Inline duct fans

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25 May 2020
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I'm currently thinking about dust extraction for my 2 car garage size workshop. Running a table saw, router table and mitre saw. I have no extraction at all at the moment- save for a shop vac which I point at the cutting blade. Yes, I know, don't go there. I'm doing something about it!

I intend to come up with a first class dust & chip extraction system. But here's the question- why aren't inline duct fans used more? I'm thinking about a 200mm duct with a high capacity inline fan (extracting to the outside).

I'll probably use this to capture fine airborne dust by having a boom arm pointing at my work, and then a conventional 100mm extractor for the machine points.

But is there a reason why I couldn't just use a cyclone separator and then have the duct fan attached to that?
Its amazing how many people spend years re-inventing the wheel, before just going out and buying a wheel (I have been guilty of that occasionally) :lol:

Theres a good reason why inline fans arent used, and several people much more technical than me will be along shortly.

Get proper extraction sorted right now, then start tinkering. :roll: The quality of life of you and your family depend on it.
They don't move enough air with a high enough pressure. Buy a proper dust extractor and vent to outside if you wish.
As sunny Bob has said, don't mess about with drainpipes, home made blast gates or any other penny pinching ideas. A two horsepower purpose made extractor preferably in a separate housing is the standard to go for. The Fox 842 features on here with a recommend on price and quality. Purpose made ducting goes together easily and is fire resistant and easily altered as required, it works out at a sensible price when compared to plastic and its static problems. Do it once and do it right! If you need any more incentive just consider your insurers reaction to a bodge job should an accident occur!
I'm assuming (!) that by inline fan you mean an axial fan (impeller looks like an aircraft engine and is perpendicular to the flow). If so be aware that they're not generally designed to handle solids passing through them nor are they generally brilliant at developing a vacuum. They can shift a lot of air and are fine for general air movement but its not the right application.

I have looked at making my own system as I wanted to mount the fan and filters separately but found the cost prohibitive - 3kw fan was going to be north of 500 quid alone.

There'd be nothing wrong with using a fume arm and axial fan as a separate system from the machine exraction and exhausting outside - as long as you don't mind all that fine dust covering whatever it may be out there - like your neighbours.

As an aside, if your machines have 100mm ports 200mm ducting seems excessive.
Here is a link to a site with a lot of information on dust collection. It is long and there is repetition to make sure people that cherry pick parts get the important stuff.

Woodwork Forums, an Australian site, has a great section on Dust Extraction in the Hand Tools & Machinery area. The Sticky threads at the top have Good info as does the rest of the forum.

Interesting.... I checked that heads up for the Fox 842 and all their models seem to be about two metres tall.
Does anyone know whether I can get a similar extract system less than a mere high please ? I'm keen to set up an extraction system in the loft space above my detached single garage with 30 degree pitched roof, as floor space down below is critical.
And is it ok to run an extraction system as an extra 13 amp circuit or does it need something beefier. I ask because i currently have a sparky about to wire my new consumer unit. Plenty of spare ways on that but always good to know what I should be planning for.
Thanks in advance
Have your cake and eat it .... etc. Yes it is possible sort of

Option 1 - have a lot of money and get a fan + filter housing custom made. P & J amongst others might be able to make it work

Option 2 - (being in a similar situation) buy a standard extractor unit; ... ine-filter , ... tor-508336 or ... actor.html) and adapt them to fit onto a wall hanging bracket. Incidentally I chose those three as it fits my needs.

In all of this you haven't mentioned anything of your needs ie what machinery.

Electrical requirements, who cares - you need to decide what you need first but frankly if in doubt get him to wire for 16A the cost difference is not large.
Thanks for the replies, guys.

I've seen Bill Pentz's site and read it all the way through a few times, but one thing I'm still not clear on is what sort of pressure differential is needed at the 100mm outlets in order for good suction to be created. Bill talks about (in metric terms) about 1200m³/hr.

What I'm trying to achieve is a 100mm flexi hose from an assembly table (with integrated cyclone separator) to a fixed duct, which then expels outside. I'm a bit pushed for floor space in my workshop, so the thought of having a large dust extractor wasn't attractive. I've no chance to site the extractor outside, but I can certainly vent outside. No neighbours close by to annoy with fine dust!

My biggest concern- after effectiveness- was the risk of flammable fine dust in close proximity to a potentially unsuitable fan. I want dust extraction, not a bomb!!! :)
A single phase extractor small enough to run off a 13A plug won't shift a lot of air.
There are plenty of discussions on here that advise the real world achievable performance of extractors are far less than the numbers the manufacturers quote. Some mfrs are more realistic than others, but whatever you think you need, you'll end up wanting bigger !
The most you can pull through a 4" pipe is about 700 cubic meters per hour with any DC. It is a limitation of the pipe that size. That is why Bill recommends 6" pipe (flows up to 2,000 cubic meters/hr) and at least a 3hp DC with a 13" or bigger impeller. A 1 1/2hp DC can't pull that much air over any distance with a pipe of that size. Add flex hose and a cyclone and there is even more resistance to flow.

