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.     

Nucleate Boiling on Porous Coated Surfaces

Boiling test results and theoretical evaluations related to predictive models for the boiling coefficient with porous coatings are critically surveyed. Consideration is limited to boiling pure fluids and coatings made of spherical particles of uniform diameter. The data for seven different porous coatings suggest that particle diameter has very little effect on performance. The preferred coating thickness is in the range of three to four particle diameters. Detailed consideration of cavity stability requirements suggests that a dynamic model is needed to rationally predict the boiling coefficient. A modified pore classification is proposed for use in the development of a dynamic model. The assumption and derivation of O'Neill's static boiling model is presented. Its ability to predict the boiling coefficient is evaluated by a comparison with the values predicted by an empirical correlation developed by Nishikawa and Ito.     

Modeling of Particulate Fouling on Heat Exchanger Surfaces: Influence of Bubbles on Iron Oxide Deposition

Studies of iron oxide deposition on Alloy-800 heat exchanger tubes have been part of a continuing research program at the University of New Brunswick (UNB); the present work formulates mechanisms for the effect of bubbles on deposition in water under boiling conditions. To supplement results from earlier deposition experiments in a fouling loop at UNB, measurements of bubble frequency and departure diameter as a function of heat flux were performed. High-speed movies of bubbling air/water systems indicated that a pumping action moved particles from adjacent areas at the surface to bubble nucleation sites. To explain the observations, the model considers deposition and concomitant removal. Deposition includes microlayer evaporation and filtration through the porous deposit. The deposit is sparse in the first stage, when the dominant process is microlayer evaporation including particle trapping and pumping, creating spots of deposit. Filtration becomes more important as the deposit thickens to a stage when microlayer evaporation becomes negligible. Chimney effects then control. Turbulence due to detaching and collapsing bubbles affects removal. In subcooled boiling, collapsing bubbles generate enough turbulence to maintain much of the deposit labile, while in bulk boiling bubble detachment from the nucleation site is dominant and a smaller portion of the deposit is labile and subject to removal. Model predictions are presented and shown to agree quite well with experimental data.     

Fouling Propensity of Modified Heat Transfer Surfaces

Abstract

As much as attention that has been paid to surface treatment as an efficient, and environmentally friendly approach toward fouling mitigation, the characterization of many innovative modified surfaces has become a matter of much debate. The latter is closely associated with the intermolecular interaction energies which would profoundly influence the adhesion of precursors onto the modified surfaces. In this study, based on the extended Derjaguin, Landau, Verwey and Overbeek (DLVO) theory, a new criterion is proposed to predict the propensity of a surface when prone to crystallization fouling or biofouling. Thereafter, the proposed criterion is examined against the present experimental results as well as those from previous studies where the required information for the determination of new criterion is available. The comparison shows that deposit formation onto heat transfer surfaces decreases with increasing the new proposed fouling propensity indicator criterion. Moreover, nearly 75% of the collated crystallization and biological fouling data points are predictable with this criterion and reasons for those that are not in compliance with the proposed criterion are discussed.     

Evaluation of Crude Oil Heat Exchanger Network Fouling Behavior Under Aging Conditions for Scheduled Cleaning

Heat exchanger network (HEN) fouling is an endemic operational challenge prevalent in many process industries. Its impact on both plant operating cost and productivity is significant and can be compounded by aging effects of the foulant. In this paper, we model and simulate the effect of aging on tube-side fouling and cleaning dynamics in a crude oil refinery preheat train (PHT) comprising a 14-unit HEN. A prescient, HEN modeling and dynamic simulation were performed wherein the transients of fouling and aging as well as the interactions between individual units were captured. To assess the temporal effects, different crude oil deposit (gel) aging scenarios (no aging vs. slow, medial, and fast aging) in the downstream units were considered for the PHTs’ overall heat recovery, cleaning options, and operability. The results show that the deleterious impact of fouling and concomitant aging, quantified in terms of thermal resistances, was significantly reduced by fast aging as opposed to medial, slow, or no aging of the gel deposit. Faster aging rate reflected improved heat recovery and a lesser demand for and lower cost of PHT cleaning. The concomitant higher growth of coke deposit due to aging, however, resulted in greater hydraulic resistance, which is inimical to operability.     

