Why is spiral ducting not more optimized?

**mwells** · 20th November 2022, 05:54 AM

I don't understand why spiral ducting is not more optimized for air flow.

I know in Australia metal ducting is very expensive so nearly all the installations I have seen on this forum use PVC. In the US, PVC is about the same price as metal spiral ducting so I have been evaluating both.

The spiral pipe is female and the fittings are male. There are good 1.5 CLR elbows, tapers, and wye available for spiral, but they end up reducing the diameter of the pipe from 6" to 5-7/8". PVC, on the other hand, has male pipe and female connectors, so the full diameter of the pipe is maintained.

PVC is also slightly bigger. The outside diameter of 6" ASTM D2729 pipe is 6.275" and the inside is about 6.075". With spiral, the inside is 6" and the fittings reduce it to about 5.875". So the inside cross section of PVC is about 5% larger.

PVC is much smoother. Bill Pentz says the friction factor for spiral is 90-100 while for PVC it is 146. (Bigger is better.)

Then we get to blast gates. I see lots of examples of homemade blast gates for PVC that allow 100% of air flow, but the metal blast gates typically used with spiral have a restricted diameter because they fit inside the pipe. What's worse is the door doesn't open all the way. The example I have has an inside diameter of about 5-9/16" and the door protrudes about 13/32".

IMG_20221119_072855.jpg

I did a measured drawing and my drawing program says the open area of the gate is about 23.5 sqin while the open area for a D2729 PVC pipe is about 29.0 sqin, so the blast gate restricts flow over 23% compared to a wide open PVC gate. The gate restricts the spiral pipe itself over 15%.

BlastGateComparison2.jpg

This forum has drummed into my head "optimize, optimize, optimize" with a dust collection system.

After looking at all this, I'm considering using PVC, despite the fact I have some pieces of spiral already. But I feel like I must be missing something because spiral appears to be the industry standard (after quick-clamp, which is way too expensive). However spiral isn't as optimized for dust collection as I would expect.

Am I splitting hairs? Does none of this matter in a real installation and hooking up flex hose, etc.? Has anyone measured the air flow of PVC vs spiral?

Mark

**BobL** · 20th November 2022, 09:41 AM

Not splitting hairs at all. Most DC systems are compromised by a bunch of factors so optimising as may as possiblel reduces the amount that is lost to capture at source.

**mwells** · 20th November 2022, 03:10 PM

Originally Posted by BobL

Not splitting hairs at all. Most DC systems are compromised by a bunch of factors so optimising as may as possiblel reduces the amount that is lost to capture at source.

I understand. It also seems that the performance of the system is difficult to measure so many people don't know whether their system performs well or not.

BobL, what would you use to determine if a system is working well? Particle counter? Pitot tubes? Hot wire anemometer?

Mark

**QC Inspector** · 21st November 2022, 01:43 AM

I believe spiral ducting has it's origins in heating and air conditioning where the speed in the ducts is lower so small differences are not as important. Always done it that way so why change?

Even with perfect ducting you're still going to run into the machine ports that are smaller.

Pete

I'll add that some of what you want to know about efficiency is in the sticky at the top of this portion of the forum. DRAFT: FAQ - Dust Extraction (Practical Aspects) There is more among the thread throughout.

**mwells** · 21st November 2022, 06:56 AM

Originally Posted by QC Inspector

I believe spiral ducting has it's origins in heating and air conditioning where the speed in the ducts is lower so small differences are not as important. Always done it that way so why change?

Even with perfect ducting you're still going to run into the machine ports that are smaller.

Pete

I'll add that some of what you want to know about efficiency is in the sticky at the top of this portion of the forum. DRAFT: FAQ - Dust Extraction (Practical Aspects) There is more among the thread throughout.

That sounds right. The quick-clamp stuff (Nordfab and the clones) seem like the purpose-build industrial answer. That ducting is really, really expensive for a hobby shop.

I clicked on that link and realized I had read those pages before. Sorry for asking a question in the FAQ. I just wish there was an easier way to measure airflow accurately, but nothing in dust collection seems easy.

Mark

**QC Inspector** · 21st November 2022, 07:19 AM

No apologies necessary. There is so much to retain that it sometimes gets lost in ones memory banks.

I think part of the reason Dust collection isn't easy is because there is so much contradictory information floating around put out by sellers and "amateur experts".

Pete

**havabeer69** · 21st November 2022, 09:33 PM

cost.

**auscab** · 21st November 2022, 10:04 PM

I used Spiral duct and did a price comparison with PVC . A lot of the spiral duct was cheaper . I think it was the long lengths that were cheaper and bends may have cost more . I don't remember but may still have the comparisons in my book of info possibly.
I mainly used 150 and 200 diameter.

Here
Questions about a second hand set up.

**auscab** · 22nd November 2022, 10:45 PM

I found my comparisons . I started writing it up but it starts to get complex comparing.
Correction . Some of the spiral is cheaper not " A lot " as I said . Overall PVC would probably be cheaper. Depends on how you do the bends. One piece or two per bend . Spiral duct bends are 90 degrees at 1.5R and PVC were 45 or 30 degrees so two or more at 45 degrees is a lot more $ but I don't remember why I didnt get 90 degree( or 88 degree) PVC bends price. Maybe because the radius was to tight .
Here's a picture of it if your interested.
The pipe is all per lineal meters . The first two are the expensive clip together expensive pipe.
Third is Trade Link PVC . They gave me the best price . And fourth is Spiral duct in Braeside where I bought my stuff from .
aaaa.jpg

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