Abstract
Modern refrigerators are equipped with fan-supplied evaporators often tailor-made to mitigate the impacts of frost accretion, not only in terms of frost blocking, which depletes the cooling capacity and therefore the refrigerator coefficient of performance (COP), but also to allow optimal defrosting, thereby avoiding the undesired consequences of condensate retention and additional thermal loads. Evaporator design for frosting conditions can be done either empirically through trial-and-error approaches or using simulation models suitable to predict the distribution of the frost mass along the finned coil. Albeit the former is mandatory for robustness verification prior to product approval, it has been advocated that the latter speeds up the design process and reduces the costs of the engineering undertaking. Therefore, this article is aimed at summarizing the required foundations for the design of efficient evaporators and defrosting systems with minimized performance impacts due to frosting. The thermodynamics, and the heat and mass transfer principles involved in the frost nucleation, growth, and densification phenomena are presented. The thermophysical properties of frost, such as density and thermal conductivity, are discussed, and their relationship with refrigeration operating conditions are established. A first-principles model is presented to predict the growth of the frost layer on the evaporator surface as a function of geometric and operating conditions. The relation between the microscopic properties of frost and their macroscopic effects on the evaporator thermo-hydraulic performance is established and confirmed with experimental evidence. Furthermore, different defrost strategies are compared, and the concept of optimal defrost is formulated. Finally, the results are used to analyze the efficiency of the defrost operation based on the net cooling capacity of the refrigeration system for different duty cycles and evaporator geometries.
Highlights
Frost is likely to build up whenever moist air flows over a chilled surface whose temperature is sub-zero and below the local dewpoint of the air stream
A niche of particular interest lies in the evaporators of household and commercial refrigerating appliances, where the performance is dramatically affected by frost accretion, which imposes an additional thermal resistance to the heat flux and diminishes the air flow passage, decreasing the air flow rate for a fixed pumping power
Frost is a porous medium comprised of ice crystals and moist air formed due to a very particular set of surface and psychrometric conditions
Summary
Frost is likely to build up whenever moist air flows over a chilled surface whose temperature is sub-zero and below the local dewpoint of the air stream. According to recent data from the International Institute of Refrigeration [2], refrigeration, air conditioning, and heat pump systems consume about 17% of the electric energy produced worldwide, with the residential sector responsible for nearly half of it. Such a large figure is due to a combination of factors, such as the enormous number of refrigerators in operation (around 1.5 billion), the relatively low efficiency of the refrigeration cycle (approximately 20% of that of a Carnot refrigerator), and operating issues such as door openings and, evaporator frosting [3]. Simulation models and optimization techniques for designing evaporators working under frosting and defrosting conditions are explored
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