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。     

翼型涡流发生器对析晶污垢抑制特性的模拟研究

为研究翼型涡流发生器的结构对析晶污垢沉积的影响,利用数值模拟的方法研究在改变其间距、攻角和翼型的情况下,污垢沉积的变化情况。通过引入抑垢率的计算,来表征涡流发生器的抑垢特性。根据实验得出的光片单位面积污垢沉积量与模拟光片得出结果对比,说明验证模拟方法的可行性。通过模拟研究发现在两种攻角下,间距与抑垢率之间的变化关系相同。当间距为10至60mm时,抑垢率随间距的增加而减小。当间距为60至80mm时,则是反向变化。在相同攻角下,流动方向投影面积与抑垢率之间呈现同相变化。投影面积的影响效果与间距相关。     

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.     

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.     

Surface effects in flow boiling of R134a in microtubes

The inner surfaces of microtubes may be influenced strongly by the process of making them due to manufacturing difficulties at these scales compared to larger ones, e.g. the surface characteristics of a seamless cold drawn tube may differ from those of a welded tube. Accordingly, flow boiling heat transfer characteristics may vary. In addition, there is no common agreement between researchers on the criteria of selecting tubes for flow boiling experiments. Instead, tubes are usually ordered from commercial suppliers, in many cases without taking into consideration the manufacturing method and its effect on the heat transfer process. This may explain some of the discrepancies in heat transfer characteristics which are found in the open literature. This paper presents a comparison between experimental flow boiling heat transfer results obtained using two different metallic tubes. The first one is a seamless cold drawn stainless steel tube of 1.1 mm inner diameter while the second is a welded stainless steel tube of 1.16 mm inner diameter. Both tubes have a heated length of 150 mm and the flow direction is vertically upwards. The tubes were heated using DC current. Other experimental conditions include: 8 bar system pressure, 300 kg/m² s mass flux, about 5 K inlet sub-cooling and up to 0.9 exit quality. The results are presented in the form of local heat transfer coefficient versus local quality and axial distance. Also, the boiling curves of the two tubes are discussed. The results show a significant effect of tube inner surface morphology on the heat transfer characteristics.     

Methodology for predicting spray quenching of thick-walled metal alloy tubes

This paper explores the parametric influences of spray quenching for thick-walled metal alloy tubes. Using the point-source depiction of a spray, an analytical model is derived to determine the shape and size of the spray impact zone, as well as the distribution of volumetric flux across the same zone. This distribution is incorporated into heat transfer correlations for all spray boiling regimes to generate a complete boiling curve for every location across the impact zone. By setting boundary conditions for both the sprayed and unsprayed portions of the tube surface, a heat diffusion model is constructed for a unit cell of the tube for both aluminum alloy and steel. This model is used to construct spray quench curves for every point along the sprayed surface and within the wall. Increasing nozzle pressure drop or decreasing orifice-to-surface distance are shown to increase the magnitude of volumetric flux, which hastens the onset of the rapid cooling stages of the quench as well as improves overall cooling effectiveness. The sprayed surface is characterized by fast thermal response to the spray, while regions within the wall display more gradual response due to heat diffusion delays. With their superior thermal diffusivity, aluminum alloy tubes transmit the cooling effect through the wall faster than steel tubes. For steel, the cooling effect is more concentrated near the sprayed surface, causing the sprayed surface to cool much faster and locations within the wall much slower than for aluminum alloy. The predictive approach presented in this paper facilitates the determination of surface temperature gradients in the quenched part to guard against stress concentration. Also, when combined with metallurgical transformation models for the alloy, it may be possible to predict material properties such as hardness and strength.     

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.     

Investigation of adhesion of CaCO3 crystalline fouling on stainless steel surfaces with different roughness

Fouling adhering experiments on AISI 304 stainless steel surfaces with different roughness had been performed in boiling supersaturated calcium bicarbonate solution. The effect of surface roughness on adhesion of fouling is limited, and the adhesion of fouling does not have a simple linear relationship with the surface roughness of samples. The surface with roughness in middle is more easily induced to form “transitional interface” which connects the fouling and matrix surface. It is also found that the crystalline types of fouling are changed in the fouling process due to the variation of metallic ions in reaction solution.     

Impact of Thermal Shock on Fouling of Various Structured Tubes During Pool Boiling of CaSO4 Solutions

Crystallization fouling during nucleate pool boiling is characterized with relatively rapid formation of a hard and brittle deposit layer. In this study, the impact of thermal shock is experimentally investigated on cleaning of various tubes when subjected to fouling of CaSO₄ solutions. The heat transfer tubes included stainless-steel plain and finned tubes with fin densities of 19 and 40 fins per inch. The tubes were used during pool boiling for heat fluxes ranging from 80 to 300 kW/m². The thermal shock is applied by the sudden decrease or increase in heat flux for approximately 30 s after the fouling layer is formed. In terms of cleaning, the experimental results showed far better performance for the plain tube. During the sudden change in heat flux the whole fouling layer cracked and peeled off, receiving again almost initial clean heat transfer performance. Contrariwise, the results for the finned tubes showed that the fouling layer was only affected marginally by the thermal shock.     

