How much space does a fan need to suck air properly?

Discussion in 'Concepts and Ideations' started by mejobloggs, Feb 7, 2019.

  1. mejobloggs

    mejobloggs Efficiency Noob
    Thread Starter

    Hi guys, is there any info/tests around regarding this?

    I'm building my own custom case with a solid front panel and sides (no airholes), but instead there's gaps to the side.

    Wondering how big I have to make the gap so I dont restrict the fan's abilities

    Added pictures to hopefully explain.

    Example 1:


    Example 2 (from Streamcom DA2):


    A nice picture of the 'front' panels I'm talking about. Except this case has them on top/bottom. Same concept. Except mine would'nt have nice airholes covering the side of the case, so I'd really want to make sure my panel isn't restricting fan/airflow

    Let me know if I'm not explaining properly and I'll try do some more diagrams/images.

    Thanks!
     
  2. bichael

    bichael Trash Compacter

    Seem to remember reading about this before somewhere but can't find it now...
    Looking at the SG06 and SG13 with solid front panels they probably have around 15mm. The solid panels do cool a bit worse than the mesh versions though (and those cases are open on the sides). Complete guess but to give minimal impact I would have thought at least 25mm.
    It's really going to depend on fan type, speed, noise, component heat, if using filters etc though. If critical and you want the smallest gap possible I think the best thing to do would be to find a way of testing it with some sort of mock up.
    Good luck!
     
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  3. tinyitx

    tinyitx King of Cable Management

    I suppose NZXT H200i is a classic example of having a solid front panel with vents on the side panel near the front for intaking air to the front fans.

    This article says the 'air gap' shown in your diagram should be, at least, equal to the diameter of the fan for close to optimal airflow.
    (https://www.hueyliew.com.au/ventilation/fan-trouble-shooting/)

    Then, more practically speaking, this guy recommends the 'air gap' should be, at least, 1/3 of the diameter of the fan.
    (https://www.rs-online.com/designspark/fan-types-why-choose-an-axial-fan)

    So, for 120mm fans, 40mm would be the gap. But then, you might want to consider other factors like aesthetics or case volume increase due to the gap. A compromise would then be made in your final design accordingly.
     
  4. Necere

    Necere King of Cable Management
    NCASE Spatial Philanthropist

    @K888D did some testing on this. These were his results:

    [​IMG]


    As you can see, performance tends to drop off below a distance of about 30-40% of the fan diameter.
     
  5. dondan

    dondan King of Cable Management
    DAN Cases

    I think the minimum distance should be the fan thickness. If you have a 120x120x25 fan the distance should be 25mm.
     
  6. Necere

    Necere King of Cable Management
    NCASE Spatial Philanthropist

    What evidence is there to support that? I would tend to think the opposite, actually: thicker fans should need less clearance than thin fans before their performance drops off. My reasoning is that low intake clearance acts as a restriction on airflow, and all else being equal, a thicker fan will have greater static pressure than a thinner one, and should therefore perform better against restriction.
     
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  7. m0t0k0

    m0t0k0 Minimal Tinkerer

    I think the crossetional area of the intake is going to have a big impact.

    This lines up quite nicely with the RS link as if you left 1/3 of the diameter gap all the way around the circumstance you would end up with about 25% more intake area. This would ensure the fan would not be choked.

    It would be difficult to match or exceed the fan intake area in a case build panel tho.
    Because you are only open at the top/bottom or sides at any one time.

    Having a vent on just one side you would need around 90mm clearance.
    But you can half that as you will be open on two side giving 45mm
    Anything less and your intake area is less than the intake area of the fan.

    So 40mm seems to be on the money as a good compromise.
     
  8. mejobloggs

    mejobloggs Efficiency Noob
    Thread Starter

    #8 mejobloggs, Feb 10, 2019 at 4:37 PM
    Last edited: Feb 10, 2019 at 4:50 PM
    Oh wow, these are very interesting replies, thank you everyone. Some good stuff to read.

    I was planning on using Noctua 20cm fans, so if a healthy airgap is based on the diameter I'll need a pretty big gap!

    Since I'm planning on 20cm fans I think I'll scrap the idea of solid front panel then. Easy enough to put mesh or airholes on a front panel.

    I was considering doing a 'vertical airflow' tower as well. With a 20cm intake fan at the bottom blowing upward. This would mean the floor is at the bottom so couldn't put mesh/airholes, so the 'vertical airflow' tower I'd probably be looking at having a healthy 10cm+ gap down the bottom

    The vertical tower should be easy enough to test though, as I can just raise it up and down with some corner blocks and measure cooling affect

    Thanks. Not too worried about having the smallest gap possible, it's just at a certain point I feel if I have to make the gap significantly large I'd rather change my design and not have a solid front panel. Just trying to find the optimal point :) (also this info is great to know for vertical airflow as I previously mentioned)
     
  9. Elerek

    Elerek Cable-Tie Ninja

    #9 Elerek, Feb 11, 2019 at 12:49 PM
    Last edited: Feb 11, 2019 at 1:14 PM
    This is just speculation, I don't have data, but I imagine both of these are correct depending on your expectations. Yes, a thicker fan should be able to move as much air as a slim fan with less breathing room. But it's also designed to move more air than the slim fan. To see that increase in airflow I would expect it to need a larger air gap. Basically I would expect a thicker fan's performance to start dropping sooner but not as harshly as a slim fan as you move a surface closer to them, but it'll probably move more air at most if not all points.

    Edit: However if the fan's thickness eats into it's breathing room it's quite possible that the thinner fan with more breathing room could move more air than the thick fan with less breathing room. Edit 2: Actually just like the yellow s red lines in your graph! The 140x25mm fan starts losing airflow immediately while the 120x15mm fan doesn't start to drop off till the very end, but it still pushes less air under most circumstances.
     
  10. Necere

    Necere King of Cable Management
    NCASE Spatial Philanthropist

    However the Prolimatech is also 15mm thick, and it has a similar curve to the 140x25mm Silverstone. My guess as to why the 120x15mm Silverstone stays flat before the drop off is that intake restriction beyond a certain distance doesn't have much affect on a fan pushing lower CFM. Note the 120x15mm is moving much less air compared to the 140x25mm across the entire graph.
     
  11. Elerek

    Elerek Cable-Tie Ninja

    Exactly, that was my point. The thicker fan moves more air at almost all points on the graph, but it starts losing cfm much earlier than the slim fan. So yes it performs better than the slim fan with the same airflow restriction, but it is still impeded before the slim fan is.

    The OP's question was "Wondering how big I have to make the gap so I don't restrict the fan's abilities". The thicker fan has it's abilities restricted before the slim fan does, it just also still out performs it.
     
  12. Necere

    Necere King of Cable Management
    NCASE Spatial Philanthropist

    You've missed my point. Three of the four fans tested are slim fans, and only the one with the lowest CFM shows no gradual decrease in airflow. The 140x15mm, especially, has a very similar curve to the 140x25mm. So this data, at least, doesn't support your hypothesis.
     
  13. Elerek

    Elerek Cable-Tie Ninja

    That's fair, I guess my assumption is that the average 25mm fan has a higher cfm than the average 15mm fan. But really all that matters is cfm and static pressure, not thickness, regardless of if there is a correlation.