Heat transfer in cable ducts and duct assemblies

STUDIES of heat transfer from cables in duct banks have indicated the need for more experimental data to establish thermal-resistivity values for several components of the duct system, particularly that portion of the heat flow path from the cable sheath to the outside of the duct wall. Theoretical considerations of heat transfer from the cable to the duct wall have treated the latter as an isothermal surface, and, generally, no consideration has been given to the effect of the thermal resistivity of the duct material. These approximations are made necessary in theoretical studies of cable systems by the complex geometry of the duct assembly, the cable sheath resting non-uniformly at the bottom of the duct. While such studies, supported by the experimental evidence available, yield reasonably accurate thermal resistivity values, the effects of a nonisothermal duct wall and of the thermal resistivity of the duct material can be taken into account by performing measurements on a duct assembly.