Influence of Surface Defects and Aging of Coated Surfaces on Fouling Behavior

To avoid the negative effects caused by fouling in heat exchanger equipment, the heat exchanger surface can be modified energetically or mechanically. Thus, mechanical, chemical, and thermal stability of the coatings with respect to the fouling and cleaning conditions is crucial. The surface is typically characterized by the measurement of the contact angles of different wetting fluids to calculate surface energy and tactile roughness measurements. The influence of several cleaning and fouling cycles on surface energy and the composition of the coatings has been investigated. The experimental investigation of different cleaning methods from acid to base solution displays the influence of the interface reactions on the surface energy. Structural analysis of the plasma-activated chemical vapor deposition (PACVD) coatings show a build-in of oxygen inside the a-C:H matrix with time, resulting in higher surface energies and an increase of polar interactions. Also, structural defects of the coatings have been analyzed by a defined disturbance of the coating process or mechanical treatment of the already coated material. These defects act as a starting point for crystallization fouling due to reduced activation energy of nucleation. Depending on the interface and process conditions, defects can enhance fouling if the crystals are able to adhere on the coated surface. The results should lead to a better understanding of interface reactions, stability of coatings, and the aging of surfaces.     

Experimental Investigation of Crystallization Fouling on Grooved Stainless Steel Surfaces During Convective Heat Transfer

The beneficial aspects of enhanced or extended heat transfer surfaces may be offset if operated under fouling conditions. In this article, preliminary experimental results for crystallization fouling of CaSO₄ solutions onto surfaces with different structures are reported. Flat stainless steel plates (50 mm × 59 mm) with “V”-shaped grooves on the side of fluid flow were used as heat transfer surfaces. Experiments were carried out under both clean and fouling conditions to discern how the same surface structures perform under such circumstances. In addition, the impact of both the direction of grooves with respect to fluid flow (crossed, longitudinal, and mixed flow grooves) and the groove dimensions has also been investigated. Fouling trends are discussed in terms of induction time and fouling rate. Significant differences have been found for the various flow conditions.     

Impact of Tube Surface Properties on Crystallization Fouling in Falling Film Evaporators for Seawater Desalination

ABSTRACT

Crystallization fouling on heat transfer surfaces is a severe problem and a complex phenomenon in multiple-effect distillation plants with horizontal tube falling film evaporators for seawater desalination. The choice of tube material affects the wettability, the adhesion forces between surface and deposit, and the induction time of crystallization fouling. The effects of surface properties on crystallization fouling from seawater have been investigated in a horizontal tube falling film evaporator in pilot plant scale. Experiments were performed with artificial seawater and various tube materials. The tube surfaces were characterized by measuring surface roughness and contact angles and by determining surface free energies. The tube materials show qualitative and quantitative differences with respect to scale formation. The interfacial defect model was applied to the system. Spreading coefficients of CaCO₃ scale on the aluminum alloys 5052 and 6060 and stainless steel grade 1.4565 were calculated to be higher than those on copper–nickel 90/10 and aluminum brass, but the quantities of CaCO₃ scale measured on the tube surfaces were much lower compared to CuNi 90/10 and aluminum brass. The application of advanced approaches such as the interfacial defect model depends on the precise knowledge of interfacial free energies, which are very difficult to find. However, results suggest that more similar values of the interfacial free energies of heat transfer surface and deposit lead to increased scale formation.     

影响换热面结垢诱导期的表面特性研究

换热面结垢是一个普遍存在的问题,而结垢诱导期的长短对污垢形成过程具有重要的影响,即使在相同实验条件下,不同材料换热面的结垢诱导期仍相差较大.因此本文通过对附着在换热面上的半球形污垢晶核进行受力分析,发现污垢晶核与换热面之间的附着力对其结垢诱导期长短起决定性作用,然后根据颗粒与平板间附着力模型,计算了污垢晶核与具有不同表面能的换热面间附着力,并与相应的结垢诱导期进行对比.结果表明:结垢诱导期的长短与换热面的表面能、污垢晶核与换热面间的附着力及表面粗糙度尺度有关.     

