If the room temperature is below 0 °C or the evaporating temperature is lower than −5 °C, the frost that builds up on the evaporator does not melt by itself. In electric defrost, rod heaters pushed into holes made in the fin block are energised at set intervals, the compressor and fans stop, and the frost melts and runs from the tray to the drain. In frozen rooms a separate tray heater is fitted under the tray; without it the melt water refreezes in the tray. Stopping the fans prevents the melt water and warm air from being blown onto the product and into the room.
The total power of the heaters ranges from a few hundred watts to tens of kilowatts depending on the size of the evaporator; roughly, it is a value between half and all of the cooling capacity. Small units use a 230 V single-phase connection, large ones a 400 V connection distributed evenly over the three phases. A typical setting is 3–6 defrosts a day and a duration of 20–30 minutes at most. Defrost should be terminated not by time but by the sensor measuring the fin temperature; the termination value is usually between +8 and +15 °C.
Most of the defrost energy stays in the room as heat. That is why defrost that is more frequent or longer than necessary increases both the electricity bill and the fluctuation in room temperature. After defrost a drip time of a few minutes and a fan delay are allowed; in this way the water droplets left on the fins are not thrown onto the product and warm air is not blown into the room. In large plants hot gas defrost is more economical than electric. Defrost times are arranged to fall in the hours when the door is opened little and the load is low.
For safety the heater circuit must have a limit thermostat that works independently of the controller; if the contactor welds, the heater can become hot enough to ignite the plastic tray and the panel. If the frost never melts in one zone, that heater may have failed open; it is found by measuring the current and testing the insulation resistance. In our catalog the defrost power and number of heaters of every evaporator are given in the model table. When a heater is replaced, one of the same length, power and voltage is fitted; a more powerful one overheats the fins and the tray.