- present participle of foul
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 or calculus 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 foulingMacro 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
Micro foulingAs 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 (calcite, aragonite usually at t > ~50 °C, or rarely vaterite)
- Calcium sulfate (anhydrite, hemihydrate, gypsum)
- Barium sulfate
- Magnesium hydroxide (brucite)
- Silicates (serpentine, acmite, gyrolite, gehlenite, amorphous silica, quartz, cristobalite, pectolite, xonotlite)
- Aluminium oxide hydroxides (boehmite, gibbsite, corundum)
- Aluminosilicates (analcite, cancrinite, noselite)
- Copper (metalic copper, cuprite)
- Phosphates (hydroxyapatite)
- Particulate fouling, i.e., accumulation of particles, typically colloidal particles, on a surface
- 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 foulingThrough 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:
- \mathsf \Longrightarrow \mathsf \downarrow + \mathsf \uparrow + \mathsf
The calcium carbonate that has formed 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 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 dependence of the solubility on temperature is shown in the figure. Calcium sulfate is a common precipitation foulant of heating surfaces due to its retrograde solubility.
Particulate foulingFouling 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 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 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. Being essentially a surface chemistry phenomenon, this fouling mechanism can be very sensitive to factors that affect colloidal stability, e.g., zeta 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. With time, the resulting surface deposit may harden through processes collectively known as "deposit consolidation" or, colloquially, "aging".
Chemical reaction foulingChemical 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 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 foulingCorrosion 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.
BiofoulingBiofouling 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 (MIC).
Bacteria can form biofilms or slimes. Thus the organisms can aggregate on surfaces using colloidal hydrogels of water and extracellular polymeric substances (EPS) (polysaccharides, 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 (or sulfur-reducing bacteria), which produce sulfide and often cause corrosion of ferrous metals (and other alloys). Sulfide-oxidizing bacteria (e.g., Acidithiobacillus), on the other hand, can produce sulfuric acid, and can be involved in corrosion of concrete.
Composite foulingComposite 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 MarsNASA Mars Exploration Rovers (Spirit and Opportunity) experienced (presumably) abiotic 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.
Quantification of foulingThe 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 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.
Progress of fouling with timeDeposit 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. 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 an "induction period", the fouling rate increases.
- 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 to the fouling curve at zero deposit loading or zero time) and the fouling rate after a long period of timing (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). 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.
Fouling modellingFouling 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 (by advection)
- Foulant transport from the bulk of the process fluid to the fouling surface (most often by molecular or eddy diffusion)
- 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.
For the modern model of fouling involving deposition with simultaneous deposit re-entrainment and consolidation, the key fouling process can be can be represented by the following scheme:
\left[\begin \text\\ \text\\ \text \end \right]= \left[\begin \text\\ \text \end \right] - \left[\begin \text\\ \text\\ \text \end \right]
\left[\begin \text\\ \text\\ \text \end \right]= \left[\begin \text\\ \text \end \right] - \left[\begin \text\\ \text\\ \text \end \right] - \left[\begin \text\\ \text\\ \text \end \right]
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):