Thread: Two 4" ducts vs 6" duct

I should add that the area of two 4" ducts (7854mm2 x 2 = 15,700mm2) is comparable to that of a single 6" duct (17,671mm2).

Originally Posted by Trav

I've got the opportunity to pick up a stack of 4" PVC ducting and junctions etc for free in the next few weeks. I know I should be looking to use 6" ducting, not 4", but could I use two parallel runs of 4" ducting? These could then go to separate 4" ports in each machine (ie at least 2 ports for each machine where possible).

I recognise that there will be more turbulence/resistance with 2 smaller tubes,but would this be too problematic?
For context, I have a generic 2hp dusty, relatively short runs and would only use one machine at a time. I plan on modifying the dusty as bob and others have suggested.

Forget about the areas, its flow rate that is important.
At normal DC pressures each 4" duct can pass about 400 cfm and 1 x 6" duct can pass about 1200 cfm the rule of thumb is that it takes 3 x 4" ducts to equate to 1 x 6" duct.

I see you plan to to modify the DC according to this thread the most you can get out of a modified version is 840 CFM with the stock needlefelt bags and 920 CFM with a pleated filter.

The question then is, since 2 x 4" ducts will draw 800 CFM, is it still worth using 6" ducting?

I reckon it is, especially on any trunk lines, because the generic 2HP DC needs every little bit of help it can get and 6" junctions and bends have much less effect on the final flow than 4" junctions and bends.

The clincher with a 6" duct on the modified generic 2HP DC is you you can get 840 cfm at the one point.
Using 2 x 4" ducts side by side will result in the two ducts partially fighting each other for the same air leading to a reduced flow.
To get the full 800 CFM the 2, 4" ducts have to be separated some distance from each other but then they are not place optimally to the dust source.

If you decide to use 2 x 4" ducting the way the 2 ducts meet in front of the DC is critical in determining the overall performance of the DC.
Certainly the crappy black plastic Y junction used on the generic 2HP DC should be thown away as this makes the two air paths smash into each other just in front of the impeller which is very inefficient.
A better way to do this is using a 6" duct with a 2 x4" to 6" Y connector about 1m in front of the impeller.

Thanks bob for your considered reply. Your expertise adds a lot to these forums.

I will make the modification as outlined in the thread you mentioned - the dusty will be in a separate enclosure vented to the outside (somehow). It has a needle felt bag, though I could look at upgrading this in due course.

I agree entirely about the useless black y junction that comes with the dusty. That will go straight in the bin. My plan was exactly as you suggested - a shortish length of 6" pipe, with a 6" 45 junction and a step down junction on each line to bring it down to 4".

But to get to your key point - should I go with 6" ducting which can move 1200cfm or go with the two 4" ducts that can pull 400cfm each? I'll need to think about this a bit more and consider how it might work in my workshop. I think I have a relatively simple system with two short runs and one longer run.

In terms of competing for the same air within a machine, I don't see how this would be different to having a larger port in the machine? Why would it be different with a 6" port into a table saw compared to two 4" ports (except that a 6" port would have a greater airflow)?

Trav

Originally Posted by Trav

But to get to your key point - should I go with 6" ducting which can move 1200cfm or go with the two 4" ducts that can pull 400cfm each? I'll need to think about this a bit more and consider how it might work in my workshop. I think I have a relatively simple system with two short runs and one longer run.

Maybe a drawing of your actual lay out would make it easier to comment.

In terms of competing for the same air within a machine, I don't see how this would be different to having a larger port in the machine? Why would it be different with a 6" port into a table saw compared to two 4" ports (except that a 6" port would have a greater airflow)?

This is something I worked out when dealing with venting whole rooms, but on reflection that was a low airspeed - high volume flow scenario which is quite different from wood dust collection from a reduced size source.

Originally Posted by Trav

In terms of competing for the same air within a machine, I don't see how this would be different to having a larger port in the machine? Why would it be different with a 6" port into a table saw compared to two 4" ports (except that a 6" port would have a greater airflow)?

Trav

Good Morning Trav

I think that you have answered your own question here. To optimise dust extractor capture rates and performance one has to optimise air flow. Essentially its air speed that moves bigger chips, and air volume that moves the truly nasty semi-visible stuff.

