I assume that what is meant by concentric heat exchanger is a double-pipe unit. One of the commercial manufacturers of these units, with and without fins, is Brown Fintube. There may be other manufacturers as well but I am most familiar with this brand. The fins add extra area and are typically longitudinal for the double pipe units.

Heat transfer outside a finned tube is composed of two mechanisms: 1) conduction along the fins and 2) convection between the surface of the fins and the fluid. To calculate the overall heat transfer coefficient for a finned tube (or bundle), one must use the standard equation of summation of resistances (corrected for area). However the outside film coefficient must be corrected for fin efficiency and the fin-bond resistance must also be included:

1/Uo = 1/hc + rw + Ao/(Ai x hi) + rfb

Uo = overall heat transfer coefficient based on outside area (finned)
hc = corrected film heat transfer coefficient for a finned tube – function of fin area, tube area, fin efficiency and a fin convection factor which is dependent on fin geometry
Ao and Ai = outside and inside areas, respectively
hi = inside film heat transfer coefficient
rfb = fin bond resistance