Fouling

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 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 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 solutions, for example
- calcite)
- gypsum)
- silica, cristoballite)
- Magnesium hydroxide (brucite)
- Aluminum oxide (corundum)
- Copper (metalic cuprite)
- Barium sulfate
Particulate fouling, i.e., accumulation of particles, typically colloidal particles, on a surface
- Iron oxides and oxyhydroxides (goethite)
Sedimentation fouling by silt and other relatively coarse suspended matter
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
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 hard water and often results in limescale.

As an example, the equilibrium between the readily soluble calcium carbonate, the following chemical equation may be written:

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

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.

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 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".

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 carbonizing of organic matter. Food industry, for example milk processing, also experiences fouling problems by chemical reactions.

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. An example of corrosion fouling can be formation of an iron oxide or oxyhydroxide deposit from corrosion of the carbon steel underneath.

Biofouling

Main article: Biofouling

Biofouling or biological fouling is the undesirable accumulation of micro-organisms, plants, algae, and animals on structures, for example ships' hulls. Water piping systems carrying untreated water are also often subject of biofouling.

Composite fouling

Composite fouling is common. This type of fouling involves more than one foulant or more than one fouling mechanism^[1] 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^[2]. 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 "heat transfer coefficient (W/m²K) with time.

If under-deposit or stress corrosion cracking) is of concern.

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-zero fouling rate is observed when the surface is new or very clean. This is often observed in biofouling and precipitation fouling. After an "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.
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.

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
Foulant transport from the bulk of the process fluid to the fouling surface
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
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^[3], 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):

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

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); λ_r is the re-entrainment rate constant (1/s); m_r is the mass loading of the removable (i.e., unconsolidated) fraction of the surface deposit (kg/m²); and λ_c is the consolidation rate constant (1/s).

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

$m(t) = {{kC\rho} \over {\lambda}} \left( t \lambda_c + {{\lambda_r} \over {\lambda}} \left( 1 - e^{-\lambda t} \right) \right)$

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^[4].

The losses initially result from impaired heat transfer, 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 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 corrosion inhibitors.

For water systems at relatively low temperatures, the applied biocides may be classified as follows: ammonium compounds.

Chemical fouling inhibitors^[5] 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, 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. 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 sponge rubber balls. Whereas chemical cleaning causes environmental problems through the handling, application, storage and disposal of chemicals, the mechanical cleaning by means of circulating cleaning balls can be a more environmentally-friendly alternative. Also ultrasonic or abrasive cleaning methods are available for many specific applications.

References

^ Hong Lu, "Composite Fouling of Heat Exchanger Surfaces", Nova Science Books, New York, 2007
^ http://mars.jpl.nasa.gov/MPF/ops/mae.html
^ C.W. Turner, S.J. Klimas, "Modelling the Effect of Surface Chemistry on Particle Fouling Under Flow-Boiling Conditions", Proceeding of Heat Exchanger Fouling: Fundamental Approaches and Technical Solutions, 2001, July 8-13, Davos, Switzerland, AECL 12171
^ H. Mueller-Steinhagen, M.R. Malayeri and A.P. Watkinson, "Fouling of Heat Exchanger--New Approaches to Solve Old Problem", Heat Transfer Engineering, 26(2), 2005
^ J.C. Cowan and D.J. Weintritt, "Water-Formed Scale Deposits. A Comprehensive Study of the Prevention, Control, Removal and Use of Mineral Scale", Gulf Publishing Company, Houston, Texas, 1976

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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fouling". A list of authors is available in Wikipedia.