Internally Enhanced Carbon Steel Tubes for Ammonia Chillers

Plate heat exchangers are used regularly in the heating, ventilating, air conditioning, and refrigeration industry. There is an urgent need for detailed and systematic research regarding heat transfer and the fluid flow characteristics of these types of exchangers. As an initiative in this respect, a literature search is presented on plate heat exchangers. New correlations for evaporation heat transfer coefficient and friction factor are introduced, which are applicable to various system pressure conditions and plate chevron angles. The correlations are based on actual field data collected during several years of installation and operation of chillers, and they are intended to serve as design tools and perhaps as a starting point for future research.     

螺纹管中实际冷却水污垢和颗粒污垢的特性研究

对一根光管和一组合有7根不同螺纹高,不同螺纹角,不同螺纹数而内径全为15.54 mm的铜质内置螺纹管中的污垢特性进行了实验研究,通过对实际冷却水污垢和颗粒污垢的实验数据的比较和分析,解释了二者之间存在差异的主要原因,并得出:螺纹管与光管中冷却水实际运行污垢热阻的比值随着面积指数和效率指数乘积的增加成线性增加,但在两个区间内(p/e>5.0和p/e<5.0)线性函数表达式不同,实验中颗粒污垢比值仅在一个区问内随效率指数的增加成线性增加关系.     

A Theory Describing Sedimentation Particulate Fouling Thresholds Inside Heat Exchanger Tubes

This paper expounds a simple, fundamental theory for predicting sedimentation particulate fouling thresholds for horizontal flows inside heat exchanger tubes. The velocities and shear stresses at the tube wall predicted by this theory for keeping particulate matter suspended compare favorably with industrial experience and proprietary Chevron data. This theory is also not inconsistent with the literature in that the shear stress required for sedimentation fouling mitigation is roughly 4–6 Pa. However, the situation where small particles become encapsulated in the viscous sublayer cannot be resolved with a simple sedimentation particulate fouling threshold theory at this time, necessitating future research.     

传热强化管管内熵产分析与热力性能评价

根据热力学第二定律,对传热强化管管内热力过程进行熵产分析,建立了基于流动与传热过程熵增原理的管内传热熵产方程,导出恒热流和恒壁温2种常见工况下的无因次熵产数表达式.在不同雷诺数和进口温差条件下,对2种螺旋槽管和光管进行恒壁温工况的熵产分析和热力性能评价,分析了传热和流动摩阻引起的熵产变化规律及2种不可逆损失占总熵产的份额.结果表明,熵产分析可用于评价传热强化管的综合热力性能,确定合理的运行工况、结构参数及强化换热形式,为强化管的应用评估及优化设计提供参考.     

Influence of the Apex Angle of Cone-Shaped Tubes on Particulate Fouling of Heat Exchangers

A two-dimensional (2D) cone shape has been added to the normal circular tubes of heat exchangers to minimize the area of stagnation and to streamline the air flow around the heat exchanger tubes. An experimental setup has been developed to study the influence of the apex angle of the cone-shaped tubes on particulate fouling of heat exchangers. Fouling experiments have been performed in which calcium carbonate particles are injected during the experiments and the deposition of particles on the tubes of the heat exchanger is monitored. Four sets of experiments have been performed, in which normal cylindrical tubes and coned tubes with an apex angle of 60°, 90°, and 120° are examined. It was found that particulate fouling ceased if the apex angle of the cone-shaped tubes is smaller than 90°. The attached cones enhance the flow around the tubes of the heat exchanger, by minimizing the stagnation area and keeping the flow attached to the tubes starting from the tip of the attached cone until separation, such that particles that deposit on the top of the tubes of the heat exchanger can be removed by the air flow.     

陶瓷膜的污染与清洗

陶瓷膜分离技术是近几年国际上最先进的膜分离技术之一.本文综述了陶瓷膜的性质、膜污染及其防治与清洗技术,并对陶瓷膜未来的发展进行了展望.     

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.     

