Why does ice not deposit in the freezer of a refrigerator equipped with defrosting technique?

The mechanism on a refrigerator involves heating the cooling element (evaporator coil) for a short period, melting any frost that has formed upon it and having it drain through a collecting duct at the back of the unit. The cycle is controlled by an electric or electronic timer: For every six to 12 hours of compressor operation it runs a defrost heater element for about 15 minutes to a half hour. The defrost heater, having a typical power rating of 350 W to 600 W, is mounted just below the evaporator coil and is protected from short circuits with fusible links. Older refrigerators ran the timer continuously, but to activate the defrost heater less often and thereby save energy, contemporary designs run the timer only when the compressor motor runs, so if the refrigerator door is left closed, the compressor and heater element will run less often. The defrost heater circuit also includes a defrost thermostat that senses when the cooling element temperature has risen to 40�F (5�C), or warmer, interrupting the current flow in the element and preventing excessive heating of the freezer compartment. The defrost timer is designed to run the compressor motor or the defrost heater, but not both at once.Inside the freezer, dry air is circulated around the cabinet using one or more fans. In a typical design the cold air from the freezer compartment is ducted to the fresh food compartment, and circulated back into the freezer compartment. Air circulation helps sublimate any ice that may form on frozen items in the freezer compartment.Instead of the traditional cooling elements assembled within the freezer liner, auto-defrost elements are compact and separated from the main cabinet space, allowing them to be heated for short periods to dispose of any ice that has formed.