Its a minefield of a subject and to his credit Pentz has at least put a fair bit of effort in - I feel he's being very conservative though. Unfortunately the equipment to test is not cheap at all or easy to rent - I tried.

When I was researching this (my real aim was to improve our hvac at work but this coincided with the woodworking which was nice) I called Axminster and Scott & Sargeant to queston their figures. Axminster told me that they themselves had tested the unit I referred to above with filter (new obviously) and 4m spiral reinforced hose - 2250 cbm. Similarly S + S had tested their unit with class M cloth filter (no hose) an got the 3000 cbm.

As to explosion in ductwork, I searched and searched but could not find an example outside od industrial processing. Read this if you want ... _221.shtml , even HSE say so . Fire from dust build is a different matter.

When you say extraction from an assembly table I'm envisioning a downdraft table - - in which case you're going to need some serious air movement.
Again, thanks for the replies guys. The thing I'm having a problem getting my head around is that if a duct fan says that it'll move, say, 1800m³/hr (via a 200mm duct), why would it not move the same volume when reduced to a 100mm duct? The 100mm duct has about ¼ of the cross sectional area of the 200mm duct, so logic would dictate that the air flows 4 times faster. As it wouldn't be flowing anywhere near the speed of sound, I don't think there would be any compressibility issues, or would there?

I'm just interested why a fan that claims to move 1800m³/hr might not be suitable for extraction. I'm already resigned to the fact I'll need a "proper" extractor, but I'm just curious now!
The fan will stall without the open duct to move the air. Much like a wing losses lift when the airplane slows down too much and the wing stalls. A wing, propeller, helicopter and a fan work on the same principles. Fans can move a lot of air but throw restrictions to the air flowing to it or from it and it doesn't work. Inability to generate much static pressure. An impeller doesn't flow as much air but it doesn't stall as easily and generates higher static pressures. Best able to handle a lot of debris too. A vacuum cleaner type fan doesn't flow a lot of air but can generate much higher static pressures than an impeller.

That's why you don't see fans used for dust collection, just ventilation of a shop. Clear as mud eh? #-o

As had been said here, it's not a good idea. A fan's performance isn't fixed to the rated numbers - these are flow rates under design conditions. All fans have a performance curve of flow rate vs differential pressure. Axial plate fans move a lot of air at very low differential pressure, but performance drops off markedly with pressure drop - you will move much less air through ducting with one than a centrifugal blower.

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Pete has made it clear! I understand fully now :)

(I'm a commercial pilot by profession so I understand the stalling wing analogy. And that's also the reason that I need to up my workshop game... Not really a great time for the aviation industry at the moment).

So... Next question, anyone have experience with SIP extractors? The Fox ones seem to get pretty good write ups on here, but I've not read much about the SIP ones.
nrg710":32om7b7z said:
So... Next question, anyone have experience with SIP extractors?

I bought a small one: It does what it says on the tin, works pretty well on a table saw, band saw, 8" planer and also general vacuum and cleanup duty. It is LOUD, but it shifts lots of air and sucks stuff up. One thing I did learn - don't let anyone use it to clean out a fire, unless the fire is out. Really, really out. Hot coals in a forced air furnace will be the result, with much amusing running in circles while flapping arms ineffectually. (I think the piloting term is "going to brown alert").
I wouldn't remember what "brown alert" was! Not looking forward to my next sim*, I think I'll have enough trouble finding the on switch, let alone dealing with the kind of things you only ever hope to see in the simulator!

I have read elsewhere that the SIP ones are loud. What are the Fox ones like for noise levels? In fact, what's the gold standard for quiet extractors?

*Sim = simulator. Actually, I'm REALLY looking forward to my next one. Because it means I'll still be in a job, rather than having to craft another one....
No such thing as quiet extraction, sorry. :roll:
High pitched screaming or low pitched humming are your choices.
I had the sip 50 litre canister (wish I still did). Extremely LOUD :shock: but once its in a sound deadening box its bearable, and will satisfy 90% of your extraction needs. Mine only bogged up when I used the thicknesser, but then again I was using only 2" hose.
nrg710":rfxmk68v said:
I wouldn't remember what "brown alert" was! Not looking forward to my next sim*, I think I'll have enough trouble finding the on switch, let alone dealing with the kind of things you only ever hope to see in the simulator!

I have read elsewhere that the SIP ones are loud. What are the Fox ones like for noise levels? In fact, what's the gold standard for quiet extractors?

*Sim = simulator. Actually, I'm REALLY looking forward to my next one. Because it means I'll still be in a job, rather than having to craft another one....

We can go to brown alert but it does mean changing the bulb.

Best thing you can do for some peace is to put extractors in a dense box or, if possible, outside your work area. So if you have a garage to work in for example, stick a hole in the wall and have the extractor on the other side of the wall in a box to keep it dry.