Perfluoropolyethers Coatings Design for Fouling Reduction on Heat Transfer Stainless-Steel Surfaces

The scope of this research is to obtain a film coating on stainless-steel surfaces in order to reduce the interaction between the metal surface and the precipitates, so as to mitigate fouling in heat exchangers. Perfuoropolyethers were used to obtain nano-range fluorinated layers in order to make hydrophobic the stainless-steel surfaces. A pilot plant with two identical heat exchangers was built to investigate the ability of the hydrophobic coating of preventing fouling. The heat exchangers, installed in parallel, operated at the same temperature and pressure conditions, namely, laminar flow regime and inlet flow temperatures of 291–293 K for cold streams and 313–333 K for hot streams. We compared the heat transfer performance of the two heat exchangers. After a 5-month operation, the decrease in the heat transferred was 56% for the coated heat exchanger and 62% for the uncoated heat exchanger. Moreover, the increase of heat transfer resistance due to scale on the uncoated heat exchanger, with respect to the coated one, was three times higher.     

Effect of Surface Temperature on Fouling of Cooling-Tower water

We will not, however, be too flattered by our victories over nature. Each such victory takes revenge on us. Each of these victories has, true, at the first turn, those consequences which were considered, but, at the second and third turn, quite different unexpected consequences that often destroy the importance of the first consequences.     

电站锅炉对流受热面积灰在线监测的研究

针对在线监测电站锅炉对流受热面积灰的需要,建立了对流受热面的污染监测模型.以HG1021/18.2-YM9型锅炉为监测对象,开发了受热面积灰在线监测系统,成功实现了锅炉对流受热面污染的在线监测.     

锅炉对流受热面积灰监测模型的灵敏度分析

在现有对流受热面积灰监测系统的基础上,以太原第一热电厂11号炉为对象,利用实际监测结果,对该模型的灵敏度进行了深入分析。利用锅炉校核程序生成的运行参数,进行灰污监测仿真实验。结果表明:三级过热器可以容忍±3%的测量误差,一级再热器可以容忍±5%的测量误差,省煤器的异常敏感参数比其它受热面要多,只能容忍±1%的测量误差。该文揭示了省煤器监测结果可靠性差的原因,为进一步改进积灰监测模型提供了依据。图7参7     

基于神经网络的锅炉对流受热面灰污监测研究

采用多层前向型神经网络,对电站锅炉对流受热面的实时污染状况建立了监测模型。模型选取合适的参数组成输入向量,利用电站数据采集系统下载的实时机组数据,经规格化处理后对神经网络进行训练。结果表明,训练后的神经网络可以较准确地实现锅炉对流受热面的积灰状态的在线监测,为吹灰方案的最优化打下了良好的基础。     

Fouling of Heat Exchanger Surface: Measurement and Diagnosis

Fouling in heat exchange equipment results in a significant energy toss by increasing heat transfer resistance and fluid frictional resistance. This paper deals with an effective way to monitor fouling and describes a potentially useful diagnostic approach for discriminating different types of deposit in situ. Information regarding deposit type would be useful in selecting an appropriate treatment procedure. Some cautions regarding the indiscriminate use of fouling factor or fouling resistance are also discussed.     

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。     

换热表面结垢对U型管蒸汽发生器传热性能的影响分析

U型管蒸汽发生器的壳侧沉积了来自二回路系统中的腐蚀产物,结垢导致热量聚积在金属换热管上,容易造成垢下热点腐蚀,危害设备安全。为了明确结垢对蒸汽发生器传热性能的影响,本研究基于仿真平台APROS建立了U型管蒸汽发生器的分布式模型,并根据已公开论文中的数据进行了模型准确性验证;推导了污垢热阻与表面换热系数之间的关系式,分析了不同结垢厚度、位置对U型管蒸汽发生器换热区域的传热管壁面温度、流体温度、传热系数、热流密度等的影响程度。研究结果表明：随着结垢程度的加剧,蒸汽发生器的换热效率不断降低,出口蒸汽品质不断下降;结垢对沸腾段换热效率的影响比对过冷段换热效率的影响更大。     

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.     

燃煤电站锅炉对流受热面积灰在线监测的研究

针对在线监测电站锅炉对流受热面积灰的需要,建立了对流受热面的污染监测模型。以某300 MW机组的1 025 t/h锅炉为监测对象,开发了受热面积灰在线监测系统,成功实现了锅炉对流受热面污染的在线监测。     

