Method for separating air by cryogenic distillation, wherein air is compressed in a first compressor, cooled in a heat exchanger and then separated in a system of columns (MP, BP), liquid oxygen is vaporized in the heat exchanger countercurrent to a flow of pressurized gas which (pseudo)condenses, a flow of gas which is air or a gas delivered from the system of columns is expanded in a first cryogenic-expansion turbine having a single wheel, and possibly in a second expansion turbine having a single wheel, the, or where applicable each, turbine having an inlet temperature lower than - 100°C, a gas which is air or a gas delivered from the system of columns is compressed in a first booster compressor having a single wheel, with an inlet temperature higher than -50°C, a gas which is air or a gas delivered from the system of columns, this gas having already been compressed in the first booster compressor, is compressed in a second booster compressor having a single wheel with an inlet temperature lower than - 100°C, the gas compressed in at least the first booster compressor cools in the heat exchanger, contributes to the vaporization of liquid oxygen by exchange of heat in the exchanger, and is (pseudo)liquefied on leaving the cold end of the heat exchanger, wherein: a) the work generated by the first expansion turbine and possibly the second expansion turbine is used for the compression step in the first booster compressor and for the compression step in the second booster compressor, b) the compression step in the first booster compressor allows work to be generated outside the cold box, and this generates cooling power for the air separation method, characterized in that c) i) the wheel of the first booster compressor, the wheel of the second booster compressor and the wheel of the first turbine and possibly the wheel of the second expansion turbine are mounted on the same rotation shaft, or ii) the first and the second booster compressor are connected to the wheel of the first expansion turbine, and possibly to the wheel of the second turbine, each by a rotation shaft, these shafts rotating at the same rotational speed, or iii) the first booster compressor and the wheel of the first expansion turbine are connected to the second booster compressor, each by a rotation shaft, these shafts rotating at the same rotational speed, d) the operating conditions for the wheel of the first expansion turbine, possibly the wheel of the second expansion turbine, the wheel of the first booster compressor and the wheel of the second booster compressor are defined such that these three, or even four, wheels have the same rotational speed, and e) of the wheel of the first expansion turbine, the wheel of the first booster compressor and the wheel of the second booster compressor, at least one has an efficiency lower than that which it would have, under the same operating conditions, at another rotational speed. a) the work generated by the first expansion turbine and possibly the second expansion turbine is used for the compression step in the first booster compressor and for the compression step in the second booster compressor, b) the compression step in the first booster compressor allows work to be generated outside the cold box, and this generates cooling power for the air separation method, characterized in that c) i) the wheel of the first booster compressor, the wheel of the second booster compressor and the wheel of the first turbine and possibly the wheel of the second expansion turbine are mounted on the same rotation shaft, or ii) the first and the second booster compressor are connected to the wheel of the first expansion turbine, and possibly to the wheel of the second turbine, each by a rotation shaft, these shafts rotating at the same rotational speed, or iii) the first booster compressor and the wheel of the first expansion turbine are connected to the second booster compressor, each by a rotation shaft, these shafts rotating at the same rotational speed, i) the wheel of the first booster compressor, the wheel of the second booster compressor and the wheel of the first turbine and possibly the wheel of the second expansion turbine are mounted on the same rotation shaft, or ii) the first and the second booster compressor are connected to the wheel of the first expansion turbine, and possibly to the wheel of the second turbine, each by a rotation shaft, these shafts rotating at the same rotational speed, or iii) the first booster compressor and the wheel of the first expansion turbine are connected to the second booster compressor, each by a rotation shaft, these shafts rotating at the same rotational speed, d) the operating conditions for the wheel of the first expansion turbine, possibly the wheel of the second expansion turbine, the wheel of the first booster compressor and the wheel of the second booster compressor are defined such that these three, or even four, wheels have the same rotational speed, and e) of the wheel of the first expansion turbine, the wheel of the first booster compressor and the wheel of the second booster compressor, at least one has an efficiency lower than that which it would have, under the same operating conditions, at another rotational speed.