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.     

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.     

An Evaluation of Heat Transfer Surface Materials Used in Fouling Applications

Fouling is a very important and complex problem that extends into many fields, including natural, chemical, medical, and industrial processes. Fouling of a surface takes place as a result of the complex reactions that cause deposits to form on process surfaces. A number of parameters influence fouling development, including flow velocity, surface temperature, surface material/finish, surface geometry and fluid properties. Fouling is a transient process that begins with a clean process surface and progresses until the surface no longer can be used effectively. The event sequence of the fouling process appears in general to be universal, beginning when fluid comes into contact with a process surface. During the induction period, the conditioning film forms with heat transfer efficiencies not changing significantly. Conditioning film development is followed by a rapid accumulation of deposit growth. It is during this growth phase that the heat transfer across the process surface starts to dramatically change. Finally, a pseudo steady-state period takes place when accumulation is almost constant. Deposit accumulation causes efficiencies to significantly decrease, and a complete surface cleaning may be required. Conclusions and observations regarding the materials/surfaces that are commonly used in designs where fouling may be a concern are presented here. Comparisons of fouling rate and deposit thickness are given for several materials.     

热交换器防垢涂层材料的研究进展

热交换器是工业中传热传质的重要设备,广泛应用于各个领域,但热交换器表面易结垢的问题严重影响了其运行效率.表面防垢涂层技术是解决热交换器表面结垢的一个重要研究方向.本文简要介绍表面涂层材料表面特性(表面能、接触角、粗糙度、耐腐蚀性)对基体表面结垢行为的影响,为揭示不同材料表面结垢行为差异提供依据;同时,对防垢涂层材料的类...     

Evolution of composite fouling on a vertical stainless steel surface caused by treated sewage

Composite biological and inorganic fouling occurs in plate heat exchangers (PHEs) using treated sewage as heat transfer medium, which lowers the heat transfer coefficient and increases the frictional resistance. In order to optimize the heat exchange process and improve the anti-fouling strategies, the dynamic behavior of composite fouling at a vertical surface of stainless steel (ANSI 316L) was investigated under typical conditions of PHEs. The growth curves of composite fouling were obtained. The evolution of composite fouling was characterized by means of environmental scanning electron microscopy (ESEM). Backscattered Electron Image (BEI) and energy dispersive X-ray spectrometry (EDS) were used as aids in interpreting the results. The experimental results show that a preliminary stage of a 6-day period with a low fouling growth rate exists during the composite fouling development. A significant change of the fouling growth rate happens after the preliminary stage during which the bacterial behaviors at the surface could be recorded clearly. After the preliminary stage, a space net-shape, mainly consisting of bacteria, extracellular products (EPS) and inorganic particles, could be established on the surface of the fouling layer. The change of fouling growth rate occurs synchronously with the evolution.     

Evolution of composite fouling on a vertical stainless steel surface caused by treated sewage

Composite biological and inorganic fouling occurs in plate heat exchangers (PHEs) using treated sewage as heat transfer medium, which lowers the heat transfer coefficient and increases the frictional resistance. In order to optimize the heat exchange process and improve the anti-fouling strategies, the dynamic behavior of composite fouling at a vertical surface of stainless steel (ANSI 316L) was investigated under typical conditions of PHEs. The growth curves of composite fouling were obtained. The evolution of composite fouling was characterized by means of environmental scanning electron microscopy (ESEM). Backscattered Electron Image (BEI) and energy dispersive X-ray spectrometry (EDS) were used as aids in interpreting the results. The experimental results show that a preliminary stage of a 6-day period with a low fouling growth rate exists during the composite fouling development. A significant change of the fouling growth rate happens after the preliminary stage during which the bacterial behaviors at the surface could be recorded clearly. After the preliminary stage, a space netshape, mainly consisting of bacteria, extracellular products (EPS) and inorganic particles, could be established on the surface of the fouling layer. The change of fouling growth rate occurs synchronously with the evolution.     

