a conventional structure of a tube connection , the sealing of which is configured according to the invention , will be described first with the aid of fig1 . a collector 2 of a motor vehicle heat exchanger is formed in two parts by a tube bottom 4 of the motor vehicle heat exchanger and a cover 6 . the tube bottom 4 has a flanged edge 8 , in which the cover 6 , designed in the shape of a pot , of the collector 2 engages and is brazed . inwardly bent collars 10 , in which ends of flat tubes ( not shown ) of the motor vehicle heat exchanger engage and are likewise brazed , can be seen on the tube bottom 4 . this is in other respects the collector of a conventionally designed flat tube heat exchanger , which need not be described in further detail . the collector 2 is used to feed the inner heat exchanger fluid of the motor vehicle heat exchanger to the flat tubes , or deliver it again from the collector after it has flowed through the flat tubes . the collector may in this case be an input collector , an output collector or even a collector which , by using a transverse wall ( not shown ), combines the functions of an input collector and an output collector , in which case the other ends of the flat tubes merely need to be connected to one another in terms of flow to form a multiple - stream arrangement . instead of flat tubes , heat exchanger tubes having a different cross section could be used . the cover 6 of the collector 2 has an outwardly directed collar 12 , into which a separate connecting sleeve 14 is inserted and in turn brazed . alternatively , the connecting sleeve 14 could also be manufactured integrally with the collector 2 , or with its cover 6 . the connecting sleeve 14 communicates with a line 16 , which feeds the inner heat exchanger fluid to the collector 2 , or delivers it from the collector 2 , according to whether the latter has an input or output function . the connecting sleeve 14 has its cross section stepped two times in the direction of the line 16 , with the corresponding symmetrical increase of the internal cross section . in the vicinity of the first step 18 , the free end , facing the collector 2 , of the line 16 engages fully or , as graphically represented , with a part of the thickness of the tube casing . in the vicinity of the second step 20 , which describes a likewise symmetrical further extension outward , a circumferential annular gap 22 is formed , with the casing of the line 16 whose cross section remains unaltered here , between the inner surface 24 of the connecting sleeve 14 and the outer surface 26 of the line 16 , in order to at least partially accommodate a seal 28 extending annularly around . in the vicinity of the first step 18 , the connecting sleeve 14 hence has a narrow cross section 30 , next to the collector , into which the free end 32 of the line 16 positively engages , and , in the vicinity of the second step 20 , has a wide cross section 34 , remote from the collector , which is used in the manner described above in order to form the annular gap 22 . the line 16 , which has an otherwise constant outer cross section in the region described here , is provided with an outer bead 36 , obtained by a longitudinal buckling , which has a nature similar to that of an annular flange and into whose flange area that projects above the outer surface 26 of the line 16 , the outwardly projecting original tube casing parts obtained by the buckling bear very tightly against one another with a double - walled configuration . the outer bead 36 interacts , in the capacity of an end flange of the line 16 , with an outer flange 38 on the free end 40 of the connecting sleeve , which is obtained by flanging of this free end outward . the outer flange 38 and the outer bead 36 have , without restriction of the generality , the same outer diameter in this case , although slight differences are still acceptable . in the final assembly state , a connection clip 42 is used to permanently clamp together the outer flange 38 , provided with the outer end surface 41 , on the connecting sleeve 14 and the outer bead 36 on the line 16 . in the case of a tube connection according to fig1 according to the internal prior art an o - ring , which represents the future seal of the tube connection , is firstly fitted undeformed onto the free end of the line 16 , in contact with the outer bead 36 . in order to obtain good radial compression in the annular gap 22 later on , the o - ring has in this case , in the undeformed state , a diameter larger than the radial width of this annular gap . together with the o - ring , the line 16 is then slid into the connecting sleeve 14 , with the free end 32 of the line , at the start of the compression of the o - ring , already engaging in the vicinity of the first step 18 of the connecting sleeve 14 . such an intermediate phase of the assembly is illustrated in fig4 where the o - ring that is provided as a seal 28 on the rounded shoulder , which constitutes the transition of the inner surface 24 of the second step 20 of the connecting sleeve 14 into the outer flange 26 , has already experienced a deformation , which on the one hand extends in the intended way into the annular gap 22 , but on the other hand also extends somewhat into the intermediate space between the outer flange 38 and the outer bead 36 , in a manner that is not actually intended . in the desired final state of the tube connection , the entire mass of the o - ring is then compressed according to fig4 a into the end state of the tube connection , in which the end surface 41 of the outer flange 38 comes into direct contact with the end surface of the outer bead 36 facing it , and the o - ring is entirely compressed with a radial compression in the annular gap 22 between the inner surface 24 of the connecting sleeve 14 and the outer surface of the free end 32 of the line 16 . in contrast to this , a typical unsuccessful tube connection is shown in fig4 b , where the o - ring does not in practice extend substantially stantially further into the annular gap 22 than in the previous assembly phase according to fig4 but instead has become compressed with a considerable section between the outer flange 38 and the outer bead 36 , so that these parts can no longer come into direct contact with one another , clamping by means of the connection clip 42 engaging over only a limited clamping range is no longer possible and the radial sealing function is to a considerable extent lost . in the case of the tube connection according to the invention — and consequently also in the three illustrative embodiments of fig2 with 2 a , 3 and 3 a , the seal 28 has an insertion sleeve 44 and a sealing flange 46 arranged on one end of the latter . in the vicinity of the other end of the insertion sleeve 44 , a first sealing element 48 , which is configured differently in fig2 and 3 a , is formed on the circumference of this sleeve . in a manner that is not shown , a plurality of two or more first sealing elements 48 may also be formed along the insertion sleeve . all such first sealing elements are intended to fulfill the function of the o - ring of the internal prior art , that is to say the function of radial sealing by radial compression in the annular gap 22 between the connecting sleeve 14 and the line 16 . in addition , the sealing flange 46 inherently fulfills yet a further axial sealing function between the outer flange 38 and the outer bead 36 . in all three illustrative embodiments shown , in order to reinforce this axial sealing action , a second sealing element 52 , which projects beyond the end surface 50 of the sealing flange 46 in the uncompressed state , is respectively also formed on the sealing flange 46 , approximately in extension of a central cross - sectional region of the insertion sleeve 44 , and a plurality of these second sealing elements may in turn also be present , in which case they are typically either radially further out on the end surface 50 and / or also on the rear side of the sealing flange 46 , as is represented by the second sealing element 52 a in fig3 . in fig3 three first sealing elements 48 a , 48 b and 48 c are also formed axially along the insertion sleeve 44 , the first sealing elements 48 b and 48 c of which are projecting annular beads circumferentially running radially inward . with reference to the first sealing element 48 a of fig3 it is shown that this may itself entirely assume the cross section of a conventional o - ring , although in this case it is not isolated but is formed integrally on the free end of the insertion sleeve 44 remote from the sealing flange 46 . however , because of the additional further sealing possibilities in the case of the seal 28 according to the invention , the radial compression and hence the radial sealing action of the first sealing element may also be attenuated compared with the compression of an isolated o - ring . fig2 shows a first attenuation possibility . in this case , the cross section of an o - ring on the free end of the insertion sleeve 44 is cut off to form an obtuse radial end surface 54 . in this case , a residual contour of an o - ring still extends radially inward , while , radially outward , a further reduction to a narrower circumferential annular bead is made in the axial direction . as an alternative , the nature of the configuration radially inward and radially outward could also be interchanged , of further modifications to the cross section of an o - ring could be made . while , however , in the design forms of the first sealing element 48 which are described and which are discussed as variants , a more or less pronounced full cross section of this first sealing element is provided , fig3 a shows that it is even possible to provide a design form that is concave on the end according to fig3 a . in the case of the latter , the cross - sectional shape of fig2 is modified by the concave design on the end and weakened in terms of radial application force so greatly as to create one sealing lip 56 a and 56 b each , radially inward and radially outward , respectively . both sealing lips are pressed via the concave recess on the end by internal pressure onto the inner surfaces of the annular gap 22 . the recess in the full cross section does not , however , extend so far that the prestress radially outward is completely lost . instead , this prestress radially outward is still intended to remain to such an extent that , even in the case of the negative pressure conditions liable to occur during operation in the space 58 to be sealed , sufficient radial compression against the mutually facing surfaces 24 and 26 of the annular gap 22 is still guaranteed . in all three first illustrative embodiments , the insertion sleeve 44 merges with wedge - shaped widening , corresponding to the curved transition of the connecting sleeve 14 from the second step 20 into the connecting sleeve 14 , into the sealing flange 46 . as can be seen from fig2 the insertion sleeve 44 further has on its radial inner surface 60 , starting from a rounded transition