Evaluation of heat exchanger surface coatings

Compact heat exchangers are very popular due to their effectiveness, small footprint and low cost. In order to protect heat exchangers in dirty applications, coatings can be applied to the heat transfer surfaces to extend effectiveness and minimize fouling. Coating selection is extremely important since the wrong coating can decrease unit effectiveness, cause more fouling, and/or erode the surface.An experimental investigation of coating effectiveness in compact plate heat exchangers is presented. New, cleaned and coated plate heat exchangers are considered in this study. Heat exchangers have been exposed to untreated lake water for various time periods. Transient effectiveness results compare the rate of fouling for coated and uncoated heat exchangers. Additional results compare deposit weight gain at the end of the test period and transient observations of heat transfer surface appearance. All heat exchanger combinations showed some deposit accumulation for the period considered.Results indicate that the thermal performance of the unit decreases with time, resulting in an undersized heat exchanger. For the conditions considered here, uncoated plates accumulate deposits up to 50% faster than coated plates and show a decrease in performance of up to 40%. Surface coating, exposure time, fluid velocity and concentration of particles can affect fouling.     

Aging and Thermal Conditioning of Modified Heat Exchanger Surfaces—Impact on Crystallization Fouling

ABSTRACT

In many research studies diamond-like-carbon coatings are used to change the wetting behavior by varying the solids´ surface free energy of heat exchanger surfaces to mitigate crystallization fouling. For future industrial application, the stability of their specific surface properties, exposed to fluidic, thermal, and chemical stresses, determines their efficiency. Therefore, fluidic thermal and cleaning stresses applied to the coating are investigated. Cleaning procedures with acid, base, and heat treatment over multiple cycles were conducted in order to investigate the solids´ surface free energy over time and thereby the stability of the coating. From this information an optimal conditioning to set constant surface properties was derived. Furthermore, the fouling behavior of CaSO₄ on new and conditioned coatings was investigated in order to identify repeatable and favorable surface properties for fouling reduction. For all coatings the cleaning treatments and fouling experiments provided changes in the energetic surface properties, dominated by the change of polar/γ^? content. Most probably these changes originate from varying elementary composition and structure of the coating.     

Experimental fouling investigation with electroless Ni–P coatings

Advanced fouling mitigation techniques include approaches to increase the duration of the induction period and/or to decrease the fouling rate during the deposition process. One such technique is to generate heat transfer surfaces with high repulsive forces to make them less attractive to the deposition of dissolved or suspended matter. The present work investigates and compares different electroless Ni–P coatings with or without boron-nitride (BN). The incorporation of boron-nitride into Ni–P coatings increases the electron donor component of surface energy which in turn reduces the propensity of the coating to fouling. A systematic set of fouling runs has been conducted to investigate the influence of these coatings on the interaction energies between CaSO₄ deposits and modified surfaces. The results show that the Ni–P coatings with Boron-nitride exhibit excellent anti-fouling behaviour compared to pure Ni–P coatings or untreated stainless steel surfaces. Surfaces having a higher electron donor component in case of Ni–P–BN produce a higher repulsive energy which causes the adhesion force between the surface and deposits to decrease. A simultaneous set of reproducibility and cleanability experiments, however, reveals that the observed surface properties of the investigated coatings are prone to significant aging after each fouling run, leading to poor abrasion resistance.     

基于格子Boltzmann方法的微通道受热表面析晶污垢模拟

针对现有的污垢析晶沉积模型不能有效模拟真实污垢生长的问题,建立了一种引入析晶沉积动力学模型的多物理场耦合数值模型。模型基于格子Boltzmann方法和有限差分方法,模拟了微通道非等温热表面上近壁面处的沉积物溶质质量浓度分布和污垢生长过程,研究了流速、壁温和沉积物溶质质量浓度对微通道热表面污垢析晶沉积的影响。结果表明：沉积初始时刻流速和壁温对近壁面沉积物溶质质量浓度分布具有不同程度的影响,随着污垢不断生长,污垢-流体界面处的析晶沉积速率减小;相比于流速,沉积物溶质质量浓度对污垢热阻的影响更为显著。     

Nonlinear flow and heat transfer dynamics of impinging jets onto slightly-curved surfaces

The nonlinear flow and heat transfer characteristics for a slot-jet impinging on slightly-curved surfaces are experimentally studied here. The effects of curved surface geometry and jet Reynolds number on the jet velocity distribution and circumferential Nusselt numbers are examined. Two different slightly-curved surface geometries of convex and concave are used as target surfaces. The nozzle geometry is a rectangular slot, and the dimensionless nozzle-to-surface distance equals to L¹ = 8. The constant heat fluxes are accordingly applied to the surfaces to obtain an impingement cooling by the air jet at ambient temperature. The measurements are made for the jet Reynolds numbers of Re = 8617, Re = 13 350 and Re = 15 415 for both curved surfaces. The velocity distributions of issuing jet from the nozzle exit to the target surface are obtained by a highly sensitive hot-wire anemometer. The T-type thermocouples are used to measure local temperatures of both the air jet and the plates. Two-dimensional velocity measurements show that the surfaces are remained out of the potential core region for all Re tested here. New correlations for local, stagnation point, and average Nusselt numbers as a function of jet Reynolds number and dimensionless circumferential distance are reported. The correlations reveal that the impinging cooling rate is higher with the concave surface and increase with increasing Re.     

