filter hurting airflow

[henmaster]

I posted a thread here about a week ago inquiring about what to use as a filter. Since then I have tried 3 different materials, and all of them hurt my airflow too much to be useful. I have experimented with dryer sheets, panty hose, and air conditioning filters. The first two are pretty thin so it wouldnt seem like they would pose too much resistance, but the fact of the matter is that whenever one of these materials is in front of the intake fan the case temperature goes up a good 2 degrees C.

I am beginning to think that perhaps this fan doesn't have enough "pressure", despite being rated at 75 cfms (which is most likely under ideal conditions). Is there a technical term for this "pressure", preferably some spec that I can look at when I purchase a fan? This one is a 120mm radio shack fan. If there's no way I can tell in advance how well a fan will react to such non-ideal conditions, could those of you that maintain good airflow even using a filter post what fan you are using and what material you are using to filter? I am looking at the panaflo 120mm fans now (considering the L1A and the M1A), so if anyone could tell me how these do with a filter i would especially appreciate it. thanks.

henmaster

 

[sharkeeper]

Vaneaxial fan airflow is always expressed under "free air" conditions. This means the fan is sitting out in the open with absolutely no resistance whatsoever. If a design calls for 75 CFM at x inches WC, then a fan with considerably higher CFM free air must be used. One must also consider the extra noise when the fan is pushing air through a filter. If the fan is pulling air through a filter, (not recommended) it will make even more noise due to eddies.

The best setup to deliver good air volume and have clean air is to use a centrifugal blower mounted in a separate enclosure often located in a different place than your pc. Air can be delivered via flexible hose and the case is pressurised a few inches WC above ambient. Having negative pressure in the case will bring contaminants in through every crack and all your removable media drives. Positive pressure with clean, cool air keeps everything dust free. Some people even use a portable A/C unit (condensor exhaust ducted outside) to keep their pc insides chilly!

Cheers!

 

[DARRIN]

Drop me an Email with your address and I'll send you some filter material that I think would work quite well.

 

[Sundog]

YG e-mail

 

[thephew]

The Lian-Li cases come with a sort of spongelike filter material in front of the intake fans. It seems to work well, and there is decent airflow. Your temps might go up a couple C with filters, but a colony of dust bunnies will cause a video card or Athlon to overheat in no time. You are better off with filtered air. My old computer was so full of dust and crap that I had to run my TNT2 Ultra at 10 MHz below stock core speed or it would crash from overheat.

 

[TunaBoo]

Human flesh makes a good filter.

 

[DARRIN]

<< YG e-mail >>



No I don't.

 

[wfbberzerker]

tunaboo just crossed the line

i dont think that a normal filter could hurt that much... 2 degrees C isn't too bad, but it all depends at what kind of cooling you have. I would rather have my system go 2 degrees higher, than having it choke on dust (i.e. dust and crap making everything overheat more than filter would do). for example, when i upgraded my system, i blew all the dust off my components, and it filled up my entire room with a huge cloud.

 

[TunaBoo]

<< tunaboo just crossed the line

i dont think that a normal filter could hurt that much... 2 degrees C isn't too bad, but it all depends at what kind of cooling you have. I would rather have my system go 2 degrees higher, than having it choke on dust (i.e. dust and crap making everything overheat more than filter would do). for example, when i upgraded my system, i blew all the dust off my components, and it filled up my entire room with a huge cloud. >>



What line?

 

[henmaster]

My input cfm is a lot greater than my output, so I do have positive pressure in the case. Do I still have to worry about dust settling on and damaging my components?

 

[henmaster]

sharkeeper: I think the airflow hit that my fan is taking is higher than normal. Other people report using these materials as filters and not losing many cfms. I would estimate that my input cfms are at least cut in half. So if some fans are better at dealing with non-free air conditions than others, what guidelines should I use in choosing a fan to do this job?

 

[sharkeeper]

The more efficient a filter is, the higher the pressure drop is going to be. Technically chicken wire is a filter, it would prevent chickens and kittens from being sucked into a large computer! Of course, dust is much smaller. Smoke particles require extremely fine filtration (HEPA) which requires considerable energy to push air through it. Foam filters can be had in different grades, their efficiency and pressure drop correlate directly to the cell size in the foam. Dust certainly won't harm computer components other than make things look untidy if dust is really bad. It may be more feasible in your case to filter the air in the entire room as well. If you computer gets dusty in a week, imagine what you're inhaling!

Choosing a fan based on backpressure is no easy task. As a fairly efficient filter gets loaded with particulates, the pressure drop will increase. Small muffin vaneaxial fans are designed to move relatively low volumes of air with little or no back pressure. If you must filter the air within the PC, several fans drawing air OUT of the case are recommended with a filter covering the inlet. Of course, this negative pressure within the case draws dust in through every little crack and cranny, and your CD and floppy drives!

Cheers!

 

[DARRIN]

So basically you should have more fans pushing air into the case so that it is not pulling air through the CR ROM and floppy. Good point.

 

[TunaBoo]

Too much + pressure can be bad too, as it makes dead spots. However, it doesnt have to be perfect. Have input and output within 20 CFM or so of each other and it will be fine, within a degree or so of as good as it will get.