Thread: 90mm ducting for dust extraction ?

define a large amount of 90mm, and distance to each of the machines?

A 2HP DE is severely throttled by using 100 mm ducting so using 90 mm makes it even worse. It's not just 10% worse but more like 20%.

It depends if you want to tackle invisible dust or you just want a chip collector. If you just want a chip collector then 90 mm will be fine. Just be aware that this ducting size will not have sufficient volume flow to capture invisible dust at source and it will spill into your shed. If you wish to vent your shed of this after making the dust, 90 mm ducting means the DC will need be run for longer after the final dust making activity.

If the 90 mm ducting is something you have come into and do not wish to spend any more money (but you will still have to buy fittings) then in terms of invisible dust using the 90 mm ducting to put your DE outside is far better than leaving the DC inside the shed. Just bear in mind that if your ducting runs are too long the 90 mm duct will compromise visible chip collection.

Even if you are not prepared to modify the ports on machines, it is still worth putting the DC outside and running a 150 mm duct as a trunk line and then using 100 mm take offs and use two take offs per machine. Near each machine connect one duct to the machine and leave the other just open near the dust making point to scavenge invisible dust. It may mean your chip collection is not quite as good but much more invisible dust will be scavenged this way. I know this is not very satisfying to go to all that trouble and still have chips laying around but remember chips don't affect your health.

Depending on how much he has laying around and factoring in the free status of it… he could do double runs of the 90mm (not as good as larger but it is free). If he only has limited lengths then probably easier to just buy bigger.
Also on 2HP probably going to need blast gates to get the most from it.

Originally Posted by HeadScratcher

[SIZE=3][FONT=Calibri]Depending on how much he has laying around and factoring in the free status of it… he could do double runs of the 90mm (not as good as larger but it is free).

Sure but with any ducting its the fittings that dominate the cost of ducting. Unless he has free fittings it might not be that different to buy all new pipe and fittings.

It is also possible to fit 100 or 90 directly to 150 mm pipe simply by cutting an oval into the 150 and then using nomoregaps to seal the join. It's not quite so easy to do this with the same size pipe.

90 mm stormwater pipe is also more fragile than DVW. It's OK if it can be setup overhead but as soon as it get near floor level it become susceptible to breakage from things dropping onto it.

Yes very good points..

Originally Posted by HeadScratcher

define a large amount of 90mm, and distance to each of the machines?

I think there's about 36 metres there, though I only need about 11, allowing for offcuts. There are various fittings but I cant be sure what because this stuff is actually at our holiday home, and I wont be going there for a week or two.

cheers
Arron

Originally Posted by BobL

A 2HP DE is severely throttled by using 100 mm ducting so using 90 mm makes it even worse. It's not just 10% worse but more like 20%.

It depends if you want to tackle invisible dust or you just want a chip collector. If you just want a chip collector then 90 mm will be fine. Just be aware that this ducting size will not have sufficient volume flow to capture invisible dust at source and it will spill into your shed. If you wish to vent your shed of this after making the dust, 90 mm ducting means the DC will need be run for longer after the final dust making activity.

If the 90 mm ducting is something you have come into and do not wish to spend any more money (but you will still have to buy fittings) then in terms of invisible dust using the 90 mm ducting to put your DE outside is far better than leaving the DC inside the shed. Just bear in mind that if your ducting runs are too long the 90 mm duct will compromise visible chip collection.

Even if you are not prepared to modify the ports on machines, it is still worth putting the DC outside and running a 150 mm duct as a trunk line and then using 100 mm take offs and use two take offs per machine. Near each machine connect one duct to the machine and leave the other just open near the dust making point to scavenge invisible dust. It may mean your chip collection is not quite as good but much more invisible dust will be scavenged this way. I know this is not very satisfying to go to all that trouble and still have chips laying around but remember chips don't affect your health.

Unfortunately its not possible to put the dust extractor outside - wish it was.

The idea of putting an open pipe near the dust making point is an interesting one - can we be confident it really removes all the micro-dust though. I suspect its a bit hit and miss ???

cheers
Arron

Originally Posted by Arron

Unfortunately its not possible to put the dust extractor outside - wish it was.

The DE itself can be inside in a cupboard as long as it vent outside.

The idea of putting an open pipe near the dust making point is an interesting one - can we be confident it really removes all the micro-dust though. I suspect its a bit hit and miss ???

If you can't vent the DE outside the shed, an open pipe near the dust maker is a waste of time. The micro is going to go straight through the DE and back into the shed anyway.

Let's assume you can vent outside.

Even a 5HP DE could not pull enough air through a 90 mm fully open duct to capture the micro dust as it is being generated. The idea of having open pipe available near the dust making activity is to create a negative pressure in the region of the dust maker to reduce the amount of micro that wafts out into the rest of the shed and to how far it goes into the shed. Even if it captures only 25% of the micro that is better than nothing because if you want to vent your shed follow your last dust making activity it means leaving the DC on for say 45 minutes instead of 90 minutes.

In a small shed (eg <20 m^2) it does not matter where the dust collecting opening is but obviously having it near the dust making activity is better. What is important is whole of shed venting. Venting through a 90mm pipe will be slow. It may be better to remove all the ducting from the DC and let it create a cross draft to pull clean air into the shed.

OK, so I spent last weekend installing ducting for my DC - using 100mm dwc pipe.

Now I'm wondering whether I should have used 150mm for the mainline. Its not a simple choice because though I already have old 150mm pipe, the ducting is on a very low ceiling so loosing 50mm will be an issue.

