Two towers, filled with a desiccant which adsorbs water vapour, provide ultra-dry air (-40°C pressure dew point) for operation of sensitive pneumatic devices. Inlet air is dried in one tower during a drying cycle. Simultaneously, the desiccant is dried in the other tower during the regenerative cycle. An electric timer automatically cycles inlet air between the towers every two minutes providing a continuous supply of dry air. Constant downstream pressure and dew point are maintained by overlapping the cycle times (both towers are pressurized briefly before changeover). During the 'regenerative cycle' the desiccant is dried by purging with a small amount of dry air from the on-line tower (Dry air passes through the desiccant and is exhausted to atmosphere). A calibrated needle valve allows the amount of air used for purging to be kept to a minimum. i.e. 5-8%