Sequential Events in Heat Exchanger Fouling: Initiation, Transport, Attachment, Removal, and Aging

Heat Exchanger Fouling: Detailed Sequential Events

Sequential Events in Fouling

From the various fouling mechanisms, it is clear that virtually all these mechanisms are characterized by a similar sequence of events. The successive events occurring in most cases are the following: (1) Initiation, (2) Transport, (3) Attachment, (4) Removal, and (5) Aging, as conceptualized by Epstein (1978). These events govern the overall fouling process and determine its ultimate impact on heat exchanger performance. In some cases, certain events dominate the fouling process, and they have a direct effect on the type of fouling to be sustained. Let us summarize these events briefly (Cannas, 1986).

Initiation of the fouling, the first event in the fouling process, is preceded by a delay period or induction period Td. The basic mechanism involved during this period is heterogeneous nucleation, and Td is shorter with a higher nucleation rate. The factors affecting Td are temperature, fluid velocity, composition of the fouling stream, and nature and condition of the heat exchanger surface. Low-energy surfaces (unwettable) exhibit longer induction periods than those of high-energy surfaces (wetta­ble). In crystallization fouling, Td tends to decrease with increasing degree of supersatura­tion. In chemical reaction fouling, Td appears to decrease with increasing surface temperature. In all fouling mechanisms, Td decreases as the surface roughness increases due to available suitable sites for nucleation, adsorption, and adhesion.

Transport of species means transfer of a key component (such as oxygen), a crucial reactant, or the fouling species itself from the bulk of the fluid to the heat transfer surface. Transport of species is the best understood of all sequential events. Transport of species takes place through the action of one or more of the following mechanisms: 

·     Diffusion: involves mass transfer of the fouling constituents from the flowing fluid toward the heat transfer surface due to the concentration difference between the bulk of the fluid and the fluid adjacent to the surface.

·     Electrophoresis: under the action of electric forces, fouling particles carrying an electric charge may move toward or away from a charged surface depending on the polarity of the surface and the particles. Deposition due to electrophoresis increases with decreasing electrical conductivity of the fluid, increasing fluid temperature, and increasing fluid velocity. It also depends on the pH of the solution. Surface forces such as London—van der Waals and electric double layer interaction forces are usually responsible for electrophoretic effects.

·       Thermophoresis: a phenomenon whereby a "thermal force" moves fine particles in the direction of negative temperature gradient, from a hot zone to a cold zone. Thus, a high-temperature gradient near a hot wall will prevent particles from depositing, but the same absolute value of the gradient near a cold wall will pro­mote particle deposition. The thermophoretic effect is larger for gases than for liquids.

·     Diffusiophoresis: involves condensation of gaseous streams onto a surface.

·     Sedimentation: involves the deposition of particulate matters such as rust particles, clay, and dust on the surface due to the action of gravity. For sedimentation to occur, the downward gravitational force must be greater than the upward drag force. Sedimentation is important for large particles and low fluid velocities. It is frequently observed in cooling tower waters and other industrial processes where rust and dust particles may act as catalysts and/or enter complex reactions.

·     Inertial impaction: a phenomenon whereby "large" particles can have sufficient inertia that they are unable to follow fluid streamlines and as a result, deposit on the surface.

·       Turbulent downsweeps: since the viscous sublayer in a turbulent boundary layer is not truly steady, the fluid is being transported toward the surface by turbulent downsweeps. These may be thought of as suction areas of measurable strength distributed randomly all over the surface.

Attachment of the fouling species to the surface involves both physical and chemical processes, and it is not well understood. Three interrelated factors play a crucial role in the attachment process: surface conditions, surface forces, and sticking probability. It is the combined and simultaneous action of these factors that largely accounts for the event of attachment.

·     The properties of surface conditions important for attachment are the surface free energy, wettability (contact angle, spreadability), and heat of immersion. Wettability and heat of immersion increase as the difference between the surface free energy of the wall and the adjacent fluid layer increases. Un-wettable or low-energy surfaces have longer induction periods than wettable or high-energy surfaces, and suffer less from deposition (such as polymer and ceramic coatings). Surface roughness increases the effective contact area of a surface and provides suitable sites for nuclea­tion and promotes initiation of fouling. Hence, roughness increases the wettability of wettable surfaces and decreases the un-wettability of the un-wettable ones.

·     There are several surface forces. The most important one is the London—van der Waals force, which describes the intermolecular attraction between nonpolar mole-cules and is always attractive. The electric double layer interaction force can beattractive or repulsive. Viscous hydrodynamic force influences the attachment of aparticle moving to the wall, which increases as it moves normal to the plain surface.

·       Sticking probability represents the fraction of particles that reach the wall and stay there before any re-entrainment occurs. It is a useful statistical concept devised to analyze and explain the complicated event of attachment.

Removal of the fouling deposits from the surface may or may not occur simulta­neously with deposition. Removal occurs due to the single or simultaneous action of the following mechanisms: shear forces, turbulent bursts, re-solution, and erosion.

·       Shear forces result from the action of the shear stress exerted by the flowing fluid on the depositing layer. As the fouling deposit builds up, the cross-sectional area for flow decreases, thus causing an increase in the average velocity of the fluid for a constant mass flow rate and increasing the shear stress. Fresh deposits will form only if the deposit bond resistance is greater than the prevailing shear forces at the solid—fluid interface.

·       Randomly distributed (about less than 0.5% at any instant of time) periodic tur­bulent bursts act as miniature tornadoes lifting deposited material from the surface. By continuity, these fluid bursts are compensated for by gentler fluid back sweeps, which promote deposition.

·     The removal of the deposits by re-solution is related directly to the solubility of the material deposited. Since the fouling deposit is presumably insoluble at the time of its formation, dissolution will occur only if there is a change in the properties of the deposit, or in the flowing fluid, or in both, due to local changes in temperature, velocity, alkalinity, and other operational variables. For example, sufficiently high or low temperatures could kill a biological deposit, thus weakening its attachment to a surface and causing sloughing or re-solution. The removal of corrosion depos­its in power-generating systems is done by re-solution at low alkalinity. Re-solution is associated with the removal of material in ionic or molecular form.

·     Erosion is closely identified with the overall removal process. It is highly dependent on the shear strength of the foulant and on the steepness and length of the sloping heat exchanger surfaces, if any. Erosion is associated with the removal of material in particulate form. The removal mechanism becomes largely ineffective if the fouling layer is composed of well-crystallized pure material (strong formations); but it is very effective if it is composed of a large variety of salts each having different crystal properties.

Aging of deposits begins with attachment on the heat transfer surface, and refers to any changes the fouling material undergoes as time elapses. The aging process includes both physical and chemical transformations, such as further degradation to a more carbonaceous material in organic fouling, and dehydration and/or crystal phase trans­formations in inorganic fouling. A direct consequence of aging is change in the thermal conductivity of the deposits.t Aging may strengthen or weaken the fouling deposits.

 A common non-fouling example of aging is the transformation of fresh, soft, fluffy snow in an open field into hard, crystalline, yellowish ice after a week or so of exposure to the sun resulting differences in its material properties.

Thanks for Reading! 

 

Reference:- Dugan P. Sekulic (2003) R K Shah