dust extraction system

[selly]

Hi

i'm planning to put in a new dust extraction system. The original idea was to use soil pipe but i have some excellent lengths of stainless steel pipe which will be cheaper.

Problem is some of this is only 3" d. although I do have a 4" piece I would rather not use it unless i have to.

Is 3" too small? I will have a table saw, p/t and bandsaw all plumbed in for it. So perhaps a quanititiy of stuff but they will never be used at the same time?

don't want to keep blocking it but maybe shavings flow better through steel than plastic and maybe I should put the p/t closest to the extractor?

[engineerone]

thing to think about is air flow rate, and hence volume, so in most cases 3inch might well be ok.

depends on the size of your machine, and the size of the waste that you produce. as you say the planer should maybe be nearer to the motor than the other machines, and obviously rarely are you going to have them all going at once. you do however need to have decent cut offs/ gates so you are not wasting the air flow by drawing air from other machines.

definately worth aiming for a cyclonic extractor since they do actually work, not least in extending the life of the various filters you might use.

thing is that you need to be cautious about thinking that you will grab all the dust, particularly on the p/t since the rotation of the blades draws stuff either up or down depending on which operation you are perfroming.

so use what you have, until you do, you will not know for sure that it doesn't work, aim for at least a 2000 watt motor, and take smaller cuts to reduce the volume of actual wood being moved.

not sure about all the technology in this, but it might well be valuable to have some earth leak leads rather like your radiators should be grounded. there are some schools of thought that think the wood can be going to fast it will cause a fire, or in the case of plastic piping, the kinetic energy may cause a heat build up. however i understand there are very few actual examples of this happening rather than "urban myth".

most important thing to try and arrange is an extended overrun on the motor after you have stopped your machining, this will reduce the chances of things getting stuck in the pipes. other thing is to make sure that the pipes angle down toward the collector again to take advantage of gravity.

think about access, and make sure that you can reach the various pipes and occassionally can run some kind of brush or similar down the pipes to ensure they stay clean.

hth

paul

[Keith]

The difference between 3 and 4 inch duct is dramatic in terms of air flow. If you work out the cross sectional area, which directly relates to volume, you get 7 square inches for the 3" duct but it almost doubles to 12.5 square inches for the 4" duct.

If you can, go for the 4" duct.

PS You don't need to worry about earthing plastic duct for a domestic system, as Paul suspected it is an urban myth

[engineerone]

however, keith you are ignoring the bernoulli effect which can allow you to increase the air flow by reducing the cross sectional area into which you force it.

it is the effect that works on the rear of racing cars when the air is moved, particularly under the car.

so in this case, i would suggest 3inch pipes at the machinery, then nearer the motor, increase to 4inches using tapered pipes.

this would speed up the air flow into the extractor itself, and probably help to increase the separation effect.

paul

[andy]

Selly what type of dust extractor will you be using

[Keith]

Paul I don't think it quite works that way. AFAIK the Bernoulli effect increases air speed locally which then alters the air pressure .

A ducted air system works more like an electric circuit wirh the fan providing the motive power ( ie Voltage); restrictions in the duct are equivalent to resistors and total air flow (the equivalent of current) is V/R.

Max air flow is more or less limited to the most restricted dimension of the pipe as air is hardly compressible at these pressures. With a 3" section by the machine the 4" section would actually have a lower air velocity.

I have been fiddling around with this for ages trying to get the cyclone to work efficiently and it seems to me that it is best to try to get the same cross sectional area for the whole of the system. The ideal duct size is then dependent on the power of the extractor.

Keith

[engineerone]

i understand your concerns, keith, but i too think you are wrong

remember that all machines have different size hoses at the production end, and it is this which causes the balancing act
that every body has to go through to get decent extraction.

for instance the axminster table saw guard come extractor
has what appears to be a 2inch dia pipe, so that will need to
be blending in.

unless you make your own extraction parts for each machine,
then you have to blend them and this will cause the kind of problems you may have been experiencing.

each machine has a different air flow capacity and requirement
and so it is difficult to balance them. i have my doubts about the air flow statistics on the american site, barry and i had lots of discussions about this before he died, and we both came to the conclusion that overall air capacity was not necessarily as important as the speed of air flow, and trust me you can compress air. ;D


paul

[andy]

Sept 11, 2008 19:25:30 GMT engineerone said:

.... and trust me you can compress air.

