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:
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.