The invention relates to a double tube system having an inner tube (10) conducting a heating medium (13). An insulating layer (21) is provided about the inner tube (10) and coaxial to the axis (15) thereof. The insulating layer is enclosed by a jacket tube (30) coaxial to the axis (15). An intermediate tube (20) is disposed coaxially between the insulating layer (21) and the jacket tube (30). A first annular space (22) which is under negative pressure is produced between the inner tube (10) and the intermediate tube (20). The insulating layer (21) is then present in the first annular space (22). A second annular space (32) which is filled with nitrogen that is under excess pressure is produced between the intermediate tube (20) and the jacket tube (30). Support elements (35) allow a displacement of the inner tube (10) relative to the jacket tube (30) along the axis (15).
