Exhausting vacuum into main dust system???

[ian maybury]

Jim N's thread about central vac systems set me thinking - so this is just to throw a pretty raw and off the wall idea out.

Just wondering if anybody has tried any creative solutions to avoid recirculating dust into the workshop air with vacuums?

I run a Bosch GAS25, and have been a bit taken aback at the high cost of dust bags and filters. That's the stock filter, I'm not even sure if there's a HEPA filter available over here - or how it might last.

There's the stock option of adding a mini cyclone, but it's fairly bulky and would cost a bit shipped over here. Do they even keep the vac filters clear?

It seems a little obvious so there's probably reasons why it's not done, but is there i wonder the option to hook up a vacuum so that it returns into the main workshop dust collection system? I for example run a ClearVue/Pentz cyclone system with Donaldson cartridges.

The main system would need to be run with a branch open to deliver enough air flow for the cyclone to function, with some sort of exhaust arrangement from the vacuum T-ed in.

Hooking up the vacuum is the interesting question. I haven't really looked at it, but the awkward bit has got to be that the fan in a vacuum is presumably not designed to pull dust and chips through it. In a stock vacuum installation the material collected drop out into the bag/lower container, and the return air seem to be filtered before the fan. (?)

If it was possible to just hook the vacuum fan exhaust to a main system duct and pull through (eliminating the vacuum's bag and filter set up) then it'd be easy - at least in principle. The main system and the vacuum fan would act in series.

Wonder if there is an arrangement of vacuum fan (maybe as used in central systems) that might allow this?

It'd be a pity to sacrifice pulling chips and dust back into the main system, but maybe one option would be to use a a mini cyclone to drop out the debris into a drum before pulling through the vacuum's fan (eliminating the vacuum's bag and filter?) into the mains system duct?

ian

[Richard Coers]

I'm confused, if you have a central dust system running, why even have a vacuum cleaner? Check out woodgears.ca for do it yourself cyclones and experiments on the benefits of running them in front of the vacuum.

[Rick Potter]

Well, I can answer one of your questions. Yes, the mini cyclone will make your vac bags last much longer. Hardly anything gets in there. I have been running my shop vac on a chop saw, along with a clearview mini, and I seem to go forever between cleanings of the filter.

You can make your own mini cyclone. There are several on youtube, including some very rudimentary ones that work well.

Rick Potter

[ian maybury]

Put another way Richard. When you run a vacuum it has its own filter and dust collection bag/drum - both of which have to be replaced/emptied (= work) Neither are cheap, and the filter sold over here is not HEPA standard meaning that it's potentially also returning dust into the air.

Adding a mini cyclone sounds like it does a decent job of extending filter life (thanks Rick), but it's not going to remove very fine dust - and the whole deal ends up quite bulky and expensive.

I meanwhile have a cyclone and set of large cartridge almost HEPA standard filters already installed in my main dust system, and am simply wondering if perhaps there are ways to use them with a vacuum.

Vacuums claim to go about 100in WG suction, while my dust system probably cranks up to around 24in WG when heavily restricted - which is why i was thinking about whether or not there might be a way to use a vacuum with it.

I've not tried dispensing with the vacuum altogether Richard - that's hooking a maybe 2in bore hose up directly to the dust system and using that with e.g. the router, or the rail saw - but now that you mention it i think somebody previously mentioned doing that. Have you tried it?

Off the cuff the available 24in WG sounds a little low - but maybe the game is to hook up say a 2in hose to the dust system. One point that could help this is that it could be that a vacuum running with an open ended dust hose and a less than fresh filter may get nowhere near to the claimed 100in WG....

ian

PS I wonder if there exists a static device (maybe something venturi based) that uses a high volume flow of relatively low suction pressure air to generate a smaller volume flow of higher suction?