An Improved Modeling Method of Water-side Fouling in Enhanced Tubes of Condensers in Application of Cooling Water Tower

Kern-Seaton fouling model of enhanced tubes and two parameters that foulant sticking probability(P) and deposit bond strength factor(ξ) in that model are analyzed theoretically. A new modeling method of fouling in enhanced tubes is proposed, which has physical meaning and a higher accuracy in comparison to traditional ones. A semi-theoretical model of fouling in internal helical-rib tubes is developed in this paper based on the long-term fouling data, targeting to improve previous fouling models. This new fouling model has three variables: area index, j-factor ratio and friction factor ratio. This triple-variable model has a maximum deviation of 5.20% and an average deviation of 1.87%, indicating a higher accuracy than previous ones. The mathematical type of this new model is in accordance with the theoretic deduction, thus makes sense in theory compared with old ones. It is also found that all current fouling models without the correction of heat transfer area index(β) have the biggest deviation when predicting the fouling resistance of enhanced tubes with p/e around 3.5, but which can be reduced by introducing the area index into the model. The new modeling method presented in this paper has outstanding advantages in modeling fouling of enhanced tubes, thus can be used in future fouling research.     

Analysis of Fouling in Six-Start Spirally Corrugated Tubes

Abstract

Fouling on heat exchanger surfaces can weaken the heat-transfer capability, increase the energy consumption, and even cause the failure of the whole system. In coaxial heat exchangers, spirally corrugated tubes perform better than smooth ones concerning heat transfer and antifouling. In this article, a parametric study on the antifouling performance of a six-start spirally corrugated tube is carried out with a solid–liquid two-phase model. First, comparisons between a smooth tube and a specific sample six-start spirally corrugated tube on the solid particle volume fraction distributions are carried out. Then, the effects of solid particle diameter, the main geometric parameters, including pitch and the corrugation depth, are investigated. Analyzing the solid particle volume fraction with different geometries, solid particle diameters and Reynolds number, the roles played by the centrifugal force, drag force, and gravity of solid particles on fouling performance in six-start spirally corrugated tubes are obtained. In addition, the corrugation depth affects the volume fraction range more while the pitch affects more on the steady range of particle volume fraction. This work is of significance for further design of spirally corrugated tubes and analysis of fouling problems in heat exchangers.     

Effect of Condensable Species on Particulate Fouling

The flue gases emanating from the combustion of fuels or gasification process invariably comprise particulate matter and many chemical species in vapor form. The temperature of the flue gases gradually reduces when passing through different sections of heat exchanger, such as the superheater, evaporator, and so on. If the temperatures of the heat exchanger tube surface and the gas phase are favorable for condensation, the chemical species in the vapor form will condense on the particles and on the tube surface. The particle deposition behavior under these conditions is drastically different from the one observed in dry particulate fouling. In order to model the particle deposition under such circumstances, it is important to evaluate the criteria for particle adhesion to the surface. Impaction experiments of particles impacting a surface coated with a thin liquid film and particles that are coated with a liquid film impacting over a dry surface are performed to evaluate the limiting parameters under which a particle sticks to the surface without rebounding. The effects of liquid viscosity, liquid film thickness, and interacting material properties are evaluated. The experimental results are compared to the results of existing models and a suitable model for fouling is proposed. Controlled fouling experiments are performed for varying liquid films coated over a deposition tube under various process conditions to mimic the condensation effects on fouling. The results are compared with detailed impaction experiments.     

凝汽器污垢特性及其在线计算模型研究

根据电厂凝汽器各类污垢的形成特点,分析了污垢对机组运行安全性和经济性方面带来的影响。建立了污垢热阻的在线计算模型,并应用于实际,以期为电厂凝汽器的优化除垢提供理论参考,提出了除垢的有效措施。     

Simulation of Thermal and Flow Phenomena in Smooth and Internally Rifled Tubes

This paper puts forward modeling thermal and flow phenomena in smooth and internally rifled tubes. The proposed model is a distributed parameter model based on solving balance equations describing the principles of the mass, momentum and energy conservation. The model enables an analysis of transient-state processes. The aim of the calculations is, among others, to find the distribution of the fluid enthalpy, mass flow and pressure in smooth and internally rifled tubes and to determine the heat transfer coefficient. The analysis concerns tubes arranged vertically and operating at supercritical steam parameters. The numerical calculation results will be compared to values obtained from computational fluid dynamics.     

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.     

