An attractive method of energy conservation in industrial settings is to utilize waste heat from largescale power generating equipment. In one such scheme, the exhaust gases from the diesel engine of an industrial power generator are channeled through a cross- flow, compact, waste-heat recovery boiler to produce process steam. In one such heat exchanger, a finned tube bundle is configured with tubes of 2.5 cm outer diameter that are arranged in a staggered equilateral- triangular array with tube center-to-center pitch of 7.5 cm. Engine exhaust gas (thermal properties of which are taken to be the same as those for air) at 400°C is in cross-flow over the tube bank with a free stream velocity of 2.0 m/s. In order to enhance the gas-side convective heat transfer, the tubes have thin circular

Question

fins on their outer surface and the finned surface extension ratio ?=1.15. Due to the steam generation process inside the tubes, their outer surface temperature is 175°C. If there are 10 rows of tubes in the flow direction (NL = 10), calculate the average heat transfer coefficient and pressure drop for the gas flow stream.



GIVEN



FIND

The average heat transfer coefficient (c h) and pressure drop in gas flow stream.

ASSUMPTIONS

Steady state

Tube wall temperature is uniform and constant

SKETCH



PROPERTIES AND CONSTANTS

Answer 1

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

velocity is



The Reynolds number is



Applying for finned tube bundle in cross flow ReD=1160 for staggered tube

arrangement



Where for ReD=1160



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