Question Additional dust filters for case?

Jun 28, 2020
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I recently built a new PC using a Fractal Design Define 7 Compact case. The case is really good, and comes with dust filters in the front, top, and bottom, but for some reason there are no dust filters in the back. My previous PC case didn't have any dust filters except at the front, and the components gathered a lot of dust. Therefore, for my new PC, I would like to add additional dust filter and make it completely dust-proof.

The area in the back where there are no filters consists of:
  1. The fan mount, 14x12cm.
  2. The expansion slots under the GPU, 10x10cm.
  3. The area above the expansion slots, 17x7cm.
For the fan I can actually screw a filter in, and I found some on Amazon that I am considering buying (although I would still like tips on what would be the best choice). But for the other two areas, I think I will have to get magnetic filters I can just stick to the back of the case, and I can't find filters in the appropriate sizes - I could only find 8x8, 12x12, 14x14, and 24x12.

Any advice?
 

tennis2

Honorable
Hardly any cases come with a dust filter for the rear fan since that's always an exhaust fan. No dust is going to get into a hole that a fan is blowing out of.

In relation to the expansion slots - you generally want to have more air being pushed into your case (through filtered fan locations) than out (positive pressure). In that way, there will be some airflow out of every opening. So not having filtered expansion slots isn't an issue. If you end up with the opposite (negative pressure, more air being drawn out of the case than in) then air will be sucked in through any/all available openings which brings dust with it.
 
Jun 28, 2020
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Thanks for your reply! You're right, it makes sense that the exhaust fan won't need a dust filter.

But regarding positive pressure - I hear that's less good for cooling purposes. I'm aiming for neutral pressure, so I have two intake fans in the front (12 and 14 cm) and two exhaust fans, 12 cm in the back and 14 cm at the top.

Actually, now that I think about it, I suspect there is actually negative pressure, since one exhaust fan has no filter and the other 3 fans all have filters, so I'm getting more air pushed out than in!

So this leads me to two questions:

1. If I want to ensure positive pressure, should I rearrange the fans as two 14 cm intake and two 12 cm exhaust (with all of them except one of the 12 cm going through filters)?

2. If I do have negative pressure, then I think although dust won't come in through the exhaust fan hole, it could still come in through the expansion slots and the space above them. So I would still like to know where to find a dust filter that would cover those openings (10x10cm and 17x7cm).
 

tennis2

Honorable
Yes, filters reduce a fans CFM (to varying degrees), but I doubt its' reducing each intake to less than 1/3 its original rated CFM.

You also have to pay attention to fan RPMs. Especially with variable rpm fans. On the package it may say 80CFM, but that's usually at 100% fan speed. Some manufacturers will have a rpm vs CFM graph on their webpage.

2. - No dust is going to go through a fan against it's airflow direction, regardless of case pressurization. If you have negative air pressure in the case, air/dust will be sucked in though the slotted expansion slots, air gaps in the case panels, etc.
 
Jun 28, 2020
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Thanks, but I think you may have misread my previous reply - perhaps I wasn't clear.

1. Without filters I would have equal intake and exhaust, because I am using two sets of identical fans (same size, same company) - doesn't matter what the CFM is. With filters, even if they reduce CFM just by 10% (for example), it still means I am getting 10% less air in vs. 5% less air out, so overall I have negative pressure. Therefore, I am still wondering whether using the two smaller fans as exhaust makes sense in order to avoid negative pressure.

2. Yes, I understand that no dust is going through the exhaust fan. As I said, I am not worried about dust going in through the exhaust fan, I am worried about dust going in through the expansion slots, and therefore my original question still remains - where can I buy dust filters to cover those holes?
 
Install 2x140mm (or 3x 120mm) intake fans at front and
1x 120mm fan exhaust fan for positive pressure and forget about dust.

Neutral pressure is very hard to achieve (close to impossible with variable rpm fans). You can forget about it. To set up neutral pressure, you'll have to run fans at constant speed.
 
Reactions: tennis2
Jun 28, 2020
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Install 2x140mm (or 3x 120mm) intake fans at front and
1x 120mm fan exhaust fan for positive pressure and forget about dust.
That sounds like a lot of positive pressure. Won't that significantly affect cooling efficiency? I would rather have neutral pressure and install additional dust filters...
 

tennis2

Honorable
With positive pressure, excess air is being forced out of the vented expansion slots, etc.

Yes, if you have ALL intake fans and no exhaust, then cooling could be impacted. 2 front intakes and 1 rear exhaust is very standard.
 
Jun 28, 2020
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With positive pressure, excess air is being forced out of the vented expansion slots, etc.

Yes, if you have ALL intake fans and no exhaust, then cooling could be impacted. 2 front intakes and 1 rear exhaust is very standard.
I see. So with my 4 fans, what would be the optimal arrangement? The two 14 cm as intake and the two 12 cm as exhaust?
 

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