Longitudinal-fin tubes are in common use on two types of applications: heating or cooling of viscous liquids, and heating or cooling of low-pressure gases. They are also used in a variety of other applications. An advantage is that shell-side flow areas of longitudinal finned-tube units are large compared with typical shell-and-bare-tube units. Another is that the crossflow component is minimal using longitudinal fins except for those designs with punched holes in the fins. Both types are used in tank heater services.