Abstract: An analytical study is conducted on energylosses of a fully developed convective heat transfer inside tubes. With theassumption of a fully developed inside tube flow subjected to constant wallheat flux or constant wall temperature, approximate expressions fordimensionless energy losses describing effects of heating, pumping power andthe variation of viscosity are obtained. The present model is applicable forthe energy analysis on fully developed laminar and turbulent flows insidetubes of a variety of cross-sectional shape.