Single-structured tubes (SSTs) and cross-structured tubes (CSTs) with helix-based structures are investigated with respect to their heat transfer properties and pressure drop based on the measurements of a non-isothermal gas flow inside the tubes. The examined Reynolds number range corresponds to relevant shell-and-tube heat exchanger applications. After a detailed description of the measuring methods used, the test facilities and their validation-appropriate wall geometries with a high potential for heat transfer improvements are discussed. The experimental results show that the Nusselt number can be improved by varying the structure depth and angle of the helix, which can also influence the pressure drop in the tube. Numerical simulations demonstrate that the tangential velocity in the SSTs strongly varies with the structure angle, while the flow inside the CSTs only rotates locally.