External heat transfer coefficient

The external heat transfer coefficient (or outside film coefficient) is a convection coefficient, calculated from the Nusselt number of the ambient fluid.

If the flow is due to free convection, the heat transfer coefficient and the Nusselt number depend on the Grashof number. In this case, the approximate heat transfer coefficient can be determined using the following numerical equation. (Source:

When the velocity of surrounding fluid is less than approximately 0.05 m/s in water, natural convection will have the dominating influnece and a value of 200 W/(K.m$^2$) may be used.

$\frac{\mathrm{Nu}_w k_w}{D_{ext}}$pipe in water
$2100\sqrt{V_w}+350$approximation for free convection