Influence of Injected Projectiles on the Induction Period of Crystallization Fouling

Projectiles of various shapes and hardness are increasingly used in process industries to mitigate fouling in tubular heat exchangers. It is a common practice to inject the projectiles at the early stage of fouling, though laboratory results are quite scarce in the open literature to assess whether this is an appropriate operating procedure. The present work aims at investigating the influence of injected projectiles on the induction period of CaSO₄ crystallization fouling. Fouling experiments have been performed in a plain heated tube. The projectiles were of spherical shape with diameter of 20.2 mm, that is, 1% bigger than the inner diameter of the heated tube, and were injected at various intervals. It has been observed that overall the attempted projectile reduced the induction period and thus expedited the fouling process. The asymptotic behavior of crystallization fouling is also approached more quickly but much less so than that of no injection. The induction period increased linearly with the flow velocity in case of no injection, while it was independent of the flow velocity when the projectile was injected as long as the injection rate was kept constant. Increasing the injection rate decreased the induction period and started the fouling process earlier. This is because the propulsion of projectiles induces air bubbles into the heat exchanger tube, which would in turn promote fouling to occur more quickly, and thus shorter induction periods are expected. Therefore, it is highly recommended to inject projectiles only after the induction period, to make use of the fouling-free operation during the induction period.     

Influences of bubble formation on different types of heat exchanger fouling

In this paper, a systematic comparison is performed to investigate fouling of suspended particles under forced convective and subcooled flow boiling heat transfer. For this purpose, two different types of fouling are separately considered: crystallization fouling of dissolved CaSO₄ particles in water and particulate fouling of suspended Al₂O₃ particles in n–heptane. The effect of hydraulic parameters such as fluid velocity and also bubble generation under subcooled flow boiling are studied. Results of the experiments demonstrate that creation of boiling condition in the heat exchanger has opposite influence in these two types of fouling. It means that bubble generation on the heat transfer surface promotes scale formation under crystallization fouling. This is due to the fact that increased bubble generation creates higher supersaturation beneath the vapor bubble, therefore, increasing the crystal concentration in the boundary layer. On the other hand, boiling condition inhibits the scale formation under particulate fouling because the suspended particles are repelled from the boundary layer by the strong turbulences created by the swarm of bubbles.     

Minimization of CaSO4 Deposition Through Surface Modification

Among the various fouling mitigation strategies, surface modification is gaining increased attention due to its environmental compatibility, i.e., reduced consumption of potentially harmful fouling inhibitors. One approach is to decrease the surface energy, which could give rise to lower adhesive strength of deposited crystals on surfaces. The present work aims at investigating the influence of various novel coatings on the interaction energies between CaSO₄ deposits and modified surfaces. Investigated coatings are (i) solvent based, (ii) water based, and (iii) electroless Ni-P-BN. A systematic set of fouling runs has been conducted under similar operating conditions for all coatings. The experimental results show that the deposition process is strongly affected by altering the surface properties, particularly the electron donor component. Furthermore, the contribution of the Lifshitz–van der Waals energy to the total interaction energy is marginal in comparison to that of the Lewis acid–base energy under the range of operating conditions in this study. Overall, in terms of reduced stickiness of deposits onto the surface, the solvent-based coatings performed best.     

Parametric study of turbulent slot-jet impingement heat transfer from concave cylindrical surfaces

Numerical investigation of convective heat transfer process from concave cylindrical surfaces due to turbulent slot-jet impingement is performed. Constant heat flux condition is specified at the concave surfaces. The flow and thermal fields in the vicinity of the surfaces are computed using the RNG k–? turbulence model with a two-layer near wall treatment. Parametric studies are carried out for various jet-exit Reynolds numbers, surface curvature, and nozzle-to-surface spacing. Results presented include streamlines, isotherms, velocity and temperature profiles in the wall-jet region, and the local Nusselt number distribution on the impingement concave wall for various parameter values in the study. The results indicate that while the jet-exit Reynolds number and the surface curvature have a significant effect on the heat transfer process, it is relatively insensitive to the jet-to-target spacing. A correlation for the average Nusselt number at the concave surface as a function of the parameters considered in the study is also derived.     

