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Thread: DRAFT Ducting Recommendations
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25th February 2020, 08:29 PM #16GOLD MEMBER
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Fien/renovator/multi tool I found is the best thing to cut pvc pipe. Nice clean cut and pretty much a safe tool to use one handed as you roll the pipe following a line
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25th February 2020 08:29 PM # ADSGoogle Adsense Advertisement
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25th February 2020, 08:46 PM #17.
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These are indeed safe to use and are also really good for cutting rectangular slots and holes in plastic pipe. I also have a medical plaster cast cutting saw that does the same thing and use that to cut holes for screens/displays etc in plastic electronics instrument boxes. These types of tools are great if you already have them otherwise I’m not sure they’re worth purchasing just to install ducting. I will add this to the recommendations above.
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28th February 2020, 03:27 PM #18.
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Those of you that read the above recommendations may have noticed the "Static" section was missing. This has now been completed, along with an associated FAQ thanks to the assistance of member dmorse and I have added some picture to aid in the explanations. I hope you find this useful.
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28th February 2020, 09:13 PM #19Senior Member
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Have had the thought - NOT council friendly -or possibly neighbors either - of a basic cyclone on the end of the system and NO filtration. Should be more efficient - would obviously depend on the fineness of dust - sanding or thicknesser. Was the old school thing till regulated away (possibly for good reasons)
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28th February 2020, 09:41 PM #20.
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2nd September 2020, 02:24 PM #21New Members
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Hi Bobl, wondering if you ever got a chance to test this semi rigid duct? If suitable I was considering using in a retractable dropper situation from an overhead trunk line. Overhead trunk being used to service multiple mobile workstations containing table saw, router, drum sander and also mobile thicknesser and bandsaw. So I was considering not servicing every station but have a few droppers and this duct would Ben better than using flex? I also have some static machines being drill press and lathe which the trunk will also service. Would like multiple drop downs to service multiple machines but because if necessity to move machines around shed want retractability to get them up out of the way when necessitated.
Thinking this semi rigid may be part of a solution - join to trunk on wyes, blast gate at each Wye, and fabricate some form of easy on/off at machine end.
I will be making mods to a 2hp dc using 150mm dimensions as you have described in the related thread with no cyclone but with both particle collector and venting on outside of shed with no filter to aid flow.
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22nd February 2021, 03:46 AM #22Intermediate Member
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Storm water 90º bends have a very tight radius and are hence also restrictive but the standard DWV 90º bend has twice the radius of curvature and although is more expensive has a much lower flow resistance. This about the only DWV junction needed for 150mm and larger ducting. (my emphasis added)
All the 150mm (6") "sweep" PVC 90s that I can find here in the U.S. are only about 1.25R, not 2R, so I've been hesitant to use them.
Related to this, if the curvature of this DWV plain 90-degree bend is less than 2R, would you still recommend using this over using a pair of 45-degree bends separated by a short bit of straight pipe?
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22nd February 2021, 09:23 AM #23.
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Sorry no picture appeared - this is probably because you are a first time poster and it will take about 5? posts before you are approved - email the pic to me @ [email protected]
All the 150mm (6") "sweep" PVC 90s that I can find here in the U.S. are only about 1.25R, not 2R, so I've been hesitant to use them.
Related to this, if the curvature of this DWV plain 90-degree bend is less than 2R, would you still recommend using this over using a pair of 45-degree bends separated by a short bit of straight pipe?
Red bars are for a single 45º bend - average is also ~1%
Green is for 2 x 45º bends average loss is a bit over 2%
Purple is for 1R bends ie average is about 7%!
Where the 1.25R bend would sit is somewhere in between the 2 x 45º and the 1R so I would go ahead and use the 2 x 45º.
150mmbends2.jpg
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22nd February 2021, 10:04 AM #24Senior Member
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First, in the US DWV is not normally used for dust collection. Sewer and Drain is what most people use. DWV has an OD that's compatible with pressure pipe, i.e., Schedule 40 or 80. S&D is ASTM-2729 or ASTM-3034 (e.g., SDR35). Different OD and different fittings.
