Use fans in parallel, not in series?

LaidbackluxLaidbacklux New Haven
edited June 2003 in Hardware
I'm designing a case that was going to have a 172mm rotron exhaust and a 172 rotron intake, but then I read this on siliconacoustic

Use fans in parallel, not in series
For example, if your case has four possible fan locations then using two fans as exhaust (parallel) is better then using one as exhaust and one as intake (series). In a free air condition two identical fans in parallel will almost double the airflow. Two fans placed in series will produce only slightly greater flow then one.

So now i am thinking that i'll just have 2 exhaust rotrons? Has anyone done this and can confirm that this would be better than in and out fans?

The case will be ventless, just open for intake and exhaust as well as for the psu. So unfiltered air won't really get in from all the negative pressure.

Suggestions please

Comments

  • EnverexEnverex Worcester, UK Icrontian
    edited June 2003
    Your front exposed drives will get filled with crap and so will every single point in which air can possibly get into the case as you would have serious negative preasure and nothing countering it, even tiny points which you may not even notice, but the drives are the biggest issue, unless you are in some sort of pure clean room or something.

    NS
  • Geeky1Geeky1 University of the Pacific (Stockton, CA, USA)
    edited June 2003
    1. It depends on the location of the fans; 2 poorly placed fans can be worse than 1 well-placed one.
    2. Having 2 exhaust fans is nowhere near free-air conditions. Check out the flow curves on those fans on Comair-Rotron's website (www.comairrotron.com) and you'll find that as soon as they have to push/pull with any kind of force, their airflow drops like a rock. This is true of all axial fans to some degree. So, unless you have enough air holes to allow them to pull in air at very low pressure, you're probably better off with the standard intake/exhaust configuration. You said the case would be ventless. So then how in the h*ll are you planning on getting air into it if the fans are all exhaust fans?!? See above ^ for details, but you have the same problem- you'll have virtually no airflow.

    BTW, those rotron fans are sweet. I bought 4 of them, and I love 'em...
  • Ed-ChigliakEd-Chigliak West Yorks (UK)
    edited June 2003
    If each fan is 200 CFM

    200CFM supply & 200CFM extract = 200CFM through and neutral case pressure. Fit a filter on the supply and the case pressure will go negative although with the PSU extracting you are probably negative already.

    200CFM extract & 200CFM extract = maybe 300CFM through and negative case pressure and make up air will be by infiltration. Have large openings specifically for air infiltration and the air will choose the path of least resistance you have provided. This will significantly reduce the infiltration through drives etc but not eliminate it entirely so over time dust will accumulate to some degree. If you fit filters to the infiltration openings you will reduce the free area significantly and more air will pull through the drives. Choose not to fit filters and your main board will get dusty. I've got an old 486 full of dust and works OK. Dust smells bad when it burns!! I don't advise this arrangement.

    200CFM supply & 200CFM supply = maybe 300CFM through and positive case pressure. Air will leak out of the case. Have large openings for air leakage and again the air will choose the path of least resistance you have provide. Fit bag filters to the back of the fans and you have yourself a clean system but the extra resistance will lower the throughput. Bag filters will need changing and fans will get dusty unless you prefilter before the fans. Bag filters have increased surface area and present less resistance to the air flow. I'm sure there is something wrong with this arrangement probably dead air or something.

    I think that's how it works. Others should validate this or correct me if I am wrong. I run one supply & one extract and I am happy with this.

    Two fans in parallel will not be double the volume because close together their 'pull' will overlap although this could be improved with a divide. Two in parallel will be more air through than two in series so long as the infiltration / leakage openings you provide are nice and big. How big I would guess the free area should be atleast equal to the size of the fans. Calculations too difficult so numbers are made up.
  • LaidbackluxLaidbacklux New Haven
    edited June 2003
    excellent points thus far, I was a bit vague in my descripotion of the case before. The original design was to have a horizontal divide about 8 cm from the bottom of the case where the intake fan would sit blowing air upward. Intake vents (area about 130% of area of rotron to make up for filters) would be on the sides of the case below the divide. This was designed with the goal of filtering the intake without slowing down the fan, front intake just isn't big enough.
    When i said the case would have no vents i was referring to the non fan vents that many cases hace (my fong kai has a series of holes next to the pci slots and also in front of the hd cage).
    BUT, then this parallel/series thing came up. A positive pressure case with dual intake seems preferable as long as the case has an easy way out for the air. I am thinking that if the open vent area is equal to that of the fan, then there should not be major static pressure effects.
    BUT, having such large open vents would mean large amounts of sound could escape. This is largely subdued by the quietness of the rotrons i suppose but i wonder if there is a good material to use to cover the large vents keeping them porous yet swallowing some of that sound??
    ANYWAY, many things to tackle, but i think puting two fans on the horizontal division as intake could be a good idea. I could also easily put another division between the two fans so they don't draw from the same air.

    I really do appreciate everyone's comments.
Sign In or Register to comment.