The external heat transfer coefficient (or outside film coefficient) is a convection coefficient, calculated from the Nusselt number of the ambient fluid.
Acc. to a publication from Bai2005, empirical correlations are commonly used to determine the value of h for unburied pipes in conditions of natural or forced convection. These correlations normally assume that the pipe is suspended in an infinite medium. This assumption, reasonable for pipes in free span, is questionable for embedded or trenched pipelines.
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 |
