Sawtooth curve

Fouling

[fou-ling]
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consists of either living organisms (biofouling) or be a non-living substance (inorganic or organic).

Other terms used in the literature to describe fouling include: deposit formation, encrustation, scaling, scale formation, crudding, and deposition. The last four terms are less inclusive than fouling; therefore, they should be used with caution.

Fouling phenomena are common and diverse, ranging from fouling of ships, 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 industrial heat exchanger systems, 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

The following lists examples of components that may be subject of fouling and the direct effects of fouling:

  • heat exchanger surfaces - reduces thermal efficiency, increases temperature, creates corrosion, increases use of cooling water
  • piping, flow channels - reduces flow, increases pressure drop, increases energy expenditure, may create flow oscillations
  • ship hulls - increases fuel usage, reduces maximum speed
  • turbines - reduces efficiency, increases probability of failure
  • solar panels - decreases the electrical power generated
  • reverse osmosis membranes - reduces efficiency of water purification, increases pressure drop, increases energy expenditure
  • electrical heating elements - increases temperature of the element, increases corrosion, reduces lifespan
  • nuclear fuel in pressurized water reactors - axial offset anomaly
  • injection/spray nozzles (e.g., a nozzle spraying a fuel into a furnace) - incorrect amount injected, malformed jet, component inefficiency, component failure
  • venturi tubes, orifice plates - inaccurate or incorrect measurement of flow rate
  • pitot tubes in airplanes - inaccurate or incorrect indication of airplane speed
  • teeth - promotes tooth disease, decreases aesthetics

Macro fouling

Macro fouling is caused by coarse matter of either biological or inorganic origin, for example industrially produced refuse. Such matter enters into the cooling water circuit through the cooling water pumps from sources like the open sea, rivers or lakes. In closed circuits, like cooling towers, the ingress of macro fouling into the cooling tower basin is possible through open canals or by the wind. Sometimes, parts of the cooling tower internals detach themselves and are carried into the cooling water circuit. Such substances can foul the surfaces of heat exchangers and may cause deterioration of the relevant heat transfer coefficient. They may also create flow blockages, redistribute the flow inside the components, or cause fretting damage. Examples:

Micro fouling

As to micro fouling, distinctions are made between:

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

Precipitation fouling

Scaling or precipitation fouling involves crystallization of solid salts, oxides and hydroxides from solutions. These are most often water solutions, but non-aqueous precipitation fouling is also known.

Through changes in temperature, or solvent evaporation or degasification, the concentration of salts may exceed the saturation, leading to a precipitation of salt crystals. Precipitation fouling is a very common problem in boilers and heat exchangers operating with hard water and often results in limescale.

As an example, the equilibrium between the readily soluble calcium bicarbonate - always prevailing in natural water - and the poorly soluble calcium carbonate, the following chemical equation may be written:

mathsf {Ca(HCO_3)_2}(aqueous) Longrightarrow mathsf {CaCO_3}downarrow + mathsf {CO_2}uparrow + mathsf {H_2O}

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.

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

Particulate fouling

Fouling by particles suspended in water ("crud") or in gas progresses by a mechanism different than precipitation fouling. This process is usually most important for colloidal particles, i.e., particles smaller than about 1 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".

The common particulate fouling deposits formed from aqueous suspensions include:

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.

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 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.

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 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 fouling

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

Fouling on Mars

NASA Mars Exploration Rovers (Spirit and Opportunity) experienced (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 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 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.

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 great 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. 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.
  • 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 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). 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. The periodic sharp decreases in the apparent fouling amount often correspond to the moments of system shutdowns or other transients in operation. The periodic decreases 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). However, other reasons are possible, e.g., trapping of air inside the surface deposits during a boiler shutdown, resulting in enhanced steam nucleation (therefore improved boiling heat transfer) during the initial period of subsequent operation.

