hiTRAN® Wire Matrix Inserts in Fouling Applications

Fouling characteristics are dictated largely by the properties of the thermal and hydrodynamic boundary layers. As a result, fouling mitigation strategies must take into account the conditions in this region. hiTRAN wire matrix tube inserts are a useful tool in altering the conditions near the tube wall, especially in the laminar and transition flow regions. This review article considers particle image velocimetry and laser doppler velocimetry measurements, which were employed in order to show the hydrodynamic differences between plain tubes and those containing inserts. Measurements indicate that the wall shear rate in tubes containing hiTRAN inserts operating in the laminar flow regime is similar to that for plain bore tubes operating in the turbulent flow regime. Moreover, the increased tube-side heat transfer coefficient that results from the reduction of the thermal boundary layer allows operation with smaller Effective Mean Temperature Differences (EMTDs). This enables the designer to reduce the tube wall temperature to a level below the fouling threshold temperature, e.g., to combat crude oil fouling. The results from the laser analyses into the hydrodynamic boundary layer are backed up by recent research data investigating the effect of hiTRAN inserts on sedimentation and particulate fouling. The thickness of the fouling layer was measured by applying a combination of photographic and laser measurement techniques. The results are compared to plain tube data and are reported as a function of both flow rate and hiTRAN insert packing density. The impact of altering the hydrodynamic and thermal conditions near to the wall is subsequently demonstrated for different fouling mechanisms. Studies of the impact of hiTRAN inserts on biological and chemical reaction fouling in crude oil processing are also reviewed. A better understanding of the threshold shear rates and wall temperatures for different fouling mechanisms is required for any study into the impact of fouling. Combining this knowledge with the principles outlined in this article clearly emphasizes the benefit of using hiTRAN wire matrix inserts as a powerful tool to mitigate fouling.     

A Theory Describing Asphaltene Adhesion Fouling Inside Heat Exchanger Tubes

A theory for mathematically modeling asphaltene adhesion fouling in heat exchanger tubes was derived and its agreement with experiment suggested feasibility. The premise of this theory is that asphaltene adhesion fouling requires the formation of a chemical bond—modeled here as a sulfur–sulfur bond—which is strong enough to resist fluid dynamic forces. This theory suggests that once an asphaltene monolayer is adsorbed onto a heat transfer surface, shear stress alone may be insufficient for preventing further fouling unless the asphaltene flocculate size distribution can be manipulated, the chemically labile heteroatoms can be deactivated, or the asphaltenes can be kept in solution. A method for calculating fouling threshold shear stresses and flocculate diameters is expounded.     

Calcium Sulfate Fouling-Precipitation or Particulate: A Proposed Composite Model

Though it is of great importance, the majority of predictive models tend not to incorporate water chemistry in their formulations. The ionic diffusion model which was developed for CaCO₃, is based purely on crystallization, and is one of the few models that incorporates water chemistry. This model does not provide satisfactory predictions for CaSO₄ fouling. In this article, a new model is proposed for CaSO₄ fouling which takes into account the effect of both crystallization and particulate fouling and is capable of predicting the fouling resistance during the cleaning cycle as well as the fouling cycle. A removal term is incorporated into the model, as the occurrence of particulate fouling for CaSO₄ tends to weaken its crystalline structure and makes it more prone than CaCO₃ to removal. Properties of the electrolyte were evaluated using MINTEQA2 computer code, which is approved by the U.S. Environmental Protection Agency. In this model, particulate fouling is estimated using the physical mechanism for particle transport and adherence, crystallization is estimated by ionic diffusion, and the removal term is approximated using hydrodynamics of flow and deposit properties. The inclusion of both crystallization and removal terms incorporates the effects of both water chemistry and hydrodynamics of the flow and provides a relationship which not only can predict fouling but also can predict dissolution, by change of water quality and/or stopping the operation, or removal by shear stress. The proposed model was assessed using published experimental data. The results indicate that this model provides good predictions: the slope of predicted rates as a function of the experimental rates is 1.05. The experimental results, though limited in number, suggest that crystallization is not the main or only mechanism contributing to CaSO₄ fouling. Particulate fouling seems to be a major contributor. Further experimentation is in process to confirm the degree of particulate fouling and to substantiate or to modify the model accordingly.     

