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2006-10-31, 3:24pm
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Max free air CFM of 12in round duct
Can anyone tell me the maximum free air flow through an open smooth 12 inch round duct?
THanks!
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Lenora
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2006-10-31, 3:46pm
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Gentleman of Leisure
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Quote:
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Originally Posted by lenorasdesigns
Can anyone tell me the maximum free air flow through an open smooth 12 inch round duct?
THanks!
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Zero..... Unless you have low pressure on one end of duct and air pressure (atmospheric) on the other end there will be no air flow....
Now if you have a fan pulling air out of a room at the rate of 500cfm (creating low pressure in room) .... The air flow through duct (fresh air in) to fill the room will be 500cfm.... There is no real free flow as it is, there must be some force pushing (or pulling) air through duct to get flow...
At some point it gets ridiculous to try and draw (force) a extremely large volume of air through duct, but I don't think you will ever come near that extreme building a glass studio...
Dale
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2006-10-31, 4:16pm
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Okay, I have a 900 CFM fan pulling air out of the room. Is a 12" open duct large enough for my makeup air?
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Lenora
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2006-10-31, 8:43pm
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Yes... If it has very little restrictions...
Dale
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2006-11-01, 5:25am
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A 12" duct can handle approximately 700 CFM of fresh air intake.
An 8" duct can handle approximately 350 CFM of fresh air intake.
A 12" duct has twice the square inch area as a 8" duct (pi r squared).
So, no, a 12" duct cannot handle 900 CFM of fresh air.
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2006-11-01, 9:00am
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Then are we to assume the 900cfm fan can only effectively move 700cfm of air through area?
Dale
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2006-11-01, 9:20am
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I'd use the word efficiently instead of effectively.
You have to consider the whole system - fresh air in, bad air out. What will happen in a situation like this where there is a 200 CFM (I hate to use the word restriction) difference in powered exhaust and free flowing fresh air is that there will be an increase in SP somewhere in the system.
The fan will still pull/push 900 CFM, and 900 CFM will flow into the space, but it won't be free flowing, and there will be SP issues in the entire system.
In a large space, such as a garage or good sized room, this difference is rarely noticable, but in a small room it can make a difference.
The key point is "free flow".
In this case, what I'd do would be to duct the fresh air using a rectangular duct that measures at least 160 square inches to get to the 900 CFM free flow number. You can also use a 12" round and an 8" round, or a pair of 12" round ducts, but I'd rather make one hole than two.
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2006-11-01, 10:33am
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Thanks guys! I think I got the info I needed!!!
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Lenora
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