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The Fouling of Alloy-800 Heat Exchanger Tubes by Nickel Ferrite Under Bulk Boiling Conditions

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The Fouling of Alloy-800 Heat Exchanger Tubes by Nickel Ferrite Under Bulk Boiling Conditions ECI Symposium Series, Volume RP2: Proceedings of 6th International Conference on Heat Exchanger Fouling and Cleaning - Challenges and Opportunities, Editors Hans Müller-Steinhagen, M. Reza Malayeri, and A. Paul Watkinson, Engineering Conferences International, Kloster Irsee, Germany, June 5 - 10, 2005 THE FOULING OF ALLOY-800 HEAT EXCHANGER TUBES BY NICKEL FERRITE UNDER BULK BOILING CONDITIONS J.L. Cossaboom1 and D.H. Lister2 1 Centre for Nuclear Energy Research, 2 Garland Court, Enterprise UNB, Fredericton, NB, Canada, E3B 6C2 [email protected] 2 Department of Chemical Engineering, University of New Brunswick, PO Box 4400, Fredericton, NB, Canada, E3B 5A3 [email protected] ABSTRACT can provide an environment for corrosion by the A laboratory program at UNB Nuclear is investigating accumulation of aggressive chemicals. the deposition of corrosion products from cooling water There are reported studies of the mechanisms of onto heat transfer surfaces. A study of the deposition of deposition of oxide particles of various types onto heat nickel ferrite particles from suspension in water at transfer surfaces (Basset et al., 2000 (a); Turner, Lister and atmospheric pressure onto heated Alloy-800 tubes has been Smith, 1990; Turner and Smith, 1992; Turner and Godin, undertaken. 1994; Charlesworth, 1970; Mizuno, Wada and Iwahori, Nickel ferrite particles of a given stoichiometry and two 1982). It has been postulated that under non-boiling particle sizes – 0.8 μm and 9 μm – were synthesized by conditions, particle deposition proceeds in two steps solid-state and solution methods, respectively. Their occurring in series (Epstein, 1988). The initial step, the deposition from water under bulk boiling conditions was transport step, involves the transport of particles from the studied as functions of pH and time. Almost linear bulk of the liquid to the vicinity of the surface. This is deposition with time allowed deposition velocities to be followed by the attachment step, involving the attachment used to characterize the process. of particles onto the surface. The deposition rate is related The smaller particles deposited more readily than the to the rate of particle transport and the rate of particle larger ones and, for both, maximum deposition occurred attachment by the following equation: when the surface charges on the particles and the test tube were of opposite sign. 1/Kd = 1/Kt + 1/Ka (1) A long-term radiotracing experiment allowed particle release processes as well as deposition to be quantified. where Kd is the deposition coefficient, Kt is the particle About seventy percent of the deposit appeared to transport coefficient and Ka is the particle attachment consolidate and become unavailable for release. A high coefficient. If one assumes a linear dependence with the release coefficient then indicated that the unconsolidated concentration in the bulk liquid (Cb), the deposition flux deposit was very labile. Overall, the deposit accumulated (Φd) can be expressed mathematically as the product of the up to 300 hours was linear. This behaviour was different bulk concentration times the deposition coefficient (Epstein, from that of magnetite in a previous experiment under 1988). similar conditions but with subcooled boiling, suggesting Φd = Cb·Kd (2) that the mode of heat transfer is important in determining the deposition mechanism. Two classes of surface interaction are considered to be the most important: London-Van-der-Waals attractions and INTRODUCTION electrical double-layer interactions. The resulting London- Fouling, the accumulation of undesired solid material at Van-der-Waals forces which arise between a particle and a phase interfaces, is a phenomenon which is deleterious to a surface are always attractive in a single-phase fluid. For wide variety of industrial operations. The ability to predict particles with a diameter less than 100 μm, London-Van- and control fouling rates has the potential for significant der-Waals forces will always dominate over the effect of cost savings. gravity (Epstein, 1988). Particulate fouling is the accumulation of solid particles The electrical charges commonly acquired by particles suspended in a fluid onto a heat transfer surface; it plays an in an electrolyte and the compensating diffuse layer of important role in determining the optimum operating counter-ions in the liquid adjacent to the particle surfaces conditions and the selection of materials in many systems. give rise to electrical double layer interaction forces that are The growth of deposits causes a decline in the thermal and strongly dependent on the pH of the solution. These forces hydrodynamic performance of equipment with time by are attractive when the particle and the wall have unlike impeding the transfer of heat and increasing the resistance charges; the deposition rate will then be mass-transfer to fluid flow. In addition, if the deposit is thick enough, it controlled: Produced by The Berkeley