An intercooler (3) is provided with heat exchanger surfaces (6) and is arranged in the gas stream between two compressors (1, 2) or two compressor stage groups for compressing a gas. An aftercooler (5) is also provided with heat exchanger surfaces (7) and is arranged after the second compressor (2). To utilize the waste heat generated in the coolers (3, 5), the heat exchanger surfaces (6, 7) of the intercooler (3) and of the aftercooler (5) are integrated in a steam generator (9) in such a way that the heat exchanger surfaces are connected with the feedwater pump (40, 59) of the steam generator (9).