Only if the dust extractor can create a vacuum ie a Camvac

if it is a twin bag type then any reduction in pipe size will throttle the air flow not increase it therefore the pipe size should be as large as possible for as long as possible with the reduction if necessary at the machine IMHO

[Keith]

i understand your concerns, keith, but i too think you are wrong

I don't

remember that all machines have different size hoses at the production end, and it is this which causes the balancing act
that every body has to go through to get decent extraction.

Not necessarily so; most machines need a high volume extractor, which normally means a chip extractor (HVLP) and a 4" outlet and duct (as a minimum). At these pressures I repeat air is hardly compressible.

Some small machines have outlets less than 4" but these would be better served with a vacuum extractor, such as the Camvac. This type of extractor is far less sensitive to restrictions in ducting as the air pressure (negative) is very much higher than that of a chip extractor.

for instance the axminster table saw guard come extractor
has what appears to be a 2inch dia pipe, so that will need to
be blending in.

If you try to use a chip extractor with a 2" duct there will be negligible air flow anyway. I know that you can get 4" fittings with a 2" take off but these would only be any use with a high power vacuum extractor.

unless you make your own extraction parts for each machine,
then you have to blend them and this will cause the kind of problems you may have been experiencing.

I didn't have problems exactly, I was experimenting with the system. That is one advantage of the cyclone I made, as it is translucent I can clearly see what is happening with the overall airflow in the system.

[jake]

Sept 11, 2008 19:36:27 GMT andy said:

Sept 11, 2008 19:25:30 GMT engineerone said:

.... and trust me you can compress air.

Only if the dust extractor can create a vacuum ie a Camvac

if it is a twin bag type then any reduction in pipe size will throttle the air flow not increase it therefore the pipe size should be as large as possible for as long as possible with the reduction if necessary at the machine IMHO

Quite. The same argument I used to have with Barry (RIP). He never did get it - now I know why - he trusted Paul!

[engineerone]

ouch jake ;D

since barry and i indeed were not using twin bags, it is a different thing.

we were never convinced that the camvac did what it suggested either.

one of the most interesting problems about all of this is that in fact your efficiency of extraction increases as the filters load up with dust, by definition therefore that slows down the airflow.

so how do you explain that enigma???

i have never said my or indeed barry's way was the only way, and i certainly do not agree with the american guy who pushed the argument for cyclones so much that he was always right, the whole problem is actually the fact that very few woodworking machines have properly designed exhaust paths, which would allow one to collect the whole dust in a kind of venturi which would immediately accelerate the air and push it through the filter and into the waste unit. by drawing the air rather than pushing it then you have i agree little compression, but putting in venturis will in fact cause either acceleration or deceleration and help in moving the dust to where you want to collect it.

however there are a couple more caveats, when your extraction is moving both dust and chips, then the most effective extraction requires the cyclonic separation, and then a drop in air pressure to allow the heavy stuff to drop out, whilst keeping the air flow of the dust across toward the filters.

the plan is to separate the various air flows so that you get chips in a big bag, the dust on top of it, and clean air flowing through the filters, cause they are the most expensive thing after the motor to change.

paul
paul

[selly]

Thankyou chaps. i think i had better stick with the 4" system then. My extractor is this:

www.axminster.co.uk/product.asp?pf_id=21167&name=extractor&user_search=1&sfile=1&jump=0

And I use a scheppach 260 p/t, the Axy AW10BSB and a scheppach basato 3 bandsaw. Previously been swopping extractor from one to the other but now fancy having a plumbed in system.

Is this good enough you reckon? I think i'll plumb it in so they are all along one side of the workshop and Y piece each item of machinery off with a blast gate?

[Keith]

Sept 12, 2008 0:09:03 GMT engineerone said:

one of the most interesting problems about all of this is that in fact your efficiency of extraction increases as the filters load up with dust, by definition therefore that slows down the airflow.

so how do you explain that enigma???

paul

As the filter clogs with dust, as you say, the filter efficiecy increases, the trouble is, with the added resistance caused by the the less air permeable filter,the air flow through the filter decreases. Which decreases the overall performance of the system.In fact if it clogs up completely then you would have a 100% effective filter with a 0% effective system.

This why cyclones are the Holy Grail as they do not block the filter so much. The best filter is the one I have, I just blow the exhaust outside ;D

[Keith]

Sept 12, 2008 6:59:13 GMT selly said:

Thankyou chaps. i think i had better stick with the 4" system then. My extractor is this:

www.axminster.co.uk/product.asp?pf_id=21167&name=extractor&user_search=1&sfile=1&jump=0

And I use a scheppach 260 p/t, the Axy AW10BSB and a scheppach basato 3 bandsaw. Previously been swopping extractor from one to the other but now fancy having a plumbed in system.

