a 0024 fig2 shows the structural unit 1 according to fig1 partly in an exploded representation with the collecting tube 2 and the collector 3 being shown as separate parts in the lower part of the drawing the collecting tube has a cover part 2 a which in its lower area has two collar shaped apertures 8 and 9 corresponding to these apertures 8 and 9 the tube piece 6 has two bores 10 and 1 1 which in terms of their internal diameter correspond to the external diameter of the collars 8 and 9 since the collecting tube 2 is designed in two parts that is to say consists of the base part 2 b and the cover part 2 a the collars 8 and 9 can be relatively easily produced by a punching embossing operation the tube piece 6 has a concave bearing surface 6 a which is adapted to the external contour of the cover part 2 a 0025 fig2 a shows a further illustrative embodiment of the invention in which instead of the circular apertures 8 9 in fig2 only projections 101 102 are provided in the shell 100 of the collecting tube the projections are here arranged in such a way that they engage in the openings 110 111 of the collector 120 and fix the latter to the collecting tube 100 two projections 101 lying opposite one another can advantageously be provided in this case in another illustrative embodiment it is also possible to provide more than two projections per fluid connection between collecting tube and collector such as is shown for example for the projections 102 and the opening 111 four projections arranged in a star shape are provided here in a further illustrative embodiment not shown only one projection is.