of the sealing flange 46 , a for example conical gradual taper toward the free end of the insertion sleeve 44 . a wedge shape is thereby obtained overall between the free end of the insertion sleeve 44 and the sealing flange 46 . the first sealing element 48 or , in the case of several first sealing elements , the first sealing element 48 a nearest the collector , causes fully or approximately a radial thickness compression in relation the radial dimension of the annular gap 22 of 20 to 40 %. this corresponds to the usual radial compression in the case of the conventional o - rings as well , and is still guaranteed in the lower limit range even in the case of the embodiment of fig3 a . the only one - sided radial compression on the inside of the further first sealing elements 48 b and 48 c according to fig3 is smaller by comparison , but always still significantly more than 0 %. following on from the single first sealing element 48 , or the first sealing element 48 c still furthest from the collector , additional extensive compression , typically in the 0 to 20 % range , occurs in the wedge region of the transition of the insertion sleeve 44 into the sealing flange 46 . with unfortunate coincidence of tolerances , this extensive compression may be entirely or close to 0 in this case , or may even lead to slight free gaps . in all these cases , there is the risk of a capillary creep flow along the desired extensive compression region . in order to stop leakage due to capillary action here , a circumferential capillary break 62 in the form of a circumferential groove is provided in the outer surface of the seal 28 , following on axially in the direction toward the sealing flange 46 , radially outward ( as shown )— and optionally instead of this or additionally radially inward ( not shown ). in the final connection state of the tube connection , unlike in the case of sealing by means of the conventional o - ring , direct contact of the outer flange 38 on the outer bead 36 does not occur , but instead the sealing flange 46 always remains in between according to fig2 a . because of the axial compression forces into the final position of the tube connection , the sealing flange 46 swells radially outward somewhat in this case , as is illustrated by the radially outer bulge 64 in fig2 a . fig2 with fig2 a describes in this regard the case of an entirely normal annular flange design of the sealing flange 46 . the same applies to the second embodiment according to fig3 . the sealing flange , however , is modified slightly in this case by the respective way in which at least one second sealing element is arranged . according to fig3 a , a return projection , which in the normal case runs radially around , but may also be divided into sections distributed in the circumferential direction , is provided on the side of the sealing flange 46 facing the collector , on this flange &# 39 ; s radially outer end . the return projection may likewise , unlike in fig3 a , have a cross section that is different than rectangular . what is essential is that it can externally engage around the outer flange 38 of the connecting sleeve 14 . the typical assembly procedure in the case of the tube connection according to the invention is as follows : the insertion sleeve 44 of the seal 28 is firstly slid as far as possible into the second step 20 on the connecting sleeve 14 , expediently until the side of the sealing flange 46 facing the collector is in contact with the free end surface 41 of the outer flange 38 . the line 16 is then slid into the insertion sleeve 44 , expediently in turn until the outer bead 36 is in contact with the free end surface 50 of the sealing flange 46 . this is possible , in comparison with the assembly procedure for a conventional o - ring , explained with reference to fig4 to 4 b , with much smaller assembly forces and with substantially greater reliability in achieving a correct tube connection with strong sealing function and without inadvertent omission to include a seal 28 . in the state of the final tube connection , it is then possible to fix the final assembly state using the connection clip 42 or another clamping device , without forces that pull apart again having to be taken into account in this regard , as they were in the case of the o - ring , so long as one neglects the relatively slight deformation forces of the sealing flange 46 , which are illustrated with the aid of the radial bulge 64 when fig2 is compared with fig2 a . apart from the axial stretching also mentioned above , the position of the insertion sleeve 44 in the annular gap 22 remains virtually unchanged from then on during the assembly process , unlike in the case of the o - ring . in general , and especially in the case of all three illustrative embodiments , the radial compression is promoted yet further by the fact that in addition , in the vicinity of the first step 18 of the connecting sleeve 14 , a step 68 extending radially inward is also formed in the free end 32 of the line 16 , which step typically has a ramp with a small assembly angle , typically of 10 to 20 °, along which the shoulder , which constitutes the transition of the first step into the second step of the connecting sleeve 14 , can run down during the radial compression of the insertion sleeve 44 . in addition , the free end 32 of the line 16 is provided on the outside with a chamfer 70 serving as an introduction slope , so that introduction into the insertion sleeve 44 does not damage the latter from the inside .