Reconsider the heat exchanger and explore the effect of tube center-to- center spacing in the equilateral array of the finned tubes. Consider the tube spacing of 8.75 cm and 10 cm. What is the extent of change in the average heat transfer coefficient and pressure drop? Likewise, what is the extent of change in the thermal and hydrodynamic performance if a square pitch (inline arrangement is considered) with tube center-to-center spacing of 7.5 cm?
GIVEN
Heated air flowing over an equilateral staggered tube bank
Exhaust gas temperature
Tair,in = 400°C
Number of tube rows (NL) = 10
Tube diameter (D) = 2.5 cm= 0.025 m
Center to center pitch(S) = 7.5 cm=0.075 m
Free stream velocity (Ua) = 2 m/s
Tube surface temperature (Tt) = 175°C
Finned surface extension ration (?)= 1.15
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
For spacing of 8.75 cm
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=1083 for staggered tube
arrangement
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
Thus there is reduction in heat transfer by 4.4% and decrease in pressure drop by 12% with increase in
spacing compared
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=1033 for staggered tube
arrangement
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.
Nusselt number is applicable to tube banks of ten or more rows. Since there are 10 rows in this case,
correction is not required.
Thus there is reduction in heat transfer by 7% and decrease in pressure drop by 20% with increase in
spacing compared to
When square pitch of 7.5 cm spacing is considered.
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 inline tube arrangement
(b) The pressure drop is given by
When inline tube arrangement is considered instead of staggered tubes there is reduction in heat
transfer by 16% and increase in pressure drop by 7.4 %.
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