[Jeff Duncan]

I know they sell adapters for attaching a vacuum hose to your duct work for just what your describing....I've thought of adding one myself. However, I've always felt the airflow on my cyclone is different than that of a vacuum. Not sure if a normal dust collector would have enough 'pull' when reduced to the really small vacuum hose sizes, especially when connected to tools? Hopefully someone here has tried it and can shed some light!

As for the mini cyclones they do indeed work very well and get most of the fine dust as well. I bought the little oneida several years ago when they first came out and have been impressed with how well they do work. I find the best use is at a bench or station where they stay put. I have mine set up with an extended length hose and use it at my bench for all my different sanders, biscuit jointer, and lipping planer. I have not had to empty the vacuum since installing it, just empty the drum about once a year or so.

Lastly I have a larger 7-1/2 hp cyclone, so not really efficient to have it running for sanding and biscuit jointing operations. As such the vacuum is a better solution for the way I work in my shop.

good luck,
JeffD

[Jim Neeley]

Richard,

A DC system can move a large volume of air (600-2000 CFM) through a large (4-8") line at low suction pressure but only has a limited maximum suction (~12-15", or about 1/2 of a PSI) which occurs when it's moving very little air.

A ShopVac moves a much smaller volume of air (~100cfm) through a smaller line (1-1/2 to 2-1/2") at low suction pressure but generates a high suction (90-100" or about 3 PSI) which occurs when moving very little air.

If you hook your DC up to the 1" port on your ROS, the small line will block nearly all flow (comapred to a 4" or 6") and the 1/2PSI will leave little air flowing.

If you hook your SV up to that same tool, it too will neck down the air from the larger line but will have ~90-100" of suction to "pull" with. Therefore the SV will pull much more air than the DC.

Just my $0.02..

Jim in Alaska

[Jim Neeley]

Ian,

Your post has gotten me to thinking too... especially since my intent is to place my ShopVacs in the same enclosure I'll be building for my Clearvue, it would be an eaqsy implementation for me. I like the idea, so have tried to think through the ramifications. In developing your idea, a couple of thoughts come to mind.

- The SV's fan is much smaller than the DC's (my SV impeller is about 6", my ClearVue is 16") and it's distance between vanes are much smaller. I would be concerned that wood chunks, nails, screws, plane shavings, etc. would block up or damage the impeller. For me, that killed the option.

- This lead me to thinking about just using a small cyclone and then discharging (filterless) into the DC. This would let the SV cyclone catch the big stuff and let the DC filters handle the fines. This seems like a plausible option. The unknown for me and my shop vacs are based on how MY vacs are built. Yours may be different.

- My vacs have the typical inlet and outlet ports however the SV motor appears to be cooled by directing some of the inlet air (after the filters) over the motors, exhausting through the top and into the room. Without a filter, this air would contain the fines not cought by the <missing> filter. Therefore it would seem the DC would need to capture this air as well. There is also the unknown as to whether or not the fines in the cooling air would damage the motor. My ShopVacs are disposable (Cra*sman), so this wouldn't overly worry me.

- We size out DC runs to maintain sufficient velocity in the lines as to ensure the material does not "drop out" and plug the lines. A shop vac wouldn't provide sufficient air to accomplish this for a 4" or 6" line. To account for this, a blast gate "upstream" of the shop vac connection to the DC could be at least partially opened to guarantee the minimum volume. If this were in the same enclosure as the shop vacs, this would kill two birds with one stone. This gate would need to be opened when the vac weas started and closed when the DC were used for other services. Additionally, this would, for me, require that my sound enclosure be partitioned to separate the dirty air (SV fan discharge) from the clean air (DC filter discharge).

