Device and method for separating air by cryogenic distillation

The invention relates to a device for separating air by cryogenic distillation comprising: a system of columns; a first turbine; a first compressor coupled to the first turbine; a heat exchanger; means for sending air cooled in the heat exchanger to an intermediate temperature of the latter to the first compressor; means for sending expanded air from the first turbine to the system of columns; means for sending air compressed in the first compressor to an intermediate point of the heat exchanger and then at least in part to the system of columns via a valve; means for sending air compressed inthe first compressor to the inlet of the first turbine via a valve without passing through the heat exchanger, wherein the device comprises means for sending air from the first compressor to the system of columns without passing either through the heat exchanger or through the turbines, these means being constituted by a bypass line provided with an expansion valve. The invention also relates tomethod for starting the de