where a flat pipe may be inserted into slot 41 the frame element not shown is positioned to rest against the header pipe such that its outer surface contacts projection surfaces 33 pointing in the direction of the slot 41 i e outwardly in the plane of the drawing the end portion of the frame element adapts to the header pipe profile and in this way upward or downward movement of the frame element is prohibited or restricted transverse movement is also substantially restricted or prohibited through the cooling fins inserted during manufacturing alternatively or in addition to the foregoing inclines or projections inside the header pipe may be provided to restrict movement of the frame element fig9 is a top view of the end portion of the header pipe flat pipe 5 is introduced from above to rest against the projections 33 from the rear persons of ordinary skill will recognize that more than two projections may be provided and that the profile of the individual projections may be different than that shown in fig9 furthermore one or more of the projections may be designed as an annular protrusion around the header pipe fig1 is a top view of the header pipe along the line c c in fig5 i e in a region where no slot is present it can be seen that the header pipe comprises two portions 54 and 55 extending substantially horizontally i e in the longitudinal direction of the flat pipes and further includes a curved or.