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I recently had the need to quickly build a fume extractor for soldering, and since at the place there was a ton of 80mm CPU fans lying around, I used those to build something like this (not to scale):

enter image description here

Where the components were (all airtight "sealed" with duct tape)

  1. Intake of an old clothes dryer hose (100mm diameter, a bit more than 1m long when extended, fixed to some big metal lump to have it stay in place). This is positioned dynamically to suck in soldering fumes
  2. the hose itself
  3. A cut off pyramid shaped cardboard construction
  4. A 3x3 stack of 80mm CPU fans. Directly behind it, the air comes out (future enhancement might be some coal filter)

So the airflow is from 1 to 4.

This thing did its job, but not in any terrific way. So I was wondering if anyone with experience in fluid dynamics could tell me how to improve it, or more specifically which of my ideas to improve it might make sense.

For the sake of this question, lets say the resources are limited to about the items described, reasonable additions that most people have in their workshop included (so e.g. hot glue is fine, but I am not looking for "add a bigger fan" kind of enhancements).

The items are:

  • The hose (2m of it, 100mm diameter)
  • Enough cardboard
  • 10 fans, 80mm diameter, all assumed to be equal in performance

My ideas now included:

  • instead of making 3x3 fan configuration, I take two 2x2 configurations and put them "in series" (that is, a box like 4 to its right, with reasonable space in between, whatever that space needs to be)
  • at point 3, add an additional 80mm fan or
  • at point 2, add an additional 80mm fan or
  • at point 1, add an additional 80mm fan (all assuming for that additional point I would only have one last fan left in case of a 3x3 configuration)

Which of these ideas (or combinations of one additional fan and the other fan configurations) would noticeably improve the performance of the device (that is, move more air)?

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If the fans have high air flow but low static pressure capabilities you would be better off with the 2x2 in series to overcome the flow restriction of the dryer hose.

There are other ways to improve flow too...

If the pyramid looks like the picture and assuming your fans are mounted on point 4, and that point 4 is the intake, the cone of the pyramid needs to be much longer. If the fan is mounted right next to a 45deg bend, you are losing airflow just from that. The longer the cone, the less the angle the air needs to change, the better your flow will be.

Also your using expanding clothes dryer hose that is likely ribbed on the inside, if possible run 2 hoses in parallel since I think that hose is restricting your flow.

For the best flow, if you have a large amount of cardboard and tape available, just build a duct out of cardboard that keeps at least the same cross sectional area.

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  • $\begingroup$ Oh, I guess that by mentioning that it is a fume extractor and the way I would use it I thought the airflow direction was clear, but I now see that it isn't. I will update the question accordingly. $\endgroup$
    – PlasmaHH
    Sep 23 '15 at 14:14

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