温度对纳米氧化镁颗粒污垢特性的影响

为了探讨温度对纳米氧化镁颗粒污垢结垢特性的影响,通过改变循环工质入口温度和水浴温度实验研究了温度对纳米颗粒污垢在交叉缩放椭圆管中的结垢特性,并通过静置沉降实验验证了入口温度对循环工质聚沉情况的影响。结果表明,循环工质入口温度和水浴温度对纳米颗粒污垢特性都有显著的影响。随着循环工质入口温度的升高,污垢热阻渐近值明显减小,并且污垢热阻达到渐近值的时间缩短。随着水浴温度的升高,污垢热阻渐近值也随之减小,但是达到渐近值的时间略有增加。     

Influence of sintering on the growth rate of particulate fouling layers

This article addresses the question; why the gas-side temperature affects the rate of particulate fouling of heat exchangers? An experiment was carried out in a gas-cooler of a full-scale biomass gasifier to investigate the influence of the gas-side temperature on the strength, structure and growth rate of particulate fouling layers. It is observed that the particulate fouling rate in the gas cooler decreases with sintering, which is a function of the gas-side temperature. Detailed impaction experiments are carried out to investigate the influence of sintering on the removal of particles from a particulate fouling layer due to an incident particle impact as well as the sticking of an incident particle to a particulate fouling layer. Sintering of a fouling layer lowers significantly the ability of an incident particle to stick to the fouling layer or to remove particles out of the layer. However, particles that are still able to deposit on the sintered fouling layer will not sinter immediately, and can be removed due to the incident particles impact. The removal of newly deposited particles on a fouling layer due to incident particles becomes easier as sintering of the fouling layer takes place. Accordingly, it may be stated that sintering reduces the fouling rate of heat exchangers by lowering the deposition of new particles and increasing the removal rate of newly deposited particles. This explains why the growth rate of particulate fouling layers decreases with the gas-side temperature.     

Use of CFD to Determine Effect of Wire Matrix Inserts on Crude Oil Fouling Conditions

Crude oil fouling rates are strongly affected by both local surface temperature and local surface shear stress. The use of in-tube inserts (such as hiTRAN) in heat exchangers has been shown to be effective in mitigating crude oil fouling while at the same time enhancing heat transfer. However, the introduction of inserts means that there will be axial and radial distributions of both local shear stress and local heat transfer coefficient between the repeating insert–wall contact points, which could mean that there will be local variations in fouling rate. While estimation of local shear stresses and film heat transfer coefficients is facile for bare round tubes, this is no longer the case for tubes fitted with inserts. Accordingly, this article describes a possible solution to the design challenge using computational fluid dynamics (CFD) simulation, the output of which is the temperature and velocity distributions in a three-dimensional geometry of the fluid flow in a tube fitted, for example, with a hiTRAN insert. A simple algorithm is then described for calculating the overall heat transfer coefficient based on the resulting temperature distribution along the wall of the tube. Simulated values of the overall heat transfer coefficient are then compared with those obtained by experiment, showing that there is good agreement, thereby indicating that predicted local values are accurate. Use of CFD in fouling applications now allows the prediction of local conditions when inserts are used and hence can be used to predict whether, and where, fouling might occur.     

