Dissertations / Theses on the topic 'Heat exchangers - Corrosion'

The HVAC&R industry is looking to transition from copper-aluminum heat exchangers to all-aluminum microchannel technology. The want for the transition stemmed from seeing the performance improvement of all-aluminum microchannel radiators in the automotive industry. Applications differ between the two industries; therefore, applying this technology for HVAC&R use must be validated. Research towards operating modes of an all-aluminum heat exchanger in a defined corrosive environment will provide the industry with a better understanding of heat exchanger design and heat exchanger material selection. The worth in this is preventing overdesign and producing more efficient heat exchangers. Furthermore, ASHRAE members and the corrosion community will find value in a defined corrosion system and corrosion test procedure. The information gained through past research has progressed assessment of material performance; however, the methods improperly simulate and expedite natural weathering. The most common method being used is the ASTM (American Society of Testing Materials) Sea Water Acetic Acid Test. The research discussed in this paper was focused on improving a standard corrosion system by implementing system modifications to simulate heat exchanger operation while performing a modified wet-dry cyclic test (e.g. ASTM G85 Annex 5). The goal is to produce results that are more representative of natural corrosion behavior and its forms. Current results were gathered from five of ten samples that underwent initial testing. Finally, possible improvements towards the chamber system and the test method, including the salt solution, are discussed.

All-aluminum microchannel heat exchangers are designed to significantly reduce refrigerant charge requirements, weight, reduced brazed joints, and decreased potential for leakage by increasing reliability. Al 3003 alloy is corrosion resistant and can be formed, welded, and brazed but the issue with all-aluminum heat exchangers is localized corrosion (pitting) in corrosive environments. Currently, there is no universally accepted corrosion test that all coil manufacturers use to characterize their products. Electrochemical testing method of cyclic polarization was employed in this investigation and relevant parameters including electrolyte corrosive agent and its concentration, electrolyte pH, and applied potential scan rate was varied to find an optimal set of parameters. Results of cyclic polarization of Al 3003 in electrolytes containing various concentrations of NaCl were compared with those of the tests in Sea Water Acidified Accelerated Test (SWAAT) electrolyte and it is shown the SWAAT electrolyte (4.2% sea salt acidified to pH of 2.9) is by far stronger (in terms of corrosivity) than typical 3.5% NaCl solution used in most corrosion testing. Corrosion rates (g/m2yr) of Al 3003 measured in this investigation were comparable to those provided by ISO 9223 standard corresponding to C1 through CX categories. Duration of cyclic polarization test is much shorter than that of SWAAT and results obtained in this test is more reproducible compared to those of SWAAT. Scanning electron microscopy micrographs show typical pit depths of about 50 μm.

Silicon nitride (Si3N4) is a potentially good ceramic material for industrial heat exchangers. However, at elevated temperatures and in coal combustion atmospheres its lifetime is severely reduced by oxidation. To increase its corrosion resistance, the formation of a protective oxidation barrier layer was promoted by the deposition of oxide thin films. Homogeneous and crack-free oxide coatings of calcium magnesium zirconium phosphate (CMZP) and magnesium doped aluminum titanate (Mg-doped Al2TiO5) were successfully deposited on Si3N4 using the sol-gel and dip-coating technique. Coated and uncoated samples were then exposed to a sodium containing atmosphere at 1000*C for 360 hours to simulate typical industrial environment conditions. Structural post-exposure analyses based on weight loss measurements and mechanical tests indicated better corrosion resistance and strength retention for CMZP coated Si3N4 compared to as received and Mg-doped Al2TiO5 coated Si3N4. This difference was attributed to the protective nature of the corrosion layer, which in the case of CMZP, significantly impeded the inward diffusion of oxygen to the Si3N4 surface.
Master of Science

