Experimental investigations of frosting and defrosting of evaporator coils at freezer temperatures

In the field of industrial refrigeration, the heat input required to defrost coils within freezers is utilized in three parts. The first part will be used to melt the frost, while the second part warms the metal of the coil and its contents. The latter part will be transferred to the refrigerant when the evaporator switches from the defrosting mode to the next refrigeration mode. The third part of the defrost heat input transfers into the freezer space. The second and third parts represent the defrost heat loads affecting the refrigeration system.; The heat loads associated with defrosting of coils have not been fully quantified as a function of the various parameters which affect the system. The current rule of thumb is to simply assume that the coil defrosting load added to the fan heat exceeds 15% of the net refrigeration load with no clear direction as to how a designer may determine what the actual defrosting load may be.; In this research, a method is outlined for determining and quantifying the heat loads associated with freezer coil defrosting using the hot refrigerant vapor defrost method. This dissertation will outline this method in detail. It will also describe an experimental test facility which was designed and constructed for that purpose in addition to describing the experimental procedures employed in the data acquisition and reduction stages. Based on the testing results obtained for several coil entering air dry bulb temperatures (EAT), the defrost heat load is found to be in a good order-of-magnitude agreement with the 15% rule of thumb.; The dissertation also examines the time-dependent dynamics of a refrigeration/defrost system. This involves the delicate task of generating time profiles of a host of variables which are bound to change as the system cycles from the refrigeration mode to the defrosting mode and back to the refrigeration mode again several consecutive times. A visualization study of frost formation at freezer temperatures is also described in this dissertation. This study is intended to provide a qualitative assessment of psychrometric processes typically encountered in true-to-life industrial freezers. Last but not least, the dissertation describes a mathematical model for predicting the evaporation and sublimation rates on a frosted cylindrical coil cooler during hot-gas defrosting of the coil.