Summary

According to the 2007 National Brazilian Energy Consumption Habits Research, the refrigeration and air conditioning equipment are responsible by 50% of the total energy consumption of the residential sector, which correspond to approximately 10% of the Brazilian overall energy production. This figure highlights the importance of undertaking research activities focused on the energy consumption of household refrigerators. There is plenty of evidence in the literature showing that each system requires an optimum refrigerant charge in order to reach a maximum efficiency level. In other words, the energy consumption is not only affected by the system components but also by the refrigerant charge. In this study an experimental apparatus was developed to simultaneously vary the refrigerant charge and the restriction of the expansion device. This apparatus is comprised of a charging device connected to a modified household refrigerator with a larger inner diameter capillary tube, and a step motor actuated needle valve installed in series and before the capillary tube. The charging device is comprised of a refrigerant cylinder, a load cell and two solenoid valves connected to the compressor suction and discharge lines. The combination of needle valve plus capillary tube allows the modulation of the restriction of the expansion device to values lower or higher than that of the original system. A mathematical model to simulate the refrigerant flow through diabatic capillary tubes was developed, validated and used to calculate the internal diameter of a capillary tube which is equivalent in terms of mass flow rate to the restriction imposed by the capillary tube and needle valve combination. Steady-state energy consumption tests were carried out, varying the refrigerant charge and the restriction of the expansion device. Such an approach was adopted to speed up the process without losing the experimental tendency. A total of 95 measurements were made. The energy consumption was plotted in a contour map with the restriction of the expansion device on the x-axis and the refrigerant charge on the y-axis. It was shown that there is a wide region where the energy consumption reaches a minimum, which stretches over a range of expansion device restriction and refrigerant charge. It was also shown that an inappropriate combination of charge and expansion device can increase the energy consumption by up to 30%. Ultimately, an empirical correlation was proposed to estimate the energy consumption of a household refrigerator based on the restriction of the expansion device and refrigerant charge. A new methodology, based on a small amount of experimental data was proposed and validated. A minimization algorithm was also developed and used to identify the point of minimum energy consumption.

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