Airflow Questions

[Aldakoopa]

Ok, I have a few questions.

First off, is there any difference on whether you mount a dust filter in front of or behind a fan? I have the Antec 300 case, so the front fans have the filter in front, but I have a fan on the side with a filter behind the fan because I just couldn't get it to work in front. I know that filters block some of the airflow, but does it make a difference if the fan is pushing or pulling it through the filter?

Secondly, I'm assuming the air is limited by the most restrictive point, but I'm wondering if I'm wrong and if it may be cumulative. For example, if the filter blocks more airflow than the the hexagon mesh on the side of my case, will removing the mesh make a difference at all?

Last, what about turbulence? I'm assuming this is the opposite of the previous question and that it is in fact cumulative, so would removing the mesh and leaving the filter remove some of the noise, or would it be exactly the same?

I was considering doing some mods to my case and removing all the mesh. Mostly to help reduce noise so the last question is in fact the most relevant to my plans. The front mesh looks especially constricting, and I'm that's also where the most of my turbulence noise is coming from. I don't hear the fans running at all, only the sound of air rushing through the mesh/filters.

 

[Weatherlite]

I am definitely by no means an expert on airflow dynamics, but I do have a bit of experience with it in some of the things I've done over the years.

To address your main concern, in my experience the effect of multiple restrictions on noise levels will be cumulative. There are so many things to consider if you want to get technical about it, but I imagine you don't care about actual stats, just perceived levels. Also, I would imagine that the closer to the turbulence the filter is, the more noise it would produce. Just seems like a more linear flow of air would make less noise when going through the medium. But, since running ductwork just to place filters farther away from the fan is inconvenient and impractical, I guess we'll just have to live with it. lol Hmm...that does bring a thought though. I wonder if any air-cooled uber-enthusiast out there would consider setting up a bunch of ductwork and running it to another room just to eliminate fan noise? Could even keep that room chilled with AC to supply chilled air! rofl!!

Anyway, as to filter placement, I saw no discernible difference whether it was placed before or after the fans I dealt with. The only factors were whether the fan had enough static pressure to overcome the extra restriction, and the convenience of placement. Well, also there was the concern of the filter medium getting sucked into the fan if there wasn't a decent support between it and the fan, which is why we usually put them after the fan.

As for your restriction question, let's assume your case has a maximum outflow of 300CFM and a maximum inflow of 250CFM stock due to design...the fans can do 400 theoretically. Not optimum, but not bad. Now we add a restriction, the filter, and we're only getting 240CFM. If you remove the mesh you now get 265CFM. Add in the filter and you're down to 255. A net improvement with cleaner air. Obviously there are too many variables to account for, but I honestly think that small gauge wire mesh's contribution to restriction of airflow is negligible anyway. Unless you're trying to eek out every last possible CFM I would say leave it alone for airflow reasons, but for noise levels, yep, I would trim that sucker!

 

[Aldakoopa]

I am definitely by no means an expert on airflow dynamics, but I do have a bit of experience with it in some of the things I've done over the years.

To address your main concern, in my experience the effect of multiple restrictions on noise levels will be cumulative. There are so many things to consider if you want to get technical about it, but I imagine you don't care about actual stats, just perceived levels. Also, I would imagine that the closer to the turbulence the filter is, the more noise it would produce. Just seems like a more linear flow of air would make less noise when going through the medium. But, since running ductwork just to place filters farther away from the fan is inconvenient and impractical, I guess we'll just have to live with it. lol Hmm...that does bring a thought though. I wonder if any air-cooled uber-enthusiast out there would consider setting up a bunch of ductwork and running it to another room just to eliminate fan noise? Could even keep that room chilled with AC to supply chilled air! rofl!!

I actually sat and considered the possibility for a few minutes. I have a window unit A/C not too far away from my desk, and I was imagining making a duct that covered half of the A/C vent, ran across the wall for a few feet, and then dropped down and attached to my computer's intake fans.

As for your restriction question, let's assume your case has a maximum outflow of 300CFM and a maximum inflow of 250CFM stock due to design...the fans can do 400 theoretically. Not optimum, but not bad. Now we add a restriction, the filter, and we're only getting 240CFM. If you remove the mesh you now get 265CFM. Add in the filter and you're down to 255. A net improvement with cleaner air. Obviously there are too many variables to account for, but I honestly think that small gauge wire mesh's contribution to restriction of airflow is negligible anyway. Unless you're trying to eek out every last possible CFM I would say leave it alone for airflow reasons, but for noise levels, yep, I would trim that sucker!

I'm imagining this as I do intake/exhaust for a vehicle, with the flow being limited to the smallest point. In other words, if you had a 2" exhaust pipe, and it shrunk to 1 3/4" at the catalytic converter, then back to 2" afterwards, then at the muffler it shrunk to 1 1/2", the entire system would be restricted to the amount of exhaust that 1 1/2" pipe at the muffler would allow. So, even if you made the catalytic converter 2", if you left the muffler alone, it would still be restricted to that 1 1/2" airflow.

 

[robble]

Do you want your fans to get dusty quickly? If not, put the filter on the intake side of the fan.

 

[Aldakoopa]

Do you want your fans to get dusty quickly? If not, put the filter on the intake side of the fan.

With the way I have the filter attached to the fan currently, and how I clean the dust out of the filters would clean the dust off the fan as well. If that's all the difference the position of the filter makes, then I will leave it on the rear.

 

[Aldakoopa]

Just a little update on this, I have confirmed that the noise is accumulative. I removed the circular mesh from the front of my case which bottlenecked a lot of airflow and caused a lot of turbulence, and it made a world of difference in noise and there should be more airflow into the case as well. Now for those hexagon meshes...