Extending the Induction Period of Crystallization Fouling Through Surface Coating

To minimize the negative effects of scale formation in heat exchangers, new anti-fouling strategies are focusing on the modification of heat transfer surfaces. These modifications should lead to tailor-made surfaces for different technical applications. The aim of this surface modification is the extension of the induction period to minimize the negative effects of fouling and maximize the endurance of the heat exchanger. To achieve this, different surface coatings on stainless steel were investigated with respect to fouling tendency. The effects of flow velocity with respect to Reynolds number on the induction time of CaSO₄ crystallization fouling were tested in different test units. Diamond-like carbon (DLC) coatings extend the induction time at every measured flow velocity. At higher Reynolds numbers, the effect of different surface crystallization due to energetic modification is reduced because of the dominating effect of the low adhesive surface. Thus the induction time can be extended by the factor of 2 for low fluid velocities (DLC or SICON®) and by more than 14 for higher Reynolds numbers (DLC and SICON®). The combination of limited nucleation spots due to electro-chemical treatment of the substrate before coating can give a tailor-made surface with maximum induction time for crystallization fouling.     

Fluid temperature and velocity effect on fouling

A wide variety of industrial processes involve the transfer of heat energy between fluids in process equipment. As a result of this energy exchange unwanted deposits accumulate on the process surfaces causing a resistance to energy transfer. These deposits reduce the heat recovery and can restrict fluid flow in the exchanger by narrowing the flow area. Prevention and control of fouling is costly and time consuming. In many situations, fouling can be reduced but not necessarily eliminated. Fouling is a major unresolved problem in heat transfer.In general, the heat exchangers evaluated in this study were exposed to untreated lake water for typical conditions. After the prescribed time period the exchangers were taken off line and evaluated. Conclusions and observations regarding fouling of brazed heat exchangers, exposed to once-through lake water, are presented here. Transient observations and photographs of the exchanger surfaces are given. Results are presented that compare these heat exchangers to test plates, also exposed to lake water. The progressive change of surface appearance with increasing immersion times is presented.     

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.     

editorial

Fouling of heat exchangers is a chronic problem in processing industries. In addition to the appropriate selection of operating conditions and exchanger geometry, there are numerous chemical and mechanical methods to mitigate fouling and to remove deposits from the heat transfer surfaces. However, all methods to reduce fouling require some understanding of the mechanisms of the deposition process and of the structure and adhesion of deposits on the heat transfer surfaces. Almost exactly 50 years ago, D. Q. Kern and his co-author, R. E. Seaton, published a paper attempting to describe the growth of fouling deposits in terms of an unsteady-state heat and mass balance for the heat transfer surface. More or less at the same time, the TEMA fouling resistances were published based on operational and anecdotal evidence of fouling for a range of heat exchanger applications. These two approaches have since formed the basis for most heat transfer fouling models and heat exchanger designs. Increased costs of energy, raw materials, and production downtime have contributed to the growing interest in heat transfer fouling. More recently, environmental legislation has put additional pressure on fouling-related CO₂ emissions and disposal of cleaning chemicals. Despite these efforts, fouling of heat exchangers is still far from been understood in its whole complexity. The present paper documents the 2009 D. Q. Kern Award Lecture in which some selected aspects of fouling research to date have been presented and areas have been identified where significant research and development activities are still required.     

污垢对管内层流换热性能影响的热力学分析和评价

采用热力学第二定律，分别在恒壁温和恒热流两种典型工况下分析了污垢对管内层流换热性能的影响；引入单位传热量的熵增率对污垢管道的热力学性能进行了评价；讨论了管内流体雷诺数(无污垢时)、量纲为1的入口换热温差、量纲为1的热流密度和污垢层厚度等参数对单位传热量熵增率的影响；并把结果和紊流时的对应工况进行了比较。结果可为工程上换热设备的优化设计提供依据。     

Economie d'énergie en trigénération

Trigeneration is defined as the production of three useful forms of energy—heat, cold and power—from a primary source of energy such as natural gas or oil. For instance, trigeneration systems typically produce electrical power via a reciprocating engine or gas turbine and recover a large percentage of the heat energy retained in the lubricating oil, exhaust gas and coolant water systems to maximize the utilization of the primary fuel. The heat produced can be totally or partially used to fuel absorption refrigerators. Therefore, trigeneration systems enjoy an inherently high efficiency and have the potential to significantly reduce the energy-related operation costs of facilities. In this paper, we describe a model of characterization of trigeneration systems trough the condition of primary energy saving and the quality index, compared to the separate production of heat, cold and power. The study highlights the importance of the choice of the separate production reference system on the level of primary energy saving and emissions reduction.     

