Fouling Totally Explained

Everything about Fouling totally explained

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 fouling

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

Examples:

Manmade refuse
Detached internal parts of components
Algae
Mussels
Leaves, parts of plants up to entire trunks

Micro fouling

As to micro fouling, distinctions are made between:

Scaling or precipitation fouling, as crystallization of solid salts, oxides and hydroxides from water solutions, for example

Calcium carbonate (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, for example, accumulation of particles, typically colloidal particles, on a surface

iron oxides and iron oxyhydroxides (magnetite, hematite, lepidocrocite, maghemite, goethite)

Sedimentation fouling by silt and other relatively coarse suspended matter

Corrosion fouling, for example, in-situ growth of corrosion deposits, for example magnetite on carbon steel surfaces

Chemical reaction fouling, for example decomposition or polymerization of organic matter on heating surfaces

Biofouling, like settlements of bacteria and algae

Composite fouling, whereby fouling involves more than one foulant or fouling mechanism.

Precipitation fouling

Through changes in temperature, or solvent evaporation or degasification, the concentration of salts may exceed the saturation, leading to a precipitation of salt crystals. Precipitation fouling is a very common problem in boilers and heat exchangers operating with hard water and often results in limescale.
As an example, the equilibrium between the readily soluble calcium bicarbonate - always prevailing in natural water - and the poorly soluble calcium carbonate, the following chemical equation may be written:
ť

mathsf  
ight) 
ight)

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.

The economic importance of fouling

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

Fouling control

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

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