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Hey all you airflow experts!!

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Apr 21, 2001
OH-Heartland of the USA
If a duct system is constructed to port fresh cool air from outside of the case directly into the intake of the HSF, which would be better
ROUND duct???

I am reminded that the cold air return on home heating systems uses SQUARE duct!!
i would imagine that round would be best due to shear forces but the man who'd answer this question best would probably be HOOT and i was an hvac tech for 3 yrs and there is not any particular reason that we used square for intakes it just what ever we could cram "and i mean cram" in there and 90 % of the time only about 3 feet of its square "jus 4 looks" then they pipe in
The reason the house ducts are square is because of where they are usually run, between joists or studs. The best, highest volume shape for that is square. As Dozier said, nowadays the plenum is square and the rest is flexduct. Hoot is probably the most knowledgeable in this area, but I will hazard a guess. If you are running an axial fan then round is better. If it is a
squirrel cage, like what Hoot has, then square would be better.
round duct is The most efficient shape for moving air. for a given area (duct or port size), the surface area or circumference of round duct is always less than that of a square duct with/same internal area. less surface area equals less resistance. the rougher the surface the more friction the air sees. More important at these air speeds though, is how flat the duct is parallel to the airflow. corrogated or flex duct even though round, is horribly inefficient. rectangular duct depending on how severe, would come next, then square. for what you are using it for, round or square won't make much difference. I'm a sheetmetal worker for lu# 33, and i work with this every day.
Is it possible to have a duct pulling too much air, I'm thinking of hooking up a duct from HSF to a blowhole using a 120mm fan. Is this a problem?
I have used both square and round ducting in my experiments with centrifugal blowers and from my experience, with such short runs, they are a "Horse Apiece". The square duct is nice because you can fit it easily to a square fan base. My gut feeling is that with axial fans, round should be a little better aerodynamically, since the air that comes out of an axial fan is spiralling.

Rocknindy, your wording confused me as to whether the ducted blowhole will be blowing air into the case or drawing it out. In my opinion, blowing in would be a better approach if the HS fan is blowing into the HS. If it is exhausting air from the HS, then the blowhole should be blowing out.

There's no problem I can think of resulting from too much air blowing in, but the converse is not the same. If you put a powerful 120mm fan sucking air out of the case, it may exceed the airflow rate of the other fans that blow into the case. In that scenario, you would have a negative case pressure and be inviting dust to invade you case through every crack and crevice.

That's the Hoot Scoop