“Zero Fouling” Self-Cleaning Heat Exchanger

Conventional shell and tube heat exchangers sometimes have to use two severely fouling process streams, one in the tubes and one in the shell. This paper presents the design of a self-cleaning heat exchanger that applies the self-cleaning mechanism in the tubes of two parallel bundles handling the fouling process streams. For the transfer of heat between both bundles, a small circulating flow of conditioned water is used as an intermediate fluid, a fraction of which evaporates on the outside of the tubes of the high-temperature bundle and condenses on the outside of the tubes of the low-temperature bundle. This novel design, which consists of two parallel bundles in one shell, experiences very high film coefficients at the outside surface of both tube bundles and does not suffer from any fouling. Therefore, it is referred to as a “zero fouling” self-cleaning heat exchanger. In this paper, a conventional severely fouling crude oil preheater will be compared with a zero fouling self-cleaning heat exchanger for the same service.     

Performances Cost Effectiveness,and Water-Side Fouling Considerations of Enhanced Tube Heat Exchangers for Boiling Service with Tube-Side Water Flow

A case study method is employed to calculate the performance benefits and cost effectiveness of enhanced tubes for an application involving seawater on the tube side and ammonia boiling on the shell side. A porous boiling surface is used on the ammonia side, and eight basic geometries are evaluated for the water side. The analysis method selects the “optimum” enhancement dimensions for each geometry type. The optimum is de fined as the minimum material requirement for fixed heat duty and water-side pumping power. Heat exchanger size reductions of 55–67% are possible, depending on the tube-side enhancement type. The cost effectiveness of the enhanced tube designs are calculated relative to a plain tube exchanger for aluminum, copper, and titanium materials. Heat exchanger tubing cost reductions in the range of 10% are predicted for the titanium and copper tubes. Doubly enhanced aluminum tubes do not appear to provide heat exchanger cost reduction. A better application for enhanced tubes may involve increasing the UA value to provide a reduced LMTD. The reduced LMTD can be used to increase process thermodynamic efficiency, and thus offset the greater heat exchanger cost. The results of this study must be considered tentative until the water-side fouling characteristics of enhanced tubes are established.     

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.     

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.     

表面强化管外池沸腾传热特性研究

为实现节能降耗,开发了多种强化沸腾传热的高效换热管。以水为工质,在0.1MPa下对垂直光管、烧结多孔管和T槽管进行了池沸腾传热实验研究,并分析了沿管子轴向的温度分布。实验结果表明,烧结多孔管与T槽管能显著降低起始沸腾过热度、强化沸腾传热:烧结多孔管和T槽管的起始沸腾过热度比光管的低1.5K左右;烧结多孔管和T槽管的核态沸腾传热系数分别为光管的2.4～3.2倍和1.6～2.0倍。此外,烧结多孔管和T槽管能降低相同热流密度下的壁面温度,且有利于降低管子轴向的温差。     

Experimental Study on Anti-Fouling Performance in a Heat Exchanger with Low Voltage Electrolysis Treatment

An innovative physical method to control fouling in heat transfer equipment—low voltage electrolysis anti-fouling (LVEAF) technology—is introduced. The objective of the present study is to testify the effect of LVEAF treatment in forced convective system and identify the operating mechanism. A series of fouling tests were carried out with and without LVEAF treatment. During the experiments fouling resistance was monitored, and the properties of test liquid were measured, including hardness, alkalinity, electrical conductivity, and pH. The main results were as follows: (1) The fouling was effectively restrained to form on the heat exchanger surface if the circulating water was treated with LVEAF. The scale inhibition ratios exceeded 90% in most cases. (2) The LVEAF technology is an active anti-fouling technology with lower energy consumption. The properties of treated test liquid were changed. (3) Electrochemical reactions occur near the electrode due to the low current existence in the water when the LVEAF device is working. A lot of fouling deposits formed in the test liquid near the cathode or on the inner surface of the treatment unit, but did not deposit on the heat exchanger surface.     

Development and application of anti-fouling ceramic coating for high-sodium coal-fired boilers

To address the fouling problem in boilers fired with high-sodium coal (HSC), a composite ceramic coating was developed using the slurry method and applied on 15CrMo steel. After sintering, the composite ceramic coating had a dense structure and was well bonded to the substrate forming a metallurgical damascene structure. The fouling and thermal shock resistance of the ceramic coating to sodium sulfate was found be excellent. After five fouling cycles, the uncoated steel had a surface fouling rate of 21.9%, compared to 0% for the coated steel. The composite ceramic coating also demonstrated an appreciable fouling resistance to sodium sulfate. Similar to the flaky oxide film formed on the uncoated steel surface, only a few fine sodium sulfate particles penetrated the microcracks generated during the sintering of the ceramic coating. However, remarkable anti-fouling results were achieved when the composite ceramic coating was applied in an industrial boiler fired with a Zhundong HSC blend.     