- If I have not missed anything in my concept development, it would appear as if this could be done although I'm unsure it would be worth my hassle. This would be because:

* I still need the SV cyclone, which still needs dumping.
* I must open the blast gate on the DC piping for the SV cooling air before SV use and close it afterwards. (If automatic gates were installed, this could go away but it adds complexity to the system).
* In addition to the electrical expense for running the Shop Vac (~1 KW-h/hr), my 5hp DC (~4 KW-h/hr) would need to be operating. This would raise the operating expense from about $0.12/hr to about $0.72/hr, assuming $0.12/KW-h. Whether or not that is significant is an individual decision.
* In exchange for the increased installation cost (at a minimum, one additional Wye in my DC piping), operating expense (electricity) and still needing to emply the SV cyclone, I get around needing to clean the SV filters. Is it worth it? Hmm....
* I suppose one could get really extravagant and design your DC system to be able to empty your SV cyclone cannister (while the SV is off) but that adds even more complexity.
* Is it do-able? It sounds like it, although to me this is sounding more and more like a Rupe Goldberg machine <g>

- Am I missing something in my analysis?

Jim in Alaska

[david brum]

Hi Ian

I have an American style cabinet saw with an early Shark Guard splitter/overhead dust collector. The older Shark Guards used a 2 1/4 receptacle, designed for a shop van hose. At the manufacturer's suggestion, I dedicated a shop vac for this single purpose. I immediately noticed that the filter clogged after only a few minutes cutting materials which create a lot of dust, like mdf. My solution was to remove the filter, replacing it with a (shop made) cylinder of metal mesh to keep larger chunks out of the shop vac's motor. Then I routed the exhaust into my TS's dust collection pipe for the main cabinet. I'm using 6" pvc throughout. I added a secton with a 4" T, than added a 4" to 2 1/4" adapter. Pretty easy.

I've been using this for several years without problem. It definitely works, although if I forget to turn on the DC when the shop vac is running, it can exhaust back into the cavity of the TS and kick up some dust.

[ian maybury]

Thanks guys, it sounds very much like we're on the same hymn sheet regarding potential advantages and practical issues to be overcome.

It sounds like the basic principle works in fan/airflow terms David. I'm guessing that chunks stopped by your mesh strainer drop into the vac's collection drum?

Dropping the solids out with a mini cyclone is definitely an option I think too Jim, but getting the vac exhaust connected to the return dust seems likely to be awkward on my Bosch unit because there's no easily hooked up to exhaust outlet. It seems to exhaust around the periphery of the dust bucket. Plus as you say the cost adds up, and there's still a bucket to empty.

One thought is that maybe there are in line vacuum motor and fan units available (really only a fan of a suitable type) that are used for central/industrial vacuum systems (or something else) that are chunky enough to pass the chips and dust coming off any likely operations through. This would allow convenient connection of the exhaust directly into a duct, especially if the format was convenient. It should be pretty compact. You might even be able to hook two up in series if the run was fairly long and it needed some extra suction???

The need to simultaneously run the larger system would stand. I guess how big a deal this is depends on how much of the time it runs for. My fan has a 4kW motor, but it only runs for shortish periods before and after making cuts etc.

I guess if the vacuum fan was powerful enough and the distances allowed you could pipe it back to drop the collected dust and chips directly into the main system's cyclone - this would avoid the need to run the main dust system, but it would need a blast gate or something to prevent blowing backwards down the main system ducts much as David describes.

Just thinking aloud...

ian

PS there seem to be plenty of small vacuum fans available as spares. Don't know if any of them get up to enough pressure, and a packaged impeller + motor + housing unit would be handy - but they look like variations on the radial centrifugal types used in most dust systems.

http://www.onlinevacshop.com/kirby_vacuum_cleaner_fans.html
http://www.espares.co.uk/part/garden-vacuums/black-and-decker/gw350/p/1330/879/0/1131830/1133172/garden-vacuum-fan.html

[Phil Thien]

The idea I've toyed with is using the same ductwork for both the DC and the shop vac.

First, you need backflow preventers (you can build two into a plenum) for both the DC and the vac. The backflow preventers prevent air from coming backwards through the DC when you're running the vac, or through the vac when you're running the DC.

