Best fan placement for optimal air flow

[Lada]

I'm building a fume extractor for my workbench. I would like to hear your thoughts on the best placement for a fan so I get the maximal suction/airflow.

I made a sketch with the general dimensions. The fan could be either in the A position before the filter or in the B position after the filter. I forgot to draw the funnel transition between the hose and the filter box.
The hose is downsized to achieve higher air speed/suction, like on a vacuum cleaner.
I would like to avoid using two fans but it's not excluded. I still didn't get the housing but I'm planning to get a smaller plastic box from Ikea.

 

[russ_watters]

There's no impact on airflow but I'd probably put the fan after the filter to keep it clean.

 

[berkeman]

Welcome to PF.

The fan could be either in the A position before the filter or in the B position after the filter.

Why do you have a filter in a fume extractor? What kind of gasses/smoke/fumes are you going to be extracting? Anything flammable?

 

[Lada]

Welcome to PF.
Why do you have a filter in a fume extractor? What kind of gasses/smoke/fumes are you going to be extracting? Anything flammable?

Thank you!
Back in the day I was into computer hardware so I researched the best cooling methods. I read theory and real life tests about fans but most of it evaporated out of my head.
I need a fume extractor for soldering. Nothing really toxic but it is toxic. Many people don't use any at all but the fumes are not exactly healthy and they stink.
Factory fume extractors are pretty expensive for what they are and they don't work at all. Poor suction and no filtering since the filters are just a coarse "sponge" like coarse sponges for aquarium filters.

https://www.ubuy.hu/en/product/2F5Z...e-prevention-absorber-diy-working-fan-for-sol

You also need to keep them close to the work area due to the poor performance and my design should be better with actual filtering. I was planning to mount it under the desk so the hose can be bent to the side without taking much space. It also shouldn't have to be too close to the working area and I plan to use a car cabin filter or kitchen extractor filter.
Professional solutions are much better but very expensive, big and often loud.
I'll rarely use mine since I'm not a professional and this will probably be an overkill but also a fun project.

 

[berkeman]

I need a fume extractor for soldering. Nothing really toxic but it is toxic. Many people don't use any at all but the fumes are not exactly healthy and they stink.

Ah, okay. I've used several soldering fume extractors/hoods over the years, so I know what you are asking about.

At one electronics lab where I worked, we had a 2-station fume extractor that was very powerful (exhausted on the roof of a 3-story building with the HVAC), and we had to put wire mesh over the intake openings to keep from having our schematics and paperworked sucked up and out of the room. It was that powerful.

At the last couple lab locations, we use desktop fan filters much like you linked. They work well if you can position them close to your work (and the binocular microscopes that we typically use for soldering and rework):

https://www.walmart.com/c/kp/soldering-fan-filter

 

[sophiecentaur]

The hose is downsized to achieve higher air speed/suction, like on a vacuum cleaner.

A vacuum cleaner uses a small bore tube so that it can be easily stored and moved about. Do you need to move the inlet about? A wide bore will pass more air for a given pressure / fan noise . Big low speed beats small high speed in the squeaking department.

 

[Lnewqban]

The most important thing to consider is the capture velocity at, or as close to as practically possible, the point where contaminants are produced.
Please, see:
https://www.engineeringtoolbox.com/exhaust-hoods-d_1036.html

For additional ideas, do a research on laminar flow hoods.

 

[berkeman]

At the last couple lab locations, we use desktop fan filters much like you linked. They work well if you can position them close to your work (and the binocular microscopes that we typically use for soldering and rework):

BTW, one thing that can help a lot (and make it so you can set the filter/fan unit a little farther clear of your work area) is if you set up a small fan (like from a PC) to blow across your work area toward the filter/fan. That gets the solder smoke moving sideways right away, instead of partially up toward you before the filter/fan unit pulls it in.

 

[erobz]

You don't want to run a small duct, you want a large duct. If you are looking to increase the capture velocity, do the "necking down" near the work.

 

[sophiecentaur]

That gets the solder smoke moving sideways right away,

Yes: To put it more crudely: "Blow" is a lot more directional than "suck".

 

[jrmichler]

When sucking through a hose with a round end, the capture velocity drops off very fast with distance from the opening. The capture velocity at one diameter from the end of the hose is about 10% of the velocity through the hose. The sketch shows the boundary of about 10% velocity as a dashed line:

Decreasing hose diameter at the suction end increases velocity through the hose at that point, but the restriction decreases total flow through the hose. The end result is worse fume capture at the same total distance from the hose end.

 

[sophiecentaur]

the capture velocity drops off very fast with distance from the opening.

Pretty well inverse square law.