Double Perforated Impingement Plate in Shell-and-Tube Heat Exchanger

This article presents a solution to a chronic problem causing repeated tube failure at shell-and-tube heat exchangers. The problem is related to the fouling process on the tubes' surface, which accumulates downstream from the impingement plate at the exchanger inlet nozzle within the first tube rows due to low velocity and vortices production. In fouling services, the suspended deposits, fouling, accumulates on the tubes' surface downstream from the impingement plate, causing under-deposit corrosion, raising the tubes' surface temperature due to lack of cooling, and accelerating fouling process. Under-deposit corrosion attacks tubes and causes repeated tube failure, costing a lot of money in terms of material, maintenance, and production losses. Normal practice of extending tube life and delaying their failure is to upgrade the tubes' metallurgy. So the article objective is to present an economical solution option through modifying the impingement plate in the shell-and-tube heat exchangers where the impingement plate is recommended by the Tubular Exchanger Manufacturers Association (TEMA). The impingement modification is to replace the solid conventional impingement plate with double spaced plates having offset holes, called double perforated impingement plates (DPIP). The objective of this work can be met by comparing the simulation of the shell-side inlet flow distribution around the conventional and modified (DPIP) impingement plates and ensuring enhancement of the flow pattern distribution at the area behind the impingement plates. Since experimental work in flow investigation is time-consuming and costly, computational fluid dynamics (CFD) Fluent software was implemented as a cost-effective helpful tool to conduct the simulation for comparison purposes. The modified impingement plate, DPIP, will destroy vortices created behind the conventional plate, retarding fouling accumulation. DPIP will enhance shell-side flow distribution downstream from the impingement plate and will stop fouling accumulation on the tubes to prevent under-deposit corrosion.     

Nucleate pool-boiling enhancement outside a horizontal bank of twisted tubes with machined porous surface

Nucleate pool boiling of refrigerants is of important application in the flooded evaporator of refrigeration and air-conditioning system. Many surface geometries involve machined porous surface have been adopted to enhance the nucleate pool boiling heat transfer of refrigerants. Nucleate pool-boiling performance of R134a and R142b outside a horizontal bank of twisted tubes with machined porous surface (T-MPS tubes) was investigated in this paper. The experimental results showed that the T-MPS tube bank could enhance boiling heat transfer evidently. The enhancement ratios of R134a from the T-MPS tube bank were 1.4–1.7 and the maximum enhancement ratio of R142b could reach up to 4.4. Analyzing the tube bank effects of boiling heat transfer for R134a and R142b, the overall trend showed that the boiling heat transfer performance of the T-MPS tube bank was inferior to that of single T-MPS tube slightly.     

Experimental Investigations on Boiling Heat Transfer Inside Miniature Circular Tubes Immersed in FC-72

To investigate the size effect on the characteristics of boiling heat transfer, boiling behavior of FC-72 in heated vertical miniature circular tubes immersed in a liquid pool was experimentally studied. Two AISI 304 stainless steel tubes with inner diameters of 1.10 mm and 1.55 mm correspondingly, were heated by swirled Ni-Cr wire heaters and sealed in Lucite blocks by silicon adhesive. Both the top and the bottom ends of the circular test sections were open to the liquid pool. The boiling curves and heat transfer coefficients were obtained experimentally. The boiling behaviors at the outlets of the miniature tubes were also visualized with a digital video camera. Experimental results show that the tube geometry has a significant effect on the boiling characteristics. Vapor blocking at the outlet of the smaller circular tube with a diameter of 1.10 mm caused severe boiling hysteresis phenomena. The CHF decreased with reducing in tube size.     

Effect of Chemical Cleaning on Steam Generator Tube Performance

Boiling experiments were performed on new, chemically cleaned, and fouled steam generator tubes to determine the heat transfer performance of each. It was found that the heat transfer performance of the fouled tube was the best, followed by the chemically cleaned tube. The performance of the new tube was the worst. Scanning electron microscope (SEM) photographs of the boiling surfaces were taken to identify differences in surface characteristics. Results revealed the presence of significant amounts of porous deposits on the surface of the fouled tube that provided ample nucleation sites for boiling. Chemical cleaning removed most of the deposits such that the boiling performance of the cleaned surface was degraded. The new tube was very smooth and there were relatively fewer nucleation sites as evidenced in the SEM photographs. Available correlations were used to predict the heat flux for a given wall superheat and were compared with the experimental data.