When the evaporator surface runs below 0 °C, the moisture in the air settles on it as frost. As the ice layer thickens, air passage and heat transfer decrease, capacity falls, the room warms up and the compressor runs longer. Defrost clears the evaporator by melting this ice at regular intervals; the melt water is drained away through the drip tray and drain pipe.
Methods: if the room temperature is +2 °C or above, cooling is stopped and the fans run; the room air melts the ice ("air defrost", no energy cost). In rooms below 0 °C electric heating elements placed between the evaporator fins are the most common method: simple, cheap, but during defrost heat also passes into the room. Hot gas defrost routes the compressor’s hot discharge gas to the evaporator; it is fast and efficient and is used in large and low-temperature systems.
The settings are made on the controller: typically 2–6 defrosts a day, each 20–45 minutes at most, terminated when the fin temperature reaches 8–12 °C; after defrost a drip time and fan delay are observed so that melt water is not blown into the room. Too frequent defrost means wasted energy and temperature swings, too infrequent means icing; "defrost on demand" (with an ice sensor) prevents both. Our evaporator catalog states the defrost heater power of every model.