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)
  • Foulant transport from the bulk of the process fluid to the fouling surface (often by molecular or turbulent-eddy diffusion, but may also occur by inertial coasting (impaction), electrophoresis, thermophoresis, diffusiphoresis, sedimentation, evaporation, vapour diffusion with condensation, 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 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 (e.g., through Oswald ripening) or cementation
  • 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{array}{c} text{rate of} text{deposit} text{accumulation} end{array} right]= left[begin{array}{c} text{rate of} text{deposition} end{array} right] - left[begin{array}{c} text{rate of} text{re-entrainment of} text{unconsolidated deposit} end{array} right]

left[begin{array}{c} text{rate of} text{accumulation of} text{unconsolidated deposit} end{array} right]= left[begin{array}{c} text{rate of} text{deposition} end{array} right] - left[begin{array}{c} text{rate of} text{re-entrainment of} text{unconsolidated deposit} end{array} right] - left[begin{array}{c} text{rate of} text{consolidation of} text{unconsolidated deposit} end{array} 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):

left{begin{array}{c} {dm/dt}=kCrho - lambda_r m_r(t) {dm_r/dt}=kCrho - lambda_r m_r(t) - lambda_c cdot m_r(t) end{array} right.

where: m is the mass loading of the deposit (consolidated and unconsolidated) on the surface (kg/m2); t is time (s); k is the deposition rate constant (m/s); ρ is the fluid density (kg/m3); λr is the re-entrainment rate constant (1/s); mr is the mass loading of the removable (i.e., unconsolidated) fraction of the surface deposit (kg/m2); and λc is the consolidation rate constant (1/s).

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

m(t) = {{kCrho} over {lambda}} left(t lambda_c + {{lambda_r} over {lambda}} left(1 - e^{-lambda t} right) right)

where λ = λr + λc.

This model reproduces either linear, falling, or asymptotic fouling, depending on the relative values of k, λr, and λc. 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 λc=0.

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.

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

Fouling control

The most fundamental and usually preferred method of controlling fouling is to prevent the ingress of the fouling species into the cooling water circuit. In steam power stations and other major industrial installations of water technology, macro fouling is avoided by way of pre-filtration and cooling water debris filters. In the case of micro fouling, water purification is achieved with extensive methods of water treatment, microfiltration, membrane technology (reverse osmosis, electrodeionization) or ion-exchange resins. The generation of the corrosion products in the water piping systems is often minimized by controlling the pH of the process fluid (typically alkanization with ammonia, morpholine, ethanolamine or sodium phosphate), control of oxygen dissolved in water (for example, by addition of hydrazine), or addition of corrosion inhibitors.

For water systems at relatively low temperatures, the applied biocides may be classified as follows: inorganic chlorine and bromide compounds, chlorine and bromide cleavers, ozone and oxygen cleavers, unoxidizable biocides. One of the most important unoxidizable biocides is a mixture of chloromethyl-isothiazolinone and methyl-isothiazolinone. Also applied are dibrom nitrilopropionamide and quaternary ammonium compounds. For underwater ship hulls bottom paints 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 are: chelating agents (for example, EDTA), long-chain aliphatic amines or polyamines (for example, octadecylamine, helamin, and other "film-forming" amines), organic phosphonic acids (for example, 1-hydroxyethylidene-1,1-diphosphonic acid, known as HEDP), or polyelectrolytes (for example, polyacrylic acid, polymethacrylic acid, usually with a molecular weight lower than 10000).

On the component design level, fouling can often (but not always) be minimized by maintaining a relatively high (for example, 2 m/s) and uniform fluid velocity throughout the component. Stagnant regions need to be eliminated. Component is normally overdesigned to accommodate the fouling anticipated between cleanings. However, a significant overdesign can be a design error because it may lead to increased fouling due to reduced velocities. Periodic on-line pressure pulses or backflow can be effective if the capability is carefully incorporated at the design time. Blowdown capability is always incorporated into steam generators or evaporators to control the accumulation of non-volatile impurities the cause or aggreviate fouling. Low-fouling surfaces (for example, very smooth, implanted with ions, or of low surface energy like Teflon) are an option for some applications. Modern components are typically required to be designed for ease of inspection of internals and periodic cleaning.

Chemical or mechanical cleaning processes for the removal of deposits and scales are recommended when fouling reaches the point of impacting the system performance or an onset of significant fouling-induced degradation (e.g., by corrosion). These processes comprise pickling with acids and metal complexing agents, cleaning with high-velocity water jets ("water lancing"),recirculating sponge rubber 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. Also, in many processes related with heat transfer, fouling can be mechanically mitigated using dynamic scraped surface heat exchangers. Also ultrasonic or abrasive cleaning methods are available for many specific applications.

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