Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms (biofouling

Biofouling

Biofouling or biological fouling is the undesirable accumulation of microorganisms, plants, algae, or animals on wetted structures.-Impact:...

) or a non-living substance (inorganic or organic). 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 community

Fouling communities are communities of organisms found on the sides of docks, marinas, harbors, and boats throughout the world. These communities are characterized by the presence of a variety of sessile organisms including ascidians, bryozoans, mussels, tube building polychaetes, sea anemones,...

), fouling of heat-transfer

Heat exchanger

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

 components through ingredients contained in the cooling water or gases, and even the development of plaque

Dental plaque

Dental plaque is a biofilm, usually a pale yellow, that develops naturally on the teeth. Like any biofilm, dental plaque is formed by colonizing bacteria trying to attach themselves to a smooth surface...

 or calculus

Calculus (dental)

In dentistry, calculus or tartar is a form of hardened dental plaque. It is caused by the continual accumulation of minerals from saliva on plaque on the teeth...

 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 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.

Components subject to fouling

Following are examples of components that may be subject to fouling and the corresponding effects of fouling:

• Heat exchanger
  Heat exchanger
  A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
  
   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
  Drag (physics)
  In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
  
  , increases fuel usage, reduces maximum speed;
• Turbine
  Turbine
  A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
  
  s - reduces efficiency, increases probability of failure;
• Solar panels - decreases the electrical power generated;
• Reverse osmosis membranes
  Membrane fouling
  Membrane fouling is a process where solute or particles deposit onto a membrane surface or into membrane pores in a way that degrades the membrane's performance. It is a major obstacle to the widespread use of this technology. Membrane fouling can cause severe flux decline and affect the quality of...
  
   - increases pressure drop, increases energy expenditure, reduces flux, membrane failure (in severe cases);
• Electrical heating element
  Heating element
  A heating element converts electricity into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element....
  
  s - increases temperature of the element, increases corrosion, reduces lifespan;
• Nuclear fuel in pressurized water reactor
  Pressurized water reactor
  Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...
  
  s - 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 plate
  Orifice plate
  An orifice plate is a device used for measuring the volumetric flow rate. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which states that there is a relationship between the pressure of the fluid and the velocity of the fluid...
  
  s - inaccurate or incorrect measurement of flow rate;
• Pitot tube
  Pitot tube
  A pitot tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot Ulo in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy...
  
  s in airplanes - inaccurate or incorrect indication of airplane speed;
• Spark plug
  Spark plug
  A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
  
   electrodes in cars - engine misfiring;
• Production zone of petroleum reservoirs and oil well
  Oil well
  An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
  
  s - decreased petroleum production
  Extraction of petroleum
  The extraction of petroleum is the process by which usable petroleum is extracted and removed from the earth.-Locating the oil field:Geologists use seismic surveys to search for geological structures that may form oil reservoirs...
  
   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
  Senescence
  Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
  
  .

Macro fouling

Macro fouling is caused by coarse

Debris

Debris is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, in geology, large rock fragments left by a melting glacier etc. The singular form of debris is debris...

 matter of either biological or inorganic origin, for example industrially produced refuse. Such matter enters into the cooling water circuit through the cooling water pump

Water Pump

Water Pump is one of the neighbourhoods of Gulberg Town in Karachi, Sindh, Pakistan. It is near main Water Pump that supplies fresh water to the city of Karachi....

s from sources like the open sea

Sea

A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean...

, river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

s or lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

s. In closed circuits, like cooling tower

Cooling tower

Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or in the case of closed circuit dry cooling towers rely...

s, 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

Fretting

Fretting refers to wear and sometimes corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration...

 damage.
Examples:

• Manmade refuse;
• Detached internal parts of components;
• Tools and other "foreign objects" accidentally left after maintenance;
• Algae
  Algae
  Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
  
  ;
• Mussel
  Mussel
  The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...
  
  s;
• Leaves
  Leaf
  A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
  
  , parts of plants up to entire trunk
  Trunk (botany)
  In botany, trunk refers to the main wooden axis of a tree that supports the branches and is supported by and directly attached to the roots. The trunk is covered by the bark, which is an important diagnostic feature in tree identification, and which often differs markedly from the bottom of the...
  
  s.

Micro fouling

As to micro fouling, distinctions are made between:

• Scaling or precipitation fouling, as crystallization
  Crystallization
  Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
  
   of solid salt
  Salt
  In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
  
  s, oxide
  Oxide
  An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
  
  s and hydroxide
  Hydroxide
  Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
  
  s from water solution
  Solution
  In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
  
  s, for example, 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
  Corrosion
  Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
  
   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
  Biofouling
  Biofouling or biological fouling is the undesirable accumulation of microorganisms, plants, algae, or animals on wetted structures.-Impact:...
  
  , like settlements of bacteria
  Bacteria
  Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
  
   and algae;
• Composite fouling, whereby fouling involves more than one foulant or fouling mechanism.

Precipitation fouling

Scaling or precipitation fouling involves crystallization

Crystallization

Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...

 of solid salt

Salt

In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

s, oxide

Oxide

An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....

s and hydroxide

Hydroxide

Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...

s from solution

Solution

In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...

s. 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

Hard water

Hard water is water that has high mineral content . Hard water has high concentrations of Ca2+ and Mg2+ ions. Hard water is generally not harmful to one's health but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling...

 and often results in limescale

Limescale

Limescale is the hard, off-white, chalky deposit found in kettles, hot-water boilers and the inside of inadequately maintained hot-water central heating systems...

.

Through changes in temperature, or solvent evaporation

Evaporation

Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

 or degasification

Degasification

Degasification is the removal of dissolved gases from liquids, especially water or aqueous solutions, in the fields of science and engineering. There are numerous possible methods for such removal of gases from solids....

