Low-temperature energy systems are processes that operate below ambient temperatures and make use of refrigeration cycles, where the main energy consumption is due to the shaft work required to drive the compressors. Very-low-temperature energy systems, also known as cryogenic processes, operate at around -150°C and below. Due to increasing demand of products from cryogenic processes and tighter environmental regulations, existing plants need to be revamped to increase their energy efficiency or adapt to new processing capacities. So, accurate models of the performance of cryogenic processes are needed in order to optimise their operation. The present work proposes a new approach for optimising the operating conditions of existing refrigeration cycles in cryogenic processes, using pure refrigerants, for different plant operating conditions. In this work, the process conditions are considered as given and not considered as variables during the optimisation. The operational optimisation is achieved by integrating models for the part-load performance of centrifugal compressors and models for the simulation of plate-fin heat exchangers (PFHEs), into a single optimisation approach. An optimisation approach similar to the one proposed in this work was not found in the open literature. The optimisation approach varies the refrigerant evaporation temperatures, flow rates and cooling duties, minimum temperature difference in PFHEs, and rotational speed of compressors. The objective function seeks to minimise shaft work demand and the constraints consider the operational limitations of centrifugal compressors (minimum and maximum flow rates) and PFHEs (no temperature crosses and meeting the target temperatures of the process streams). In order to explore the solution space that is generated by the complex interactions between the variables and find an approximation to a global optimum, a multistart optimisation algorithm is implemented. The part-load centrifugal compressor model implemented in this work uses regressed data from their performance curves together with the fan laws. The proposed simulation model of PFHEs represents these units as a ‘fictitious’ heat exchanger network of two-stream matches. The simulation model accounts for single and two-phase streams and for the temperature-dependent physical properties of pure refrigerants (e.g. viscosity, heat capacity, etc.). In addition to the simulation model, design and rating models for PFHEs with single and two-phase streams are also proposed. The examples presented in this work for the design, simulation and rating of single and two-phase streams in PFHEs show that the models proposed can find feasible designs, and can predict the outlet temperature of the process streams within ±3°C for different inlet conditions. The example presented in this work for the operational optimisation of refrigeration cycles shows that savings of around 3% in shaft work consumption (up to £0.86 million per year), for different process throughput, can be achieved using the proposed methodology.

Este trabalho tem como objetivo propor e avaliar um novo método para controle antissurto de compressores dinâmicos. É sabido que compressores dinâmicos são sujeitos ao fenômeno de surto, o qual pode danificar gravemente os componentes do compressor e provocar distúrbios de produção. O surto pode surgir a partir da ocorrência de distúrbios (por exemplo, fechamento da válvula de descarga do compressor) os quais, sem ação antissurto, fariam com que o ponto de operação se deslocasse para uma região de baixas vazões delimitada por uma linha conhecida como linha de surto. Portanto, compressores dinâmicos sempre são equipados com mecanismos antissurto: tipicamente uma válvula de reciclo de ação rápida controlada por um controlador PI antissurto. Dado que o surto se desenvolve extremamente rápido, usualmente não se permite que o compressor opere muito próximo da linha de surto. Esse objetivo de controle (manter o compressor afastado da linha de surto) é conflitante com objetivos de eficiência energética, pois os pontos de maior eficiência estão localizados próximos a essa linha. Logo, é desejável operar o compressor utilizando a mínima margem de surto que ainda garanta que a ação antissurto seja efetiva. Este trabalho propõe um método para ativação da ação antissurto no compressor com o objetivo de atingir uma ação mais rápida que o controle PI tradicional. O método proposto se baseia no cálculo offline das aberturas necessárias para a válvula de reciclo para cada possível combinação das posições dos atuadores do sistema, considerando um conjunto discreto pré-definido de valores dentro da faixa de valores de cada atuador. Esse processo gera uma tabela auxiliar para uso online. Os valores da tabela auxiliar são utilizados para gerar valores de referência para um controlador por realimentação, o qual é responsável por garantir que a trajetória do sistema irá do estado inicial no momento da ativação do controle antissurto para o estado estacionário desejado. Diversos cenários de distúrbio são simulados para diferentes controladores por realimentação e comparados com o controle antissurto PI tradicional. Os resultados mostram que a estratégia proposta é um candidato para melhorias na prática atual de controle antissurto, mas um controle por realimentação adequado deve ser selecionado e avaliado também considerando o aspecto de robustez, o qual é brevemente considerado no escopo deste trabalho. / This work aims at proposing and evaluating a novel method for anti-surge control of dynamic compressors. Dynamic compressors are known to suffer from surge, which can severely damage compressor components and disturb production. Surge may arise by the occurrence of disturbances (e.g. compressor discharge valve closure) that would bring its operating point to a region at low flows delimited by the so called surge line. Therefore, dynamic compressors are always equipped with anti-surge mechanisms: typically a fast actuating recycle valve controlled by a PI anti-surge controller. Since surge develops extremely fast, the compressor is usually not allowed to operate too close from the surge line. This control objective (keep the compressor away from the surge line) is conflicting with energy efficiency requirements, since higher efficiency operating points are located close to the surge line. Therefore, it is desirable to operate the compressor using the smallest possible surge margin that still guarantees anti-surge action is effective. This work proposes a method for triggering the compressor anti-surge action, aiming at a faster action than traditional PI control. The proposed anti-surge control method relies on an offline computation of necessary recycle valve openings for each possible combination of the system actuators positions, considering a predefined discrete set of values from the actuators positioning ranges. This generates a look-up table for online use. The values from the look-up table are used to identify the necessary compressor flow set-point for a feedback controller, which is responsible for ensuring that the system trajectory goes from the state upon anti-surge activation to the desired steady state. Several disturbance scenarios are simulated for different feedback controllers and compared to the traditional PI anti-surge controller. Results show that the proposed strategy is a candidate for improvements in current anti-surge control practice but an adequate feedback control strategy must be chosen and evaluated also under the consideration of robustness, which is slightly considered in the scope of this work.

