6" ducting

[jussi]

So I'm finally getting around to re doing my ducting.  I had smaller DC before and used 4" SD straight pipe with pvc joints.  I got a much more powerful 3HP  1600 cfm DC a couple of months ago and want to upgrade to 6" for the main lines with 4" drops.  While I would love nordfab (or similar) it's just way to expensive for me.  I'm leaning towards spiral ducting unless someone can persuade me otherwise.  If I go with metal ducting what gauge should I get?  Doing a quick search most of the local hvac places carry the 6" diameter in 26 gauge.  Will that be substantial enough?

[wtnhighlander]

 I make no claim to be a ducting expert of any sort, but if your collector fan develops much static pressure, thin metal ductwork is at more risk of collapsing. For example, a dent from swinging a wayward board could create a weak spot that might fail as the pressure increases. Larger diameter tube would be more susceptible to such damage. I have observed this to happen in commercial installations, not hobby shops, but HVAC ducting isn't designed for vacuum applications.

PVC would not have that particular weakness, but might have others.

I haven't gone through this evaluation personally, is the metal duct signifcantly lower in cost at 6"?

[Coop]

I just reworked by dc collection using the white sewer drain pipe and connections ( not sch 40) readily available and inexpensive at HD and it works great. 

[BonPacific]

On 7/7/2022 at 5:49 PM, wtnhighlander said:

 I make no claim to be a ducting expert of any sort, but if your collector fan develops much static pressure, thin metal ductwork is at more risk of collapsing. For example, a dent from swinging a wayward board could create a weak spot that might fail as the pressure increases. Larger diameter tube would be more susceptible to such damage. I have observed this to happen in commercial installations, not hobby shops, but HVAC ducting isn't designed for vacuum applications.

PVC would not have that particular weakness, but might have others.

I haven't gone through this evaluation personally, is the metal duct signifcantly lower in cost at 6"?

I ran the numbers recently for my own region and the fittings are way cheaper in 6" metal than 6" PVC. At least 30% and some more like 50%. Also some PVC pieces, like 6x6x6 wyes are for some reason really hard to find in my neck of the woods (neither box store nor any local building/agriculture supply even stocks them) and would have to be special order.

[Mark J]

I don't have any answers.  But I'm wondering if you one could acquire a section of pipe and an end cap, and test it out?

[Chestnut]

I went plastic 5 years ago and I feel really confidant about the decision. The SDR35 pipe is readily available as well as sweep 45s and 45 degree wyes in my region. That said cost for pipe has gone up 3x. 10 foot sticks used to cost $20 they are now $60. Fittings are roughly the same though 45s are $15 and wyes are $17. My shop was $485 to plumb with plastic. Now it'd be closer to $850.

This company sells metal pipe and has a table that shows the allowable negative pressure in 26ga spiral pipe. http://airhand.com/wp-content/uploads/2015/06/AHS_Cut_Sheet_Elbow_Loss_Spiral_Pipe_Allow_Neg_Pressure.pdf It looks like the best 3hp hobby collector will do just fine with 26ga material.

My total for metal would be around $1500 (not including shipping if i had to ship it in). Have to include some specialized tools to crimp and work with metal duct. It also should be noted that cutting and working with metal may not be as easy as plastic. I used my woodworking miter saw to cut the 6" pvc or a handsaw in some situations.

 

[gee-dub]

The cost of ASTM-2729 was a driving factor back in the day.  At today's prices I am not so sure.  I bought a lot of extra back when a 10' stick was $11.  It has served me well since 2009.  Very easy to expand or change.  Everything moved with  me to the new shop.

The ridge in spiral pipe adds quite a bit of strength.

[wtnhighlander]

That link posted by @Chestnut is a good resource. A quick web search indicates common 3hp dust collectors for woodworking produce static pressure in the 14" to 17" of water column range, well below what 6", 26 gauge steel duct can withstand. My concern is not so much with the pristine round duct, but stems from the fact that metal duct can dent, which will affect the structural integrity of the tube. Keep in mind that the 40" of water column rating translates to about 1/10th of atmospheric pressure at sea level.

In all honesty, this argument is strictly theoretical. I would go whichever way is most cost effective for you.

[Chestnut]

1 hour ago, wtnhighlander said:

. My concern is not so much with the pristine round duct, but stems from the fact that metal duct can dent, which will affect the structural integrity of the tube.

I think this is an excellent point. To alleviate that concern the PDF indicates that a factor of safety of 3 was used. That tells me that even at it's highest rating of 30-40" there would still be some allowance for dings. This is with 26 ga SPIRAL pipe though. Smooth pipe is a different story and might collapse if you look at it wrong. Most DCs are around 20-25" which is a factor of safey closer to 4-6.

The interesting thing from my research is that more HP =/= more static suction. Both Oneida gorilla pro collectors have the same max suction of 23". It's complicated and comes down to impeller design. hence why a shop vac can pull a large amount of static suction but move very little air.

[drzaius]

I have 26 ga spiral and it will never collapse. I can stand on a length of 8" and it barely bends. Easy to work with too.