M.Tech. (Department of Chemical Engineering, Faculty of Engineering and Technology), Vaal University of Technology
Corrosion responses of some carbon steels, stainless steel and copper alloys in the presence of a culture of bacteria (referred to as SRB-Sulphate-reducing bacteria) found in industrial heat exchangers, was studied to recommend best alloys under this service condition, with techno-economic consideration. Water from cooling towers in three plants in a petrochemical processing complex were analysed for SRB presence. Two of the water samples showed positive indication of SRB presence. The mixed cultures obtained from plant one were grown in prepared media and incubated at 35 °C for 18 days. Potentiodynamic polarisation studies in anaerobic conditions were done on the selected alloys in aqueous media with and without the grown SRB. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were then used to study the corrosion morphology and corrosion products formation. The voltamograms show higher icorr for alloys under the SRB compared to the control media, indicating the SRB indeed increased the corrosion rates. The surface analysis showed pitting on steel alloy ASTM A106-B. Localised attack to the grain boundaries on a selective area, was seen on ASTM A516-70 dislodging the grains, and intergranular corrosion was seen throughout the exposed area of ASTM A179. Copper alloys showed pitting on ASTM B111 grade C71500 (70-30), and denickelification on ASTM B111 grade C70600 (90-10), and is a good alternative material for use apart from carbon steel alloys, recording a low corrosion rate of 0.05 mm/year. The EDS analysis supported the findings showing higher weight percent of iron and sulphur on surface of the alloys after exposure to the SRB media. This implies that the presence of the sulphur ion indeed increased the corrosion rate. ASTM A516-70 carbon steel was chosen as a suitable alternative material to the stainless steel in this environment. The Tafel plot recorded a corrosion rate of 1.08 mm/year for ASTM A516-70 when exposed to SRB media.

O mexilhão-dourado é um organismo invasor que tem causado grandes prejuízos a sistemas de captação de água e usinas hidroelétricas (UHEs). Existem diversos métodos de controle da incrustação destes organismos em tubulações de usinas hidroelétricas, porém o impacto da aplicação desses métodos ainda não é amplamente conhecido. Este trabalho visa desenvolver uma metodologia para avaliar a degradação dos sistemas de resfriamento de UHEs submetidas a tratamentos químicos para o controle da incrustação por mexilhão-dourado. Neste trabalho é usado, como caso exemplo, o sistema de resfriamento do ar do núcleo do gerador e do óleo dos mancais de uma UHE com turbina Kaplan de potência aproximada de 150 MW. A análise proposta baseia-se na aplicação de ensaios acelerados de corrosão, a fim de determinar a taxa de corrosão nas condições normais de operação. Os ensaios executados neste estudo simulam condições operacionais mais severas do que as usualmente enfrentadas pelo sistema, visando reduzir o tempo de execução dos ensaios. Os ensaios acelerados de corrosão baseiam-se no aumento da solicitação térmica e da concentração das substâncias químicas injetadas no fluxo de água que são a adição de gás ozônio e de hipoclorito de sódio. Para realização dos ensaios foi construído um circuito experimental capaz de impor condições de temperatura e concentrações das substâncias químicas. Os resultados dos ensaios acelerados fornecem subsídios para aplicação dos conceitos de confiabilidade estrutural para determinação da probabilidade de falha dos equipamentos em estudo em função do tempo de aplicação do tratamento químico. Verificou-se entre os produtos químicos utilizados neste estudo, que o hipoclorito de sódio é 50% mais agressivo para a liga de cobre níquel e 700% mais agressivo para o aço inoxidável em relação a degradação observada pela ação do ozônio.
The golden mussel (Limnoperna fortunei) is an invader organism that has caused great damage to water catchment systems and hydroelectric power plants. There are various methods of fouling control of these organisms in hydroelectric power plants pipelines, but the impact of the application of these methods is not yet widely known. This work aims at developing a methodology to evaluate the degradation of hydroelectric power plants cooling systems pieces of equipment subjected to chemical treatments for control of golden mussel fouling. This work uses as a case example, the cooling system inside generator and oil bearing of Kaplan hydroelectric turbine with 150MW nominal power output. The analysis is based on the application of accelerated corrosion tests in order to determine the rate of corrosion under normal operation conditions given the rates of corrosion under various accelerated conditions. The tests run on this thesis simulate operating conditions more severe than those usually experienced by the system, to reduce the runtime of the tests. The proposed accelerated corrosion tests are based on increased thermal loading and concentration of chemical substances injected into water flow. The chemical treatments tested in this work are the addition of ozone and sodium hypochlorite. For carrying out the tests an experimental circuit capable of imposing different conditions of temperature and concentration levels was built. The results of accelerated tests provide subsidies for the implementation of structural reliability concepts for determining the failure probability of equipment under consideration. Among the chemical products used in the analysis, the sodium hypochlorite is 50% more aggressive for copper-nickel 90/10 alloy and 700% more aggressive for stainless steel in relation to degradation observed for ozone.

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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Heat exchanger tubes are commonly made from stainless steel.In a heat exchanger both the process fluid and the cooling orheating media can be corrosive. It is therefore important to beable to select materials that do not suffer from corrosion.Current methods in this area, however, suffer from limitationsand shortcomings. This thesis concerns corrosion tests forgeneral corrosion, dewpoint corrosion and stress corrosioncracking, respectively. For evaluation of general corrosion ofsuperduplex stainless steels in hydrochloric acid and sulphuricacid the importance of activation was studied. The results showthat activation has a great influence on the test result.Further the results indicate that experimental differences canbe the explanation for previously reported differences incorrosion resistance of superduplex stainless steel that havebeen attributed to the alloying with Cu and W. Furthermore, asimple test loop for testing stainless steels under dew formingconditions with a formed condensate of 1 % hydrochloric acidhas been developed. In the work constant strain and constantload test result have been compared and the observationsindicate that the differences can be explained by differencesin the relaxation properties of the materials.

