallization on the surface (or attachment of the colloidal particle, or chemical reaction, or bacterial growth)
- Deposit dissolution (or re-entrainment of particles)
- Deposit consolidation on the surface
- Deposit spalling
Deposition consists of transport to the surface and subsequent attachment. Deposit removal is either through deposit dissolution, particle re-entrainment or deposit spalling. Fouling results from foulant generation, foulant deposition, deposit removal, and deposit consolidation.

# Quantification of fouling
The most straight-forward way to quantify fairly uniform fouling is by stating the average deposit surface loading, i.e., kg of deposit per m2 of surface area. The fouling rate will then be expressed in kg/m2s, and it is obtained by dividing the deposit surface loading by the effective operating time. The normalized fouling rate (also in kg/m2s) will additionally account for the concentration of the foulant in the process fluid (kg/kg) during preceding operations, and is useful for comparison of fouling rates between different systems. It is obtained by dividing the fouling rate by the foulant concentration. The fouling rate constant (m/s) can be obtained by dividing the normalized fouling rate by the mass density of the process fluid (kg/m3).
Deposit thickness (m) and porosity (%) are also often used for description of fouling amount. The relative reduction of diameter of piping or increase of the surface roughness can be of particular interest when the impact of fouling on pressure drop is of interest.
In heat transfer equipment, where the primary concern is often the effect of fouling on heat transfer, fouling can be quantified by the increase of the resistance to the flow of heat (m2K/W) due to fouling (termed "fouling resistance"), or by development of heat transfer coefficient (W/m2K) with time.
If under-deposit or crevice corrosion is of primary concern, it is important to note packing of confined regions with deposits or creation of occluded "crevices". The non-uniformity of deposit thickness (e.g., deposit waviness) can also be important if underdeposit corrosion of material (e.g., intergranular attack, pitting, stress corrosion cracking) is of concern.

# The economic importance of fouling
Fouling is ubiquitous and generates tremendous operational losses, not unlike corrosion. For example, one estimate puts the losses due to fouling of heat exchangers in industrialized nations to be about 0.25% of their GDP.
The losses initially result from impaired heat transfer, corrosion damage (in particular under-deposit and crevice corrosion), increased pressure drop, flow blockages, flow redistribution inside components, flow instabilities, induced vibrations, fretting, premature failure of electrical heating elements, and a large number of other often unanticipated problems. In addition, the ecological costs should be (but typically are not) considered. The ecological costs arise from the use of biocides for the avoidance of biofouling, and from the increased fuel input to compensate for the reduced output caused by fouling.
For example, "normal" fouling at a conventionally fired 500 MW (net electrical power) power station unit accounts for output losses of the steam turbine of 5 MW and more. In a 1,300 MW nuclear power station, typical losses could be 20 MW and up (up to 100% if the station shuts down due to fouling-induced component degradation). In seawater desalination plants, fouling may reduce the gained output ratio by two-digit percentages. (The gained output ratio is an equivalent that puts the mass of generated distillate in relation to the steam used in the process.) The extra electrical consumption in compressor-operated coolers is also easily in the two-digit area. In addition to the operational costs, also the capital cost increases because the heat exchangers have to be designed in larger sizes to compensate for the heat-transfer loss due to fouling. To the output losses listed above, one needs to add the cost of down-time required to inspect, clean, and repair the components (millions of dollars per day of shutdown in lost revenue in a typical power plant), and the cost of actually doing this maintenance. Finally, fouling is often a root cause of serious degradation problems that may limit the life of components or entire plants.

# Fouling control
The most fundamental and usually preferred method of controlling fouling is to prevent the ingress of the fouling species into the cooling water circuit. In steam power stations and other major industrial installations of water technology, macro fouling is avoided by way of pre-filtration and cooling water debris filters. In the case of micro fouling, water purification is achieved with extensive methods of water treatment, membrane technology (reverse osmosis) or ion-exchange resins. The generation of the corrosion products in the water piping systems is often minimized by controlling the pH of the process fluid (typically alkanization with ammonia, morpholine, ethanolamine or sodium phosphate), control of oxygen dissolved in water (for example, by addition of hydrazine), or addition of corrosion inhibitors.
For water systems at relatively low temperatures, the applied biocides may be classified as follows: inorganic chlorine and bromide compounds, chlorine and bromide cleavers, ozone and oxygen cleavers, unoxidizable biocides. One of the most important unoxidizable biocides is a mixture of chloromethyl-isothiazolinone and methyl-isothiazolinone. Also applied are dibrom nitrilopropionamide and quaternary ammonium compounds.
Chemical fouling inhibitors can reduce fouling in many systems, mainly by interfering with the crystallization, attachement, or consolidation steps of the fouling process. Examples are: chelating agents (for example, EDTA), long-chain aliphatic amines or polyamines (for example, octadecylamine, helamin, and other "film-forming" amines), organic phosphonic acids (for example, 1-hydroxyethylidene-1,1-diphosphonic acid, known as HEDP), or polyelectrolytes (for example, polyacrylic acid, polymethacrylic acid, usually with a molecular weight lower than 10000).
On the component design level, fouling can often (but not always) be minimized by maintaining a relatively high (for example, 2 m/s) and uniform fluid velocity throughout the component. Stagnant regions need to be eliminated. Component is normally overdesigned to accommodate the fouling anticipated between cleanings. However, a significant overdesign can be a design error because it may lead to increased fouling due to reduced velocities. Periodic on-line pressure pulses or backflow can be effective if the capability is  carefully incorporated at the design time. Blowdown capability is always incorporated into steam generators or evaporators to control the accumulation of non-volatile impurities the cause or aggreviate fouling. Low-fouling surfaces (for example, very smooth, implanted with ions, or of low surface energy like Teflon) are an option for some applications. Modern components are typically required to be designed for ease of inspection of internals and periodic cleaning.
Chemical or mechanical cleaning processes for the removal of deposits and scales are recommended when fouling reaches the point of impacting the system performance. These processes comprise pickling with acids and metal complexing agents, cleaning with high-velocity water jets ("water lancing"), or recirculating sponge rubber balls. Whereas chemical cleaning causes environmental problems through the handling, application, storage and disposal of chemicals, the mechanical cleaning by means of circulating cleaning balls can be a more environmentally-friendly alternative. Also ultrasonic or abrasive cleaning methods are available for many specific applications.