, the concentration of salts may exceed the saturation

Supersaturation

The term supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances...

, leading to a precipitation

Precipitation (chemistry)

Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...

 of solids (usually crystals).

As an example, the equilibrium between the readily soluble calcium bicarbonate

Calcium bicarbonate

Calcium bicarbonate , also called calcium hydrogencarbonate, does not refer to a known solid compound; it exists only in aqueous solution containing the calcium , bicarbonate , and carbonate ions, together with dissolved carbon dioxide...

 - always prevailing in natural water - and the poorly soluble calcium carbonate

Calcium carbonate

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

, the following chemical equation may be written:


The calcium carbonate that forms through this reaction precipitates. Due to the temperature dependence of the reaction, and increasing volatility of CO₂ with increasing temperature, 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

Solubility

Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...

 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 increase their solubility with increasing temperature and thus will foul the cooling surfaces. The salts with "inverse" or "retrograde" solubility will foul the heating surfaces. An example of the temperature dependence of solubility is shown in the figure. Calcium sulfate is a common precipitation foulant of heating surfaces due to its retrograde solubility.

Precipitation fouling can also occur in the absence of heating or vaporization. For example, calcium sulfate decreases it solubility with decreasing pressure. This can lead to precipitation fouling of reservoirs and wells in oil fields, decreasing their productivity with time. Fouling of membranes in reverse osmosis

Reverse osmosis

Reverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...

 systems can occur due to differential solubility of barium sulfate in solutions of different ionic strength

Ionic strength

The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as the dissociation or the solubility of different salts...

. Similarly, precipitation fouling can occur because of solubility changes induced by other factors, e.g., liquid flashing

Flash evaporation

Flash evaporation is the partial vapor that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit operations...

, liquid degassing, redox potential changes, or mixing of incompatible fluid streams.

The following lists some of the industrially common phases of precipitation fouling deposits observed in practice to form from aqueous solutions:

• Calcium carbonate
  Calcium carbonate
  Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
  
   (calcite
  Calcite
  Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
  
  , aragonite
  Aragonite
  Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
  
   usually at t > ~50 °C, or rarely vaterite
  Vaterite
  Vaterite is a mineral, a polymorph of calcium carbonate. It was named after the German mineralogist Heinrich Vater. It is also known as mu-calcium carbonate and has a JCPDS number of 13-192. Vaterite, like aragonite, is a metastable phase of calcium carbonate at ambient conditions at the surface...
  
  );
• Calcium sulfate
  Calcium sulfate
  Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...
  
   (anhydrite
  Anhydrite
  Anhydrite is a mineral – anhydrous calcium sulfate, CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the orthorhombic barium and strontium sulfates, as might be expected from the...
  
  , hemihydrate, gypsum
  Gypsum
  Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
  
  );
• Calcium oxalate
  Calcium oxalate
  Calcium oxalate is a chemical compound that forms needle-shaped crystals, known in plants as raphides. A major constituent of human kidney stones, the chemical is also found in beerstone, a scale that forms on containers used in breweries...
  
   (e.g., beerstone);
• Barium sulfate
  Barium sulfate
  Barium sulfate is the inorganic compound with the chemical formula BaSO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium and materials prepared from it...
  
   (barite
  Barite
  Baryte, or barite, is a mineral consisting of barium sulfate. The baryte group consists of baryte, celestine, anglesite and anhydrite. Baryte itself is generally white or colorless, and is the main source of barium...
  
  );
• Magnesium hydroxide
  Magnesium hydroxide
  Magnesium hydroxide is an inorganic compound with the chemical formula Mg2. As a suspension in water, it is often called milk of magnesia because of its milk-like appearance. The solid mineral form of magnesium hydroxide is known as brucite....
  
   (brucite
  Brucite
  Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and formed during serpentinization of dunites...
  
  ); magnesium oxide
  Magnesium oxide
  Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
  
   (periclase
  Periclase
  Periclase occurs naturally in contact metamorphic rocks and is a major component of most basic refractory bricks. It is a cubic form of magnesium oxide ....
  
  );
• Silicate
  Silicate
  A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
  
  s (serpentine, acmite, gyrolite, gehlenite
  Gehlenite
  Gehlenite, , is a sorosilicate, Al-rich endmember of the melilite complete solid solution series with akermanite.The type locality is in the Monzoni Mountains, Fassa Valley in Trentino in Italy, and is named after Adolf Ferdinand Gehlen by A.J...
  
  , amorphous silica, quartz
  Quartz
  Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
  
  , cristobalite
  Cristobalite
  The mineral cristobalite is a high-temperature polymorph of silica, meaning that it has the same chemical formula, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the quartz group which also include coesite, tridymite and stishovite...
  
  , pectolite
  Pectolite
  Pectolite is a white to gray mineral, NaCa2Si3O8, sodium calcium inosilicate hydroxide. It crystallizes in the triclinic system typically occurring in radiated or fibrous crystalline masses. It has a Mohs hardness of 4.5 to 5 and a specific gravity of 2.7 to 2.9. The gemstone variety, larimar, is a...
  
  , xonotlite
  Xonotlite
  Xonotlite is a mineral with the chemical formula Ca6Si6O172. It crystallizes in the monoclinic - prismatic crystal system with typically an acicular crystal form or habit. It can be colorless, gray, light gray, lemon white, or pink. It is transparent with a vitreous to silky luster. It leaves a...
  
