A tube in a fire tube boiler, designed to convey exhaust gases and in contact with the water to be heated at an outer surface of the wall (2, 6, 10, 15, 21), which has a constant thickness; the tube has, in contact with the inner surface of the wall having a constant thickness, a plurality of elements (3, 7, 8, 9, 11, 12, 13, 16, 17, 19, 20) provided with protrusions which are mutually locked by elastic deformation.