Is this good enough you reckon? I think i'll plumb it in so they are all along one side of the workshop and Y piece each item of machinery off with a blast gate?

Hi Selly, the extractor is well suited to the job in hand, but remember this is a chip extractor it will not filter out the finest dust, in fact it will blow it back into the air.

It is always a good idea to wear a decent respirator as well, I nearly ::)always do. Plus if you could build a cupboard to house the extractor with a vent to outside (this is what I have done) then the contaminated exhaust air will blow harmlessly outside. Perhaps something to plan for the future.

[selly]

Thankyou keith.

My new workshop has a roller door and so i take your point about dust. What I guess I could do is open this up and keep the extractor as close to outide as possible. When the machine tools are on and get the dust to head out that way maybe?

Or boxing it in wouldn't be hard in the future..

[mrgrimsdale]

In fact if it clogs up completely then you would have a 100% effective filter with a 0% effective system.

;D ;D I was going to say that!

I was wondering abt boxing mine in too. Perhaps 2 x 4" pipes to the outside? Would sound carry through?
The box would need a wide door (with draught proofing) for easy access as the bag fills quite fast on a busy day.

cheers
Jacob

[jake]

Sept 12, 2008 7:20:54 GMT Keith said:

Sept 12, 2008 0:09:03 GMT engineerone said:

one of the most interesting problems about all of this is that in fact your efficiency of extraction increases as the filters load up with dust, by definition therefore that slows down the airflow.

so how do you explain that enigma???

paul

As the filter clogs with dust, as you say, the filter efficiecy increases, the trouble is, with the added resistance caused by the the less air permeable filter,the air flow through the filter decreases. Which decreases the overall performance of the system.In fact if it clogs up completely then you would have a 100% effective filter with a 0% effective system.

And again, the sensitivity of extractor through-flow to clogging is determined by the pressure the extractor can pull.

Big systems shake the filters clean automatically. I'm doing a case on a massive stone crushing plant, dealing with fine dusts, and there's not a cyclone to be seen on site - there's a lot of sonic blast filter cleaning, and mechanical filter shaking.

This why cyclones are the Holy Grail as they do not block the filter so much. The best filter is the one I have, I just blow the exhaust outside ;D

Every bit as good as a cyclone-based solution. Better, in fact, because much, much cheaper, smaller, and less fuss. That's if you can get away with it (I couldn't, being in town) and you can live with the air exchange in winter.

[engineerone]

patently, there is a level at which the filter can become over loaded with waste, however, it is a proven fact that when newly installed, a filter is not as effective as it will be after a couple of hours when it has been used. the idea is to reach a balance. ;D

whilst i am still not convinced about all of the global warming
arguments, i do believe that we should do as much as possible to reduce our individual impact. exhausting dust into the atmosphere is certainly going to cause a small effect, however,
together it will have a big impact.

for the smaller shop, the cyclone does i think do the most to reduce the exhausting of dust particles, and the recycled air is pretty clean which makes it a valuable workshop aid.

with the various waste disposal acts in this country, and the clean air act, which was originally aimed at coal fires, it is probably illegal to exhaust unclean air into the local atmosphere.

so it is better to have some kind of collection device, plus i think
a workshop air box which goes to work after you stop, and cleans the floating dust from the air, since as i have said proper collection is almost impossible.

paul

[Keith]

Sept 12, 2008 8:17:13 GMT mrgrimsdale said:

In fact if it clogs up completely then you would have a 100% effective filter with a 0% effective system.

;D ;D I was going to say that!

I was wondering abt boxing mine in too. Perhaps 2 x 4" pipes to the outside? Would sound carry through?
The box would need a wide door (with draught proofing) for easy access as the bag fills quite fast on a busy day.

cheers
Jacob

Jacob I've had a boxed in extractor for years (a Rojek 2 bag model) I cut a 6" square hole through the wall fitted a cowl to keep out the rain on the outside and a metal mesh screen on the inside to keep out the rats, mice and bats. There was always a lot of fine dust on the floor in the cupboard, obviously this would have been blowing round the workshop had it not been boxed in.

I had a panel with a glass window so I could see when the bags were full and it was just clipped into position with foam strip in the rebate.

Well worth it, if you have the space, which I think you have.

And with a big workshop I doubt you will notice the loss of warm air in the winter.

Regards

Keith

[Keith]

Sept 12, 2008 9:44:28 GMT jake said:

Every bit as good as a cyclone-based solution. Better, in fact, because much, much cheaper, smaller, and less fuss. That's if you can get away with it (I couldn't, being in town) and you can live with the air exchange in winter.

