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 ?=1.12, determine the average air-flow heat transfer coefficient for this finned-tube bank. Compare the results with those for the un-finned or bare tube bank and comment upon the extent of heat transfer enhancement achieved by adding fins.

Question

GIVEN



FIND

The average heat transfer coefficient (c h) and extent of heat transfer enhancement achieved by

adding fins.

ASSUMPTIONS

Steady state

PROPERTIES AND CONSTANTS

Answer 1

Therefore, the minimum free flow area is between adjacent tubes in a row, and the maximum air

velocity



The Reynolds number is





Applying for finned tube bundle in cross flow ReD=7515 for staggered tube

arrangement



Where for ReD=7515



Since, the fin parameters are not given and only surface extension ratio is given.

We have,

Surface extension ratio (?)=Total surface area with fins/ Surface area without fins



Since all the parameters are not given we should consider this problem as design data problem and

select the suitable parameters for fin. For, surface extension ratio of 1.15 taking D0 as 2.85 we have



Thus above equation gives



Nusselt number is applicable to tube banks of ten or more rows. Since there are 10 rows in this case,

correction is not required.



(b) The pressure drop is given by



From