Experimental Study of Fouling Resistance in Twisted Tube Heat Exchanger

This paper discusses fouling of a twisted tube heat exchanger under different conditions of fluid velocity and heat input. The fluid velocity was varied from 0.5 to 2.0 m/s, whereas the heat input to the heat exchanger was varied from 200 to 800 W. The experimental results show that for low fluid velocity of 0.5 m/s, the fouling resistance showed noticeable variation with respect to heat input, whereas for high velocity ranges, that is, 1.0–2 m/s, the variation in fouling resistance is less. The fouling in twisted tube steadily increases with time for different values of heat input from 1000 min onward for fluid velocity in the range from 1.0 to 2.0 m/s. It is also observed that fouling resistance curves overlap for various values of heat input. During the initial 1000 min of the test duration, the maximum fouling in a twisted tube heat exchanger decreases with increase in fluid velocity from 1.0 to 2.0 m/s. This behavior of the fouling rate can be attributed to the fact that at higher fluid velocity, flow becomes turbulent, and this in turn flushes the fouling particles. The time-series correlations for the fouling resistance are found to be logarithmic in nature.     

Oil fouling in double‐pipe heat exchanger under liquid‐liquid dispersion and the influence of copper oxide nanofluid

Dispersions of oil in water are encountered in a variety of industrial processes leading to a reduction in the performance of the heat exchangers when thermally treating such two phase fluids. This reduction is mainly due to changes in the thermal and hydrodynamical behavior of the two phase fluid. In the present work, an experimental investigation was performed to study the effects of light oil fouling on the heat transfer coefficient in a double‐pipe heat exchanger under turbulent flow conditions. The effects of different operating conditions on the fouling rate were investigated including: hot fluid Reynolds number (the dispersion), cold fluid Reynolds number, and time. The oil fouling rate was analyzed by determining the growth of fouling resistance with time and through pressure drop measurements. The influence of copper oxide (CuO) nanofluid on the fouling rate in the dispersion was also determined. It was found that the presence of dispersed oil causes a reduction in heat transfer coefficient by percentages depending on the Reynolds number of both cold and hot fluids and the concentration of oil. In addition, the time history of fouling resistance exhibited different trends with the flow rates of both fluids and its trend was influenced appreciably by the presence of CuO nanofluid.     

Experimental observation of surface morphology effect on crystallization fouling in plate heat exchangers

In this study, stainless steel test plates with different surface roughness and textures, which are used as the heat transfer surface of a plate heat exchanger, are tested individually in calcium carbonate fouling experiments. The present experimental results clearly indicate a strong correlation between the surface roughness and the amount of crystallization fouling deposit. Through detailed image analysis, four stages of the formation of crystallization fouling are identified, and the impact of the surface morphology on the extent of crystallization fouling is described qualitatively.     

Experimental Study of Heat Transfer and Pressure Drop for H-type Finned Oval Tube with Longitudinal Vortex Generators and Dimples under Flue Gas

To enhance heat transfer and reduce fouling of the finned-tube surface in economizers of coal-fired power plants, heat transfer and pressure drop characteristics for H-type finned oval tube with longitudinal vortex generators (LVG) and dimples, both in-line and staggered arrangements, are studied experimentally under flue dust condition. In addition, the ash samples and heat exchanger surfaces after the test are analyzed to help understanding the ash fouling and tube wear mechanisms. Compared to the original H-type finned oval tube, the Nusselt number of H-type finned oval tube bank with longitudinal vortex generators and dimples is improved by 34.5–41.7% (in-line arrangement) and 28.1–31.7% (staggered arrangement) within the studied Reynolds numbers, while the Euler number is increased by 21.9–28.3% (in-line arrangement) and 19% (staggered arrangement) in the clean finned-tube surface state. In the stable fouling state, the Nusselt number is improved by 37.7–42.3% (in-line arrangement) and 27.8–45.1% (staggered arrangement), while the Euler number is increased by 22.9–25.2% (in-line arrangement) and 33.3–42% (staggered arrangement). The results show that the novel fin structures can both inhibit fouling and enhance heat transfer effectively.