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I'm about to install a proper dust extraction kit in my workshop. I've been wheeling my dust extractor or vac around to each of the powertools but I've got some pipes, junctions and gates that I'm going to route around the shop to allow me to leave my extractor in the corner (before I move it to an external lean-to).

But occasionally —even in a small shop— a 1.5HP extractor isn't enough. It's possible I might want two lines open at once when routing or planing. That's going to cause some pretty sluggish air, even down at 2½".

But something occurred to me this morning. Why can't I just add my vac to the line? I could easily spur in a t-juction at the same end as my extractor. I could even add another extractor one day.

So my question is, can you gang up multiple vacuums in parallel? My vac and extractor have completely opposite airflow profiles so do I have to consider CFMs and static pressures?

And for the sake of thinking this through, what about putting the two suction devices at opposite ends? Without any real thought, I think that'd just cause a big blockage in the middle but maybe the different profiles mean it'd actually work much better.

  • I have done a very quick test with poorly fitting pipes thrown together and with absolutely no real benchmark. The suction at the tool-end does appear to be amplified but you can see the dust bag on the extractor wilt and also hear the stress on the vac. I don't think this is a long term solution. – Oli Oct 13 '17 at 10:12
  • Yeah I wouldn't risk it. At best you're gonna get some funky air currents within the piping which may caused localised clogging and stuff. At worst you might burn out one of your units. – WhatEvil Oct 16 '17 at 19:23
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I had the same thought when I was setting up my system (since I have two units). I did quite a bit of research and came to the conclusion that using two units in parallel is not a good idea. The two units will be working against each other and it could end up damaging one or both of the systems.

It would be better to setup the second unit on a separate "circuit" of pipes. This way, you limit the length of pipes that each system is pulling from and therefore increase the effectiveness of each unit.

2

I tee'd my van into my extractor line (before the cyclone separator) and made the following casual observations without any data or :

  • It did appear to demonstrate some additional suction
  • This combined suction did not feel as effective as the two vacuums separately.
  • Both vacuums were running noisier (working harder), as if you'd suffered a small blockage.

On the last point alone, making this the default installation does not seem like a good idea.


It turns out this is something Stumpy Nubs covered in a video as part of their dust collection series. From what I can tell from the video, its comments and other random sources since I've been looking into this:

  • Parallel is a bad idea. If you can lower the resistance (a much larger shared pipe) you can certainly double the CFM but any pressure is equalised, split between both the pipe and opposing impeller/motor. More load, slower overall CFM, less efficacy. Boo.

  • Series seems to be the only way forward here. I'm not 100% up on my fluid dynamics but as impellers aren't sealed pumps, you can suck air through them, as well as letting them operate. Obviously you'll have to spin faster and most impeller motors are balanced against the load, not an arbitrary spin speed.

    ∴ If you can keep spinning [faster] you can push more air along. But that uses more power.
    ∴ You need to limit flow to limit spin speed to limit motor energy consumption.

    If you can do all that, you can gang up two blowers to improve CFM and pressure. But note this will also change the behaviour of things like cyclone separators so your dinky-little knock-off probably won't work.

  • But before you stick your nozzle in your impeller, Stumpy's video assumes identical blowers. Identical impellers, motors, CFMs, pressures. Series-ganging a high-pressure vac behind a high-CFM just won't work.

    The little motor cannot keep up with the baseline CFM from the extractor, let alone add to it. And the hose size on the shop vac makes the opposite impossible too. The extractor won't work faster at that pressure.

So it looks like my dream to build a 15hp black-hole extractor system out of junk motors is in tatters. I'll probably settle for moving the extractor out of the same air space and keeping the vac around as a secondary option.

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