Guilherme Borges Ribeiro
Jader Riso Barbosa Jr.
Date of publication
Currently, technical advances in the area of power electronics have enabled the more frequent use of frequency inverters for compressor speed control in air conditioning refrigerating systems. This reduction in compressor speed, aiming at controlling the cooling capacity, reduces the compression power, increases the evaporating temperature and decreases the condensing temperature, resulting in a higher coefficient of performance (COP) and a smaller fluctuation of the internal ambient temperature. At the same time, in response to environmental restrictions concerning refrigerants with high GWP (Global Warming Potential ), applications using propane (R-290) are becoming more frequent, since R-290 has low environmental impact and thermodynamic advantages when compared to the most used fluids R-410A and R-22. The purpose of this study is to develop a computational model that allows the analysis of variable capacity air conditioning systems via the SEER performance factor. Sub-models were created for each component of the air conditioning system, including the connecting lines. The study also contributed with the evaluation of several aspects that permit the charge reduction of R-290 in air conditioners. With focus on this latter objective, peripheral fins heat exchangers were explored and applied in the model. Also, an analysis of the irreversibilities of the system from the second law of themodynamics point of view, considering several geometrical characteristics of heat exchangers and of the compression mechanism was carried out. In order to make the analysis more realistic, the mixture R-290/POE ISO 22 was considered as working fluid. The thermodynamic properties were obtained from a departure-function approach using the Peng-Robinson cubic equation of state. The oil effects on the heat transfer coefficients and on the pressure drop were also taken into account. In order to validate the proposed model, a air-conditioner calorimeter was constructed and tested under several ambient temperatures and compressor speed. This experimental facility is proposed as an alternative test procedure for standardized air-conditioning tests. To complete the experimental activities, the oil mass flow rate was measured in a compressor evaluation facility adapted for this purpose. These measurements were applied to characterize the compressor pumping oil sub-model. The tests were performed with a high capacity calorimeter using a scroll compressor with same geometrical characteristics simulated in the model, under different operating conditions. Good agreement was observed between the computational model and the experimental results. The increase of oil concentration resulted in a decrease of the cooling capacity and an increase of the system power input. Moreover, it has been seen that the use of peripheral fins allow a reduction of the mixture mass presented in the system, despite the decrease of the SEER factor.
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