Below a supply air temperature of approximately +10°C, the air cooler is defrosted at regular intervals, since as of this temperature, the surface temperature on the air cooler can be below 0°C. During defrosting, the circulating fans in the container and the cooling circuit are stopped. The air cooler and the drip tray are electrically heated. Since hot air rises slowly in the refrigeration unit, the return air temperature sensor always shows relatively high temperatures during defrosting.
Defrosting is generally time-controlled, i.e. it occurs e.g. every six, eight or twelve hours. The end of the defrosting process is normally determined when an end temperature is reached at the defrost end sensor above the air cooler. At the same time, however, a maximum defrosting period is also specified. If this is exceeded, defrosting is also terminated.
Some controllers are able to carry out defrosting as and when required. In this case, defrosting is triggered if the supply air temperature is not reached over a specific period or if the temperature differential between the supply air and the return air is too high. Both criteria can indicate an insufficient air flow, which could in turn be caused by ice formation on the air cooler. Other causes are also possible, however, making it difficult to carry out defrosting automatically as required.

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