Removal of gas-side particulate fouling layers by foreign particles as a function of flow direction

Removal of particulate fouling layers by externally injected particles as a function of flow direction with respect to gravity is investigated experimentally. Three orientations of flow have been investigated, horizontal flow, upward flow and a downward flow. It is found that fouling starts at the point of stagnation irrespective of the flow direction, and also starts at the top point of the heat exchanger tubes. Particulate fouling grows from these two points except for the downward flow, were the flow stagnation point coincides with the top point of the heat exchanger tubes and the growth of the fouling layer starts only from one point. It was not possible to remove the fouling layer in case of a horizontal and an upward flow by the externally injected particles, however in case of a downward flow most of the fouling layers were removed by the external particles. It can be concluded that the downward flow is the best flow orientation to linger particulate fouling and for removal of fouling layers by externally injected particles.     

Optimization of Flow Direction to Minimize Particulate Fouling of Heat Exchangers

The influence of flow direction with respect to gravity on particulate fouling of heat exchangers is investigated experimentally to determine the optimal flow direction to minimize fouling. Four orientations of flow have been investigated: horizontal flow, upward flow, downward flow, and a flow under an angle of 45°. It is observed that fouling starts at the point of stagnation irrespective of the flow direction, and also at the top of the heat exchanger tubes. Particulate fouling grows from these two points till they meet and the fouling layer covers the whole surface of the heat exchanger tube. Fouling at the upper half of the tubes is much faster than the lower half of the tubes, and the fouling rate is faster at the bottom tubes of the heat exchanger section than at the upper tubes. The best orientation for lingering particulate fouling is the downward flow, where the flow stagnation point coincides with the top point of the heat exchanger tubes and the growth of the fouling layer only starts from one point.     

流场剪切力对微生物污垢影响的CFD模拟

再生水水质复杂,在再生水源热泵板式换热器中极易形成微生物污垢,严重影响换热性能和系统安全。在微生物污垢的研究中,微生物污垢所处流场与微生物污垢的受力和生长是密不可分的。利用CFD方法,借助FLUENT软件,对微生物污垢所处流场进行模拟,从改变流场和强化剪切力的角度出发,主要探究了在矩形流道的基础上,加入主动脉冲流、含有微刻痕、含有颗粒相的流场剪切力对微生物污垢生长的影响。模拟结果显示:方波形式的脉冲流以及微刻痕可以有效增加壁面剪切力,且脉冲周期越小、微刻痕尺寸越小,壁面剪切力增加越多;含有颗粒相的流场,随着颗粒粒径的增加,颗粒个体碰撞概率增加,单位质量碰撞概率减小。     

微粒污垢的特性分析

利用微粒污垢的一个预测模型，结合实验结果，研究了管壳式换热顺管内微粒污垢的积聚特性，考察了颗粒质量分数，颗粒直径和流体速度对渐近污垢热阻的影响，提出了大、中、小粒子及未知尺寸粒子的尺寸界限的参考值。     

Particulate fouling in waste incinerators as influenced by the critical sticking velocity and layer porosity

Fouling of heat transfer surfaces introduces a major uncertainty into the design and operation of heat exchange equipment. Fouling layers as observed on the tube bundles of the economizer in a Dutch waste incinerator were thin and powdery. The fouling layer showed an asymptotic growth rate with a levelling off increase of the thickness. In this study, the influence of the critical sticking velocity on the growth rate of particulate fouling layers is described. The critical sticking velocity of an incident particle hitting a powdery layer is defined as the maximum impact speed at which the particle will stick to the layer. Since the critical sticking velocity is a key parameter in the deposition mechanism, a well-defined experimental set-up has been built to assign it. Experimental results showed that the critical sticking velocity increases with the porosity of the fouling layer. Literature shows that the porosity of a thin sintered powdery layer changes with the layer thickness. Based on the experimental results and the variation of porosity with thickness for a thin sintered powdery fouling layers, a correlation is developed which shows that the sticking velocity decreases exponentially as the fouling layer thickness increases. Therefore, fewer particles are likely to stick as the fouling layer builds up and consequently the deposition rate decreases. The change in the critical sticking velocity as the fouling layer builds up contributes to the explanation of the asymptotic growth of particulate fouling layers on the tube bundle of waste incinerators.     

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.     

