Fiber-modified scaling in heat transfer fouling mitigation

The study of heat exchanger fouling using supersaturated calcium sulphate solutions has been widely reported. In this study fouling was investigated in a larger-scale heat exchange apparatus using stainless-steel pipe, and data were obtained at different flow rates, concentrations and temperature differences. The deposits were examined using a scanning electron microscope, X-ray diffraction and conventional photography. In a novel approach, wood pulp fibers were added to the fouling solution at various concentrations to mitigate fouling. Heat transfer enhancement above the solution-alone was observed initially and the onset of fouling delayed. When fouling eventually developed the final asymptotic level was lower than the fiber-free case for the experimental conditions specified. At a fiber concentration of 0.15% heat transfer augmentation occurred for 11 days. However, at 0.25% fiber concentration, heat transfer augmentation (no fouling) was sustained over the experimental duration of 45 days. It can be concluded that the service-life cycle of a heat exchanger can be prolonged with the addition of asymmetric, flexible, natural fibers. In this work it is argued that fibers modify the onset of deposition by boundary layer scavenging, and interact with turbulent eddies to reduce the rate of mass transfer of the foulant to the heated surface. When scale forms, the crystalline structure of the scale is interrupted by the fibers, which appear to roughen the heat transfer surface initially and increase the heat transfer coefficient. However, the scale deposit continues to build up very slowly, causing the thermal resistance to eventually override the turbulence augmented heat transfer effect of the fibers.     

羧甲基葡聚糖的快速沉降法阻垢特性研究

工业循环冷却水中的碳酸钙污垢一直是困扰工业生产者的重要问题,使用快速沉降法（FCP）研究了不同浓度的羧甲基葡聚糖对于碳酸钙污垢的抑制作用。控制溶液中的钙离子浓度为200 mg/L,羧甲基葡聚糖的浓度分别为0.5、1、2和4 mg/L时,对羧甲基葡聚糖的阻垢性能进行分析。结果表明,羧甲基葡聚糖对于减缓碳酸钙的成核过程有明显效果,此外,它也显著降低了碳酸钙成核后晶体的生长速率。当羧甲基葡聚糖的浓度为4 mg/L时,羧甲基葡聚糖对于碳酸钙的成核及晶面生长起到了完全抑制的作用,对于工业应用中碳酸钙的防垢处理,提升换热效率具有借鉴意义。     

板式换热器以松花江水为冷却水条件下的污垢特性

为研究板式换热器在天然水下的污垢特性,实验采用BR0.015F型板式换热器,研究了其在不同的流速和温度下的污垢特性,用扫描电镜观察了已经结垢的换热表面。结果表明：此板式换热器松花江水的污垢类型为渐进型且不存在诱导期,随着流速的增大污垢热阻降低的幅度减小。冷却水温度在27.5～28.5 ℃时污垢热阻较大,温度升高污垢热阻降低;换热表面的混合污垢成片的附着于换热面,有少量的难溶盐析出的晶体生成。     

污水换热器流态化在线防、除垢实验

采用污水源热泵供暖空调是节能减排的有效途径,但换热器结垢问题尚未得到有效解决。本文将固液流态化除垢技术应用到污水源热泵换热器的防、除垢中,并设计了一套污水换热器流态化除垢系统。该系统以沙粒作为除垢颗粒,通过理论分析和实验验证相结合的方法,研究了固液流态化除垢的最佳流速、除垢能力、强化换热效果及减少磨损的最佳工作参数。实验结果表明;当换热管内污水流速达到0.87m/s时,直径2～3mm沙粒可流化,沙粒循环回收率达到95%,实现了污水换热器在线清洗;沙粒对壁面的碰撞力可有效去除换热管壁的软垢及铁锈,除垢效果明显;该除垢系统持续运行24h后,污水换热器传热系数提高了25.6%;沙粒体积分数为4%,流速为扬动流速,是减少磨损和保证除垢效率的最佳工作参数。     