Key words:corrosion testing, heat exchanger, stainlesssteel, general corrosion, immersion tests, activation, stresscorrosion cracking, constant load, u-bends, relaxation,dewpoint corrosion

Les échangeurs de chaleur sont utilisés dans de nombreux secteurs car ils représentent des composants indispensables au déroulement de procédés industriels. Les divers projets de développement d’échangeurs innovants passent nécessairement par une étude matériau permettant de fixer la nuance d’alliage, le design et le procédé de conception optimum. C’est dans ce cadre que la présente étude « matériau » s’est déroulée à travers un projet institutionnel porté par Five’s Cryogénics et soutenu par la région Grand Est. L’étude physico-chimique visait à évaluer la résistance à la corrosion aqueuse en milieux chlorurés de nouveaux échangeurs construits en aciers inoxydables et assemblés par un procédé de brasage. L’objectif était d’acquérir la meilleure connaissance possible des processus élémentaires de corrosion qui pourraient affecter les dispositifs envisagés. L’objectif de la première étape de ce travail était de caractériser la microstructure des aciers et de la brasure avant et après traitement thermique (TH). Les résultats obtenus ont permis d’atteindre les objectifs visés et d’établir les évolutions structurales en surface et à cœur suite au TH mis en œuvre. Ils ont notamment montré que l’acier 316LTi est moins sujet au grossissement granulaire en température que le 316L. De plus, ils ont souligné la persistance de controverses au sujet de la stabilité de certaines phases intermétalliques (Cr6Ni16Si7) qui nuisent à l’établissement de modèles thermodynamiques fiables. L’objectif de la deuxième étape était de concentrer les investigations sur la réactivité chimique en milieux aqueux chlorurés des différentes phases intermétalliques identifiées au cours de l’étude microstructurale. Les mesures électrochimiques ont permis de caractériser leur résistance à la corrosion et conduit aux résultats suivants : i) les deux aciers présentent des vitesses de « corrosion généralisée » du même ordre de grandeur mais ce processus de corrosion n’est pas « impactant », ii) ils sont sensibles à la corrosion par piqûres, iii) ils sont passivables et la couche passive produite à haute température est particulièrement protectrice, iv) les phases constitutives de la brasure (avant TH) et du cordon brasé (post TH) sont cathodiques par rapport aux deux aciers et v) peuvent les protéger en jouant le rôle d’anodes solubles. L’objectif de la dernière étape était d’évaluer le comportement de différents assemblages suite à des immersions de plusieurs milliers d’heures
Heat exchangers are used in many sectors because they represent essential components for the development of industrial processes. Researches focusing on the development of innovative exchangers necessarily require a metallurgical study to determine the optimal alloy composition and design process. It is within this framework that the present study took place through an institutional project financially supported by Five's Cryogénics and the "Grand Est" region. The physicochemical study was undertaken to evaluate the resistance to aqueous corrosion in chloride media of new exchangers made of stainless steel of high purity and assembled by a brazing process performed at 1200°C. The objective was to acquire the best knowledge as possible of the fundamental corrosion processes that could affect the devices envisaged for specific service conditions. The objective of the first step was to characterize the microstructure of steels and brazing alloy before and after heat treatment (TH). The obtained results led to the determination of the microstructural transformation during heat treatment (HT). In particular, they showed that 316LTi steel is less subject to granular coarsening than 316L stainless steel. In addition, they evidenced the existence of Cr6Ni16Si7 remains controversial and that hinder the establishment of reliable thermodynamic models. The objective of the second step was to concentrate the investigations on the chemical reactivity in chlorinated aqueous media of the different intermetallic phases identified during the microstructural study. The electrochemical measurements have made it possible to characterize their resistance to corrosion and led to the following major results: i) the two stainless steels are susceptible to pitting corrosion and 316LTi substrate show higher performance that 316L alloy at 25 ° C but both exhibit identical performance at 60 ° C, ii) they are passivable and the passive layer produced at high temperature is particularly protective, iii ) the constitutive phases of the brazing alloy (before TH) and those appearing after heat treatment (post TH) are cathodic compared to both stainless steels and iv) they can protect them by playing the role of soluble anodes. The objective of the last step was to evaluate the behavior of different assemblies through immersion tests of several thousand hours. The obtained re.sults clearly show that after 2000 hours the tested devices are resistant to corrosion provided

