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Old Jul 16th 2017, 03:52 PM   #1
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Maximize heat transfer in ductwork

Yes, you read that right. In contrast to the typical goal with ductwork, I'm trying to MAXIMIZE the amount of thermal energy transferred from the air inside the ducts to the surrounding environment. Here's the situation:
I have roughly 12 feet of distance to cover with a 4" diameter duct. One end is sucking air in from the exterior of the house through a soffit vent (and I do mean SUCKING; I've installed a 120mm computer fan to the interior side of the vent). By the time the air gets to the other side, I want it to have shed as much heat as possible. This is in an attic, so I have the physical space to do whatever is necessary. I will have a high output gable fan blowing on the ductwork, which itself is sucking in air primarily from a different group of exterior soffit vents.
I considered splitting the 4" duct into two 2" ducts, then again into four 1" ducts. I'm trying to do this for a total of eight 4" ducts (maybe many more in the future) so I thought about arranging them in banks of two or even three high. The problem of course is that the ones in the front will get most of the air from the fan, however I figure there will be plenty of space between the ducts so maybe this won't be a problem. I am delivering air to computer equipment, so in addition to cooling the air, I also want to maximize flow. What sort of ducting should I use? I figured smooth, straight, thin walled aluminum would be best. Would a ribbed flexible tube be better for heat transfer, as it increases surface area? Any other thoughts on this?
What other ideas can you think of? I considered having the 1" ducts run horizontally across the floor through a shallow pool of standing water, which the fan would then blow on to dissipate heat. Would this be dramatically better than air alone? I'm sure that flowing water would be better still, but this would be logistically more difficult (although not impossible, if you guys thought that it would be worth it).
I'm interested to hear what you all think!!


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Old Jul 30th 2017, 12:30 AM   #2
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Old Jul 30th 2017, 01:18 AM   #3
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Hi Stubby .....you don't give the temperature difference between the inside and outside surface of the duct... this is most important.

But generally my instinct , from many decades dealing with heat transfer , tells me whatever you do will not make more than a few degrees of difference to the air temperature , this must be about maximising the airflow ...

I would go with large diameter smooth aluminum ...smaller diameter will only slow your flow, one 4"dia duct will restrict the flow less than 20 @1"dia ..

Think about minimising obstructions like grills on the computer equipment ... watch out for dust accumulating on the electronics ...

For a more accurate answer supply temperatures...are they the same all year ???
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