Abstract: A longitudinal sealing (20), which, as a rule, extends below the rotating tube (30), is proposed to increase the heating efficiency in a rotary tubular kiln, in which a rotating tube (30) is surrounded, like a bowl, by a heating tunnel (32) and is indirectly heated by a heating medium. It is designed in the shape of a separation wall between the entry side (38) and the exit side (40) for the heating medium in the heating tunnel and consists of a rigid part (22) and a flexible part (24), situated thereon and adaptable to the rotating tube profile.