Metal to PVC

[Bernie Kopfer]

I've pretty much designed my dust collection system and will be using pvc 2729 pipe and fittings. Reducing from 6in to 4in using supplied fittng is rather abrupt. How well do metal ducting fittngs attach to pvc? Or am I wrong in thinking the abruptness will cause a significant flow loss? Thanks for you r help. :)

[Alan Schaffter]

Where do you want to go from 6" to 4", and why?.

If you have a 4" inlet on the blower, change it out to 6" right away- you never want to have 4" inlet with 6" pipe and 4" is way too small of an inlet for most blowers. A 4" inlet with 6" pipe results in too little velocity in the pipe, and 4" inlet really restricts CFM horrendously.

If you want to drop from 6" mains to 4" at the machine or at a drop, a gentle transition is a tad better than abrupt but not that bad since flow is not hitting the shoulder- its going the other way. You just get a bit of turbulence. But, if you have a decent DC keep it 6" all the way to the machine, and open the machine ports to 6". If you will be using only one machine at a time, don't need to balance a system, and you don't need smaller to maintain velocity (4000 fpm), then there is no need to reducing to 4" which restricts CFM- go 6" all the way to the machine.

[Ray Bell]

I am sure the air flow experts will find a lot wrong with this system, but this is how I did it, and it seems to work fine. There are six home made blast gate mounted on the 4x4 along the wall. Six inch pipe at the top of the gates, and four inch coming out to 4" flex tube to the machines.

[Bernie Kopfer]

Where do you want to go from 6" to 4", and why?.

If you have a 4" inlet on the blower, change it out to 6" right away- you never want to have 4" inlet with 6" pipe and 4" is way too small of an inlet for most blowers. A 4" inlet with 6" pipe results in too little velocity in the pipe, and 4" inlet really restricts CFM horrendously.

If you want to drop from 6" mains to 4" at the machine or at a drop, a gentle transition is a tad better than abrupt but not that bad since flow is not hitting the shoulder- its going the other way. You just get a bit of turbulence. But, if you have a decent DC keep it 6" all the way to the machine, and open the machine ports to 6". If you will be using only one machine at a time, don't need to balance a system, and you don't need smaller to maintain velocity (4000 fpm), then there is no need to reducing to 4" which restricts CFM- go 6" all the way to the machine.
In an ideal world I would be able to go 6in all the way to the machine but alas i must reduce to 4in flex line for the last few feet. I'm using a 3hp Oneida so I;m not too concerned about flow loss, but i'm willing to get all I can and was thinking that the smooth reductoin of metal ducting could be a significant improvement over the abrupt shoulder in PVC. Hence the question about how well/easily does pvc and metal ducting fit together? The blast gates will be at the machines.

[Eric DeSilva]

Oddly, I just did this minutes ago. I tried to take a pic... There is some stuff there you probably don't need, but you should see a small spur of 6" PVC S&D coming out of the blast gate. The next piece is a rubber 6" > 4" PVC to PVC adapter I found at lowes. I think it might be for concrete, but it worked perfectly, and you can see a stub of 4" PVC coming out the other end...

[Alan Schaffter]

Oddly, I just did this minutes ago. I tried to take a pic... There is some stuff there you probably don't need, but you should see a small spur of 6" PVC S&D coming out of the blast gate. The next piece is a rubber 6" > 4" PVC to PVC adapter I found at lowes. I think it might be for concrete, but it worked perfectly, and you can see a stub of 4" PVC coming out the other end...

It is a "Fernco (http://www.fernco.com/plumbing/flexible-couplings)" neoprene fitting used join various sizes and types (including PVC) of pipe and available at Lowes.

http://www.fernco.com/img/products/plumbing/flexible-couplings/flexible-couplings-stock/flexible-couplings-stock-main.jpg

[Brian Cover]

... A 4" inlet with 6" pipe results in too little velocity in the pipe, and 4" inlet really restricts CFM horrendously.

A smaller diameter pipe does not reduce velocity. In fact it increases the velocity if the CFM is unchanged. What it DOES do is Increase static pressure and friction losses. When designing any exhaust system, the we need adequate velocity to suspend the material in the air flow. Too little velocity will result in debris settling in the ducts. Too much velocity and we lose cfm due to the increased (negative) static pressure and the friction caused by the walls of the duct. Also, too much velocity will result in the erosion of the ducts due to the abrasive rubbing caused by the debris traveling in the duct. Theoretically, with enough velocity, we could cause the wood chips to penetrate or wear through the bag at the end of the collection system.

[Terry Hatfield]

HVAC metal reducers are super cheap and fit inside the pipe. The have a sloping transition so not terrible for the flow. This in is on my bandsaw. I glued a piece of pipe into the end of the wye and just inserted the reducer into it. Sealed with silicone. Works perfect.

http://terryhatfield.com/album/4/73907187.jpg

You can get them at Lowes or Home Depot for a couple bucks.

Terry

[Alan Schaffter]

A smaller diameter pipe does not reduce velocity. In fact it increases the velocity if the CFM is unchanged.

I agree, but that is NOT what I said. If you read my post again, what I said is that a 4" blower inlet with 6" ducting results in lesser velocity in the duct.

[Joe Chritz]

In dust collection silicone makes almost anything fit. :D

If you are necking down at the machines the slope is against the flow and really won't cause any problems beyond being a smaller pipe.

Joe

[Curt Harms]

In an ideal world I would be able to go 6in all the way to the machine but alas i must reduce to 4in flex line for the last few feet. I'm using a 3hp Oneida so I;m not too concerned about flow loss, but i'm willing to get all I can and was thinking that the smooth reductoin of metal ducting could be a significant improvement over the abrupt shoulder in PVC. Hence the question about how well/easily does pvc and metal ducting fit together? The blast gates will be at the machines.

Please excuse me if I'm missing something, but could you go 6" flex hose to the machine, then reduce to 4"? I have that on a 10' 6" flexible hose (must use hose because machines are mobile) necked down at the machine attachment. A 6" X 4" PVC reducer fits over the hose and over 4" PVC stubs on each machine. I figure the larger hose somewhat offsets the added turbulence & resistance of flex vs. smooth wall. The 4" portion of the install is only a few inches and there doesn't seem to be a problem with material settling out of the airstream. I know it's optimal to have 6" all the way but I don't see a way to enlarge the Jet JJP-12's dust hood.