the present description only the heat exchanger operating in countercurrent between the air to be separated and the gas to be supplied to the network i e nitrogen will be considered thus only passages 8 for cooling air and passages 9 for warming nitrogen have been represented in the drawings a duct 10 for the supply of compressed air to be separated from which water and co 2 have been removed is connected at the hot ends of the passages 8 a duct 11 connects the cold end of these passages 8 to the air inlet of the apparatus 3 a duct 12 connects the nitrogen outlet of the apparatus to the cold end of the passages 9 and the duct 1 is connected to the hot end of these passages 9 the apparatus also comprises a regulating throttle valve 13 associated with the duct 12 and in addition to the cold box 2 a stop valve 14 called production valve mounted on the duct 1 upstream of this valve a duct 15 leading to the air diverted from the duct 1 and provided with a valve 16 the latter operates as an overflow i e as an upstream pressure regulator a flow meter 17 measuring the flow of gas in the.