Heat Exchanger Fouling Mitigation And Cleaning Techniques PdfBy Emeterio R. In and pdf 25.03.2021 at 15:26 4 min read
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- Heat Exchangers Fouling, Cleaning, and Maintenance
- heat exchanger fouling
- Heat Exchangers Fouling, Cleaning, and Maintenance
- fouling in heat exchanger pdf
Show all documents Design enhancement and analysis of pigging device for heat exchanger fouling cleaning There are many methods used to control the fouling problem which is classified to two techniques. These techniques are online and offline respectively Garrett-Price et al.
Heat Exchangers Fouling, Cleaning, and Maintenance
Fouling is the accumulation of unwanted material on solid surfaces. The fouling materials can consist of either living organisms biofouling or a non-living substance inorganic or organic. Fouling is usually distinguished from other surface-growth phenomena in that it occurs on a surface of a component, system, or plant performing a defined and useful function and that the fouling process impedes or interferes with this function. Other terms used in the literature to describe fouling include deposit formation, encrustation, crudding, deposition, scaling, scale formation, slagging, and sludge formation.
The last six terms have a more narrow meaning than fouling within the scope of the fouling science and technology, and they also have meanings outside of this scope; therefore, they should be used with caution.
Fouling phenomena are common and diverse, ranging from fouling of ship hulls, natural surfaces in the marine environment marine fouling , fouling of heat-transfer components through ingredients contained in 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 primarily devoted to the fouling of industrial heat exchangers, although the same theory is generally applicable to other varieties of fouling.
In cooling technology and other technical fields, a distinction is made between macro fouling and micro fouling. Of the two, micro fouling is the one that is usually more difficult to prevent and therefore more important. 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. As to micro fouling, distinctions are made between: . 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.
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. 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:.
The calcium carbonate that forms through this reaction precipitates. Due to the temperature dependence of the reaction, and increasing volatility of CO 2 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.
Salts with the "normal" solubility increase their solubility with increasing temperature and thus will foul the cooling surfaces. Salts with "inverse" or "retrograde" solubility will foul the heating surfaces. An example of the temperature dependence of solubility is shown in the figure. Calcium sulfate is a common precipitation foulant of heating surfaces due to its retrograde solubility.
Precipitation fouling can also occur in the absence of heating or vaporization. For example, calcium sulfate decreases its solubility with decreasing pressure. This can lead to precipitation fouling of reservoirs and wells in oil fields, decreasing their productivity with time. The following lists some of the industrially common phases of precipitation fouling deposits observed in practice to form from aqueous solutions:.
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. Particles are transported to the surface by a number of mechanisms and there they can attach themselves, e.
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 ", P: . The value of P for colloidal particles is a function of both the surface chemistry, geometry, and the local thermohydraulic conditions. An alternative to using the sticking probability is to use a kinetic attachment rate constant, assuming the first order reaction:  .
Being essentially a surface chemistry phenomenon, this fouling mechanism can be very sensitive to factors that affect colloidal stability, e. 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.
Particles larger than those of colloidal dimensions may also foul e. With time, the resulting surface deposit may harden through processes collectively known as "deposit consolidation" or, colloquially, "aging".
Fouling by particles from gas aerosols is also of industrial significance. The particles can be either solid or liquid. The common examples can be fouling by flue gases , or fouling of air-cooled components by dust in air. The mechanisms are discussed in article on aerosol deposition. 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.
Corrosion fouling should not be confused with fouling corrosion, i. 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. The 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 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 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. Zebra mussels serve as an example of larger animals that have caused widespread fouling in North America. 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.
This illustrates the universal nature of the fouling phenomena. The most straightforward way to quantify fairly uniform fouling is by stating the average deposit surface loading, i. It is obtained by dividing the fouling rate by the foulant concentration. 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.
If under-deposit or crevice corrosion is of primary concern, it is important to note non-uniformity of deposit thickness e. Such deposit structures can create environment for underdeposit corrosion of the substrate material, e.
Porosity and permeability of the deposits will likely influence the probability of underdeposit corrosion. Deposit composition can also be important - even minor components of the deposits can sometimes cause severe corrosion of the underlying metal e. 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 concern in almost any application. 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:.
Deposition consists of transport to the surface and subsequent attachment. Deposit removal is either through deposit dissolution, particle re-entrainment, or deposit spalling, erosive wear, or exfoliation. 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 fouling process can be represented by the following scheme:. 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 :.
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.
heat exchanger fouling
The fouling of heat exchangers may be defined as the accumulation of unwanted deposits on heat transfer surfaces. The foulant layer imposes an additional resistance to heat transfer and the narrowing of the flow area, due to the presence of deposit, results in an increased velocity for a given volumetric flow rate. Furthermore, the deposit is usually hydrodynamically rough so that there is an increased resistance to the flow of the fluid across the deposit surface. Therefore, the consequences of fouling are, in general, a reduction in exchanger efficiency and other associated operating problems including excessive pressure drop across the exchanger. It is only in recent years that the problem of heat exchanger fouling has attracted scientific and theoretical treatment and many aspects remain to be investigated.
Heat Exchangers Fouling, Cleaning, and Maintenance
Fluid temperature and velocity effect on fouling - ScienceDirect. For this purpose a linear correlation is developed to predict fouling resistance based on velocity as well as temperature difference. The fouling resistance observed by this correlation is within the limit shown in the literature Muller-Steinhagen and Branch, It reduces overall heat transfer coefficient by
fouling in heat exchanger pdf
For this reason, you should use the agency link listed below which will take you directly to the appropriate agency server where you can read the official version of this solicitation and download the appropriate forms and rules. Fouling of heat exchangers is a serious and long-standing problem that can result in decreased heat transfer efficiency, higher resistance to fluid flow, increased energy consumption, decreased heat exchanger lifetime, and increased downtime necessary to replace or clean fouled parts. Biological fouling is the accumulation of microorganism, plants, algae or animals on the interior of the tube and is the type of fouling most experienced. Turf-like algae growths are increasingly found when operating in warm seawater environments.
The last six terms have a more narrow meaning than fouling within the scope of the fouling science and technology, and they also have meanings outside of this scope; therefore, they should be used with caution. Keywords: heat exchanger, fouling, fouling mitigation, green technology, cleaning of heat exchangers 1. Only the pulse echo technique yielded promising results.
mitigation techniques of fouling are harsh to the environment. measure to clean off the deposits from heat exchanger surfaces to provide.
The formation of process-related deposits on the heat transfer surfaces is probably the least understood phenomenon in heat exchangers, causing severe problems in design and operation of the equipment. In recent years, the importance of this phenomenon has received further attention due to the increasing cost of energy, climate changes as a result of energy conversion processes, technical advancement requiring more efficient thermal management, and changes in the nature of feed materials, such as:. Nevertheless, present design procedures still involve massive uncertainties. The predictions of fine-tuned correlations and computer models for clean heat transfer coefficients need to be corrected by crudely estimated fouling resistances.
Handbook of Thermal Science and Engineering pp Cite as. Fouling results in a significant cost to the process and power industry, and a better understanding of the fouling behavior and mitigating measures is of interest to both personnel in operating facilities and research organizations. Fouling reduces the heat transfer rate and increases the pressure drop of heat exchangers. Fouling mechanisms are broadly classified as sedimentation, chemical reaction, crystallization, and biological. Heat exchanger designs should accommodate these fouling mechanisms and guidelines based on the vast operating experience that are discussed.