Reconsider the heat exchanger of Problem 6.38 with an externally finned-tube bundle used to enhance the air-side heat transfer coefficient in the pre-heater. If thin circular fins are attached on the outside of the tubes such that the surface extension ratio ε = 1.12,  determine the average air-flow heat transfer coefficient for this finned-tube bank. Compare the results with those for the unfinned or bare tube bank and comment upon the extent of heat transfer enhancement achieved by adding fins.