Abstract: A condensation air drying apparatus comprising a refrigeration system, the evaporator of which is used for cooling an air flow to or below its dew point whereby the moisture in the air as drawn through the cooler by a fan is condensed on the cooler and drained off. A switch over valve enables the cooler to be temporarily connected as a condenser in the refrigerator system, whereby the cooler is heatable for defrosting. A temperature sensor mounted direct on the cooler serves to control the fan power for optimal economy in normal operation and to detect frost formation and control start and stop of the defrosting cycles in response to such detection. The control equipment may include an air temperature sensor adjusting the working characteristic of the cooler temperature sensor in an optimal manner according to the temperature of the air to be dried.