# OHTC of pipe wall

Overall heat transfer coefficient (OHTC) of the pipe wall and coatings.

In cables, induced currents produce ohmic losses in the screen/sheath and armour and an alternating voltage causes dielectric losses in the insulation. In GIL on the other hand, induced currents produce ohmic losses in the metal enclosure. In both cases, the heat flow between conductor and surface needs multiple steps to calculate. The value of $U_{wall}$ is representing only the outer sheath for cables, the insulation for heat sources and the protective cover for GIL. In case of cables in ducts, the insulation of the air inside the duct and the duct wall are both incorporated into $U_{wall}$ as well.

Symbol
$U_{wall}$
Unit
W/(K.m$^2$)
Formulae
 $\frac{1}{D_{wall} T_{3} \pi}$ cable not in duct $\frac{1}{D_{wall} \pi \left(T_{3} + T_{4i} + T_{4ii}\right)}$ cable in duct $\frac{1}{D_{wall} T_{ins} \pi}$ heat source $\frac{1}{D_{wall} T_{prot} \pi}$ GIL $\frac{2}{D_{ext} \left(\frac{\ln{\left(\frac{do_{pipe}}{di_{pipe}} \right)}}{k_{pipe}} + \frac{\ln{\left(\frac{d_{ins}}{do_{pipe}} \right)}}{k_{ins}}\right)}$ pipe acc. Incropera1996
Related
$D_{ext}$
$d_{ins}$
$D_{wall}$
$di_{pipe}$
$do_{pipe}$
$k_{pipe}$
$T_{4i}$
$T_{4ii}$
$T_{ins}$
$T_{prot}$
Used in
$T_{wall}$
$U_{buried}$
$U_{inwall}$