A feedwater heater is a typical component in power plants which increases the cycle efficiency. Over the last decade, renewable energies have significantly developed and been employed in the power grid. However, weather conditions are inconsistent and therefore produce variable power. Fossil fuel power stations are often required to supplement the variable renewable energies, which increased the rate of power cycling to an unforeseeable extent over the past decade. Power cycling results in changes in the flow rate, pressure, and temperature of a feedwater heater’s inlet flows. In a tubesheet-type feedwater heater, these transients induce cycling stress in the tubesheet and failures due to thermal fatigue occur. The header-type feedwater is currently employed in high pressure applications as it is more resistant to thermal fatigue compared to the tubesheet-type. However, the tubesheet-type is more cost effective to construct and maintain. It would be advantageous if the cyclic thermal stresses in the tubesheet can be better analysed and alleviated to support the use of the tubesheet-type. A detailed transient temperature distribution of the tubesheet is required to understand the thermal fatigue. Normally, engineers opt towards a full CFD to obtain such results. However, the size and complexity of a feedwater heater is immense and cannot be simulated practically solely using CFD spatial elements. This study developed a multiscale approach that thermally couples 1D network elements, CFD spatial elements, and macroscopic heat transfer correlations to reduce the computational expense substantially. The combination of the various selected techniques and the specific application of this methodology is unique. This approach is capable of obtaining the detailed transient temperature distribution of the tubesheet in a reasonable time, as well as include the effects of the upstream and downstream components within the network model. The methodology was implemented using Flownex and Ansys Fluent for the 1D network and CFD solvers, respectively. The internal tube flow was modelled using 1D network elements, while the steam was modelled with CFD. Thermal discretisation, mapping, and convergence were considered to create a robust methodology not limited to feedwater heaters only. Additionally, a method was developed to analyse flow maldistribution in tube-bundles using the coupled 1D-3D approach. The implementation of the methodology consists of two parts, of which one is for development purposes, and the other serves as a demonstration. The development was done on a simple TEMA-FU heat exchanger which is representative of a feedwater heater. The methodology was tested by varying the primary fluid’s flow rates, changing the fluid media, and conducting transient simulations. The temperature distributions obtained were compared against a full CFD model and corresponded very well with errors less than 4%. A reduction in computational time of more than 40% was achieved but is highly dependent on the specific problem. Improvements to be made in future studies include the accuracy of the laminar case method and the stability of the flow maldistribution algorithm. The methodology was demonstrated by applying it to an existing industrial feedwater heater. No plant data was available to use for input conditions and therefore were assumed. The steam in the DSH was modelled using 3D CFD elements and the tube flow with 1D network elements. The condensing zone’s heat transfer was approximated using an empirical correlation. A steady state case was simulated and the outlet temperatures corresponded well with the manufacturer’s data. The temperature distribution of the tubesheet and surrounding solids were obtained. Finally, assumed sinusoidal transient perturbations to the inlet conditions were imposed. It was evident that the thermal gradients of both sides of the tubesheet were misaligned which highlights the thermal lag and inertia that cause differential temperatures. The 1D-CFD methodology was developed successfully with results that proved to correspond well, for a wide range of conditions, to full CFD. The methodology was applied and can be, in future work, validated with experimental results or extended by modelling upstream and downstream components in the network solver. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2019. / Mechanical and Aeronautical Engineering / MEng (Mechanical Engineering) / Unrestricted

This master thesis focuses on the issue of tightness evaluation of gasketed tubesheet in U tube exchangers, which meets strength requirements of ČSN EN 13 445-3 and ČSN EN 1591-1 standards. It deals with the forces acting on the gasket when measuring the effects acting on the entire heat exchanger. A separate part is devoted to the calculation of the heat profile in the heat exchanger and verification of the strength of the heat exchanger. The final part introduces model of a heat exchanger, on which a possible approach to solving the tightness of the tubesheet in heat exchangers is presented. In thesis temperature field evaluation was conducted, the results of which were used in stress strain analysis, in order to obtain gasket contact pressure. Extreme gasket contact pressures were used to estimate leak rate limits.