  );
• Aluminium oxide hydroxide
  Aluminium oxide hydroxide
  Aluminium hydroxide oxide or aluminium oxyhydroxide, AlO is found as one of two well defined crystalline phases, which are also known as the minerals boehmite and diaspore...
  
  s (boehmite
  Boehmite
  Boehmite or Böhmite is an aluminium oxide hydroxide mineral, a component of the aluminium ore bauxite. It is dimorphous with diaspore. It crystallizes in the orthorhombic dipyramidal system and is typically massive in habit. It is white with tints of yellow, green, brown or red due to impurities...
  
  , gibbsite
  Gibbsite
  Gibbsite, Al3, is one of the mineral forms of aluminium hydroxide. It is often designated as γ-Al3 . It is also sometimes called hydrargillite ....
  
  , diaspore
  Diaspore
  Diaspore is a native aluminium oxide hydroxide, α-AlO, crystallizing in the orthorhombic system and isomorphous with goethite. It occurs sometimes as flattened crystals, but usually as lamellar or scaly masses, the flattened surface being a direction of perfect cleavage on which the lustre is...
  
  , corundum
  Corundum
  Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...
  
  );
• Aluminosilicate
  Aluminosilicate
  Aluminosilicate minerals are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a major component of kaolin and other clay minerals....
  
  s (analcite
  Analcite
  Analcime or analcite is a white, grey, or colourless tectosilicate mineral. Analcime consists of hydrated sodium aluminium silicate in cubic crystalline form. Its chemical formula is NaAlSi2O6·H2O. Minor amounts of potassium and calcium substitute for sodium...
  
  , cancrinite
  Cancrinite
  Cancrinite is a complex carbonate and silicate of sodium, calcium and aluminium with the formula Na6Ca2[2|Al6Si6O24]·2H2O. It is classed as a member of the feldspathoid group of minerals; the alkali feldspars that are poor in silica. Yellow, orange, pink, white or even blue, it has a vitreous or...
  
  , noselite);
• Copper (metallic copper
  Copper
  Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
  
  , cuprite
  Cuprite
  Cuprite is an oxide mineral composed of copper oxide Cu2O, and is a minor ore of copper.Its dark crystals with red internal reflections are in the isometric system hexoctahedral class, appearing as cubic, octahedral, or dodecahedral forms, or in combinations. Penetration twins frequently occur...
  
  , tenorite
  Tenorite
  Tenorite is a copper oxide mineral with the simple formula CuO.-Occurrence:Tenorite occurs in the weathered or oxidized zone associated with deeper primary copper sulfide orebodies. Tenorite commonly occurs with chrysocolla and the copper carbonates, azurite and malachite. The dull grey-black color...
  
  );
• Phosphates (hydroxyapatite);
• Magnetite or nickel ferrite (NiFe₂O₄) from extremely pure, low-iron water.

The deposition rate by precipitation is often described by the following equations:

Transport:

Surface crystallisation:

Overall:

where:

m - mass of the material (per unit surface area), kg/m²

t - time, s

C_b - concentration of the substance in the bulk of the fluid, kg/m³

C_i - concentration of the substance at the interface, kg/m³

C_e - equilibrium concentration of the substance at the conditions of the interface, kg/m³

n1, n2 - order of reaction

Order of reaction

In chemical kinetics, the order of reaction with respect to certain reactant, is defined as the power to which its concentration term in the rate equation is raised .For example, given a chemical reaction 2A + B → C with a rate equation...

 for the crystallisation reaction and the overall deposition process, respectively, dimensionless

k_t, k_r, k_d - kinetic rate constants for the transport, the surface reaction, and the overall deposition reaction, respectively; with the dimension of m/s (when n1 and n2 = 1)

Particulate fouling

Fouling by particles suspended in water ("crud

CRUD

CRUD or Crud may refer to:* A sticky substance, especially when dirty and/or encrusted.* Create, read, update and delete, basic functions of a computer database* Crud , played on a billiard table...

") 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 μm 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

Flocculation

Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...

 or coagulation

Coagulation

Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...

. 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

Sticking probability

The sticking probability is the probability that molecules are trapped on surfaces and adsorb chemically. From Langmuir's adsorption isotherm, molecules cannot adsorb on surfaces when the adsorption sites are already occupied by other molecules, so the sticking probability can be expressed as...

", P:


where k_d and k_t are the kinetic rate constants for deposition and transport, respectively. The value of P for colloidal particles is a function of both the surface chemistry, geometry, and the local thermohydraulic conditions.

An alternative to using the sticking probability is to use a kinetic attachment rate constant, assuming the first order reaction:

and then the transport and attachment kinetic coefficients are combined as two processes occurring is series:

where:

• dm/dt is the rate of the deposition by particles, kg m^-2 s^-1,
• k_a, k_t and k_d are the kinetic rate constants for deposition, m/s,
• C_i and C_b are the concentration of the particle foulant at the interface and in the bulk fluid, respectively; kg m³.

Being essentially a surface chemistry phenomenon, this fouling mechanism can be very sensitive to factors that affect colloidal stability, e.g., zeta potential

Zeta potential

Zeta potential is a scientific term for electrokinetic potential in colloidal systems. In the colloidal chemistry literature, it is usually denoted using the Greek letter zeta, hence ζ-potential...

. A maximum fouling rate is usually observed when the fouling particles and the substrate exhibit opposite electrical charge, or near the point of zero charge of either of them.

Particles larger than those of colloidal dimensions may also foul e.g., by sedimentation ("sedimentation fouling") or straining in small-size openings.

With time, the resulting surface deposit may harden through processes collectively known as "deposit consolidation" or, colloquially, "aging".

