Summary

Household refrigerators are labeled as frost-free when frost forms on the evaporator coil only, from where it is periodically removed by the action of electrical heaters. However, when certain environmental and operating conditions take place, frost can also be formed on the cabinet inner liners, increasing the field call rates and consequently the maintenance and quality costs. In order to avoid this inconvenience, manufacturers have been trying to improve their refrigerator designs, but that increases costs and penalizes the energy performance. Albeit several distinguishing publications in the field of frost formation can be found in the open literature, none of them actually focused on the typical working and geometric conditions of household refrigerators and freezers. Therefore, this dissertation aimed to devise a methodology for analyzing the frost formation on the inner liners of frost-free refrigerators focused on the most influencing factors that drive the frost nucleation, growth and densification processes, such as psychrometric (temperature and humidity), flow (velocity), and surface conditions (roughness and contact angle). In order to accomplish that, a refrigerator was tested inside an environmental air temperature and humidity controlled chamber using an automatic door-opening device. In addition, samples of different liner materials were extracted from the refrigerated cabinet in order to have their contact angles and surface roughness measured. These samples were tested using a purpose-built testing facility for measuring both the mass and thickness of the frost deposited over a flat horizontal surface. The results were used to devise a criterion that establishes whether the frost nucleation would start. In addition, a mathematical model based on the mass and energy conservation principles, was developed to predict the mass and thickness of the frost layer over time. The model was validated against experimental data, showing deviations for the predicted frost thickness within a ±10% error band. Although the methodology was applied to a specific household refrigerator, it can be easily extended to any kind of refrigerated room subject to frost formation.

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