Evaporator frosting in refrigerating appliances: Fundamentals and applications Christian J. L. Hermes, Joel Boeng, Diogo L. da Silva, Fernando T. Knabben, Andrew D. Sommers Energies, 2021 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.
Cooling kinetics and mass transfer in postharvest preservation of fresh fruits and vegetables under refrigerated conditions T. Hoffmann, A. Ronzoni, D. Silva, S. Bertoli, C. K. Souza Chemical Engineering Transactions, 2021 Fruits and vegetables are fresh products that are highly perishable and refrigeration is widely applied to extend food shelf-life under postharvest conditions. The main phenomena associated with the refrigeration process are heat and mass transfer, which directly influence food decay. For this reason, the objective of this research is to evaluate the thermal and mass profiles of fresh strawberry and lettuce, under refrigeration. Strawberry and lettuce samples were harvested from a hydroponic system and stored at 5 ± 1 °C for 5 days under low relative humidity conditions (50-60 %). In order to correlate the two main phenomena, the mass transfer and cooling kinetics parameters were quantified. The mass loss results demonstrated a greater reduction for lettuce (21.7 %) than for strawberry (16.7 %) samples, which is related to the larger surface to mass ratio of lettuce. The strawberry transpiration rate presented a stable behavior after the first day of storage (1.31 g kg-1 h-1), which provides a linear reduction in the strawberry mass, while lettuce had a higher transpiration rate at the beginning (4.25 g kg-1 h-1) and showed a gradual reduction during cold storage until reaching 1.81 g kg-1 h-1. Water loss in food occurs through evaporative heat from respiration and a reduction in the water content leads to an increase in the internal food temperature. A gradual increase in the food temperatures was observed for both lettuce and strawberry (by 0.5 °C and 0.1 °C, respectively) during storage due to vegetable physiology. Based on the thermal history, a faster thermal response was observed for lettuce. Also, the cooling rate was higher for lettuce (8.7 °C h-1) than for strawberry (6.9 °C h-1) and the half-cooling times were 0.2 h and 0.3 h for the lettuce and strawberry samples, respectively. These findings aid a better understanding of postharvest food behavior and could lead to novel preservation technologies.