Particulate Fouling and Challenges of Metal Foam Heat Exchangers

ABSTRACT

In recent years, open-cell metal foam has gained attention for utilization for exhaust gas recirculation coolers due to its large surface area and porous structure. Theoretically, the porous foam structure would have better transfer heat through conduction and convection processes. However, the exhaust gases that enter the cooler would carry particulate matter, which may deposit within the foam structure. The existing fouling studies cannot explain the underlying mechanisms of particulate deposition thoroughly within the foam structure. This study reviews the particulate fouling of heat exchangers, particularly in the exhaust gas recirculation system. Some past approaches to investigate fouling, particle transport, and deposition in the metal foam heat exchangers for many different applications are also included. In addition, this study also includes the challenges that lie ahead in implementing the metal foam heat exchangers in the industries.     

Fouling thresholds in bare tubes and tubes fitted with inserts

Maya crude oil fouling reveals a straightforward dependency of initial fouling rate on surface temperature but a rather complex dependency on velocity in bare tubes, the initial fouling rate showing a maximum and then decreasing significantly towards zero as the velocity is increased. Surface shear stress clearly is an important parameter. CFD simulation of fluid flow in a tube fitted with a hiTRAN® insert reveals a complex distribution of surface shear stress. To compare the insert situation with the bare tube, an equivalent velocity concept is introduced on the basis that at a given average velocity the fluid flow results in the same average wall shear stress regardless of whether the tube is bare or is fitted with an insert. Using the equivalent velocity concept, the fouling data obtained using both a bare tube and a tube fitted with inserts can be correlated using a single model. Moreover, the fouling threshold conditions below which fouling is negligible, can be predicted for both situations.     

Effect of Geometry and Flow Conditions on Particulate Fouling in Plate Heat Exchangers

This article describes particulate fouling experiments performed on small-scale and full-scale plate heat exchangers for three different corrugation angles (30 deg, 45 deg and 60 deg). The velocity effect has been studied as well as the particle type and concentration effects. The test duration ranges between 20 and 1,500 h in order to reach asymptotic behavior. The results clearly indicate that the corrugation angle has a major influence on the asymptotic fouling resistance. Increasing the corrugation angle leads to lower values for the fouling resistance. Furthermore, for a given corrugation angle, the asymptotic fouling resistance is inversely proportional to the velocity squared. Finally, the asymptotic fouling resistance is proportional to the particle concentration. Fouling mitigation can be obtained by taking into account at the design stage the heat exchanger geometry and fluid velocity.     

Influence of gas velocity on particulate fouling of exhaust gas recirculation coolers

The objective of this research is to study the influence of gas flow velocity on particulate fouling of exhaust gas recirculation (EGR) coolers. An experimental setup has been designed and constructed to simulate particulate fouling in EGR coolers in diesel engines. The setup consists of soot generator, gas/particle flow heater, testing section for EGR coolers and finally an exhaust system. Two sets of fouling experiments have been performed with and without water injection, and the gas velocity in each set has varied between 30, 70 and 120 m/s. The concentration of soot particles in the gas flow is 100 mg/m³, and the average diameter of the particles is 130 nm with a standard deviation of 55 nm. It has been found that the thermal resistance and thickness of the fouling layer and the fouling rate decrease as the gas velocity in the EGR cooler increases. If EGR coolers are operated with a gas velocity, which is just lower than the critical flow velocity for the largest particle in the flow, quick deterioration of the thermal performance of the heat exchanger will nevertheless occur. This strongly indicates that the gas velocity should exceed a certain critical flow velocity in order to prevent particulate fouling. In addition, the presence of water vapour in the gas flow improves the thermal performance of the cooler and decreases the fouling rate, and its influence decreases as the gas velocity increases.     

