Abstract: An extensional flow static heat exchanger with at least two components of tubular cooling elements arranged in a flow passage, wherein the components are successively arranged in the flowing direction, each component having a first and a second set of at least two cooling elements, the cooling elements of the first set being angularly offset to the cooling elements of the second set, the cross sections of the cooling elements have a convex profile on the upstream and downstream side or a droplet shape, the cooling elements of the first and the second set, respectively of the adjacent component are arranged at least partially in the space between two cooling elements of the first and second set, respectively of the previous component, a process for producing a low density extruded thermoplastic foam material and a low density extruded thermoplastic foam material.