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Performance and safety of centrifugal chillers using hydrocarbons.Tadros, Amir, The University of New South Wales. School of Mechanical & Manufacturing Engineering, UNSW January 2008 (has links)
The high ozone depletion and global warming potentials of fluorocarbon refrigerants have resulted in prohibitions and restrictions in many markets. Hydrocarbon refrigerants have low environmental impacts and are successfully used in domestic refrigerators and car air conditioners but replacing fluorocarbons in centrifugal chillers for air conditioning applications is unknown. Hydrocarbon replacements need a heat transfer correlation for refrigerant in flooded evaporators and predictions for operating conditions, capacity and performance. Safety precautions for large quantities of hydrocarbon refrigerants are needed and control of overpressure in plantrooms requires accurate prediction. Reliable correlations exist for forced convection in a single phase flow, condensation outside tubes and evaporation off sprayed tubes. For flooded evaporators this thesis proposes a new correlation for forced convection boiling of any refrigerant. An enhancement factor is combined with a modified Chen coefficient using recent pool boiling and forced convection correlations outside tubes. This correlates within typically a factor of two to known boiling literature measurements for CFC-113, CFC-11, HCFC-123, HFC-134a and HC-601. The operating conditions, capacity and performance of replacement hydrocarbons in centrifugal chillers were predicted using fluorocarbon performance as a model. With the new heat transfer correlation hydrocarbon predictions for flooded evaporators were made. For any fluorocarbon refrigerant there exists a replacement mixture of hydrocarbons which with a rotor speed increase about 40% gives the same cooling capacity in the same centrifugal chiller under the same operating conditions. For example replacing HCFC-123 in a flooded evaporator with HC-601/602 [90.4/9.6] and increasing the rotor speed by 43% will increase the coefficient of performance by 4.5% at the same cooling capacity. The maximum plantroom overpressure considered was from leakage and ignition of a uniform air/refrigerant mixture with maximum laminar burning velocity. Flow was modelled using a turbulence viscosity due to Launder and Spalding and turbulent deflagration using a reaction progress variable after Zimont. These partial differential equations were solved approximately for two and three dimensional geometries using finite volume methods from the Fluent program suite. Simple overpressure predictions from maximum flame area approximations agreed with Fluent results within 13.7% promising safe plantroom design without months of computer calculation.
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Performance and safety of centrifugal chillers using hydrocarbons.Tadros, Amir, The University of New South Wales. School of Mechanical & Manufacturing Engineering, UNSW January 2008 (has links)
The high ozone depletion and global warming potentials of fluorocarbon refrigerants have resulted in prohibitions and restrictions in many markets. Hydrocarbon refrigerants have low environmental impacts and are successfully used in domestic refrigerators and car air conditioners but replacing fluorocarbons in centrifugal chillers for air conditioning applications is unknown. Hydrocarbon replacements need a heat transfer correlation for refrigerant in flooded evaporators and predictions for operating conditions, capacity and performance. Safety precautions for large quantities of hydrocarbon refrigerants are needed and control of overpressure in plantrooms requires accurate prediction. Reliable correlations exist for forced convection in a single phase flow, condensation outside tubes and evaporation off sprayed tubes. For flooded evaporators this thesis proposes a new correlation for forced convection boiling of any refrigerant. An enhancement factor is combined with a modified Chen coefficient using recent pool boiling and forced convection correlations outside tubes. This correlates within typically a factor of two to known boiling literature measurements for CFC-113, CFC-11, HCFC-123, HFC-134a and HC-601. The operating conditions, capacity and performance of replacement hydrocarbons in centrifugal chillers were predicted using fluorocarbon performance as a model. With the new heat transfer correlation hydrocarbon predictions for flooded evaporators were made. For any fluorocarbon refrigerant there exists a replacement mixture of hydrocarbons which with a rotor speed increase about 40% gives the same cooling capacity in the same centrifugal chiller under the same operating conditions. For example replacing HCFC-123 in a flooded evaporator with HC-601/602 [90.4/9.6] and increasing the rotor speed by 43% will increase the coefficient of performance by 4.5% at the same cooling capacity. The maximum plantroom overpressure considered was from leakage and ignition of a uniform air/refrigerant mixture with maximum laminar burning velocity. Flow was modelled using a turbulence viscosity due to Launder and Spalding and turbulent deflagration using a reaction progress variable after Zimont. These partial differential equations were solved approximately for two and three dimensional geometries using finite volume methods from the Fluent program suite. Simple overpressure predictions from maximum flame area approximations agreed with Fluent results within 13.7% promising safe plantroom design without months of computer calculation.
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Análise numérica e experimental de sistema de ar condicionado em edifícios verdesPEREIRA, Gutenberg da Silva 22 February 2016 (has links)
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Previous issue date: 2016-02-22 / Com a atual escassez de energia, alta demanda e aumento do custo da energia elétrica,
verifica-se que uma questão importante é a melhoria da eficiência dos sistemas de
climatização dentro de um padrão de eficiência certificado possibilita-se, assim, para uma
comparação real de qual é a melhor opção de layout e operação dos sistemas, já que os
sistemas de climatização são responsáveis por uma alta parcela do consumo energético.
O presente trabalho se propõe a comparar dois sistemas de refrigeração diferentes, usando
chiller de compressão de vapor, para a climatização de ambientes. Foram propostas
diferentes modalidades para a comparação em operações isoladas e ações combinadas.
Essas operações são avaliadas nos conceitos de edifícios verdes. Foi desenvolvido um
modelo matemático baseado nos princípios de conservação de massa e energia,
complementado por várias funções, de modo a determinar as propriedades termofísicas e
eficiências dos compressores. O modelo avalia a influência dos parâmetros operacionais
principais do sistema de refrigeração a água gelada operando em três configurações
diferentes. As equações do modelo foram resolvidas pelo programa EES (Engineering
Equation Solver). Os resultados mostraram que o sistema com compressão diferenciada
apresenta COP igual ao do sistema com chiller parafuso na faixa de 0 a 300 TRs, e um
COP superior em média de 9% na faixa de 400 a 800 TRs. / With the current energy shortages, high demand and rising cost of electricity, it turns out
that an important issue is the improvement of the air conditioning efficiency within a
standard of efficiency certificate for a real comparison of what is the better layout option
and operation of systems, as the HVAC systems account for a high share of energy
consumption. This study aims to compare two different cooling systems using vapor
compression chiller for air conditioning environments. They were different proposals
modalities for comparison in isolated and combined action operations. These transactions
are valued on the concepts of green buildings. A mathematical model was developed
based on the principles of conservation of mass and energy, supplemented with various
functions in order to determine the thermophysical properties and efficiencies of the
compressors. The model evaluates the influence of the main operating parameters of the
cold water cooling system operating in three different configurations. The model
equations were solved by EES program (Engineering Equation Solver). The results
showed that the system presents differentiated compression COP equal to the screw
chiller systems in the range from 0 to 300 RTs and a higher COP (average of 9%) in the
range of 400 to 800 TR.
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