The dusty is a 2hp with a filter cartridge on top and plastic bag on bottom. The main line goes up the wall for about 1.8 mtrs, then across the ceiling for about 3 metres. There it has a 90 degree turn which is also its first inlet, and runs for a further two metres but with 4 more inlets. It will later need to be extended for about another 2.5 metres with two more inlets but that is another project. The inlets are simply T joins with their open ends pointing downwards and blocked by turned wooden plugs (couldnt use blast gates because of the configuration). Because each machine needs to be wheeled out from its home against the wall to be used, I cant attach any permanently to the ducting, so I just reach for a short lenght of flexible pipe when I need to use.

I should also point out that the 3 ninety-degree turns involved are done with 600mm lengths of flexible hose so are gentle curves. I couldnt use stock fittings because of the different elevations of the pipes.

So my question is, could I expect a big or a small difference if I change the main-line (up to the first inlet) to 6 inch ?

cheers
Arron

Originally Posted by Arron

The inlets are simply T joins with their open ends pointing downwards and blocked by turned wooden plugs (couldnt use blast gates because of the configuration).

Instead of T joins I recommend using Y's. If you use a 6" main the 6"-4" Y's seem expensive but then again so are 6"-4" T's, besides, what is your health worth?

I should also point out that the 3 ninety-degree turns involved are done with 600mm lengths of flexible hose so are gentle curves. I couldnt use stock fittings because of the different elevations of the pipes.

According to the BP staticalc.xls, @500 cfm (which is all you can expect with 4" ducting and a 2HP system) 2 ft of 4" flex loses 0.42" of static pressure whereas a 90º bend loses 0.23". If you use 6" ducting all the way to a machine (and the machine has a 6" duct) the flow should double (1000cfm). Now 2 ft of 6" flex loses 0.26" whereas a 90º loses 0.21", still worth using the 90º. The 90º bends are for smooth steel with radius or curvature relative to duct diam ratio of 2, PVC will be slightly (maybe 5% better). If two 45º and PVC are used to make the 90º bend the static press loss drops even further (maybe 15%?)

So my question is, could I expect a big or a small difference if I change the main-line (up to the first inlet) to 6 inch ?

It will make no difference unless you use more than 1, 4" connection to each machine. If you only have one 4" connection on a machine then another 1 or 2 open ducts located near the dust making area of a machine is worth considering. Bear in mind that your big chip collection efficiency will be reduced but they are not a problem as it is the invisibles that operators should be more concerned about. Escaped chips can be always be swept up or vacuum later.

If you can post a picture of your system, there may be other improvements that can be identified.

Ok,so I understand that there is no advantage going to 6 inch pipe when I dont currently intend to have either 6 inch ports on any machines or free air extraction working near any machine. - but now I'm thinking about it from another angle. Is there any DOWNSIDE to having a 6 inch main line and then 4 inch from the first inlet onwards.

I' thinking that I may be able to introduce the free air extraction later, as well as have two ducts to some machines (tablesaw and drum sander would benefit). So as I have the 6 inch pipe and I'm all keen, should I put it in now?

Cheers and thanks for guidance offered so far
Arron

I'm also curious as to why the flexible ducting attenuates the air flow to a greater degree then a 90 degree fitting. Assuming the ducting is smooth on the inside, its all pretty much the same to a molecule of air travelling along a pipe, is't it ?

arron

Originally Posted by Arron

Ok,so I understand that there is no advantage going to 6 inch pipe when I dont currently intend to have either 6 inch ports on any machines or free air extraction working near any machine. - but now I'm thinking about it from another angle. Is there any DOWNSIDE to having a 6 inch main line and then 4 inch from the first inlet onwards.

If your DC vents outside the shed there is one BiG advantage I did not mention and that is in venting your shed after a dust making activity.
With 4" ducting your have to leave the DC on a factor of ~3 times longer than with 6" ducting. With 6" ducting, if you have 3 (or more) 4" gates you can open after your last dust making activity then the shed will vent much quicker and the ports don't have to be in the vicinity of the dust making area.

If you DC vents inside the shed there is no benefit and no disadvantage.

I' thinking that I may be able to introduce the free air extraction later, as well as have two ducts to some machines (tablesaw and drum sander would benefit). So as I have the 6 inch pipe and I'm all keen, should I put it in now?

That is eminently sensible.

Originally Posted by Arron

I'm also curious as to why the flexible ducting attenuates the air flow to a greater degree then a 90 degree fitting. Assuming the ducting is smooth on the inside, its all pretty much the same to a molecule of air travelling along a pipe, is't it ?

arron

The pressure differences I quoted are for 2 ft (600 mm) of straight flex V a 90º bend with R/D ratio of two. If the 2ft flex is bent thru 90º then the pressure drop is even greater. If a 90º bend is made from a 1ft piece of flex then the R/D ratio changes and even though less flexy is used the decrease in the R/D ratio will kick in and change the pressure loss. The a 90º bend pressure loss should be added to the flexy loss.

A better comparison is 4" flexy V 4" straight pipe.
AT 500 cfm the Flex drops 0.21"/ft whereas the pie only drops 0.07"/ft

RE: Why is flexy so lossy.
There are two factors, one is microscopic smoothness (which is what I'm thinking you are referring to) and the other is a mm and greater scale of the regularity of the duct. The diam of flexy pipe is determined by the constancy of the diam which a wire coil can maintain over its length and this not the same all the way through the pipe even when new let alone after it has been squished etc during use. This level of irregularity sets up instabilities which increases the thickness of the boundary layer and reduces air flow