Tubes can be finned on both the interior and exterior.
This is probably the oldest form of heat transfer
enhancement. Finning is usually desirable when the fluid
has a relatively low heat transfer film coefficient as does a
gas. The fin not only increases the film coefficient with
added turbulence but also increases the heat transfer
surface area. This added performance results in higher
pressure drop. However, as with any additional surface
area, the fin area must be adjusted by efficiency. This fin
efficiency leads to an optimum fin height with respect to
heat transfer. Most of the heat transfer and film coefficients
for finned tubes are available in the open literature and
supported in most commercial heat exchanger rating
packages. Recent papers also describe predicting finned
tube performance.