This article attempts to update the rather sketchy methods for double pipe exchangers in the present literature, especially those for longitudinal finned tubes. Areas and conditions for the most useful application of double and multitube exchangers are outlined. Calculation methods are presented for plain double pipe units as well as finned tube units, with a new development in the important transition region with ＇＇cut-and-twist＇＇ turbulency promoters. Equations for the mean temperature difference for units with flow in series-parallel are also given.