Effect of Hygienic Design and Operational Parameters on Frosting and Defrosting of Evaporators in Refrigerated Food Processing and Storage Facilities

Effect of Hygienic Design and Operational Parameters on Frosting and Defrosting of Evaporators in Refrigerated Food Processing and Storage Facilities

Frank Theo Moerman (Catholic University of Leuven, Belgium) and Kostadin Fikiin (Technical University of Sofia, Bulgaria)
DOI: 10.4018/978-1-4666-8398-3.ch018
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Abstract

Proper control and performance of evaporators in food refrigeration facilities are vital to provide a suitable temperature regime, safety, quality and wholesomeness of refrigerated products at minimum electricity costs. When humid air passes along the surfaces of a low-temperature evaporator, frost is usually formed, which decreases the heat transfer efficiency. Frosting and defrosting phenomena have been extensively investigated for different industrial scenarios and extensive literature exists in the matter. However, no studies have been published so far to address in a comprehensive way the methods and patterns of evaporator defrosting as affected by hygienic design implications and criteria. This book chapter is intended to fill in this gap by enforcing hygienic imperatives in the evaporator design. Various design solutions and conditions of operation are considered as decisive in determining the amount, thickness and structure of the frost build-up. Advantages and drawbacks of diverse defrost methods are outlined with regards to contamination risks in refrigeration facilities.
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Frosting Of Evaporator Coil Surfaces By Ablimation Of Water Vapour

Transfer of Moisture to the Frost Surface (Frost-Air Interface)

Both heat and mass transfer processes occur between the air stream and the deposit interface. As air moves along the evaporator tubes and fins, the air closest to the tube and fin walls behaves differently than the air farther away from the tube and fin surfaces. Air closest to the tube and fin walls tends to move slower than air farther away, setting up a hydraulic boundary layer where heat transfer becomes more difficult. Boundary layers act as an insulator and interfere with heat transfer. Even in turbulent flow, a laminar sublayer forms and heat moves slower through these hydraulic/thermal boundary layers. The air in the hydraulic/thermal boundary layer is cooled by convection to the frost-air interface and dehumidified by diffusion of water vapour across the frost-air interface.

Key Terms in this Chapter

Unfavourable Frost: Snow-like, low density frost, which is more insulating, has a higher impact on air flow rate through the coil, and is more difficult to defrost than more “favourable” frost types.

Hygienic Design and Engineering: Design and engineering of equipment and premises to assure the control of food safety hazards, to maintain product quality and to enhance cleanability.

Ablimation: De-sublimation, water vapour undergoing phase change to ice crystals.

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