d 0 06 to 0 4 u00d7 dd rr 0 5 to 2 u00d7 t1 and or rr 0 1 to 0 6 u00d7 h 45 u00b0 u2266 a u2266 75 u00b0 dd 0 05 to 0 6 u00d7 l1 dde 0 05 to 0 6 u00d7 l1 position of a row of protuberances 10 carried by the longitudinal line of symmetry of the tube u00b1 10 mm protuberances distributed over a row if 10 u2266 w1 u2266 30 mm protuberances distributed over two rows if 26 u2266 w1 u2266 45 mm where h height of the protuberance 10 d diameter of the contact surface 12 of the protuberance 10 rr connection radius relative to the contact surface 12 and to the surface of the tube 2 a stamping angle of the protuberance 10 t1 thickness of the tube 2 w1 width of the tube 2 l1 length of the tube 2 dd distance between protuberances 10 of the same row dde distance between the centre of the first protuberance 10 of a row and an of the tube 2 according to a first embodiment for the casing 3 illustrated in fig8 and 9 a casing 3 has been illustrated with a single group of two rows of protuberances 11 the definition of the protuberance pattern 11 is determined.