Critical heat transfer of an insulated duct often occurs at small convection effects of
ambient air, thus heat radiation must be considered. It is found in this study that the critical heat transfer characteristics of an insulated circular duct considering heat radiation are big different from those conventional results neglecting heat radiation. The conventional critical heat transfer characteristics only depend on circular duct size, external convection heat transfer coefficient, and insulation conductivity. In addition to above parameters, the actual critical heat transfer characteristics considering
heat radiation are also related to the surface emissivities of circular duct and insulation,surrounding temperature, and the internal convection heat transfer coefficient.Meanwhile, it is found that the critical heat transfer does not only apply to small insulated duct and also to big size compound metal duct as well.