板式换热器Ni-P-TiO2复合纳米镀层微生物污垢特性

换热器微生物污垢问题普遍存在于能源化工领域,污垢的聚集会导致设备的流动阻力、燃料消耗和维护成本支出大幅度增加。本文采用复合纳米镀层来抑制和减轻换热表面的微生物污垢的附着和沉积。首先采用化学镀的方式,在板式换热器的不锈钢316板上镀覆 Ni-P-TiO₂复合纳米镀层和对照性的Ni-P 镀层。基于板式换热器的微生物污垢在线监测实验系统,研究了镀覆Ni-P-TiO₂复合纳米镀层的板式换热器微生物污垢特性。结果表明,清洁状态下,镀覆两种镀层的板式换热器其摩擦系数（f）和Nusselt数（Nu）相较未镀覆板式换热器均略有增加;微生物污垢实验后,相比较未镀覆的板式换热器,镀覆Ni-P镀层的板式换热器污垢热阻减少了8.36%~23.07%,而镀覆Ni-P-TiO₂复合纳米镀层的板式换热器污垢热阻减少了16.6%~30.96%;在相同微生物污垢实验工况下,镀覆Ni-P-TiO₂复合纳米镀层的板式换热器的摩擦系数（f）相比Ni-P镀层的低2.54%~11.82%,但Nu却明显高于Ni-P镀层达8.47%~9.45%,并且污垢热阻明显小于Ni-P镀层达10.66%~18.18%,镀覆Ni-P-TiO₂复合纳米镀层的板式换热器展现了优异的强化传热性能和抑垢性能。     

板式换热器铁细菌生物污垢特性的实验分析

为探讨铁细菌在板式换热器里的污垢规律,对不同流速、温度及浓度下铁细菌在板式换热器内的污垢特性进行了实验研究。结果表明：随着流速的增加,铁细菌的污垢热阻渐近值逐渐减小;随着温度的升高,铁细菌的结垢诱导期明显缩短,结垢速率加快,污垢热阻的渐近值呈减小的趋势;而随着浓度的增加,污垢热阻值呈现明显的增长。     

Impact of deposit aging and surface roughness on thermal fouling: Distributed model

A kinetic model was recently proposed to describe the effect of aging on deposit thermal conductivity and the thermal performance of a shell‐and‐tube heat exchanger undergoing crude oil fouling. The model is adapted for implementation within a dynamic, distributed system with spatial and temporal distributions, relaxing several of the previous assumptions. The evolution of surface roughness is also considered, using conjectural linear and asymptotic functions. Simulations are performed for a single tube representative of a refinery exchanger. The results demonstrate the substantial effects over time of aging and roughness on heat transfer and pressure drop. Roughness effects yield apparently negative initial fouling resistances, as reported in some experimental tests. The importance of accounting for roughness dynamics in short time scale pilot plant scale tests and aging over longer time scales in industrial applications is highlighted. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Investigation of flow boiling in circulating three-phase fluidised bed: Part I: Experiments and results

In flow boiling apparatus, fouling is frequently a problem. Mechanical methods to mitigate fouling include the impact of solids on the deposit to remove it. Solid particles fluidised by the two-phase boiling mixture may accomplish sufficient deposit removal to keep boiling surfaces clean. This results in a self-cleaning fluidised bed boiling heat exchanger. The particles additionally enhance the heat transfer. In this paper, a comprehensive investigation of three-phase fluidised flow boiling in a circulating system is presented. A test apparatus which is a three-phase circulating fluidised bed was built with glass construction to visualise boiling phenomena in a water system. It could also be operated as a two-phase system and this was investigated to provide the basis for comparison and also to verify reproducibility and confirm well-established flow boiling results. For the three-phase system (where steel particles were added), a range of liquid flowrates and heat fluxes was used and three different particle sizes were investigated. Three-phase results were compared with two-phase results. This work is primarily a study of boiling heat transfer enhancement as a result of addition of particles, but the effect of the particles creating a self-cleaning heat exchanger is a significant operational advantage for industrial application.As expected, heat transfer coefficients were higher overall for the three-phase system than for the two-phase system. The onset of nucleate boiling was independent of heat flux and the heat transfer coefficient initially increased with increasing Reynolds number whereafter there was some deterioration of heat transfer. Visualisation of boiling heat transfer phenomena for the three-phase system is also provided which allows mechanistic explanations of the measured phenomena.In Part II of this paper, a correlation is developed, based on boiling heat transfer modelling, to describe heat transfer during boiling in a three-phase circulating fluidised bed. A flow-dependent function is added as is a boiling heat transfer enhancement factor. This correlation is validated against this experimental data and found to show agreement within about 20% and better agreement with the data than an existing correlation.     

脱硅中液固循环流化床清洁传热

考察了铝土矿熟料溶出粗液脱硅加热过程中的结垢行为,采用有机-无机复合材质的惰性固体颗粒研究了循环流化床对脱硅加热过程中硅渣结垢的清除及防止性能.结果表明：硅渣结垢曲线为具有诱导期的渐近式曲线,硅渣结垢机理为结晶结垢和微颗粒沉积结垢混合机理;循环流化床不仅能有效防止硅渣结垢的形成,而且能完全清除已有的硅渣垢层,硅渣结垢的清除速率随操作流速及固体颗粒浓度的增大而增大;惰性固体颗粒的引入不影响硅渣结垢的机理.根据液固流化床的防垢机理建立了结垢模型,其预报值与实验值吻合较好.     