Extruded Al-Mn alloy are used in heat exchanger applications due to their light weight and good thermal conductivity. Depending on the application, the units may be subjected to external corrosion, which can lead to perforation of the tube. The industrial test most commonly used to assess heat exchanger alloys is the seawater acetic acid test (SWAAT). This is a cyclic fog at 40°C and pH 2.9. In the present study, it was found that pits developing in extruded Al-Mn tubes during the SWAAT test are purely crystallographic. Furthermore, a mechanistic understanding for crystallographic pitting has been developed. The SWAAT test can be of relatively long duration and, typically, does not yield information on the underlying corrosion initiation and propagation mechanisms. In the present study, alternate methods to assess pitting corrosion were elaborated. A drop testing procedure has been successfully implemented to study the mechanism of pit initiation. It was revealed that pits initiated within the aluminium matrix in the vicinity of grain boundaries. A close link between large second-phase particles and pit initiation was established. No preferred grain orientation for pit initiation was evident. Scanning electron microscopy and associated tomography were undertaken for the first time to clarify the mechanism of pit propagation. The pit walls were oriented {100}, while the fast-dissolving planes were {110} and {111}. The findings were in accordance with previous literature. Corrosion penetrated deeper into the alloy when the corrosion front was close to a grain boundary. Pit walls were cathodic to the aluminium matrix, possibly due to enrichment of alloying elements at pit walls. The effect of alloy additions on the corrosion behaviour of extruded aluminium alloys was investigated. Alloys with varying copper, iron and manganese contents were compared. Shot noise analysis and post-mortem analyses were undertaken. The increased amount of manganese in solid solution delayed the transition from micropits to stable pitting. This delay is attributable to second-phase particles that are less cathodic to the aluminium matrix in alloys with increased manganese content. Increasing copper decreased the size of the dissolved polyhedra during stable pitting. Furthermore, pits propagated faster in alloys rich in copper. This could be attributed to an increased level of copper enrichment at the pit walls. Finally, more second-phase particles were present in alloys with increased iron levels. Additionally, pits located in those alloys propagated deeper than pits located in alloys with low levels of iron. A competition between two different types of cathodes, enrichment layer and second-phase particles, is suggested. In conclusion, the effect of microstructure and alloy additions on the corrosion mechanism for crystallographic pitting developed during the project was clarified.

Power generation through decentralized small scale CHP would facilitate the use of biomass as an energy source, with the externally fired gas turbine (EFGT) being a promising technology due to its high electrical efficiency. In an EFGT hot flue gases are heat-exchanged with an air cycle, driving the turbine. The operation requires higher flue gas temperatures than other technologies, for example steam turbines, to achieve optimal performance.  The operating conditions subjects the high temperature heat exchanger (HT-HE) to both physical and chemical stress, with the corrosion related issues yet to be solved. Problems concerning deposit formation and corrosion, on for example super heaters and heat exchangers, when firing biomass are important issues even in commercially available technologies, where the choice of fuel and fuel additives together with component design and choice of material plays important roles in order to minimize the problems. The significantly higher temperatures of the heat transferring surfaces for an EFGT entails combustion deposit related problems less studied. The evaluation of turbine control, deposit formation and corrosion as well as design of the HT-HE and system integration will enable the development of the EFGT technology for applications with small- and medium-size biomass combustion. In this work four potential HT-HE alloys of various grades have been evaluated with respect to corrosion resistance, when exposed to alkali salts and salt mixtures in the KCl-K2CO3-K2SO4 system. The exposures were done in a tube furnace during 24 h for each experiment at four temperature levels between 700–1000oC. Morphological and elemental analysis of the alloy surface and corrosion layers was performed with SEM-EDS. The presence of KCl in the salt caused the most severe corrosion attacks while the corrosion attacks of the pure sulfate and carbonate were more modest. Significant differences between the four materials were observed. X20 experienced severe corrosion, with corrosion scale formation in most cases. The KCl-containing salts caused 253MA to form corrosion scales at all temperatures, while the corrosion resistance to other salts was fairly good. Inconel 600 had the second best overall corrosion resistance. However, it should be pointed out that in some cases the alloy was surpassed by 253MA. Kanthal showed the best overall performance, with limited corrosion scale formation and surprisingly high corrosion resistance to the KCl-containing ternary salt mixture at 900°C and 1000°C.