Electronic Press, 2016 110 Heat Exchanger Fouling and Cleaning - Challenges and Opportunities [2005], Vol. RP2, Article 18 therefore only unconsolidated deposit is subjected to re- 1/Kd ~ 1/Kt (3) entrainment. The consolidation model for amount of deposited foulant, mf, proposed by Turner and Klimas If the charge between the particle and wall are the same, the (2001) is given below: net surface force will be repulsive and the deposition rate will be attachment controlled: CK bd ⎛ λ r ()λ+λ− t ⎞ m = ⎜ t +λ − e1 cr ⎟ (5) f ⎜ c ()⎟ ()λ+λ cr ⎝ ()λ+λ cr ⎠ 1/K ~ 1/K (4) d a where λ is the removal rate constant and λ is the r c consolidation rate constant. Therefore a particle will either adhere or be repelled from The onset of boiling enhances the particle deposition the surface depending on the forces of attraction and rate and the contribution to the deposition rate from boiling, repulsion when a particle is in close proximity to the K , can be added to the single phase convective rate to give surface. b a deposition rate coefficient for two-phase flow, K (2φ), Interactions between particles and a liquid result in a d (Turner and Godin, 1994): thermophoretic particle force when there is a non-uniform temperature environment (Papavergos, 1984). This is the K (2φ) = K (1φ) + K (6) phenomenon whereby small particles in temperature d d b gradients experience a thermal force and consequently where K (1φ) is the deposition rate coefficient for single- move in the direction of the lower temperature. Hence cold d phase flow and is defined by Eq. (1). However, this walls attract and hot walls repel particles (Kern and Seaton, approach would appear to be an over-simplification of a 1959; McNab and Meisen, 1973). The thermophoretic complex interaction. effect increases with increasing temperature gradient and The most commonly invoked mechanism in nucleate decreases with increasing particle size. Deposition of boiling is the evaporative mechanism, which relies on the particles due to thermophoresis is an effective mechanism presence of a thin liquid layer at the base of the bubble, the for particles less than 2 μm and has been shown to be more microlayer. During bubble growth, a thin liquid film is on important in gases then liquids. Because temperature the heater surface beneath the bubble. In sub-cooled gradients are small in boiling heat transfer, thermophoretic boiling, latent heat is removed by evaporation of the film effects are likely to be small in the experiments reported resulting in a temperature decrease below the bubble while here and will not be discussed further. the top of the bubble is projected into the cooler bulk liquid It is well established that there are five steps involved resulting in condensation. This simultaneous evaporation in the fouling of heat transfer surfaces: initiation, transport, and condensation in the bubble is the mass transfer attachment, re-entrainment and ageing. Fouling models mechanism and was first experimentally verified by Moore reported in the literature have primarily focused on the and Mesler (1961) who were also the first to observe the transport, attachment and re-entrainment steps as initiation presence of a microlayer at the base of a bubble (Robin Jr. does not apply to particulate fouling and little is known and Snyder, 1970). During the initial stages of bubble about how ageing of the deposit affects the overall fouling growth the bubble grows rapidly and the microlayer is rate. An extensive investigation of particulate fouling under formed underneath the bubble. The bubble continues to boiling conditions in high temperature water has been grow as the liquid evaporates from the microlayer until it conducted at Chalk River Laboratories. The results have totally evaporates and a dry region is formed leaving the been used to develop a model of particulate fouling that solid content of the microlayer to deposit on the surface. In takes account of the transport, attachment, re-entrainment the final stage the bubble reaches a maximum diameter and and ageing steps that contribute to the overall fouling leaves the heated surface. process (Turner and Klimas, 2001). Usually, microlayers are supposed to be wedge-shaped Turner and Klimas (2001) believe that once a particle in cross-section, increasing in thickness from the center of has been deposited it will be subjected to either re- the bubble. According to Cooper and Lloyd (1969) the entrainment or consolidation. The process whereby a microlayers beneath growing bubbles are stagnant and particle becomes chemically bonded to either the heat evaporation of the microlayer would cause it to become transfer surface or pre-existing deposit is known as thinner with time. The microlayer center diminishes with consolidation or ageing. The precipitation of dissolved time, accompanied by dry-patch growth, whereas the outer species within the pores of the deposit is the driving force edge increases in thickness with time. This indicates that for consolidation while the rate of consolidation is the microlayer is being replenished by fluid exceeding the dependent on particle size, the solubility of the deposit, and rate of liquid evaporation (Jawurek, 1969).
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