The common particulate fouling deposits formed from aqueous suspensions include:

• iron oxides and iron oxyhydroxides
  Iron oxide
  Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
  
   (magnetite
  Magnetite
  Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
  
  , hematite
  Hematite
  Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
  
  , lepidocrocite
  Lepidocrocite
  Lepidocrocite , also called esmeraldite or hydrohematite, is an iron oxide-hydroxide mineral. Lepidocrocite has an orthorhombic crystal structure, a hardness of 5, specific gravity of 4, a submetallic luster and a yellow-brown streak. It is red to reddish brown and forms when iron-containing...
  
  , maghemite
  Maghemite
  Maghemite is a member of the family of iron oxides. It has the same structure as magnetite, that is, it is spinel ferrite and is also ferrimagnetic.Maghemite can be considered as an Fe-deficient magnetite with formula...
  
  , goethite
  Goethite
  Goethite , named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low-temperature environments. Goethite has been well known since prehistoric times for its use as a pigment. Evidence has been found of its use in paint pigment samples...
  
  );
• Sedimentation
  Sedimentation
  Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
  
   fouling by silt
  Silt
  Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
  
   and other relatively coarse suspended matter.

Fouling by particles from gas aerosol

Aerosol

Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...

s is also of industrial significance. The particles can be either solid or liquid. The common examples can be fouling by flue gas

Flue gas

Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants...

es, or fouling of air-cooled components by dust in air. The mechanisms are discussed in article on aerosol deposition

Deposition (Aerosol physics)

In aerosol physics, Deposition is the process by which aerosol particles collect or deposit themselves on solid surfaces, decreasing the concentration of the particles in the air. It can be divided into two sub-processes: dry and wet deposition. The rate of deposition, or the deposition velocity,...

.

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. Also, the geometry of the metal-oxide and oxide-fluid interfaces may allow practical distinction between the corrosion and fouling deposits. An example of corrosion fouling can be formation of an iron oxide or oxyhydroxide deposit from corrosion of the carbon steel underneath. Corrosion fouling should not be confused with fouling corrosion, i.e., any of the types of corrosion that may be induced by fouling.

Chemical reaction fouling

Chemical reactions may occur on contact of the chemical species in the process fluid with heat transfer surfaces. In such cases, the metallic surface sometimes acts as a catalyst. For example, corrosion and polymerization

Polymerization

In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

 occurs in cooling water for the chemical industry which has a minor content of hydrocarbons. Systems in petroleum processing are prone to polymerization of olefin

Alkene

In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...

s or deposition of heavy fractions (asphaltenes, waxes, etc.). High tube wall temperatures may lead to carbonizing

Carbonization

Carbonization or carbonisation is the term for the conversion of an organic substance into carbon or a carbon-containing residue through pyrolysis or destructive distillation. It is often used in organic chemistry with reference to the generation of coal gas and coal tar from raw coal...

 of organic matter. Food industry, for example milk processing, also experiences fouling problems by chemical reactions.

Fouling through an ionic reaction with an evolution of an inorganic solid is commonly classified as precipitation fouling (not chemical reaction fouling).

Solidification fouling

Solidification fouling occurs when a component of the flowing fluid "freezes" onto a surface forming a solid fouling deposit. Examples may include solidification of wax (with a high melting point) from a hydrocarbon solution, or of molten ash (carried in a furnace exhaust gas) onto a heat exchanger surface. The surface needs to have a temperature below a certain threshold; therefore, it is said to be subcooled in respect to the solidification point of the foulant.

Biofouling

Biofouling

Biofouling

Biofouling or biological fouling is the undesirable accumulation of microorganisms, plants, algae, or animals on wetted structures.-Impact:...

 or biological fouling is the undesirable accumulation of micro-organisms, algae and diatoms, plants, and animals on surfaces, for example ships' hulls, or piping and reservoirs with untreated water. This can be accompanied by microbiologically influenced corrosion

Microbial corrosion

Microbial corrosion, also called bacterial corrosion, bio-corrosion, microbiologically-influenced corrosion, or microbially-induced corrosion , is corrosion caused or promoted by microorganisms, usually chemoautotrophs...

 (MIC).

Bacteria can form biofilms or slimes. Thus the organisms can aggregate on surfaces using colloidal hydrogels of water and extracellular polymeric substances (EPS) (polysaccharide

Polysaccharide

Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...

s, lipids, nucleic acids, etc.). The biofilm structure is usually complex.

Bacterial fouling can occur under either aerobic (with oxygen dissolved in water) or anaerobic (no oxygen) conditions. In practice, aerobic bacteria prefer open systems, when both oxygen and nutrients are constantly delivered, often in warm and sunlit environments. Anaerobic fouling more often occurs in closed systems when sufficient nutrients are present. Examples may include sulfate-reducing bacteria

Sulfate-reducing bacteria

Sulfate-reducing bacteria are those bacteria and archaea that can obtain energy by oxidizing organic compounds or molecular hydrogen while reducing sulfate to hydrogen sulfide...

 (or sulfur-reducing bacteria

Sulfur-reducing bacteria

Sulfur-reducing bacteria get their energy by reducing elemental sulfur to hydrogen sulfide. They couple this reaction with the oxidation of acetate, succinate or other organic compounds....

), which produce sulfide and often cause corrosion of ferrous metals (and other alloys). Sulfide-oxidizing bacteria (e.g., Acidithiobacillus

Acidithiobacillus

Acidithiobacillus is a genus of Proteobacteria. Like all Proteobacteria, Acidithiobacillus is Gram-negative. The members of this genus used to belong to Thiobacillus, before they were reclassified in the year 2000....

), on the other hand, can produce sulfuric acid, and can be involved in corrosion of concrete.

Zebra mussel

Zebra mussel

The zebra mussel, Dreissena polymorpha, is a small freshwater mussel. This species was originally native to the lakes of southeast Russia being first described in 1769 by a German zoologist Peter Simon Pallas in the Ural, Volga and Dnieper rivers. They are still found nearby, as Pontic and Caspian...

s serve as an example of larger animals that have caused widespread fouling in North America.