Composite Fouling of Heat Transfer Equipment in Aqueous Media - A Review

Recent research on fouling where composite fouling in aqueous media may be present is reviewed. In practical industrial applications, usually several types of fouling occur simultaneously; however, due to the complexity of fouling, various types of fouling are studied in isolation. Crystallization (precipitation) fouling is the most-studied type of fouling. Other types of fouling are studied in various degrees. Not much attention has been paid to the relative significance and the interactive effects of these processes when they occur simultaneously. This is specifically the case for composite inorganic (precipitation and particulate) and biological fouling. In general, there is lack of attention to the presence, mechanism, modeling, and mitigation of composite fouling.     

A review of heat exchanger fouling in the context of aircraft air-conditioning systems,and the potential for electrostatic filtering

This paper presents a focused literature review to understand the common problem of fouling of air-conditioning heat exchangers aboard aircraft, with the academic consideration to employ electrostatic precipitation to remove airborne particulate matter.Particulate matter suspended in air, is carried through the matrices of aircraft environmental cooling systems. The deposition and build up of such contaminants affects the thermal performance of cooling systems and leads to component failure, expensive repairs and loss of service of an aircraft.Although there have been many publications of material pertaining to heat exchangers and fouling, very little publications specifically to aircraft air-conditioning systems or failures have been published. Nonetheless, the literature review indicates that sizes and distribution of particulate matter including Reynolds numbers and rates of deposition have been established in previous papers.The novel approach to this industrial problem has been to evaluate the operational problem of aircraft air-conditioning systems, identify local factors, and to consider the use of means of protection employed in other non-aerospace industries. It is believed the application of electrostatic precipitation could potentially aid prevention of fouling particulate matter on aircraft air-conditioning heat exchangers.     

Fouling Management of Thermal Cracking Units

ABSTRACT

Visbreakers and other thermal cracking units are thermal process units in crude oil refineries that upgrade heavy petroleum, usually residual oils produced from atmospheric or vacuum distillation of crude oil. The associated process streams of these units consist of heavy hydrocarbons with very high viscosities and impurities, resulting in fouling of the heat exchangers used to cool or heat these streams. This paper presents a practical fouling analysis for thermal cracking units in a refinery in Germany. Fouling management at this refinery was initiated as part of the refinery energy-saving program. Following similar analysis of the refinery's crude preheat trains, heat exchanger networks associated in the thermal cracking units were modeled by entering the plant monitoring data, network topology, and heat exchanger geometries into a commercial heat exchanger network simulator, SmartPM. Fouling behaviors of vacuum residue streams and thermal cracker residue streams were identified and quantified. Both chemical reaction fouling and particulate fouling mechanisms were identified to be responsible for the fouling in these streams. Dynamic fouling models were fitted and used to predict fouling of these heavy petroleum streams, which fouled on both the shell and tube sides of the shell-and-tube heat exchangers.     

Observation of an Isokinetic Temperature and Compensation Effect for High-Temperature Crude Oil Fouling

The initial fouling rates of four crude oils were determined at a nominal bulk temperature of 315°C, an initial heated wall shear stress of 13 Pa, and initial surface temperatures between 375 and 445°C. These initial fouling rates ranged from 1.3(10^{? 6}) to 7.8(10^{? 5}) m² K/kJ. Corresponding Arrhenius plots were linear with the initial fouling rates passing through an isokinetic temperature of 407.5°C. A plot of the natural logarithm of the pre-exponential factors [7.6(10⁴)–5.2(10¹⁵) m² K/kJ] versus the apparent activation energies (128–269 kJ/mol) was also linear, confirming the validity of the isokinetic temperature and the presence of the compensation effect. Below the isokinetic temperature, the relative fouling rates were Crude Oil C > Crude Oil A > Crude Oil D > Crude Oil B; above the isokinetic temperature, the relative fouling rates were reversed (Crude Oil B > Crude Oil D > Crude Oil A > Crude Oil C). Chemical characterization of a fouling deposit suggested that the dominant fouling mechanism at these conditions was coking, with significant contributions from sedimentation (iron sulfide) and corrosion (～ 340 μ m/yr) of the 304 stainless steel test material.