The internal heat transfer coefficient (or inside film coefficient) is also a convection coefficient, calculated from the Nusselt number of the internal fluid. It is the heat transfer by convection at the inner surface of the pipe wall in contact with the pipeline product (gas, oil, water, and possibly solids.

In cables, there is no fluid flow at the inner surface of the insulation. In GIL, the convection of the gas inside the chamber is considered separately. In both cases, the value of $h_{in}$ is set to quasi-infinity in order to eliminate the its influence.

Symbol
$h_{in}$
Unit
W/(K.m$^2$)
Formulae
$1e+101$cable, GIL
$\frac{N_{fluid} k_{fluid}}{D_{in}}$pipelines
Related
$D_{in}$
$k_{fluid}$
Used in
$U_{buried}$
$U_{inwall}$