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 Rover

Mars Exploration Rover

NASA's Mars Exploration Rover Mission is an ongoing robotic space mission involving two rovers, Spirit and Opportunity, exploring the planet Mars...

s (Spirit

Spirit rover

Spirit, MER-A , is a robotic rover on Mars, active from 2004 to 2010. It was one of two rovers of NASA's ongoing Mars Exploration Rover Mission. It landed successfully on Mars at 04:35 Ground UTC on January 4, 2004, three weeks before its twin, Opportunity , landed on the other side of the planet...

 and Opportunity

Opportunity rover

Opportunity, MER-B , is a robotic rover on the planet Mars, active since 2004. It is the remaining rover in NASA's ongoing Mars Exploration Rover Mission...

) experienced (presumably) abiotic fouling of solar panels by dust particles from the Martian atmosphere. Some of the deposits subsequently spontaneously cleaned off

Cleaning event

A cleaning event is a phenomenon whereby dust is removed from solar panels, particularly ones on Mars, by the action of wind. The term cleaning event is used on several NASA webpages; generally the term is used in reference to the fact that Martian winds have blown dust clear of the solar panels of...

. This illustrates the universal nature of the fouling phenomena.

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 (μm) 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

Crevice corrosion

Crevice corrosion refers to corrosion occurring in confined spaces to which the access of the working fluid from the environment is limited. These spaces are generally called crevices...

 is of primary concern, it is important to note non-uniformity of deposit thickness (e.g., deposit waviness

Waviness

Waviness is the measure of the more widely spaced component of surface texture. It is a broader view of roughness because it is more strictly defined as "the irregularities whose spacing is greater than the roughness sampling length"...

), localized fouling, packing of confined regions with deposits, creation of occlusions, "crevices", "deposit tubercles", or sludge piles. Such deposit structures can create environment for underdeposit corrosion of the substrate material, e.g., intergranular attack

Intergranular corrosion

Intergranular corrosion , also known as intergranular attack , is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides...

, pitting

Pitting corrosion

Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the creation of small holes in the metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic while an unknown but potentially vast area becomes cathodic, leading...

, stress corrosion cracking

Stress corrosion cracking

Stress corrosion cracking is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when...

, or localized wastage. Porosity

Porosity

Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

 and permeability

Permeation

Permeation, in physics and engineering, is the penetration of a permeate through a solid, and is related to a material's intrinsic permeability...

 of the deposits will likely influence the probability of underdeposit corrosion. Deposit composition can also be important - even minor components of the deposits can sometimes cause severe corrosion of the underlying metal (e.g., vanadium in deposits of fired boilers causing hot corrosion).

There is no general rule on how much deposit can be tolerated, it depends on the system. In many cases, a deposit even a few micrometers thick can be troublesome. A deposit in a millimeter-range thickness will be of concern in almost any application.

Progress of fouling with time

Deposit on a surface does not always develop steadily with time. The following fouling scenarios can be distinguished, depending on the nature of the system and the local thermohydraulic conditions at the surface:

• Induction period
  Induction period
  An induction period in chemical kinetics is an initial slow stage of a chemical reaction; after the induction period, the reaction accelerates. Ignoring induction periods can lead to runaway reactions....
  
  . Sometimes, a near-nil fouling rate is observed when the surface is new or very clean. This is often observed in biofouling and precipitation fouling. After the "induction period", the fouling rate increases.
• "Negative" fouling. This can occur when fouling rate is quantified by monitoring heat transfer. Relatively small amounts of deposit can improve heat transfer, relative to clean surface, and give an appearance of "negative" fouling rate and negative total fouling amount. Negative fouling is often observed under nucleate-boiling heat-transfer conditions (deposit improves bubble nucleation) or forced-convection (if the deposit increases the surface roughness and the surface is no longer "hydraulically smooth"). After the initial period of "surface roughness control", the fouling rate usually becomes strongly positive.
• Linear fouling. The fouling rate can be steady with time. This is a common case.
• Falling fouling. Under this scenario, the fouling rate decreases with time, but never drops to zero. The deposit thickness does not achieves a constant value. The progress of fouling can be often described by two numbers: the initial fouling rate (a tangent
  Tangent
  In geometry, the tangent line to a plane curve at a given point is the straight line that "just touches" the curve at that point. More precisely, a straight line is said to be a tangent of a curve at a point on the curve if the line passes through the point on the curve and has slope where f...
  
   to the fouling curve at zero deposit loading or zero time) and the fouling rate after a long period of time (an oblique asymptote to the fouling curve).
• Asymptotic fouling. Here, the fouling rate decreases with time, until it finally reaches zero. At this point, the deposit thickness remains constant with time (a horizontal asymptote
  Asymptote
  In analytic geometry, an asymptote of a curve is a line such that the distance between the curve and the line approaches zero as they tend to infinity. Some sources include the requirement that the curve may not cross the line infinitely often, but this is unusual for modern authors...
  
  ). This is often the case for relatively soft or poorly adherent deposits in areas of fast flow. The asymptote is usually interpreted as the deposit loading at which the deposition rate equals the deposit removal rate.
• Accelerating fouling. Under this scenario, the fouling rate increases with time; the rate of deposit buildup accelerates with time (perhaps until it becomes transport limited). Mechanistically, this scenario can develop when fouling increases the surface roughness, or when the deposit surface exhibits higher chemical propensity to fouling than the pure underlying metal.
• Seesaw fouling. Here, fouling loading generally increases with time (often assuming a generally linear or falling rate), but, when looked at in more detail, the fouling progress is periodically interrupted and takes the form of sawtooth curve
  Sawtooth wave
  The sawtooth wave is a kind of non-sinusoidal waveform. It is named a sawtooth based on its resemblance to the teeth on the blade of a saw....
  
