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I'm sorry if this sounds like a beginner question. I've tried to search around, but I may be looking for the wrong terms. I have basic knowledge of fluid-dynamics, but not enough to properly measure a ventilation system.

I've been redirected here from Physics Stack Exchange - I hope this is the right place for my question.

I want to build a fume extractor. Basically, I will have a fan sucking air into a flexible duct, which I will send out of the window, approximately at the same height as the fan, or possibly slightly higher (no more than a couple of meters).

I've learned that if I use a duct that is too small (with regards to cross section) the airflow of the fan will be limited to the point that air may even be pushed back towards the fan. However, I'm afraid that, if I use a duct with a cross section much larger than the section of the fan, inefficiencies may arise and limit the air intake of the system.

I'm already thinking of a larger fan (140 mm instead of the usual 120 mm), but what should be a good size for the duct? Would it be bad to use a 150mm one? Should the sizes match? Should it be larger? Does it matter how long the duct is?

I'm also aware that a rigid duct would perform better than the flexible one, but I'm not really trying to optimize the performance. I'm just trying to discover most of the stupid things I should avoid doing. Thanks for understanding.

EDIT: More details

Here's simple schematics for my project:

Schematics of my project

The fan is secured to a box (made of cardboard, or possibly 3D-printed plastic), and at the other end a duct guides away the extracted air, out of the window or into a carbon filter. In a separate room of the box, cables and electronics don't interfere with the air flow to avoid turbulence.

Does the size of the box matter? Should it just fit the fan and the duct, or could it be any larger? Should it be tapered?

Does the shape of the folds in front of the fan have any importance? Are they really helpful or should I skip them?

I may be overthinking this: I see many people who just glue some cardboard to a fan and they're done. But I see their designs sometimes don't actually extract air well, and I would like to make sure I'm not overlooking important details. Again, I'm not trying to squeeze any little bit of performance out of the system.

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  • $\begingroup$ Length of the duct work will definitely matter because the longer the duct, the more pressure drop you will have. Have you done any of the sizing calculations yet and/or what details about the system and fan do you have? Gas density, required flow rate... $\endgroup$ – J. Ari Jul 18 '17 at 18:04
  • $\begingroup$ @J.Ari Actually I have no idea where to start with that :) I only know that a larger fan moves more air, so I'm leaning towards a 140 mm one. As to the duct, it will probably be between 100-150 mm, and probably 2-4 m long - but I don't plan to extend it completely, the system will be portable and I may be closer to the window than 3 meters. I assume it's better to have a shorter duct, but to keep it straight rather than making it bend hard, right? Would an almost horizontal duct sill create a pressure drop, or do I need to put the far end higher up? $\endgroup$ – Simone Jul 19 '17 at 6:55
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    $\begingroup$ For a fume extraction you don't need any significant pressure drop, just constant flux. Yes, long or twisty duct will reduce the efficiency - by a small margin. Duct more than a little narrower than the fan will cause efficiency drop too - but both effects are nearly insignificant, because as soon as fumes are past the fan, you may get rid of them at your leisure, there's no hurry. Just make sure they aren't sucked back in through the window once released from the pipe end. $\endgroup$ – SF. Jul 19 '17 at 8:20
  • $\begingroup$ What you should worry about is that air is sucked throughout the whole volume of the fume hood, not forming pockets where fumes coalesce, or whorls and back-currents that would blow fumes out from under the hood. THIS is the tricky part - with an easy and failproof (but noisy, heavy, expensive and energy-inefficient) solution of very powerful fan, or a difficult, tricky but optimal solution of engineering the correct air flux through the hood to a fan of throughput just sufficient to remove fumes as they are created and not more. $\endgroup$ – SF. Jul 19 '17 at 8:24
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    $\begingroup$ @Simone: Oh, if that's merely soldering, and not some noxious chemical reactions, there's little to worry about. A little escaping won't cause problems. The two possible causes of back-currents are 1) fan and other effects causing negative pressure differential between the room and the outside, fumes drawn in from outside through cracks between the window and the duct (in your case negligible), 2) eddies in air flow under the hood (before the fan) splitting the air flow, and pushing some of it back down the sides of the hood (again, in your case negligible amounts.) $\endgroup$ – SF. Jul 19 '17 at 9:19

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