When operating at very low temperatures, below approximately -10°C, small fluctuations in temperature are insignificant. The cooling compressor used to regulate the temperature is therefore switched on and off in this mode, to save energy. The circulating fans are operated at low speed, thereby reducing their power consumption by a factor of 8. The temperature is regulated by means of the return air temperature, i.e. the temperature of the air coming out of the cargo. If this temperature exceeds the nominal value by e.g. 0.2°C, cooling starts (the compressor is switched on), and when the temperature falls e.g. 0.2°C below the nominal value, it is switched off again. To prevent the compressor from constantly switching on and off, a minimum period must have elapsed between two switching operations.
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