  . The periodic sharp variations in the apparent fouling amount often correspond to the moments of system shutdowns, startups or other transients in operation. The periodic variations are often interpreted as periodic removal of some of the deposit (perhaps deposit re-suspension due to pressure pulses, spalling due thermal stresses, or exfoliation due to redox transients). Steam blanketing has been postulated to occur between the partially spalled deposits and the heat transfer surface. However, other reasons are possible, e.g., trapping of air inside the surface deposits during shutdowns, or inaccuracy of temperature measurements during transients ("temperature streaming").

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 (most often by advection
  Advection
  Advection, in chemistry, engineering and earth sciences, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries...
  
  );
• Foulant transport from the bulk of the process fluid to the fouling surface. This transport is often by molecular
  Diffusion
  Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
  
   or turbulent-eddy diffusion
  Eddy diffusion
  Eddy diffusion, eddy dispersion, or turbulent diffusion is any diffusion process by which substances are mixed in the atmosphere or in any fluid system due to eddy motion...
  
  , but may also occur by inertial coasting/impaction, particle interception by the surface (for particles with finite sizes), electrophoresis
  Electrophoresis
  Electrophoresis, also called cataphoresis, is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. This electrokinetic phenomenon was observed for the first time in 1807 by Reuss , who noticed that the application of a constant electric...
  
  , thermophoresis
  Thermophoresis
  Thermophoresis, thermodiffusion, or Soret effect , is a phenomenon observed when a mixture of two or more types of motile particles are subjected to the force of a temperature gradient and the different types of particles respond to it differently. The term "Soret effect" normally means...
  
  , diffusiphoresis
  Diffusiphoresis
  Diffusiophoresis is a motion of dispersed particles in a fluid induced by a diffusion gradient of molecular substances that are dissolved in the fluid. This gradient affects structure of the particles in an interfacial double layer and causes sliding motion of the fluid relative to the particle...
  
  , Stefan flow
  Stefan flow
  Stefan flow, occasionally called Stefan's flow , is a transport phenomenon concerning the movement of a chemical species by a flowing fluid that is induced to flow by the production or removal of the species at an interface...
  
   (in condensation and evaporation), sedimentation
  Sedimentation
  Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
  
  , Magnus force (acting on rotating particles), thermoelectric effect
  Thermoelectric effect
  The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference...
  
   , and other mechanisms.
• Induction period, i.e., a near-nil fouling rate at the initial period of fouling (observed only for some fouling mechanisms);
• Foulant crystallisation on the surface (or attachment of the colloidal particle, or chemical reaction, or bacterial growth);
• Sometimes fouling autoretardation, i.e., reduction (or potentially enhancement) of crystallisation/attachment rate due to changes in the surface conditions caused by the fouling deposit;
• Deposit dissolution (or re-entrainment of loosely attached particles);
• Deposit consolidation on the surface (e.g., through Oswald ripening or differential solubility in temperature gradient) or cementation
  Cementation (geology)
  Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material within sediment pores; this is how "sediment" becomes "rock". The new pore-filling minerals form "bridges" between original sediment grains, thereby binding them together. So sand becomes...
  
  , which account for deposit losing its porosity and becoming more tenacious with time;
• Deposit spalling
  Spallation
  In general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection or vaporization of material from a target during impact by a projectile...
  
  , erosion wear
  Wear
  In materials science, wear is erosion or sideways displacement of material from its "derivative" and original position on a solid surface performed by the action of another surface....
  
  , or exfoiliation.

Deposition

Deposition (chemistry)

In chemistry, deposition is the settling of particles or sediment from a solution, suspension and mixture or vapor onto a pre-existing surface...

 consists of transport to the surface and subsequent attachment. Deposit removal is either through deposit dissolution, particle re-entrainment, or deposit spalling, erosive wear, or exfoliation. Fouling results from foulant generation, foulant deposition, deposit removal, and deposit consolidation.

For the modern model of fouling involving deposition with simultaneous deposit re-entrainment and consolidation, the fouling process can be represented by the following scheme:





Following the above scheme, the basic fouling equations can be written as follows (for steady-state conditions with flow, when concentration remains constant with time):



where:

• m is the mass loading of the deposit (consolidated and unconsolidated) on the surface (kg/m²);
• t is time (s);
• k_d is the deposition rate constant (m/s);
• ρ is the fluid density (kg/m³);
• C_m - mass fraction of foulant in the fluid (kg/kg);
• λ_r is the re-entrainment rate constant (1/s);
• m_r is the mass loading of the removable (i.e., unconsolidated) fraction of the surface deposit (kg/m²); and
• λ_c is the consolidation rate constant (1/s).

This system of equations can be integrated (taking that m = 0 and m_r = 0 at t = 0) to the form:



where λ = λ_r + λ_c.

This model reproduces either linear, falling, or asymptotic fouling, depending on the relative values of k, λ_r, and λ_c. The underlying physical picture for this model is that of a two-layer deposit consisting of consolidated inner layer and loose unconsolidated outer layer. Such a bi-layer deposit is often observed in practice. The above model simplifies readily to the older model of simultaneous deposition and re-entrainment (which neglects consolidation) when λ_c=0. In the absence of consolidation, the asymptotic fouling is always anticipated by this older model and the fouling progress can be described as:



where m^* is the maximum (asymptotic) mass loading of the deposit on the surface (kg/m²).

The economic and environmental 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

Gross domestic product

Gross domestic product refers to the market value of all final goods and services produced within a country in a given period. GDP per capita is often considered an indicator of a country's standard of living....

