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Heat dissipation coefficient for black surfaces in free air

All cables are assumed to be served cables or having a non-metallic surface, and are thus considered to have a black surface. All ducts are also considered to have a black surface, except for metallic ducts, in which case a value of $h_{bs}$ equal to 88% of the value for a black surface is assumed.

It is also the heat dissipation coefficient for cable surface to tunnel wall.

Symbol
$h_{bs}$
Unit
W/m$^2$/K$^{5/4}$
Formulas
$\frac{Z_{bs}}{{D_o}^{g_{bs}}}+E_{bs}$
Related
$D_o$
Outer diameter [m]
$E_{bs}$
Installation constant E
$g_{bs}$
Installation constant g
$Z_{bs}$
Installation constant Z
Used in
$K_A$
Coefficient K in air
$T_{4iii}$
Thermal resistance ambient [K.m/W]
$T_{sa}$
Thermal resistance convection object—air [K.m/W]