Nos últimos anos houve, no Brasil, um significativo investimento em usinas de álcool para suprir a demanda. Todavia, surgiram problemas de natureza econômica e ambiental, já que a tecnologia atualmente utilizada nas destilarias de álcool produz em média 13 litros de vinhaça para cada litro de álcool. Na busca de uma solução para reduzir o volume deste resíduo, desenvolveu-se um equipamento para concentração de vinhaça, denominado Ecovin, pela empresa Citrotec localizada em Araraquara-SP. Para cada Ecovin são utilizados cerca de 4000 tubos com costura nos trocadores de calor, e para reduzir seu custo de fabricação, avaliou-se como opção o uso de tubos da liga AISI 444. Inserido neste contexto, o presente trabalho procurou caracterizar os mecanismos de corrosão e mostrar de forma comparativa o desempenho dos tubos nas juntas tubo-espelho. Estudo-se, portanto, dois tipos de junta, a do projeto atual, composta por chapa e tubo da liga AISI 316L; e a nova proposta, composta por chapa da liga AISI 316L e tubo da liga AISI 444. Para tanto, iniciou-se com a avaliação da soldabilidade das ligas estudadas, através da caracterização microestrutural, qualificação do procedimento de soldagem da junta tubo-espelho e de ensaios de sensitização nas soldas. Em seguida, ensaios de perda de massa por imersão foram realizados nas soluções de 0,5 M \'H IND.2\'SO IND.4\' e 0,5 M \'H\'CL\', nas temperaturas de 30°C, 50°C, 70°C e 90°C, de acordo com o intervalo de temperatura de operação do Ecovin. Nas mesmas soluções, e na temperatura ambiente, foram realizados ensaios eletroquímicos de polarização potenciodinâmica em amostras que reproduziram o ciclo térmico de soldagem das juntas tubo-espelho estudadas, nas regiões do metal de base, zona afetada pelo calor (ZAC) e no metal de solda. Na junta tubo-espelho a corrosão ocorreu preferencialmente na ZAC formada entre a interface tubo-metal de solda, e no cordão de solda à margem do tubo, com a atuação de mecanismos de corrosão generalizada e localizada. Na junta tubo-espelho dissimilar observou-se que o processo corrosivo foi predominante na superfície do tubo AISI 444, típico do mecanismo de corrosão galvânica, onde o tubo AISI 444 caracterizou-se como a região anódica. O desempenho da liga AISI 316L, assim como a junta soldada composta somente por essa liga, apresentou um melhor desempenho em corrosão, porém, na solução contendo cloreto, a variação da temperatura exerceu uma influência proporcional para ambos os casos avaliados nesse estudo. Destaca-se ainda que a liga AISI 316L sofreu corrosão por pite na ZAC e no metal de solda, em ambas as soluções, com maior severidade do que a liga AISI 444. Os resultados obtidos indicam que para a aplicação requerida, o tubo AISI 444 pode ser utilizado na temperatura de 50°C com satisfatório desempenho e similar à junta composta somente por AISI 316L. / Recently in Brazil there has been constant investments in ethanol plants, to supply the internal and external market, resulting at a high increase of the volume of this product. However, economical and environmental problems arose because of this new demand, whereas the current technologies produce thirteen liters of vinasse for each liter of alcohol. Each ECOVIN uses nearly 4000 welded tubes in the heat exchangers, and there is an option to use welded AISI 444 alloy tubes in order to reduce the manufacture costs of the ECOVIN. In this context, this work determines the corrosion mechanisms and analyzes the performance of welded tubes and the tube-to-tubesheet welded joints. The following types of tube-to-tubesheet were evaluated: current welded joints of this project, with all components manufactured with AISI 316L alloy; and the welded joint suggested for the project, using the welded tubes of AISI 444 alloy and the plate of AISI 316L alloy. For this purpose, the first step was to evaluate the weldability of the studied alloys, through microstructural characterization, welding procedure qualification and the intergranular corrosion test. After that, mass loss tests were conducted in 0,5 M \'H IND.2\'SO IND.4\' and 0,5 M \'H\'CL\' solutions, at 30°C, 50°C, 70°C and 90°C, according to the temperature range of the equipment operating. Electrochemical polarization tests were made in the same solution concentration used in the mass loss tests, but only in room temperature. These tests were made in samples that were welded with the same thermal cycle of the tube-to-tubesheet welded joints, and the evaluations were made on the base metal, heat affected zones and weld metal. The uniform and localized corrosion process occurred preferentially in the interface formed between tube and weld metal, and on weld metal near the tubes. In the tube-to-tubesheet dissimilar welded joints, it was observed that the corrosion process was predominant on the surface of AISI 444 alloy, probably because of the galvanic corrosion process, where the AISI 444 was an anodic region. The best performance on corrosion process was observed in AISI 316L, both the base metal and the similar welded joints. However, for the tests in chloride environment, in this process, the temperature caused a proportional influence in the corrosion rate of AISI 316L and AISI 444. In the AISI 316L alloy it was observed nucleation of pitting in HAZ and in weld metal, in both solutions, and more aggressive than observed in the AISI 444 alloy. The results showed that the AISI 444 alloy can be applied for the initial ranging temperatures of the equipments operation, with satisfactory performance.