. Another analysis estimated (for 2006) the economical loss due to boiler and turbine fouling in China utilities at 4.68 billion dollars, which is about 0.169% the country GDP .

The losses initially result from impaired heat transfer, corrosion

Corrosion

Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

 damage (in particular under-deposit and crevice corrosion

Crevice corrosion

Crevice corrosion refers to corrosion occurring in confined spaces to which the access of the working fluid from the environment is limited. These spaces are generally called crevices...

), increased pressure drop, flow blockages, flow redistribution inside components, flow instabilities, induced vibrations (possibly leading to other problems, e.g., fatigue

Fatigue (material)

'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...

), fretting

Fretting

Fretting refers to wear and sometimes corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration...

, 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 biocide

Biocide

A biocide is a chemical substance or microorganism which can deter, render harmless, or exert a controlling effect on any harmful organism by chemical or biological means. Biocides are commonly used in medicine, agriculture, forestry, and industry...

s for the avoidance of biofouling, from the increased fuel

Fuel

Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 input to compensate for the reduced output caused by fouling, and an increased use of cooling water in once-through cooling systems.

For example, "normal" fouling at a conventionally fired 500 MW

Watt

The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

 (net electrical power) power station

Power station

A power station is an industrial facility for the generation of electric energy....

 unit accounts for output losses of the steam turbine

Steam turbine

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....

 of 5 MW and more. In a 1,300 MW nuclear power

Nuclear power

Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...

 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

Desalination

Desalination, desalinization, or desalinisation refers to any of several processes that remove some amount of salt and other minerals from saline water...

 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

Gas compressor

A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...

-operated coolers is also easily in the two-digit area. In addition to the operational costs, also the capital cost

Capital cost

Capital costs are costs incurred on the purchase of land, buildings, construction and equipment to be used in the production of goods or the rendering of services, in other words, the total cost needed to bring a project to a commercially operable status. However, capital costs are not limited to...

 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

Root cause

A root cause is rarely an initiating cause of a causal chain which leads to an outcome or effect of interest. Commonly, root cause is misused to describe the depth in the causal chain where an intervention could reasonably be implemented to change performance and prevent an undesirable outcome.In...

 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

Power station

A power station is an industrial facility for the generation of electric energy....

 and other major industrial installations of water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 technology

Technology

Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...

, macro fouling is avoided by way of pre-filtration

Filtration

Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...

 and cooling water debris filters

Filter (water)

A water filter removes impurities from water by means of a fine physical barrier, a chemical process or a biological process. Filters cleanse water to various extents for irrigation, drinking water, aquariums, and swimming pools.-Methods of filtration:...

. Some plants employ foreign-object exclusion program (to eliminate the possibility of salient introduction of unwanted materials, e.g., forgetting tools during maintenance). Acoustic monitoring is sometimes employed to monitor for fretting by detached parts. In the case of micro fouling, water purification is achieved with extensive methods of water treatment, microfiltration

Microfiltration

Microfiltration is a membrane technical filtration process which removes contaminants from a fluid by passage through a microporous membrane. A typical microfiltration membrane pore size range is 0.1 to 10 micrometres...

, membrane

Artificial membrane

An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of twentieth century. A...

 technology (reverse osmosis

Reverse osmosis

Reverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...

, electrodeionization

Electrodeionization

Electrodeionization is usually considered a water treatment technology that utilizes an electrode to ionize water molecules and separate dissolved ions from water...

) or ion-exchange resins. The generation of the corrosion products in the water piping systems is often minimized by controlling the pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 of the process fluid (typically alkanization with ammonia

Ammonia

Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

, morpholine

Morpholine

Morpholine is an organic chemical compound having the chemical formula O2NH. This heterocycle, pictured at right, features both amine and ether functional groups. Because of the amine, morpholine is a base; its conjugate acid is called morpholinium...

, ethanolamine

Ethanolamine

Ethanolamine, also called 2-aminoethanol or monoethanolamine , is an organic chemical compound that is both a primary amine and a primary alcohol . Like other amines, monoethanolamine acts as a weak base...

 or sodium phosphate), control of oxygen dissolved in water (for example, by addition of hydrazine

Hydrazine

Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

), or addition of corrosion inhibitor

Corrosion inhibitor

A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy. The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime...

s.

For water systems at relatively low temperatures, the applied biocide

Biocide

A biocide is a chemical substance or microorganism which can deter, render harmless, or exert a controlling effect on any harmful organism by chemical or biological means. Biocides are commonly used in medicine, agriculture, forestry, and industry...

s may be classified as follows: inorganic chlorine

Chlorine

Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 and bromide

Bromide

A bromide is a chemical compound containing bromide ion, that is bromine atom with effective charge of −1. The class name can include ionic compounds such as caesium bromide or covalent compounds such as sulfur dibromide.-Natural occurrence:...

 compounds, chlorine and bromide cleaver

Cleaver

-Implement or device:*Cleaver is a type of large knife*Cleaver is a type of prehistoric stone tool*Cleaver is a propeller design, esp...

s, ozone

Ozone

Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

 and oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 cleavers, unoxidizable biocide

Biocide

A biocide is a chemical substance or microorganism which can deter, render harmless, or exert a controlling effect on any harmful organism by chemical or biological means. Biocides are commonly used in medicine, agriculture, forestry, and industry...

s. One of the most important unoxidizable biocides is a mixture of chloromethyl-isothiazolinone and methyl-isothiazolinone. Also applied are dibrom nitrilopropionamide and quaternary ammonium

Ammonium

The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...

 compounds. For underwater ship hulls bottom paint

Bottom paint

Anti-fouling paint or bottom paint is a specialized coating applied to the hull of a ship or boat in order to slow the growth of organisms that attach to the hull and can affect a vessel's performance and durability...

s are applied.

