the use of the profiles 44 45 the fluid 5 is diverted around the heat exchanger tubes 12 so that in particular a lower pressure loss is generated than in the case of circular heat exchanger tubes 12 as a result of the placement of structures 23 on the guide elements 13 and or on the heat exchanger tubes 12 see fig4 the heat transfer in the region between the heat exchanger tube 12 and guide element 13 is improved the structures 23 can generate more intense turbulence with greater pressure loss in that region than in the case of circular heat exchanger tubes 12 because the pressure loss as the flow impinges on and flows around the profiles 44 45 without structures 23 is lower fig4 shows a portion of fig3 with which the flow around the heat exchanger tubes 12 will be explained in more detail herein the fluid 5 flows from the inner tube 6 through openings 11 into an intermediate space between the inner tube 6 and the outer tube 19 in this case openings 11 are provided where heat exchanger tubes 12 are disposed between guide elements 13 those regions 42 of the inner tube 6 which are disposed exclusively between the guide elements 13 are closed in this case in such a way that at these points no fluid 5 can flow into the regions 42 the guide elements 13 make contact with one another in this case and are thus supported this leads to additional mechanical fixing of the position of the guide elements 13 the fluid 5 is guided around the heat exchanger tubes 12 in the flow direction 16 by the guide elements 13 in this case the fluid flows only in.