Jake I was meaning blowing out post filtered air rather than straight saw dust ;D

I have actually done that though, by mistake, the filter had torn and it must have run for months like that. The dust was extracted through a vent in the roof and it was only the gradual change in appearance of the roof tiles that gave me a clue something was wrong. When I took the tiles up though it was a different matter, the space between the tiles and the roof felt was absolutely packed with hardened dust.

I doubt there is any HSE issue though as wood dust is not classed as hazardous unless in concentration, so unless you were actually causing a nuisance I doubt anyone would bother. In fact a lot of the really big comercial systems do blow the very fine dust to atmosphere quite legally.

[Keith]

Sept 12, 2008 10:18:16 GMT engineerone said:

patently, there is a level at which the filter can become over loaded with waste, however, it is a proven fact that when newly installed, a filter is not as effective as it will be after a couple of hours when it has been used. the idea is to reach a balance. ;D;)

No, this is a ploy the unscrupulous manufacturers use, they quote air flow with a new bag and filtration with one so clogged up it hardly passes wind ;D

whilst i am still not convinced about all of the global warming
arguments, i do believe that we should do as much as possible to reduce our individual impact. exhausting dust into the atmosphere is certainly going to cause a small effect, however,
together it will have a big impact.

I agree, if we all, 60 million of us, started blowing out sawdust we would go back to the smog days, but industry creates far more dust than us and nature creates vastly more than that. We used to regularly have a few millimeters of Sahara dust laid right across the UK. I wonder if that has stopped? I have never noticed it since moving out to the sticks.

for the smaller shop, the cyclone does i think do the most to reduce the exhausting of dust particles, and the recycled air is pretty clean which makes it a valuable workshop aid.

Yes I think the cyclone is the way forward as there are no filters to clog, but the cyclone itself does create quite an overhead on the extraction system and is pretty expensive.

with the various waste disposal acts in this country, and the clean air act, which was originally aimed at coal fires, it is probably illegal to exhaust unclean air into the local atmosphere.

You would need to define what was "clean". There are rules on hazardous and toxic fumes but wood dust is neither unless in concentration.

so it is better to have some kind of collection device, plus i think
a workshop air box which goes to work after you stop, and cleans the floating dust from the air, since as i have said proper collection is almost impossible.

Yes even the best systems are never 100% effective. A lot of machinery has dust extraction added as an afterthought and is largely ineffective. Which is why I usually wear a respirator.

An air cleaner is a good idea too, I have mine running all the time I am in the shop.

[davyowen]

Sept 11, 2008 10:19:50 GMT engineerone said:

but it might well be valuable to have some earth leak leads rather like your radiators should be grounded.

They most certainly should not be 'grounded' You should have main bonding at the point where the service pipes enter your property and depending on whether your installation meets the new 17th edition or the older 16th edition, you should have supplementary bonding in the bathroom - this can be ommited under the 17th edition provided that all the circuits are RCD protected and main bonding is in place.

Please do not go around saying that you need to 'ground' things unnecessarily, its immensly unsafe to start introducing potential differences in a domestic property without knowing what you are talking about.


Onto dust extraction, as has been said, if you want to move lots of air and your blower can only produce low pressures you will need large diameter ducting. Considering the rated airflow of your extractor is only 1200m³/h you shouldn't have any problem with 4", but if you want to move any more (with a larger blower) you need to consider going up to 5" or 6".

[jfc]

If you make your ducting clear you can watch the saw dust wizz around the tubes

[jake]

Sept 12, 2008 13:36:11 GMT Keith said:

Sept 12, 2008 9:44:28 GMT jake said:

Every bit as good as a cyclone-based solution. Better, in fact, because much, much cheaper, smaller, and less fuss. That's if you can get away with it (I couldn't, being in town) and you can live with the air exchange in winter.

Jake I was meaning blowing out post filtered air rather than straight saw dust ;D

Yeah, I know, what I meant was not bothering with fine filtering (which is where you start doing in the flow rate).

The only place I could put it would be in our side alley between us and the next house - I don't think they'd thank me for the (fine) dust, and the extractor would block our path completely.

[engineerone]

davy i am not trying to suggest i know these things in relation to leccy, however my mid 80's installed central heating has earth loops in every conceivable place, and even some not conceivable ;D

so it is pleasing to know someone has looked and seen it might be less required. however my comments were based on the various parts of the urban legends about waste igniting in various forms of ducting.

paul