汽-水换热器内流体诱导振动强化传热试验

提出利用换热器内流体诱导振动实现强化传热的方法 .在利用传热表面振动提高对流换热系数的同时 ,利用振动变形减少积垢 ,降低污垢热阻 ,实现了复合强化传热 .对管内外流体流动诱导弹性管束的振动特性、强化传热特性和污垢特性进行了试验研究     

换热器防垢除垢实验研究

考察了二甲基甲酰胺中的盐在换热器换热冷却过程中的结垢行为,分别采用内置弹簧和液固流态化以及两者结合的方法对换热器在线防垢除垢进行研究.采用测定总传热系数法研究污垢热阻随时间的变化规律,并用污垢称重法测定不同条件的总污垢量.研究了流体流速、传热温差、弹簧结构、惰性固体颗粒数量和大小等诸因素对结垢的影响,结果表明:直径与管内径比为0.8,螺距和管内径比为1.5的弹簧在0.6～1.0 m-s-1的流速范围内防垢除垢效果最佳,随着颗粒数量的增加和粒径的增大,污垢生成速率降低,结垢量减少,相比同条件下单纯颗粒流态化除垢,内置弹簧和颗粒流态化结合法可以使污垢量减少20%～50%.实验研究结果对实际的工业过程具有一定的参考意义和指导价值.     

积灰与酸耦合作用下套管换热器传热特性热态实验

烟气露点是低温腐蚀的关键指标,制约着锅炉尾部排烟余热的深度利用。换热器表面积灰是传热特性恶化的主要原因,电站锅炉尾部烟气环境中,积灰与酸凝结共同作用影响换热器的传热特性。在含尘、含酸的烟气环境下对换热器传热特性进行热态实验研究,以套管换热器为基础,得到具有更高使用价值的烟气工程应用露点,结合传热特性随外壁温的变化情况,得到换热管传热特性随积灰及酸耦合的变化规律;并通过对不同外壁温下灰样的分析,得到不同酸凝结状况下积灰规律,得到实验环境下的工程应用酸露点温度为72℃,较传统热力学酸露点低35℃以上。     

Hydraulics and Mass Transfer Efficiency of a Commercial-Scale Membrane Extractor

Abstract

In recent years there has been significant interest in utilizing microporous hollow fiber membranes for liquid-liquid extraction. The membrane extractor resembles the shell and tube heat exchanger with the tube section composed of 1000–2500 fibers/in². The diameter of each fiber is approximately 300 microns. In this process, the feed may be passed through the shell side, while the solvent is passed through the fiber side, or vice versa. Mass transfer occurs across the liquid-liquid interface formed in the pores of the fiber wall. The advantages of this technology are high throughput capacities, independence of density difference between the feed and solvent, and potentially high mass transfer areas. The mass transfer performance of an available commercial scale nonbaffled membrane extraction module was determined to be lower than expected from results obtained in smaller scale modules. Mass transfer studies of a commercial-scale membrane extraction module at the Separations Research Program have shown that a significant portion of the fibers are bypassed by the shell side fluid and consequently only a fraction of the total fiber surface area is utilized. A hydraulic study using a dye tracer technique verified this finding with an aqueous flow on the shell side. A model which incorporates mass transfer correlations reported by others has been developed and shown to have excellent agreement with the experimental data obtained. In this paper, the efficiency of the membrane extractor is compared with conventional spray, sieve tray, and packed columns; the effect of shell side bypassing is also presented.     

换热器因应结垢慢时变的控制系统重构分析

换热器作为化工过程中重要的热量传递设备,一般需要持续运行较长时间,期间由于结垢热阻的不断累积,换热器的换热效率将随时间逐渐下降,直至无法满足工艺要求。在实际的工业换热过程中,工艺人员通常会对换热器进行裕量设计,然而,当换热器需要运行的时间增长或者裕量设计不足时,面对不可避免的结垢增长,常用的流量控制策略在换热器运行末期的调节效果逐渐变差,旁路控制的方案无法实现全周期的持续控制。因此,本文首先在渐近增长模型的基础上建立了换热器结垢累积的积分渐近模型,该模型考虑了过程参数对结垢速率的影响;其次,以一个用于水循环的小面积换热器为例,分析了考虑结垢增长的条件下流量控制和旁路控制的调节效果;最后,针对结垢过程的慢时变和持续性等特点,设计了基于流量与旁路开度的控制系统重构方案。示例的运行结果表明,对于同样面积裕量有限的换热器,这一控制方案能延长其使用寿命,实现其全周期持续可控的目标。     

