2. FOULING - Definition
Fouling is the aggregation and settling of
insoluble material [suspended in the fluid
through physical processes] into the surfaces
of the equipment where the fluid flows .
17. CORROSION – Causes (factors)
fluid chemistry
fluid or metal temperature
fluid flow rate
material selection
metal working (forming or welding)
maintenance or cleaning frequency
fouling
film formation
Fouling is the piling up of insolubles in a surface
This here is a real-life example of fouling. Pass it around, see and smell it so you know why it’s called fouling. That is a bottle sterilizer that was already being used 19 years ago. It holds many memories so please don’t hold it rashly.
From our daily experience in calculating fouling factors we know that it is significant in increasing thermal resistance. Aside from that, we want to avoid it for practical reasons such as those listed.
There are different types of fouling by location like shell-side fouling, tube-side fouling, etcetera etcetera. Just about any surface of the equipment exposed to fluid experience fouling.
Fouling can also be classified by its cause.Corrosion fouling – sounds oxymoronic, but corrosion sometimes produces a precipitate which causes fouling.
Biofouling. Occurs when organisms in the fluid stream attach to the surface and grow forth and multiply. They are perhaps most common in marine equipment. They not only lower heat transfer coefficients, but even contaminate the fluid. There are two subgroups calledmicrobiofoulants such as slime and algae; and macrobiofoulants such as snails,barnacles, and mussels.Chemical reaction fouling (ex.—Coking). Chemical reaction of the fluid takes place on the heat-transfer surface producing an adhering solid product of reaction.
Particulate fouling. Suspended particleswill deposit out on the heat-transfer surface in areas of sufficiently lower velocity such as those right after an orifice or a plug. The smoke belcher shows precipitation fouling as the hot exhaust gas cools downPrecipitation fouling (ex.—Scaling). The fluid solution becomes supersaturated as the fluid cools. Disturbances like vibrations force crystallization of the solute. Freezing fouling. Overcooling of a fluid below the fluid’s freezing point at the heat-transfer surface causes solidification and coating of the heat-transfer surface.
From the types of fouling and their mechanisms, we can predict the causes“Fouling is the direct result of temperature extremes at the heat-transfer surface”Suspended Solids — road dust, suspended silt, clay depositsOrganic Contamination— Oil fouling in refinery systems is a common exampleIron and Manganese — Soluble Fe + 2 and Mn + 2 ions are found in well water. These ions may be oxidized in processes and form particulate material.Corrosion Products — Corrosion fouling, may precipitate in place or somewhere elseTreatment Products — Overdosing some treatment products is primarily a scale forming mechanism due to precipitation of the treatment with calcium or other positive ions in solution.Microbial Growth — Slime formed by bacteria acts as a binder for other foulants such as silts and scales.
asymptotic fouling: build-up of fouling resistance decelerates and approaches some asymptotic value. As fouling decreases the diameter, the velocities increase which scrapes off the fouling. This is commonly found in temperature-driven fouling. Etoyungina-assume natinna fouling resistance valuelinear fouling: build-up of fouling resistance increases linearly. This could be experienced in case of severe particulate fouling where the accumulation of dirt during the time of operation did not appreciably increase velocities to mitigate the problem. falling rate fouling, is neither linear nor asymptotic but instead lies somewhere between these two extremes. The rate of fouling decreases with time but does not appear to approach an asymptotic maximum during the time of operation. This is the most common type of fouling in the process industry and is usually the result of a combination of different fouling mechanisms occurring together.
While fouling is the accumulation of insolubles on a surface, corrosion wears away the surface. Normal corrosion rates for equipments range from 5–25 μm per year
Rusting is a form of corrosion we are all very familiar with.
