Abstract: Grooved metal tubes, with a groove-bottom thickness Tf and outside diameter De, internally grooved with N helical ribs having an apex angle ?, height H, base width LN, and helix angle ?, two consecutive ribs being separated by a groove, generally flat-bottomed, having a width LR, with a pitch P equal to LR+LN, are characterized in that: a) the thickness Tf of the tube is such that Tf/De is equal to 0.023±0.005, Tf and De being expressed in mm, with De ranging between 4 and 14.5 mm; b) the ribs have a height H such that H/De is equal to 0.028±0.005, H and De being expressed in mm; c) the number N of ribs is such that N/De is equal to 2.1±0.4, and the corresponding pitch P is equal to p·Di/N, with Di equal to De?2·Tf, and De being expressed in mm; d) the base widths LN and LR are such that LN/LR is between 0.20 and 0.80; e) the apex angle ? ranges from 10° to 50°; and f) the helix angle ? ranges from 20° to 50°.