Molecular Modeling Approach on Fouling of the Plate Heat Exchanger: Titanium Hydroxyls,Silanols, and Sulphates on TiO2 Surfaces

Molecular modeling is a novel approach in the field of fouling research. A method was used to calculate fouling reactions and molecular level interactions between heat transfer surface and flowing fluid. The focus was on the comparison of the reaction mechanisms of Ti(OH)₄ and Si(OH)₄ on a rutile (101) surface. The calculated reaction energies indicate strong chemical bonding via condensation reaction of titanium(IV) hydroxyls and weak hydrogen bonding of silanols without a chemical reaction on the surface. The chemical composition and structural properties of fouling layers from a real process were characterized. On the heat transfer surfaces, deposits containing titanium had dense structure and were very difficult to clean while silica was porous and amorphous, causing less severe problems in cleaning. Molecular modeling was found to be an effective tool in predicting reaction mechanisms and interaction forces between the fouling fluid and heat transfer surface at a molecular level.     

Mineral scale formation and mitigation on metals and a polymeric heat exchanger surface

An experimental set-up was built to study heat transfer fouling of different pipe materials used in heat exchangers. Fouling mitigation investigations using wood pulp fibres in suspension in the fouling liquid were also performed. The new set-up allows progressive visual observation of fouling with time together with a recorded history under the same solution conditions. On completion, the tube under investigation could be removed to obtain quantitative data on the progressive build up of the deposit as well as the composition of the deposit.The experimental technique involved a pipe test specimen being centrally located in a cylindrical tank concentric with a vertical agitator to give constant and uniform flow conditions near the tube surface. The investigation of calcium sulphate deposition on four different metal surfaces (copper, aluminium, brass and stainless steel SS 316 respectively) and a polycarbonate surface reveals that the fouling increases with time but at a decreasing rate. The deposition on a metal surface can be seen to increase with increasing thermal conductivity and decreasing total surface energy over the range of experiments. Low surface energy material such as polycarbonate causes less attraction to the floating crystals and receives less deposition in comparison to the SS surface.Bleached Kraft softwood fibres at various concentrations were added to the solution to examine their effects on fouling. The results indicate that fouling is reduced as fibre concentration increases. It was also found that the fouling on stainless steel, brass and copper surfaces were all retarded in presence of fibre in the solution.     

换热器腐蚀污垢的形成机理及其防治措施

换热器表面形成的腐蚀污垢使换热器面的传热热阻增加、传热系数降低 ,从而增加了换热器的总费用。本文主要对腐蚀污垢的定义、形成机理、数学预测模型及其防治措施作了详细介绍。在防治措施中 ,特别介绍了涂层和非金属材料的防治效果。     

基于污水换热器污垢不同生长阶段的除垢试验研究

为了提高污水换热器的除垢效果,文章以管壳式污水换热器为研究对象,以沙粒作为除垢粒子,以污垢热阻变化率表征除垢效果,在利用烘干灼烧失重法、能谱分析法和微观结构分析法分析污垢成分的基础上,运用污垢热阻法进行除垢试验研究。通过试验研究了在污垢生长的诱导期、生长期和渐近期3个阶段除垢对污垢热阻值的影响。试验结果表明:管壳式污水换热器管内污垢的主要成分为含水量较高的有机物;结垢工况下,污垢热阻渐近值为0.74×10^-3(m^2·K)/W;在污垢生长的诱导期、生长期和渐近期进行除垢后,污垢热阻渐近值分别为0.4×10^-3,0.42×10^-3,0.6×10^-3(m^2·K)/W,与结垢工况相比,污垢热阻渐近值分别降低了45.9%,43.2%,18.9%,除垢工况下污垢热阻增长速率较结垢工况明显减缓。     

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.