Chemical fouling inhibitors can reduce fouling in many systems, mainly by interfering with the crystallization, attachment, or consolidation steps of the fouling process. Examples for water systems are: chelating agents (for example, EDTA

EDTA

Ethylenediaminetetraacetic acid, widely abbreviated as EDTA , is a polyamino carboxylic acid and a colourless, water-soluble solid. Its conjugate base is named ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ligand...

), long-chain aliphatic amines or polyamine

Polyamine

A polyamine is an organic compound having two or more primary amino groups .This class of compounds includes several synthetic substances that are important feedstocks for the chemical industry, such as ethylene diamine , 1,3-diaminopropane , and hexamethylenediamine...

s (for example, octadecylamine, helamin

Helamin

Helamin is a group of water treatment chemicals. The name is a registered trademark of Filtro, SA, Geneva, Switzerland. Chemically, most of the Helamin types are stated by the manufacturer to be a "mixture of polyamines and polycarboxylates in aqueous solution", but some also utilize volatile...

, and other "film-forming" amines), organic phosphonic acids (for example, etidronic acid

Etidronic acid

Etidronic acid or 1-hydroxyethane 1,1-diphosphonic acid is a bisphosphonate used in detergents, water treatment, cosmetics and pharmaceutical treatment....

), or polyelectrolyte

Polyelectrolyte

Polyelectrolytes are polymers whose repeating units bear an electrolyte group. These groups will dissociate in aqueous solutions , making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes and polymers , and are sometimes called polysalts. Like salts, their...

s (for example, polyacrylic acid

Polyacrylic acid

Poly or PAA or Carbomer is a type of anionic polymer. The monomer of poly is acrylic acid. In a water solution at neutral pH, many of the side chains of PAA will lose their protons and acquire a negative charge. This makes PAA a polyelectrolyte.Dry PAA is a white solid...

, polymethacrylic acid, usually with a molecular weight lower than 10000). For fired boilers, aluminum or magnesium additives can lower the melting point of ash and promote creation of deposits which are easier to remove. See also process chemicals.

Magnetic water treatment

Magnetic water treatment

Magnetic Water treatment usually by the application of a changing Magnetic flux is a proposed method of water softening The flux change may be deliberately caused by an electromagnet or the result of water flowing near a permanent magnet, usually in a pipe surrounded by the magnetic water treatment...

 has been a subject of controversy as to its effectiveness for fouling control since 1950s. The prevailing opinion is that it simply "does not work". Nevertheless, some studies suggest that it may be effective under some conditions to reduce buildup of calcium carbonate deposits.

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. Components are 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 that cause or aggravate fouling. Low-fouling surfaces (for example, very smooth, implanted with ions

Ion implantation

Ion implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into another solid. This process is used to change the physical, chemical, or electrical properties of the solid...

, 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. On-line fouling monitoring systems are designed for some application so that blowing or cleaning can be applied before unpredictable shutdown is necessary or damage occurs.

Chemical or mechanical cleaning processes for the removal of deposits and scales are recommended when fouling reaches the point of impacting the system performance or an onset of significant fouling-induced degradation (e.g., by corrosion). These processes comprise pickling with acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

s and complexing agents, cleaning with high-velocity water jets ("water lancing"), recirculating ("blasting") with metal, sponge or other balls, or propelling offline mechanical "bullet-type" tube cleaners. Whereas chemical cleaning causes environmental problems through the handling, application, storage and disposal of chemicals, the mechanical cleaning by means of circulating cleaning balls or offline "bullet-type" cleaning can be a more environmentally-friendly alternative. In some heat-transfer applications, mechanical mitigation with dynamic scraped surface heat exchangers is an option. Also ultrasonic or abrasive cleaning methods are available for many specific applications.

See also

• Boiler
  Boiler
  A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
  
  
• Cooling tower
  Cooling tower
  Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or in the case of closed circuit dry cooling towers rely...
  
  
• Corrosion
  Corrosion
  Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
  
  
• Fouling Mitigation
  Fouling mitigation
  Fouling Mitigation describes the various ways to eliminate fouling that reduces the efficiency of cooling towers, heat exchangers, pipelines and control equipment....
  
  
• Hard water
  Hard water
  Hard water is water that has high mineral content . Hard water has high concentrations of Ca2+ and Mg2+ ions. Hard water is generally not harmful to one's health but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling...
  
  
• Heat exchanger
  Heat exchanger
  A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
  
  
• Steam generator (nuclear power)
  Steam generator (nuclear power)
  Steam generators are heat exchangers used to convert water into steam from heat produced in a nuclear reactor core. They are used in pressurized water reactors between the primary and secondary coolant loops....
  
  
• Tube cleaning
  Tube cleaning
  For the article about the tube cleaning of steam locomotive boilers see: Tube cleaning .Tube cleaning describes the activity of, or device for, the cleaning and maintenance of fouled tubes....
  
  
• Water treatment
  Water treatment
  Water treatment describes those processes used to make water more acceptable for a desired end-use. These can include use as drinking water, industrial processes, medical and many other uses. The goal of all water treatment process is to remove existing contaminants in the water, or reduce the...
  
  

External links

• Crude Oil Fouling research
• On-line & In-situ fouling/biofilm/scales/slime monitoring
• A 'green' Antifouling alternative to Chlorine
• fouling monitor instrumentation to quantify problems and solutions
• http://economictimes.indiatimes.com/articleshow/msid-3720536,flstry-1.cms The Economic Times: Cleantech firms gearing up for bigger opportunities in biofouling