Abstract: The compressed air flow in an air drier is directed to either one of its two drying towers by a switch-over valve whose switch-over function is performed with the air drier unloaded by an unloader valve arranged between the air drier and a compressor. The unloader valve is caused to perform its unloading function not only at a predetermined pressure in an air tank to which the air is fed but also--by an electronic control device--after a predetermined time during periods of prolonged continuous air loading from the compressor through the air drier even if the predetermined pressure is not reached.

Abstract: A switch-over valve for directing a compressed air flow to either one of two drying towers of an air drier, the other one being vented to the atmosphere through the valve. A positively operated valve body is movable between two valve seats in a central chamber. A valve sleeve arranged at either side of the valve body is axially movable in a dividing wall between the central chamber and a respective outer chamber connected to the drying tower, is spring biased in a direction towards the valve body to a position where a radial valve sleeve flange in the outer chamber is in sealing contract with the wall, and is biased in the opposite direction by the valve body for lifting the flange from the wall and venting out the air from the outer chamber to the atmosphere through a channel in the wall.