Dynamic Wetting Behavior and Water Drops on Microgrooved Surfaces

The retention of water on the air-side surface of heat exchangers is important in air conditioning, refrigeration, and heat pumping applications. Recently, surfaces with grooves tens to hundreds of micrometers deep and wide have been proven highly effective in promoting water drainage from heat exchangers. Dynamic wetting behavior governs the motion of drops on such surfaces, and this work provides the first data on the dynamic wetting behavior on microgrooved metallic surfaces. Moving drops on microgrooved surfaces can be in a Wenzel or Cassie–Baxter wetting state. In a Wenzel wetting state, drops are elongated and have a tail; the length of the tail increases with Wenzel roughness. In a Cassie–Baxter wetting state, drops become more elongated as the Cassie–Baxter roughness increases. Drops in the Wenzel and Cassie–Baxter wetting states have dynamic advancing contact angles that increase with drop velocity and dynamic receding contact angles that decrease with velocity. In the Cassie–Baxter state, dynamic contact angles vary linearly with velocity and have larger hysteresis for larger Cassie–Baxter roughness. The behavior is more complicated for drops in the Wenzel state, showing nonlinearity in dynamic contact angles, with generally larger hysteresis for larger Wenzel roughness.     

变负荷工况下锅炉对流受热面污染的监测

针对在线监测大型电站锅炉受热面污染的需要,讨论了稳定负荷的计算模型无法用于变负荷工况监测的原因,分析了变负荷过程中各物理参数的变化机理,从热量平衡出发,考虑了金属管壁蓄热及蒸汽蓄热对受热面污染计算的影响,推导了变负荷工况下的污染监测模型及实时计算方法.以某电厂300MW机组锅炉的低温对流过热器为计算对象,采用现场实时数据进行了变负荷工况下受热面污染监测计算.提出了考虑金属及工质蓄热的锅炉变负荷污染监测模型与计算方法,基本能够消除负荷变化对受热面实际污染率计算的影响,得到比较合理的污染率计算结果,对改进现有的根据热平衡算法的锅炉受热面污染在线监测模型有重要的参考价值和工程实际意义.     

Functional Correlation of Surface Temperature and Flow Velocity on Fouling of Cooling-Tower Water

Experimental fouling data have been analyzed on the basis of the change in overall heal transfer coefficient of the fouling test section. It is assumed that thermal hydraulic conditions in the test section remain reasonably constant for the duration of a fouling test. The model ofTaborek et al.|I| is used, and two parameters,/8,. and Rf?, that can be determined for each fouling test are derived by regression analysis. The parameter Rf? contains all the factors that influence fouling, while 1/ ?_c, contains shear stress, deposit thickness, and bonding strength of the deposit. The parameter R?is the asymptotic fouling resistance and ? is the lime constant of the fouling resistance-time curve. These parameters were determined as a function of surface temperature. Limited data were available to indicate the effect of velocity on the parameters. The parameters 1/8, and R? may be used to predict the history of fouling in a heat exchanger. Until more.data are obtained and analyzed in this fashion, the values of 1/ ?_c. and R? obtained in this paper should be applied at conditions for which the fouling data were obtained. A numerical example is presented.     

锅炉受热面积灰监测方法的研究

根据积灰机理,用多变量统计过程监控方法,对DCS系统采集的数据进和主元分析,计算其Ti2值,对废热锅炉积灰状态和故障进行在线监测,可以排除故障的干扰,为合理确定吹灰周期时间提供科学的依据.     

Composite Fouling on Heat Exchange Surface in Australian Sugar Mill Evaporator

Composite scale of amorphous silica and calcium oxalate is very intractable; it imposes a significant fraction of scale management cost in sugar mill evaporators. This work evaluates silica and calcium oxalate composite fouling using dynamic fouling-loop experiments under different operating conditions with and without sugar. A novel closed-loop setup simulating in a single run the effect of feed concentration in successive stages of evaporation cycle was used. Experiments under constant composition and varying thermal hydraulic effect were conducted. Results indicated synergistic effects for initial silica and calcium oxalate supersaturation values of 2.6 and 1.7. Sugar effects became significant for concentrations in the third effect and above. The fouling mechanism was particulate deposition of silica and calcium oxalate colloidal species strengthened by consolidation; fouling rate increased with decreasing interfacial energy barrier between the surface and foulant, assessed by zeta-potential analysis. Deposit characteristics and deposition rates were concentration and thermal hydraulics dependent. Results confirmed the need that the 5 × 5 (types × stages of fouling) matrix of Epstein needs to be expanded to a 6 × 6 matrix to include composite fouling as a type of fouling. In this study the composite fouling was a surface-controlled process for which the activation energy was similar to typical chemical bond strengths, explaining the role of consolidation in composite fouling. The results contribute to advancement of fundamental understanding of heat exchanger fouling and to development of scale control strategies for composite fouling minimization in sugar mill evaporators.