So like fouling, we don’t want corrosion because it’s not ideal. As you can see, “blocked fluid flow” is a bit ironic since corrosion wears away the surface. Peroparang corrosion fouling langyan
Corrosion has two mechanisms of action:General or Uniform- slow, uniform attack over the chamber material Pitting - localized corrosion of a metal surface confined to a point or small area that takes the form of cavities, common in bends
Stress corrosion cracking - attacks grain boundaries(changes in the crystalline structure of the metal) in stressed areasGalvanic (electrical current) corrosion- occurs when dissimilar metals are joined in the presence of an electrolyte, such as acidic water
Crevice corrosion - originates in and around hidden and secluded areas - between baffles and tubes, or under loose scale or dirtIntergranularcorrosion- occurs when the grain boundaries are depleted of the corrosion-inhibiting compound
So based on these types of corrosions, we have lots of factors affecting it, like fluid properties, flow properties, material properties, surface geometries, etc. Since corrosions involve chemical reactions these factors simply boil to reaction kinetics: compounds, surface area, reactant concentration, and temperature.
normal corrosion rate: 5–25 μm per year. (operating life of 20 years)
Cleaning and Maintenance: Mechanical and ChemicalThe optimum operating period between cleanings depends upon the rate and type of fouling, the heat exchanger used (i.e. baffle type, use of extended surface, and velocity and pressure drop design constraints), and the ease with which the heat exchanger may be removed from service for cleaning.Chemical removal of fouling can be achieved in some cases by weak acid, special solvents, and so on. Other deposits adhere weakly and can be washed off by periodic operation at very high velocities or by flushing with a high-velocity steam or water jet or using a sand-water slurry. These methods may be applied to both the shell side and tube side without pulling the bundle. Many fouling deposits, however, must be removed by positive mechanical action such as rodding, turbining, or scraping the surface. These techniques may be applied inside of tubes without pulling the bundle but can be applied on the shellside only after bundle removal. Even then there is limited access because of the tube pitch and rotated square or large triangular layouts are recommended. In many cases, it has been found that designs developed to minimize fouling often develop a fouling layer which is more easily removed.Smooth surfaces are often less susceptible to particle deposition compared to rough surfaces but rough surfaces on the other hand provide greater area for heat transfer [by the creases inherent in the rough surfaces].Of course, specific solutions have been devised by engineers in order not just to address fouling but other equipment problems related to fouling and usually those with solutions that promote fouling (e.g. use of rough surfaces for larger heat transfer area)
Like any other field, knowledge and prediction are two crucial factors in prevention.Knowledge - Once the combination of mechanisms contributing to a particular fouling problem are recognized, methods to substantially reduce the fouling rate may be implementedcorrosion fouling:choose a less corrosive material of construction balancing material cost with equipment lifebiofouling:use copper alloys and/or chemical treatment of the fluid stream to control organism growth and reproductionparticulate fouling: ensuring a sufficient flow velocity, avoid stagnant flows. For water, the recommended tube-side minimum velocity is about 0.9 to 1.0 m/s. Not always be possible for moderate to high-viscosity fluids. polymerization, precipitation, and freezing — Lower temperature difference between surface and fluid: Increase velocity, Use extended surface. This not only reduces the rate of fouling but also makes the heat exchanger generally less sensitive to the effects of any fouling that does occur. It should be noted that fouling buildup and eventual mechanical cleaning are inevitable, and extended surface should be used only when the exchanger construction allows access for cleaning.PredictionThere are no published methods for predicting fouling resistances a priori. The accumulated experience of exchanger designers and users was assembled more than 40 years ago based primarily upon segmental-baffled exchanger bundles and may be found in the Standards of Tubular Exchanger Manufacturers Association (TEMA). In the absence of other information, the fouling resistances contained therein may be used.CORROSIONproper material selection use material coatings plating, galvanizing or anodizing adjust material thickness monitor flow ratedrain & dry heat exchangers when not in use
![FOULING - Definition
Fouling is the aggregation and settling of
insoluble material [suspended in the fluid
through physical processes] into the surfaces
of the equipment where the fluid flows .](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)