低压静电用于循环冷却水阻垢的实验研究

对低压静电处理循环冷却水的阻垢性能进行了实验研究与机理分析。采用动态监测污垢热阻的方法进行了结垢与阻垢实验,并对溶液的水质进行了分析。结果表明,低压静电处理循环冷却水对换热表面的结垢起到很好的抑制作用,实验中仅需2.2W功率就可达到100%的阻垢率。低压静电水处理不同与其它物理水处理技术,它可降低溶液的硬度、碱度、电导率及pH值,是一种主动的抗垢技术。被处理溶液中发生的微电解反应使更多的污垢晶体在处理器中聚集并生长,而不是沉积在换热表面,从而起到阻垢作用。     

Heat Exchanger Network synthesis with Detailed Heat Exchanger Design

Pressure drop and fouling are important issues in heat exchanger network synthesis, which are usually neglected. Heat exchangers were designed in detail during the heat exchanger network synthesis, and pressure drop and fouling effects were taken into account. Pinch analysis combined with exergoeconomic analysis was used for determining optimal minimum approach temperature (ΔT_min) for heat exchanger network synthesis. Exergy consumption of heat transfer in HENs was calculated using a subsection integral on balanced composite curves. The heat transfer coefficients of all heat exchangers in the network were calculated iteratively to meet the requirements of optimal area and allowable pressure drops. The proposed method was applied to an industrial case. Numerical results indicate the importance of the detailed design of heat exchangers in HENs synthesis.     

流动过冷沸腾传热强化及阻垢

在流动沸腾传热实验中,考察了CaCO3污垢溶液的形成过程及各种工艺条件对流动过冷沸腾传热的影响. 研究条件包括流体速度、溶液温度、CaCO3溶液浓度及热通量,实验中发现了一些规律. 同时还考察了不同阻垢剂[聚天冬氨酸(PASP)、2-膦酸丁烷-1,2,4-三羧酸(PBTCA)及氨基三甲叉膦酸(ATMP)]对流动过冷沸腾传热的影响. 结果表明,所选阻垢剂均能抑制污垢的生成并降低了污垢热阻,而且存在最佳浓度范围. 但不同阻垢剂的阻垢效果不尽相同,在本实验条件下,ATMP的阻垢效果最好,PBTCA次之,PASP的阻垢效果较差.     

Numerical method for heat transfer and fouling analysis of a shell and tube heat exchanger using statistical analysis

Through proper monitoring, problems can be identified and isolated well before the economics of the process are threatened. In contrast to most conventional methods, fouling can be detected when the heat exchanger operates in transient states. Statistical analysis is used to develop a fouling growth model of a heat exchanger subjected to fouling. The statistical analysis was considered for four different types of distributions out of which the lognormal distribution was found to be most suitable. Experiments were conducted with a single pass shell and tube heat exchanger with water both as the hot and cold fluids. The results show that the proposed tool is very effective in detecting critical fouling in a heat exchanger, which can be utilized for predicting the optimal maintenance schedule. Hence, the results of this work can find application in predicting the reduction in heat transfer efficiency due to fouling in heat exchangers that are in operation and assist the exchanger operators to plan cleaning schedules.     

考虑关键换热器备用的原油预热系统清垢周期优化

原油结垢是原油预热过程面临的一个严峻问题,它使换热器的传热系数降低,影响正常的生产过程。应对原油预热过程的结垢问题最普遍的方法就是对换热器进行周期清洁,并优化清洁周期。许多结垢严重的行业都采用换热器备用的方式来减少因换热器离线造成的热回收减小,但原油预热系统尚未考虑,因而进一步考虑了关键换热器的备用来进行原油预热系统清垢周期优化。首先选取关键换热器,再通过模拟退火算法进行清洁周期优化。应用所提出的方法对简化的原油预热系统进行了清垢周期优化,结果显示,考虑换热器备用时所得到的清垢周期经济效益更好。     

LAMINAR FLOW HEAT TRANSFER WITH INLINE MIXERS INSERTS

Heat transfer coefficients were measured for heat transfer to a Newtonian fluid flowing in laminar flow through a tube with inline mixer inserts. Kenics “Static” mixer and Ross LPD mixer inserts were studied as heat transfer augmentation devices. The mixer inserts were in the inner tube of a concentric tube heat exchanger. Steam was condensed in the annulus of the exchanger. Significant heat transfer enhancement was obtained with both inserts at the expense of even greater pressure drop increases. The use of the Ross mixer insert gave greater augmentation than did the use of the kenics insert. An analysis using the analogy between momentum and heat transfer allowed the prediction of heat transfer coefficients from pressure drop measurements. The predicted coefficients were in good agreement with experimentally measured heat transfer coefficients for laminar flow.