Planned Air Defrost
Sometimes the standard off cycle is not long enough to defrost the evaporator. This problem usually occurs when the box temperature is maintained between 34°F and 36°F. The evaporator can also develop too much frost when the box receives very heavy usage from warm product, bad door gaskets, or just excessive door openings.
Technicians have to plan when, and for how long, to shut off the compressor in order to clear frost from the coils. For this reason, it is called a “planned” defrost and uses a time clock to shut off the compressor long enough to accomplish the necessary air defrosting.
Normally, defrosts are scheduled for times when the box is not in use. For example, technicians usually set the clock to go into defrost at 2:00 in the morning for one or two hours. This gives the coil enough time to melt the frost accumulated during the day. The product temperature in the box may rise by a few degrees, but not enough to cause food spoilage. When the defrost period is complete, the compressor quickly restores the product to its original temperature.
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