三维变形管与三维内肋管在管式空气预热器上的应用研究

本研究对比分析了三维内肋管及三维变形管的结构特点和强化传热机理,在相同工况下,揭示了光滑圆管、三维内肋管及三维变形管用于管式空气预热器时的传热性能和流动阻力性能,三维内肋管和三维变形管传热性能均优于光滑圆管,三维变形管管内传热系数和流动阻力系数随短长轴B/A的减小而增大,三维内肋管可增加每米肋数、肋宽和肋高以强化传热效果,但流体流动阻力也将增加,低Re下,三维变形管管内综合传热性能优于三维内肋管,三维变形管管外自支撑而防止管束振动的特点可以实现在管内外的冷热流体纯逆流动,提出一种传热温差高的逆流三维变空间管式空气预热器,在相同工况条件下,空气预热器重量减轻,体积减小约65%,节省大量的生产和运输成本以及安装空间,三维变形管空气预热器在烟气余热利用中具有推广应用价值。     

Pulsed Flow Cleaning of Whey Protein Fouling Layers

This article reports on the use of intermittent pulsing superimposed on a slow steady flow to enhance the rate of cleaning of a model food soil—namely, a whey protein deposit—in a well-characterized flow geometry. Whey protein deposits were generated by recirculating 3.5 wt% WPC solutions through an electrically heated annular test section and then cleaned using recirculating solutions of 0.5 wt% NaOH, simulating industrial cleaning-in-place operations. Protein removal was monitored by local measurements of fouling resistance (at low heating power) and a total protein assay. Bulk flow velocities of 0.1–0.3 m/s and waviness ratios (amplitude of velocity pulse/baseline flow velocity) of 0.33–5.0 were studied at room temperature. Cleaning at these temperatures is a relatively slow process and allows the effect of flow regime to be followed readily. The resulting cleaning profiles showed that protein was removed in two stages: an initial rinsing stage, followed by protein swelling and gradual dissolution. Only the rinsing stage was observed in the absence of NaOH at a noticeably lower rate. Slow flow pulsing enhanced the overall cleaning rate, which exhibited a noticeable increase when the waviness of the flow exceeded unity and backflow of the fluid occurred. The results are discussed in terms of cleaning enhancement as a function of extra flow rate and extra energy input to the process.     

Management of Acute Particulate Fouling in a Titanium Dioxide Reactor System

ABSTRACT

The gas-phase manufacture of titanium dioxide is subject to acute fouling in the cooler unit located directly downstream of the reactor which quenches the reaction. A model of the cooler system was constructed, incorporating aspects of compressible flow, multimode heat transfer, fouling, and changes in geometry. This indicated that deposition could be very rapid. The effect of deposit layer buildup required measurement of the thermal conductivity of the porous layer; this was achieved using a novel testing device similar to that reported by Tan et al. (2006), for measuring the thermal conductivity of surface coatings. Active mitigation techniques are employed to reduce the effect of rapid fouling. The effectiveness of adding an erodent, in this case sand, to the flow was appraised by studying the breakup of deposit layers by impinging particles. The experimental conditions (high-temperature chlorine gas, high flow velocities) were simulated in cold experiments by matching the inertia and size of test particles to those of the sand. These studies showed that sand at the feed size would detach deposits, but could result in breakage of the sand particles. Mitigation efficiency is then determined by sand distribution and redistribution.     

锅炉管内腐蚀结垢过程的实验研究

管内腐蚀结垢是影响锅炉安全运行的主要因素之一,为了研究高温高压下锅炉管内的腐蚀结垢过程,在一定工况下进行挂片实验,利用SEM,EDS,XRD等分析手段,结合称重法,得出以下结论:金属表面的腐蚀结垢层分3层,依次为:氧化层、中间层、结垢层;腐蚀结垢过程也分为3个阶段:氧化膜形成期,结垢迅速成长期,稳定期;浓度的增加会促进金属的腐蚀和结垢速率;表面粗糙度增加对腐蚀的影响不大,但会促进结垢。     

钛管CaCO3污垢特性的实验研究

实验研究了钛管的污垢特性,并和不锈钢管、紫铜光管的污垢特性进行了比较.结果表明,钛管的污垢诱导期随着浓度的增大而增大,随着流速的增大而减小;但污垢热阻值随着浓度和流速的变化规律较复杂.在同种工况下,钛管的诱导期比不锈钢管短、却比铜管长,污垢热阻渐近值比铜管小、却比不锈钢管大,说明了钛管的抗垢性能虽好于铜管,但比不锈钢管差.     

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.