Cette étude concerne un réchauffeur électrique de fluides corrosifs conçu à partir d'un échangeur de chaleur en graphite, à courants croisés. Les premiers essais ont montré une mauvaise répartition de la température du bloc d'échange et du fluide en sortie d'appareil. Nous avons donc étudié les éléments chauffants électriques, le bloc d'échange et la répartition du fluide dans celui-ci. Les caractéristiques des éléments chauffants utilisés ne pouvant pas être obtenues expérimentalement, nous les avons déterminées en utilisant une modélisation que nous avons validée sur d'autres éléments dont les résultats expérimentaux étaient connus. L'étude du bloc d'échange a révélé l'existence d'une limite supérieure du rapport des vitesses maximale et minimale dans l'appareil. Nous avons recherché la valeur de ce rapport en étudiant expérimentalement et par modélisation la répartition du débit. La valeur trouvée étant trop élevée nous avons cherché à homogénéiser l'écoulement du fluide dans l'appareil. Nous avons obtenu expérimentalement de bons résultats en plaçant un obstacle dans la boite à eau. Nous avons proposé une loi représentative de la répartition du fluide qui, sur la base d'hypothèses simplificatrices, nous permet de déterminer l'obstacle qui permet une utilisation optimale de l'appareil. Nous avons montré que l'utilisation d'un logiciel permet également de bonnes prévisions. Nous avons modifié notre prototype et nous avons repris nos essais avec succès. Les résultats obtenus pourraient être extrapolés à des réchauffeurs de fluides ordinaires dont le bloc d'échange serait en un matériau moins noble et donc moins couteux
This report deals with the study of an electrical heater for corrosive fluids that is built from a crossflow graphite heat exchanger. The first tests have shown a maldistribution of the temperature in the heater core and in the fluid in the outlet header. We have consequently studied the heating elements, the heater core and the fluid distribution. As we cannot know experimentally the thermal characteristics of the heating elements that have been used, we have found them by using a simulation based on well-known heating elements. It results from the study of the heater core that the ratio of the highest speed to the smallest speed is a fundamental parameter and must stay under an upper limit to keep the heater in good working order. We have determined the actual value of this ratio experimentally and by using a software. As we have found this value up the upper limit, we have looked for a way to homogenize the fluid flow in the core of the heater. We have experimentally reached good results by placing a porous baffle in the plenum chamber of the inlet header. We have suggested a law for the fluid distribution that helps, with simple hypothesis, to determine the porosity of the baffle that leads to an optimal use of the heater. We have also shown that a software can also correctly predict the improvement due to the porous baffle. We have modified our prototype and carried on new tests with success. The results that have been obtained could be extrapolated to « normal fluids » heater built around a core that could be in a less expensive material

M. Tech. Mechanical Engineering.
Discusses the economiser design to absorb as much heat as possible within the flue gases. Fly ash particles, a product of combustion, entrained in the flue gas of the furnace part of the boiler in coal fired boilers, affect economiser thermal performance by causing erosive wear and scale on the outer surface of the economiser tubes along the flow path, which in turn increases the thermal resistance characterisation of coals in relation to combustion behaviour traditionally relies on staged quenching of the reaction and subsequent gravimetric analysis of the remaining sample. Three typical steam-raising coals are compared with regard to reactivity and broadly examined relative to their petro-graphic constituents and other standard laboratory tested samples. A significant correlation was found between the ignition temperature and the hydrogen or carbon ratio determined by the ultimate analysis, inter alia, the erosion rate and the heat transfer rate.

Improving the efficiencies of thermal power plants requires an increase of the operating temperatures and thus of the corrosion resistance of heat exchanger materials. Therefore, the present study aimed at developing protective coatings using the pack cementation process. Two types of heat exchanger steels were investigated: a 17%Cr/13%Ni austenitic steel and three ferritic-martensitic steels with 9 (P91 and P92) and 12 %Cr (HCM12A). The austenitic steel was successfully aluminised at 950 °C. For the ferritic-martensitic steels, the pack cementation temperature was decreased down to 650 °C, in order to maintain their initial microstructure. Two types of aluminides, made of Fe2Al5 and FeAl, were developed. A mechanism of the coating formation at low temperature is proposed. Furthermore, combining the pack cementation with the conventional heat treatment of P91 allowed to take benefit of higher temperatures for the deposition of a two-step Cr+Al coating. The corrosion resistance of coated and uncoated steels is compared in simulated coal firing environment for durations up to 2000 h between 650 and 700 °C. It is shown that the coatings offer a significant corrosion protection and, thus, an increase of the component lifetime. Finally, the performance of coated 9-12 % Cr steels is no longer limited by corrosion but by interdiffusion between the coating and the substrate.