About 5-6 years ago I built a nice workbench with an integrated (diy) fume extractor complete with an anti-static Loc-Tite hose, on/off sw, and fan speed control. Problem is that it sucks... in that it doesn't suck fumes very well. I want to make it suck more.

The current design is simple. Two 120mm fans are housed in a 6" cube steel enclosure, one on the inlet and other on the outlet. A reducer is attached to the case to reduce the tube size to that of the hose's coupling, then there's the hose. The filter is cheap piece of activated carbon that fits in a hose piece that is designed for a piece of plastic to shut off airflow (thin black rectangle on image). The fans are computer fans (Silverstone FM121) which have one of highest CFM ratings I could find (110CFM). I wasn't aware of static pressure when originally built, but these have a higher pressure spec than most "static pressure" fans at 3.26mmH2O.

I researched professional designs and see that most of them are like the image above. The air enters the center of a 3-stage HEPA filter and is drawn through the filter, around platform to hold the filter into a blower then out. The specs on these are generally 100-150CFM and 150-350mmH20.

This comparison raises a few questions:

• It would seem the pro design would be less efficient with all of airflow direction changes, so would it be best to just get more powerful fans and keep my original design? If so, how much fan do I need? I don't mind spending money but I don't want to start trying $50+ fans with 10x the pressure of my current ones and find it's still not enough.
• Should I be looking at blowers/centrifugal fans instead of axial types? Research seems to say "yes" for this type of high resistance application. The problem there is I'm not finding many blowers that even approach the "Pro" specs (EBM-Papst has a couple), but I can find plenty of axial ones that do.
• On that same note, if an axial and centrifugal fans have the same specs, will they perform equally?
• Is the second fan actually hurting the airflow?
• With electrical designs, we can calculate and be relatively sure the design will work as intended. Are there sources that can help me do that same thing here?

After seeing the pro designs, I am leaning towards modifying so I can include a HEPA filter. I have around 16" of width to play with but 6"H and maybe 8"D. The hose will have to be rotated 90deg on the pro image (airflow coming in on the side of the filter as drawn) but I think that could still work just by adding a second 6" cube chassis to hold a filter and add a more powerful fan. Any more thoughts on how to improve?

Thanks for helping me suck.

 

The proliferation of marijuana grow rooms means that you will find a lot of fans and filters on Amazon cheapish with high capacities. I would search for that. Even Home Depot gives some results, such as:

https://www.homedepot.com/p/Duct-Booster-6-in-Inline-Duct-Fan-M-6/302247738#product-overview

I needed a fan to dry out a flooded ceiling and put together a 4" flexible duct and an inline fan that worked very well. The other thing is that standard dust collection fittings are 4" so flanges and connectors and the like are also readily available.

 

Thank you, Yaakov! Actually, I never thought about using a duct fan and that's what this application is (sort of). The big problem here will be that the hose is built into the desk and it's 2.5" dia. I'm sure we could find a reducer, but I think the reducer may be a part of the problem in my original design because when I feel around the opening of the reducer (with the hose detached) it almost feels like the fan is blowing close to the wall of the reducer, but sucking in the center. I assume that is turbulence.

 

Are your fumes produced by soldering?
My soldering iron has a controlled temperature so it is always at the correct soldering temperature. The rosin in Name Brand solder makes a pleasant aroma not smoke.
Your soldering iron is Way Too Hot and incinerates the rosin.

 

Yes, soldering. I use a Weller WESD51 station with the digital temp control and a Hakko FR301 for desoldering. My solder is Wonder lead free as that's the best LF solder I've found to date. It's quite possible my temps are too high, I'm set to 740deg F on the Weller and I haven't messed with the Hakko's temp. I rarely have any black rosin on the boards though after soldering.

 

I don't eat solder so I have never used lead-free.
My Weller tip is 700 degrees F and a solder joint takes less than one second. I do not carry melted solder around on the tip of a dirty soldering iron like the fools "over there" on You Tube videos.

 

You could set up the fans so they blow over the top of the cone shaped 'reducer' you show in your drawing. You can stack as many as you need small computer fans and have them blow over the top of that so it'll 'suck' at the other end of the tube. They would be situated so the air flow would be at 90 degrees to the tube.

Since you've ready built that assembly, that would be an easy inexpensive way to increase the draw. In this design, the filter would need to be after the fans.

I would look for a small squirrel cage fan and attach the reducers output to the input of the fan. They are much quieter than most fans, move on the order of 16 CFM to 35 CFM...I'm not finding any computer fans that move 110 CFM, did you maybe mix up the SIZE of "110 mm square" size with the CFM?

 

Thank you Jim@HiTek! So, just to understand, the fans would be arranged like you are blowing over a bottle to make it whistle? Or basically, just take the box housing the fans and rotate it 90 deg while keeping everything else where it is in the drawing?

As for the fans I'm using, nope, they're 120mm 110CFM (supposedly). Here's the link: Silverstone FM121 Fan.

 

Yea, you understand correctly about orientation. Worth a try. What I'd be shooting for with that setup is less restriction of air flow from the source to the output of the fans which in turn would provide more draw of the fumes.

Also I was thinking that a squirrel cage fan would be the best at having a 'vortex' type air flow at the source of the fumes once you'd attached and sealed the reducer to the fan's input. That would PULL the fumes up, into the fan, then exhaust them at 90 degrees from the reducer. So it's not the same as blowing over the neck of a bottle like the first idea I had.

Ah, the Silverstone...a ball bearing style fan. It's kinda pretty. I'm doubtful that it really moves 110 CFM based on it's similarity to thousands of other fans that only claim 35 to 55 CFM in the same 110 mm package, but hey, maybe? It is higher RPM and bunches noiser so...

Hey, I found another 110 CFM fan! At Walmart of all places. It's $40 USD, 110 Vac.

I'm not a big fan of designing stuff that uses exotic parts, but these 110 CFM fans may take over the market too. Who knows?

You could just hide this one in the ceiling, it's rated at 429 CFM. Only 6" dia. Now that I think about it, having spent 55 years hunched over a work bench soldering now and again, this would be best, not those little baby fans you're considering...6" Mixed Flow Inline Fan

 

Good stuff, Jim@HiTek! I'm leaning towards your 2nd idea because that will (should) allow for a HEPA filter. Plus that will fit my space better, as to expand I can only widen the box (L shape, right angle to the hose). My board house did some assembly work but stenciled all of the SMD parts, not just the ones they were doing. So as a result I have a bunch of SMD pads full of solder that I have to remove. I don't know what they used but the stuff stinks and the smell lingers in a largish basement. Even when the stuff smells good it can't be great to be inhaling.

Ya, those Silverstone fans are loud. Definitely wouldn't tolerate it in a computer. But as I've been researching it appears I want what the equivalent of power would be for airflow, i.e. CFM and pressure. So they may do 110 CFM but only when there's not much resistance to flow.

That 6" inline fan would be ideal but the room is drywalled over concrete with nowhere to vent unfortunately. I'm curious if a 4" one wouldn't fit the space and somehow allow for a filter. I'm finding a ton of those like Yaakov said I would on Amazon.