Thread: more Dust questions

Originally Posted by man from uncle

My shed is approximately. 2.2×4.5×1.8m = 17.82m3

So, if I install an extractor fan capable of 300m3 per hour, that should change the air 16.8. times every hour, including all the fi e dust. I am refereeing to a standard bathroom fan vented outside.
Would this work.? I only use a lathe.

Exasperated in ACT

While I encourage people to install extractor fans your maths doesn't work quite that simply in practice. When you exhaust some dusty air the new air that comes in mixes with the dusty air inside the shed and pretty soon some of the new air is being exhausted along with some of the dusty air remaining inside the shed.

The degree of mixing and the effectiveness of dust extraction depends on where the openings or vents to the room are located in relation to the extractor fan and the primary dust source. Having the extractor as close as possible to the dust source is pretty obvious but bear in mind that 300 m^3 per hour is only 300 x 0.6 = 180 cfm which is a factor of >5 below the standard spec so there is no way it will capture the fine dust at source and the shed will fill up with fine dust while undertaking dust making activities.

Now to room changes per hour.
If the air intake is right next to fan the fresh air comes in and goes out and there will be little dilution of the fine dust and you may not even get one complete room air change per hour. Worse still some of the dusty air being extracted and removed will be sucked back in again!

For the extractor to remove the entire air in the room as quickly as possible the vents should be spaced all around the room with none on the wall near the extractor and more on the wall opposite the extractor fan.

Even very well distributed vents work roughly on an exponential scale so approximately half the dusty room air is vented in the time taken by the extractor to exhaust a room's worth of air. In the time taken to exhaust the next room's worth of air about a quarter of the dust is removed etc. So it takes about 4 times longer to vent (0.5 +0.25 +0.125 + 0.0625 = 0.9575) 96% of the room air. Remember this is for perfect vent placement so in reality (EG one of you walls may be a brick wall up against a house) it may take 2 or 3 time longer again than this. This means that it will take somewhere between 4 and 12 times longer than you think to exhaust 96% of the dusty air originally in the shed.

A cheap way to equilibrate the fresh and dusty air is to have one largish vent in the wall opposite the extractor fan and also run a ceiling or pedestal fan inside the shed - this is equivalent to having good vents but some people do not like to have these fans operating all the time.

I realise I sound a bit pessimistic (sorry - that's the way it is) nevertheless extractor fans are still worth doing just make sure to keep using them on for a good 15 - 20 minutes after you stop making dust.

Thanks mate.

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One more dust question. I have brougt a preloved carbatec dust extractor. When i switch on i get a puff of dust from the top bag.
I was considering building a box round the whole Lot. A Bit like putting it in a cupboard? Are there any problems i should anticipate? Or design features to consider?

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Originally Posted by man from uncle

One more dust question. I have brougt a preloved carbatec dust extractor. When i switch on i get a puff of dust from the top bag.
I was considering building a box round the whole Lot. A Bit like putting it in a cupboard? Are there any problems i should anticipate? Or design features to consider?

Or put the DE outside the shed.

If you put the DE in a box or cupboard remember the air has to escape out somehow - if you do this inside the shed then this is somewhat pointless. Venting the box outside the shed is another way to go.

Oh yeah and the magnetic switch will need to be relocated otherwise you have to open the box to turn it on.
OR
maybe you rig up a stick to poke through a hole in the box to turn it on?

Originally Posted by man from uncle

One more dust question. I have brougt a preloved carbatec dust extractor. When i switch on i get a puff of dust from the top bag.

A good indication of its effectiveness in trapping dust. Get it outside and make sure it is not near an open door or window so the dust cannot re-enter the workshop.

I have no mag switch problem as this is an oldermodel with no mag switch. Ill box it and vent it through the window using 4inch pipe?
Will 4 inch pipe be sufficient?

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Originally Posted by man from uncle

I have no mag switch problem as this is an oldermodel with no mag switch. Ill box it and vent it through the window using 4inch pipe?
Will 4 inch pipe be sufficient?

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Whether you add more 4" pipe to the front or back end of the system is the same so adding more 4" pipe just slows your system down. How far is it to the window? If it is not too far 2 x 4" pipes would be better

It is about 1metre from the window. If i put in 2 or more 4inch pipes, that should do the trick?

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I reckon the inside of the cabinet is going to get very dusty. When the bags have to be emptied it would be better if the cabinet was outside as opening up and removing the bags is going to mean a lot of dust will be disturbed and get back into the workshop with you breathing the dust. It will be bad enough outside of the workshop, putting it inside does not make sense.

I have to work to my limitations so, inside it has to go.

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Originally Posted by Mini

I reckon the inside of the cabinet is going to get very dusty. When the bags have to be emptied it would be better if the cabinet was outside as opening up and removing the bags is going to mean a lot of dust will be disturbed and get back into the workshop with you breathing the dust. It will be bad enough outside of the workshop, putting it inside does not make sense.

I agree it is better to have the DC outside but even if the cabinet gets very dusty it doesn't really matter in the overall scheme of things. Exposure is concentration multiplied by time and as the time is short the exposure in this case is low. Once the bags are back on just run the DC for 20 minutes or so and it will clear the workshop of the fine dust.

Thank you all for the advice and comments - really made me think hard. This is my final solution. Extractor has to stay inside - close to the lathe and window to minimise length of pipe work. I am going to box in only the top half. I will vent the top out through the window using two 4 inch pipes. I will also open up the whole of one side of the box and cover it with a needle felt fabric.
This set up should not impede air flow too much and as the bottom is not boxed, I will not release dust when emptying/changing the bag.
Finally - dust free - ahh

PS - This extractor has a cloth bottom bag - can it be replaced with plastic? for ease of changing, or is the cloth bag part of the filteration??

Originally Posted by man from uncle

Thank you all for the advice and comments - really made me think hard. This is my final solution. Extractor has to stay inside - close to the lathe and window to minimise length of pipe work. I am going to box in only the top half. I will vent the top out through the window using two 4 inch pipes. I will also open up the whole of one side of the box and cover it with a needle felt fabric.

If this opening is inside the shed then all this will do is release very fine dust into the shed. Needlefelt is only good to about 99% at 5 microns so in the long run this will pump a lot of dust into the shed.

PS - This extractor has a cloth bottom bag - can it be replaced with plastic? for ease of changing, or is the cloth bag part of the filteration??

If it cloth it is part of the filtration system and will leak fine particles into the shed. If you change that to a plastic bag it will slow down the air flow. It sounds like you need to box the whole thing up

After much thought, pain and consternation I struck upon a simple and I think elegant solution to my problem. I took a step back and reminded my self of the following. 1. I will never ever achieve a 100% dust free environment in wood turning. 2. I need to balance effort to reduce dust with cost and time. 3. Pareto principle, that 20% effort(cost) achieves 80% result, the remaining 20% requires 80% effort (cost). 4. That my original problem was to deal with the puff of fine dust on start up of the dust extractor. Solution – I purchased 2 metres of felt for 10 bucks from the craft shop and stitched up an over bag. This bag goes over the top of the existing bag, hence trapping the puff of fine dust and not impeding air flow. I plan on replacing this bag every six months or so. Perfect.

And thank you to everyone for the advise and comments, without which I might still be coughing dust..