Nos últimos anos houve, no Brasil, um significativo investimento em usinas de álcool para suprir a demanda. Todavia, surgiram problemas de natureza econômica e ambiental, já que a tecnologia atualmente utilizada nas destilarias de álcool produz em média 13 litros de vinhaça para cada litro de álcool. Na busca de uma solução para reduzir o volume deste resíduo, desenvolveu-se um equipamento para concentração de vinhaça, denominado Ecovin, pela empresa Citrotec localizada em Araraquara-SP. Para cada Ecovin são utilizados cerca de 4000 tubos com costura nos trocadores de calor, e para reduzir seu custo de fabricação, avaliou-se como opção o uso de tubos da liga AISI 444. Inserido neste contexto, o presente trabalho procurou caracterizar os mecanismos de corrosão e mostrar de forma comparativa o desempenho dos tubos nas juntas tubo-espelho. Estudo-se, portanto, dois tipos de junta, a do projeto atual, composta por chapa e tubo da liga AISI 316L; e a nova proposta, composta por chapa da liga AISI 316L e tubo da liga AISI 444. Para tanto, iniciou-se com a avaliação da soldabilidade das ligas estudadas, através da caracterização microestrutural, qualificação do procedimento de soldagem da junta tubo-espelho e de ensaios de sensitização nas soldas. Em seguida, ensaios de perda de massa por imersão foram realizados nas soluções de 0,5 M \'H IND.2\'SO IND.4\' e 0,5 M \'H\'CL\', nas temperaturas de 30°C, 50°C, 70°C e 90°C, de acordo com o intervalo de temperatura de operação do Ecovin. Nas mesmas soluções, e na temperatura ambiente, foram realizados ensaios eletroquímicos de polarização potenciodinâmica em amostras que reproduziram o ciclo térmico de soldagem das juntas tubo-espelho estudadas, nas regiões do metal de base, zona afetada pelo calor (ZAC) e no metal de solda. Na junta tubo-espelho a corrosão ocorreu preferencialmente na ZAC formada entre a interface tubo-metal de solda, e no cordão de solda à margem do tubo, com a atuação de mecanismos de corrosão generalizada e localizada. Na junta tubo-espelho dissimilar observou-se que o processo corrosivo foi predominante na superfície do tubo AISI 444, típico do mecanismo de corrosão galvânica, onde o tubo AISI 444 caracterizou-se como a região anódica. O desempenho da liga AISI 316L, assim como a junta soldada composta somente por essa liga, apresentou um melhor desempenho em corrosão, porém, na solução contendo cloreto, a variação da temperatura exerceu uma influência proporcional para ambos os casos avaliados nesse estudo. Destaca-se ainda que a liga AISI 316L sofreu corrosão por pite na ZAC e no metal de solda, em ambas as soluções, com maior severidade do que a liga AISI 444. Os resultados obtidos indicam que para a aplicação requerida, o tubo AISI 444 pode ser utilizado na temperatura de 50°C com satisfatório desempenho e similar à junta composta somente por AISI 316L. / Recently in Brazil there has been constant investments in ethanol plants, to supply the internal and external market, resulting at a high increase of the volume of this product. However, economical and environmental problems arose because of this new demand, whereas the current technologies produce thirteen liters of vinasse for each liter of alcohol. Each ECOVIN uses nearly 4000 welded tubes in the heat exchangers, and there is an option to use welded AISI 444 alloy tubes in order to reduce the manufacture costs of the ECOVIN. In this context, this work determines the corrosion mechanisms and analyzes the performance of welded tubes and the tube-to-tubesheet welded joints. The following types of tube-to-tubesheet were evaluated: current welded joints of this project, with all components manufactured with AISI 316L alloy; and the welded joint suggested for the project, using the welded tubes of AISI 444 alloy and the plate of AISI 316L alloy. For this purpose, the first step was to evaluate the weldability of the studied alloys, through microstructural characterization, welding procedure qualification and the intergranular corrosion test. After that, mass loss tests were conducted in 0,5 M \'H IND.2\'SO IND.4\' and 0,5 M \'H\'CL\' solutions, at 30°C, 50°C, 70°C and 90°C, according to the temperature range of the equipment operating. Electrochemical polarization tests were made in the same solution concentration used in the mass loss tests, but only in room temperature. These tests were made in samples that were welded with the same thermal cycle of the tube-to-tubesheet welded joints, and the evaluations were made on the base metal, heat affected zones and weld metal. The uniform and localized corrosion process occurred preferentially in the interface formed between tube and weld metal, and on weld metal near the tubes. In the tube-to-tubesheet dissimilar welded joints, it was observed that the corrosion process was predominant on the surface of AISI 444 alloy, probably because of the galvanic corrosion process, where the AISI 444 was an anodic region. The best performance on corrosion process was observed in AISI 316L, both the base metal and the similar welded joints. However, for the tests in chloride environment, in this process, the temperature caused a proportional influence in the corrosion rate of AISI 316L and AISI 444. In the AISI 316L alloy it was observed nucleation of pitting in HAZ and in weld metal, in both solutions, and more aggressive than observed in the AISI 444 alloy. The results showed that the AISI 444 alloy can be applied for the initial ranging temperatures of the equipments operation, with satisfactory performance.

Diploma thesis is fucused on strength calculation of the heat exchanger according to ČSN EN 13 445. Introductory part introduce problems with the design of heat exchangers. In other parts there are a strength calculation of the heat exchanger according to ČSN EN 13 445, the results of the analysis with their appreciation. Annexes included output logs from the program Sant 'Ambrogio, and drawings documentation.

This thesis is focused on the thermal-hydraulic and construction design of heat exchanger with floating head. The introductory part is dedicated to the design of heat exchangers. Next part is focused on the thermal-hydraulic design created in HTRI software (module Xist. The next section is the strength calculation of selected components of the heat exchanger according to EN 13445 in software Sant'Ambrogio. The following part describes each of the analysis in software Ansys Workbench. The final part contains complete manufacturing documentation of heat exchanger.