Abstract: A heat exchanger is provided with a plurality of fins connected in thermal communication with a tube. The tube is shaped to conduct the flow of the first fluid and the fins are arranged to be disposed within a stream of flow of a second fluid. The fins are provided with a plurality of holes, or perforations, through which the second fluid is directed to flow. The holes through the fins are generally oblong in shape, being somewhat elliptical. The oblong hole has two generally semicircular ends and two connecting long sides, wherein one of the long sides is generally flat and the other long side is provided with a cusp surface which divides the oblong hole into two generally equal portions. The relative position of the cusp surface to the long surface creates counter rotating vortices in the flow of fluid as it passes through the hole. These counter rotating vortex pairs wipe the surface of an adjacent fin when the flow is directed to impinge against a non-perforated portion of that adjacent fin.