No description available.

Orientador: Luiz Felipe Mendes de Moura / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-28T12:01:47Z (GMT). No. of bitstreams: 1 Nobrega_ClaudiaRosadoEspiritoSanto_M.pdf: 49731796 bytes, checksum: 231227e66649d1f3e29b15b3a64c3913 (MD5) Previous issue date: 2015 / Resumo: O presente trabalho refere-se a uma investigação experimental do comportamento de um sistema de refrigeração utilizando um compressor de rotação variável. Diante da necessidade do uso racional de energia elétrica, estudos entorno da eficiência energética são desenvolvidos como alternativas de redução do desperdiço em equipamentos, como desenvolvimento de dispositivos que necessite de menos consumo de energia sem comprometimento de sua eficiência, e possibilitando a acessibilidade destas alternativas para comunidade em geral. O uso do compressor de rotação variável tem sido estudado como alternativa de redução no consumo de energia por ser responsável por 80% do consumo total de energia consumida pelo sistema de refrigeração, que está cada vez mais presente nas indústrias e residências. Foi implementada uma bancada de teste composta por um subsistema de refrigeração por compressão a vapor e um subsistema secundário de resfriamento. Este estudo visa investigar experimentalmente a potência consumida, potência de compressão, coeficiente de performance (COP) e eficiência do sistema de refrigeração variando-se a carga térmica solicitada utilizando um compressor de rotação variável e o compressor em rotação fixa máxima fornecida pelo fabricante. Os resultados encontrados experimentalmente com a utilização do compressor em rotação variável apresentaram valores satisfatórios para os parâmetros avaliados e um melhor desempenho do sistema se comparado aos valores obtidos com o compressor em rotação fixa variando-se a carga térmica solicitada / Abstract: The present work relates to an experimental investigation of the behavior of a refrigeration system using a variable speed compressor. Faced with the need for rational use of energy, environment studies of energy efficiency are developed as wasting reduction alternatives in equipment such as developing devices that need less power consumption without compromising its efficiency and allowing the accessibility of these alternatives community. The use of variable speed compressor has been studied as alternative reduction in energy consumption to be responsible for 80 % of total energy consumption consumed by the cooling system, which is increasingly present in industry and households. A test bench comprising a subsystem vapor compression refrigeration and secondary cooling subsystem has been implemented. This study aims to experimentally investigate the consumed power, compression power, coefficient of performance (COP) and the refrigeration system efficiency by varying the thermal load applied using a variable speed compressor and the compressor at maximum fixed speed provided by the manufacturer. The results experimentally using the variable speed compressor showed satisfactory values for the parameters evaluated and improved system performance compared to the values obtained with fixed speed compressor in varying the required heat load / Mestrado / Termica e Fluidos / Mestra em Engenharia Mecânica

A one-dimensional, stage-by-stage axial compression system mathematical model has been constructed which can describe system behavior during post-stall events such as surge and rotating stall. The model uses a numerical technique to solve the nonlinear conservation equations of mass, momentum, and energy. Inputs for blade forces and shaft work are provided by a set of quasi-steady stage characteristics modified by a first order lagging equation to simulate dynamic stage characteristics. The model was validated with experimental results for a three-stage, low-speed compressor and a nine-stage, high-pressure compressor. Using these models, a parametric study was conducted to determine the effect of inlet resistance, combustor performance, heat transfer, and stage characteristic changes due to hardware modification on post—stall system behavior. / Ph. D. / incomplete_metadata

