Fouling
Precipitation fouling is a very common problem in boilers and heat exchangers operating with hard water and often results in limescale.Through changes in temperature, or solvent evaporation or degasification, the concentration of salts may exceed the saturation, leading to a precipitation of solids (usually crystals).[8] Fouling of membranes in reverse osmosis systems can occur due to differential solubility of barium sulfate in solutions of different ionic strength.An alternative to using the sticking probability is to use a kinetic attachment rate constant, assuming the first order reaction:[10][11] and then the transport and attachment kinetic coefficients are combined as two processes occurring in series: where: Being essentially a surface chemistry phenomenon, this fouling mechanism can be very sensitive to factors that affect colloidal stability, e.g., zeta potential.For example, corrosion and polymerization occurs in cooling water for the chemical industry which has a minor content of hydrocarbons.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.Biofouling or biological fouling is the undesirable accumulation of micro-organisms, algae and diatoms, plants, and animals on surfaces, such as ships and submarine hulls, or piping and reservoirs with untreated water.Thus the organisms can aggregate on surfaces using colloidal hydrogels of water and extracellular polymeric substances (EPS) (polysaccharides, lipids, nucleic acids, etc.).In practice, aerobic bacteria prefer open systems, when both oxygen and nutrients are constantly delivered, often in warm and sunlit environments.Zebra mussels serve as an example of larger animals that have caused widespread fouling in North America.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.[16] NASA Mars Exploration Rovers (Spirit and Opportunity) experienced (presumably) abiotic fouling of solar panels by dust particles from the Martian atmosphere.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.This model reproduces either linear, falling, or asymptotic fouling, depending on the relative values of k, λr, and λc.For example, one estimate puts the losses due to fouling of heat exchangers in industrialized nations to be about 0.25% of their GDP.[26] Another analysis[27] 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.Finally, fouling is often a root cause of serious degradation problems that may limit the life of components or entire plants.Some plants employ foreign-object exclusion program (to eliminate the possibility of salient introduction of unwanted materials, e.g., forgetting tools during maintenance).The generation of the corrosion products in the water piping systems is often minimized by controlling the pH of the process fluid (typically alkalinization 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 fired boilers, aluminum or magnesium additives can lower the melting point of ash and promote creation of deposits which are easier to remove.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.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).
Foul (disambiguation)Membrane foulingfowlingHeat exchangerpower plantCondenserbiofoulingBucklingCorrosionCorrosion fatigueFatigueFractureHydrogen embrittlementImpactLiquid metal embrittlementMechanical overloadMetal-induced embrittlementStress corrosion crackingSulfide stress crackingThermal shockYieldingmarine foulingheat-transfercooling waterplaquecalculuscalcium carbonatethermal efficiencyTurbinesReverse osmosis membranesheating elementsFirearmmuzzleloaderspressurized water reactorsVenturi tubesorifice platesPitot tubesSpark plugpetroleum reservoirsoil wellspetroleum productionsenescencecoarserefusewater pumpsriverscooling towersheat transfer coefficientfrettingMusselsLeavestrunkscrystallizationoxideshydroxidessolutionsParticulatecolloidalcarbon steelbacteriaLimescalehard waterevaporationdegasificationsaturationprecipitationcalcium bicarbonatesolubilityreverse osmosisionic strengthflashingcalcitearagonitevateriteCalcium sulfateanhydritehemihydrategypsumCalcium oxalateBarium sulfatebariteMagnesium hydroxidebrucitemagnesium oxidepericlaseSilicatesserpentineacmitegyrolitegehlenitesilicaquartzcristobalitepectolitexonotliteAluminium oxide hydroxidesboehmitegibbsitediasporecorundumAluminosilicatesanalcitecancrinitenoselitecoppercupritetenoritehydroxyapatitenickel ferriteorder of reactionflocculationcoagulationsticking probabilitythermohydraulicsurface chemistryzeta potentialpoint of zero chargeiron oxides and iron oxyhydroxidesmagnetitehematitelepidocrocitemaghemitegoethiteSedimentationaerosolsflue gases