But the way this basically works is when you're using the vac, the velocity drop that occurs once the air is in the 4" - 6" ducting allows the dust to settle in the bottom of the ducts. And it just sits there until you need to use the DC again. When you power-up the DC, it clears out the ducts.

[Chris Parks]

I have been thinking about the same issue for some time. Ditch the bags and filters and the canister of the vac, build a plenum/box to hold the remains of the vac, that is the motor and fan unit and duct that box into the cyclone system. Seems pretty simple to me or have I missed something? The vac will not supply enough air to the box/plenum so make up air would be supplied via a blast gate built into the box and used to control the amount of make up air. You could have the vac switch fire up the cyclone via a current sensor so it all becomes automatic.

[Jim Neeley]

Chris,

As long as you could be assured that you never pick up a piece of wood, large shaving or a bolt, screw, etc., big enough to plug or damage the small, rapidly spinning vacuum fan blades... and that the slipstream of now-dusty air used for vac cooling didn't damage the motor, I think you'd be in business.

It'd be straightforward to check the motor for its sealing rating, but as for the others, I'm open to ideas. Perhaps if it were dedicated to a sander or the ilk???

[Chris Parks]

Chris,

As long as you could be assured that you never pick up a piece of wood, large shaving or a bolt, screw, etc., big enough to plug or damage the small, rapidly spinning vacuum fan blades... and that the slipstream of now-dusty air used for vac cooling didn't damage the motor, I think you'd be in business.

It'd be straightforward to check the motor for its sealing rating, but as for the others, I'm open to ideas. Perhaps if it were dedicated to a sander or the ilk???

I am not sure I am following you there Jim, do you mean the vac picking up something? It would be no different to if it was operating normally. Perhaps you are seeing something that escapes me.

[ian maybury]

There's various options.

The real move if you could pull it off would be some sort of a venturi powered vacuum/suction intensifier with no moving parts. That you would hook on to a drop/flex hose from the main dust system, and attach your vacuum hose to.

It may be possible in theory at least - in that a high volume/CFM airflow at a lower in WG suction/vacuum level should deliver enough energy to create a low volume (say 150CFM = 10% of the flow from a big cyclone type system) higher suction flow. (say 125in WG+ compared to the low pressure maybe 20in WG + = x6 increase)

I can't remember enough fluids engineering to know if it's a possibility, but if it is there might just be a stock device out there to do it already. The pitfall may be the relatively low airspeeds available from low pressure air - the vacuum generators I've seen in industry typically use compressed air or steam...

has anybody seen anything of that sort?

ian

[Jim Neeley]

I am not sure I am following you there Jim, do you mean the vac picking up something? It would be no different to if it was operating normally. Perhaps you are seeing something that escapes me.

Yes, Chris. Currently any block, screw or bolt entering the vacuum is blocked from getting to the fan by the filter, at least on my vacs. Without a filter and expecially if the vacuum head were mounted in a plenumless area however... Suck... BANG!

Ian,

It's been 30 years since I had fluid mechanics myself... and I do have a one of Joe Woodworker's (Veneer Supplies) venturi vacuum press kits.. but doubt that there is sufficient energy density in the DC air stream to practically make this happen. When I hook up my shop vac to the press, the 90psi air stream is approaching sonic (speed of sound) velocity while the 4000 FPM in a DC is < 40MPH.

I'd gladly be found wrong, however! :)

Jim

[ian maybury]

There should be enough energy Jim, in that there's maybe 4HP going into creating the flow in a 6in duct on a big dust system. Trouble is as we've both said is probably the low airspeed...

I'd like to be shown to be wrong too...

ian

[Chris Parks]

Just put an open mesh guard over the impeller of the vac to stop impact happening, why complicate what is a fairly easy thing to do.

[Myk Rian]

Seems it would be easier to build a box around the vacuum, and vent that box to the outdoors.

For clarity, it is x" WC (water column).

