Modeling the Cooling Process Path of a Dehumidifying Coil Under Frosting Conditions

Abstract

Whenever humid air comes in contact with a cooling coil whose temperature is below both the dew-point of water vapor in air and the freezing point, frost will form. The nature of the frost forming on the coil will depend to a large measure on the psychrometric conditions prevailing inside the freezer and whether the air around the coil is subsaturated or supersaturated. Psychrometric theory and the apparatus-dew-point calculating procedure assume that the cooling process path as the air passes through the coil is a straight-line on the psychrometric chart. The actual path is however a result of a much more complex series of processes and is therefore a curve. While researchers have calculated the actual process path on a dehumidifying coil, none has attempted to do the same for a frosted, multi-row coil. It is believed that determining the actual conditions leaving a given row in a multi-row freezer coil is a crucial step in identifying the coil location in the vicinity of which the transformation from the subsaturated zone to the supersaturated zone occurs. This will prove a key step in identifying a demarcation line between the unfavorable snow-like frost and the more traditional (and more favorable) frost formation patterns. Thus, the objective of this paper is to calculate the air path on an actual industrial-size finned-tube, multi-row coil utilizing experimentally derived data and correlate the shape of the path with the prevailing psychrometric conditions in the freezer in the hope of identifying the demarcation line in question.