4772 - Implications of Coil Frosting on System Designs for Low-Temperature Applications PDF

Air relative humidity (RH) in a refrigerated facility is determined by the balance between moisture entry (principally through doors) and moisture removal (principally as frost on the air-cooling coil). It has been shown that coil frosting is “unfavorable,” leading to very rapid decline in performance, if the air becomes super-saturated as it is cooled. The conditions for the transition to unfavorable frost formation can be predicted from coil design information including air-on temperature and RH, evaporation temperature, and heat load sensible heat ratio (SHR). Using advanced models for air infiltration through doors plus standard models for other heat loads, and assuming that air cooling follows a straight line approach from the air-on condition to the saturation at the coil surface temperature, the load SHR, coil SHR, and, hence, the balanced air RH were estimated for a typical refrigerated facility with a warehouse, environmental loading area (ELA), and blast freezer. It was shown that unfavorable frost formation is likely to occur in low-temperature facilities even if ambient conditions are not that extreme and moisture entry is reduced by doorway protection. Unfavorable frosting can be best avoided by a combination of improved coil design (e.g., smaller air to refrigerant temperature difference, TD) and enhanced door protection (especially doors opening to the ambient), but occasionally active dehumidification or special coil design (e.g., staggered fin spacing) must be used to avoid frosting problems.

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