[Chris Parks]

I can't see anything wrong with that either.

[ian maybury]

Fair point Myk, but would that still require using the filter and original chip collection drum to protect the fan?. Not sure if its feasible, but the real trick might be a reliable and zero maintenance vacuum generating device that the dust and chips can blow through - whether fan based or something else. This could then exhaust outside, or into a dust system and wouldn't need filters or cleaning.

There's seemingly lots of vacuum motor sets like this available one (at various CFM and suction pressure capabilities) that give lots of suction pressure compared to a portable shop vacuum (maybe good for a built in system with long piping runs like Jim is thinking of), but there's not much information with them. Looks though like the fan is unlikely to pass dust and chips without problems though: http://www.grainger.com/Grainger/AMETEK-LAMB-ACDC-Vacuum-Motor-4M921?Pid=search

I've sent enquiries to a couple of companies doing vacuum systems, although i'm not expecting a lot of information back...

ian

[Clint Baxter]

I also use a mini-cyclone like some of the previous posts and find that pretty much everything is captured there, fine dust as well as larger particles. I've been using a Festool vac and have yet needed to change out the bag in the vacuum, yet have emptied the 10 gallon main catcher many times. I like it so well that I just ordered, (and received), the Oneida Ultimate Dust Deputy so it can be a permanent addition to the Festool vac.

It would be another item to dump, but is a lot easier and cheaper to empty than using the vacuum alone.

Good luck no matter which way you end up going.

Clint

[Michael W. Clark]

Ian,
When I first read the original post, I thought you wanted additional filtration on the SV exhaust. If you use the SV as is factory set-up, and you are after the very fine dust that gets through its less than HEPA filters, it seems like you could simply duct the SV outlet into the duct between the cyclone fan outlet and filters. Obviously, this would not be good if you are not getting good filtration of the large particulate in the SV.

If you want to avoid multiple drums to dump, a common dust bin may be the best solution. One where the SV motor and cyclone discharge are attached to the top of a common bin. The SV would discharge into the duct after the cylone system fan as above. I think you would still need some type of filter on the SV motor intake for protection of the motor and keeping big stuff out of the SV fan and out of the cyclone after filters. In addition, you would need a check valve such that you do not pull through the SV when operating your cyclone system. This system must be sealed or you get re-entrainment in the cyclone and reduce the cyclone efficiency.

Personal preference, but I would not want to run the cyclone system everytime I ran the SV. I would rather empty multiple dust bins or keep the systems separate.

Regarding the venturi, I have seen them that use a LV compressed air supply to generate a HV flow. I have not seem them that go the other way (HV/LP to LV/HP).

Mike

[Jim Neeley]

IF I was going to go this way I would install a SV cyclone in front of the SV fan and route the air into the DC system (with the supplemental air blast gates, etc.) and filter removed. I'd them have it interlocked such that when the SV fan turned off the DC kept running, opening an actuated pair of DC blast gates into the SV cyclone's catch bin, one to draw the cyclone-catched heavies into the DC system and another vented to atmosphere to provide makeup air. After a few seconds of operation the DC would then turn off and the gates would close.

Might be a fun science project and it appeals to the engineering nerd in me but it really sounds like an expensive solution looking for a problem! :) :) :).

Jim in Alaska

Do I hear a bit of Rube Goldberg here? <g>

[Chris Parks]

Might be a fun science project and it appeals to the engineering nerd in me but it really sounds like an expensive solution looking for a problem! :) :) :).

Jim in Alaska

Do I hear a bit of Rube Goldberg here? <g>

Jim as you say a complicated but fun project. me, I would build a three sided box put the whole vacuum in it and a duct off the box into the DE. The sheer volume of air being drawn out of the box would trap all the exhausted fines from the vacuum which is what Ian is after. Granted you would have to empty the vac, tip the box over, fire up the DE and empty it into that, done carefully very little would escape. Have you been reading the Ozzie forum Ian?