The passage shock wave end wall boundary layer interaction in a transonic compressor was investigated with a laser transit anemometer. A two stage transonic compressor designed without inlet guide vanes was used in this flow field investigation. Measurements of the flow velocity were made within the first stage rotor passage of this transonic compressor. Laser measurements were made in two blade passages at six axial locations from 10% of the axial blade chord in front of the leading edge to 30% of the axial blade chord into the blade passage. At three of these axial locations, laser traverses were taken at different radial immersions to investigate the flow behavior near the tip end wall. Twenty-six different locations were traversed circumferentially. The measurements reveal that the end wall boundary layer in this region is separated from the core flow by what appears to be a shear layer where the passage shock wave and all ordered flow seem to end abruptly. / Ph. D. / incomplete_metadata

The objective of this research is the computer simulation of the vibrations of the suspension system of a two-cylinder reciprocating compressor. A theoretical model is developed to describe the various steps undertaken to calculate the response of this six-degree-of-freedom rigid system. The response, which is in the form of a displacement vector, serves as the input to a computer animation of the motion of the orbit of the compressor with respect to the four suspension system springs. The theoretical model is developed by calculating (1) the System Mass and Inertial Matrix, (2) the Gyroscopic Matrix, (3) the Total Assembly Stiffness Matrix, and (4) the Shaking Forces and Moments Matrix. Experimental and finite element methods used to evaluate the parameters required to calculate these matrices are also discussed. An eigenanalysis is performed to calculate the eigenvalue frequencies and eigenvectors for the system. The force analysis is performed to calculate the forcing function in the time domain for the first 40 harmonics. The Fast Fourier Transform method is used to transform the forcing function from the time domain to the frequency domain. The validity of the results are checked by simultaneously developing another model using IMP (Integrated Mechanisms Program). The response is then calculated in original coordinates, after performing a modal transformation. Finally, the response, which is a displacement vector, is utilized by an animation program in PHIGS (Programmer's Hierarchical Interactive Graphics Standard) to animate the motion of the orbit of the compressor. / Master of Science

The objectives of this research are the prediction of the thermodynamic behavior of a multi-stage intercooled reciprocating compressor and its progressive loss of performance due to leakage. A theoretical model is developed to simulate the thermodynamics of the compressor system and the lubricating condition and wear of the piston ring pack for a multi-stage intercooled reciprocating compressor. A first law of thermodynamics approach is used to determine the thermodynamic properties of the gas inside the cylinders, the intercoolers and the inlet and discharge manifold. The compressor valves are modeled as single degree-of-freedom, spring-mass=damper systems. The flows through the valves are calculated based on the steady flow equations for equivalent orifices. The lubricating condition of the piston ring pack are determined on the basis of hydrodynamic lubrication theory. The wear of the piston rings is assumed to occur when the hydrodynamic oil film between the piston ring and cylinder bore breaks down. Based on the theoretical model, a computer program is developed. This program is tested on an Ingersoll-Rand Model 242, two stage aircooled reciprocating air compressor. The comparison of the experimental values of the pressure variations in the first cylinder with the value predicted by the computer program shows a reasonable match. The computer program predicts the pressure, temperature and mass flow rates for each cylinder and the intercooler. Also predicted is the wear rate of each piston ring. The progressive loss in the compressor mass discharge, and hence the loss in its performance, is determined by calculating the leakage losses several times, updating the leakage area each time based on the wear rate of the piston rings. The result shows a drop of about 15 percent in the discharge rate of the Model 242 compressor after 8000 hours of running time. / Master of Science

An experimental investigation of the performance characteristics of a solid wall linear compressor cascade was conducted. The purpose of the experiments was to determine the effects of the blade row configuration parameters stagger and solidity on the pre-and post-stall behavior of the flow in the cascade. Tests were conducted at a solidity of 1.5, and for two stagger angles, 36.4 degrees and 25 degrees. The investigation included the use of high speed motion pictures with smoke flow visualization in the cascade, measurements of the total pressure and velocity of the flow upstream and downstream of the cascade, and measurements of the blade surface pressures. The experiments were conducted for a range of angle of attack from 0 degrees to 45 degrees. To determine the effects of solidity on the pre- and post-stall behavior of the flow in the cascade, the results obtained for the present 1.5 solidity cascade were compared to previous results from the same cascade tested at a solidity of 1.0. The flow in the two cascades was observed to be similar in nature, but the influence of the reduced blade loading in the high solidity cascade was apparent. For the higher solidity cascade, flow losses at low angle of attack were found to be larger, but stalling behavior was delayed. / Master of Science

No description available.