Dryer for compressed air including an air inlet and an air outlet, two drying units (22', 22"), and two respective circuits (18', 18"), inside which air to be treated circulates from said air inlet to said air outlet, a first of said drying units comprising a first cooling apparatus, configured to cool the compressed air to positive temperatures, a few degrees above zero and near to freezing temperature, and a first freezing apparatus, disposed downstream of said first cooling apparatus and configured to cool the compressed air to abundantly negative values, a second (22") of said drying units comprising a second cooling apparatus (12") and a second freezing apparatus (14"), each of said freezing apparatuses (14', 14") having a first freezing operating condition and a second defrosting operating condition, wherein - a first segment of the circuit, passing through said first cooling apparatus and said first freezing apparatus, comprises at least one return segment through which the air exiting from said first freezing apparatus flows in an opposite direction in said first cooling apparatus in order to generate a lowering of temperature in the air entering into said first cooling apparatus and/or into said second cooling apparatus (12"); - a second segment of the circuit (18"), passing through said second cooling apparatus (12") and said second freezing apparatus (14"), comprises at least one return segment through which the air exiting from said second freezing apparatus (14") flows in an opposite direction in said second cooling apparatus (12") in order to generate a lowering of temperature in the air entering into said second cooling apparatus (12") and/or into said first cooling apparatus; - wherein said first and second (18") segments of the circuit are connected to each other by valve means (27', 27"); - wherein said valve means (27', 27") have at least one first operating condition in which the stream of compressed air entering is sent first along said first segment of the circuit through said first cooling apparatus and said first freezing apparatus put in its defrosting condition, and then along said second cooling apparatus (12") and said second freezing apparatus (14") put in its freezing condition; and a second operating condition in which the stream of compressed air entering is sent first along said second segment of the circuit (18") through said second cooling apparatus (12") and said second freezing apparatus (14") put in its defrosting condition, and then along said first cooling apparatus and said first freezing apparatus put in its freezing condition; characterized in that said first drying unit and/or said second drying unit (22") comprise, along said circuit and downstream of the respective freezing apparatus (14', 14"), at least one deflector element (123a, 123b), which can be selectively activated to choke the passage of the compressed air to be sent into one and/or the other cooling apparatus (12', 12"). - a first segment of the circuit, passing through said first cooling apparatus and said first freezing apparatus, comprises at least one return segment through which the air exiting from said first freezing apparatus flows in an opposite direction in said first cooling apparatus in order to generate a lowering of temperature in the air entering into said first cooling apparatus and/or into said second cooling apparatus (12"); - a second segment of the circuit (18"), passing through said second cooling apparatus (12") and said second freezing apparatus (14"), comprises at least one return segment through which the air exiting from said second freezing apparatus (14") flows in an opposite direction in said second cooling apparatus (12") in order to generate a lowering of temperature in the air entering into said second cooling apparatus (12") and/or into said first cooling apparatus; - wherein said first and second (18") segments of the circuit are connected to each other by valve means (27', 27"); - wherein said valve means (27', 27") have at least one first operating condition in which the stream of compressed air entering is sent first along said first segment of the circuit through said first cooling apparatus and said first freezing apparatus put in its defrosting condition, and then along said second cooling apparatus (12") and said second freezing apparatus (14") put in its freezing condition;