Another ay of analysing it is too look at the relevant surface areas of the ducts. Two 100mm ducts will have about 33% more surface area than one 150 mm duct - that is, much more friction. And against the wall of each pipe this friction creates turbulance, read minimal suction, for about 5mm or so, so that the effective cross-sectional area is significantly reduced. This is why two smaller pipes of the same total sectional area cary less airflow than one larger duct. [Ok, Bob, I have over-simplified a little.]

Once you have added your effort into the equation, the costs of buying a couple of lengths of 150mm duct will be a very small part of the project costs.


Fair Winds

Graeme

Thanks bob and graham.

Your wisdom - and the fact that my 4" pipe seems to be falling through - has swayed me. I think I'll focus on doing it once and doing it right and using 6" for the trunk lines and looking at how to either get two or three 4" lines to each machine or even 6" direct to each machine.

I need to find a good supplier of the junctions etc. seems plumbing suppliers here in Canberra should wear masks and carry guns - daylight robbery for some of the junctions. I'll keep hunting.

Trav

daylight robbery for some of the junctions.

Try Masters

Originally Posted by Trav

I need to find a good supplier of the junctions etc. seems plumbing suppliers here in Canberra should wear masks and carry guns - daylight robbery for some of the junctions. I'll keep hunting.

Trav

Got all my PVC pipe and fittings from Total Eden, at about 40% less than the plumbing supplies shops. I note there is not an outlet in ACT, but if the NSW prices are as good as the Strathpine store, it might pay to have the goods delivered.

This place can be a little spooky! I had read about total Eden, even looked it up once to find that there was no store in Canberra. I wrote it off at the time.

But I'll be heading up to Brisbane on a family holiday soon, and driving back with an empty car. Now that I've decided to go with 6" pipe, I tried to find total Eden again but forgot the name and couldn't find it. Then you chime in with EXACTLY the detail I wanted. Spooky. Will now stop in on the way through to pick up the junctions etc. I'll but the piping here, but that is cheap enough at bunnings.

Thanks again.

Trav

Originally Posted by DavidG

Try Masters

They only sell end caps and straight joiners at masters. About the same as bunnings. Not sure why they even bother. I can't imagine there would be much demand for long straight runs of 150mm pipe...

Trav

Originally Posted by Trav

This place can be a little spooky! I had read about total Eden, even looked it up once to find that there was no store in Canberra. I wrote it off at the time.

But I'll be heading up to Brisbane on a family holiday soon, and driving back with an empty car. Now that I've decided to go with 6" pipe, I tried to find total Eden again but forgot the name and couldn't find it. Then you chime in with EXACTLY the detail I wanted. Spooky. Will now stop in on the way through to pick up the junctions etc. I'll but the piping here, but that is cheap enough at bunnings.

Thanks again.

Trav

You might like to give the Total Eden Strathpine store a ring and get a quote. Tell them you have been referred to them and get some prices. When I did that the bloke in the store emailed me a list of prices for the bits I wanted within 30 minutes.

He quoted me (a year ago now) 6m lengths pipe $48.36 ... 45 degree bend F:F $12.46 ... Wye Junction F:F:F $18.50

Originally Posted by John Samuel

You might like to give the Total Eden Strathpine store a ring and get a quote. Tell them you have been referred to them and get some prices. When I did that the bloke in the store emailed me a list of prices for the bits I wanted within 30 minutes.

He quoted me (a year ago now) 6m lengths pipe $48.36 ... 45 degree bend F:F $12.46 ... Wye Junction F:F:F $18.50

JS

Thanks for that info as I am about to head down a similar pathe to Trav, but have been phased by the hideous cost of some of the fittings in 150mm.

Regards
Paul

Trav, Reece Plumbing has a few stores in the ACT.

Originally Posted by Bushmiller

JS

Thanks for that info as I am about to head down a similar pathe to Trav, but have been phased by the hideous cost of some of the fittings in 150mm.

Regards
Paul

My pleasure Bushmiller,

It was a delight to find them after reeling away from the plumber's supplies in shock. One of the nice things about this forum is that both good and bad news gets around pretty fast ... and Total Eden were very good news for me.

I note you do some milling. I might be interested in buying some timber if you had some nice stuff going through the saw that was surplus to your needs.

Cheerio!

John