Second, US curvature names are not the same as Bob's. Bob gives the ratio of curvature radius to pipe inside radius. In the US it's referenced to inside diameter. So, Bob's 1R is .5R in the US.
A common S&D sweep 90 is .833R US or 1.67 Bob. It will have less loss than two .5R/D 45s.
Ninety_75rD_2.jpgDave
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22nd February 2021, 11:37 AM #25.
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Here in Oz we have two types of pipe commonly used for dust extraction, DWV, used for sewer pipe (grey coloured) and comes in different wall thicknesses (pressure ratings)
The other pipe is Storm Water (SW) which is cream/white coloured and is not defined by wall thickness (it's thinner than DWV) but stiffness.
For 4"/100mm or less, DWV and SW are not dimensionally compatible but above this size they are. DWV fittings are thicker, have longer overlaps on pipe fittings, and bends have longer radii of curvature. There is no 100mm SW available, the nearest is 90mm which is rubbish for dust collection.
Many stores will not stock the full range of fittings for 6" or larger - they many only stock DWV fittings since these cover both applications
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22nd February 2021, 01:07 PM #26Intermediate Member
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Bob, thanks for your reply. The image to which I was referring is in your post #4 of this thread. Your charts are most helpful, thank you.
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22nd February 2021, 01:18 PM #27Intermediate Member
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dmorse, thanks for clarification on the terminology. I'm working with D-2729 and SDR35 pipe for our dust collection system. I was not aware that the terms "S&D" vs "DWV" held a meaningful distinction.
So, to be clear: in the U.S., you believe that the sweep 90 in your drawing will have less loss than paired 45s? If so, this is not jiving with what I think I'm seeing for 6" in the Cincinnati Fan Engineering Data pdf (page 9) which I've been trying to understand. Not being argumentative at all - just trying to understand what to use!
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22nd February 2021, 10:48 PM #28Senior Member
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The table on that page does not go below 1.5 R/D. How are you extrapolating that to .5 and .833?
ASHRAE/SMACNA tabulate the following
Code:R/D Loss coefficient 0.5 0.71 .75 .33 1.0 .22
You can only compare loss coefficients qualitatively to Bob's data. His numbers apply only to his setup. The ordering remains the same but relative values can be different. Loss coefficient is the ratio of pressure loss to velocity pressure, not the ratio of two flow rates.Dave
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23rd February 2021, 06:24 AM #29Intermediate Member
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Hi Dave, thanks for your continued efforts to educate me on this. I hope the conversation can help others also trying to figure out best practices for our dust collection ducting.
I'm looking at the Cincinnati Fan data very much as a lay person with no background in the engineering, and certainly none in fluid dynamics. I've spent a lot of time reading Bill Pentz's materials and I understand the desirability of using long sweep 90s (2R or greater) versus shorter radius 90s. I'm trying to get some firmer test data or engineering data that supports whether to use some given sweep radius 90 versus pairs of 45s. My focus is on 6" pipe.
When I look at the Cincinnati chart (shown below for convenience), I note that with 6" pipe has a coefficient of 12 at 1.5R and 7 at 2R. A 45 degree elbow is stated to be one-half each of these. So, I'm thinking if I use a pair of 45s spaced by some straight pipe, that combination will still have a loss coefficient less than a .833 R.D 90 even if not as good as a 1.5R 90.
Cincinnati Fan Resistance Loss Chart for Elbows.jpg
(open in new tab to see full size)
So what am I not understanding here in making my leaping guess? Is it that I'm not taking into account the R/D of the 45-degree elbow I may be using and that a tighter radius 45 will be more similar to the loss resistance of the .833 no matter how I space them apart? In other words, to make my leap I'd need to be sure I'm using 45-degree elbows manufactured to that same 1.5R or 2R radius?
I suspect that, just as there are no 1.5R or 2R radius D-2729/SDR35 90-degree elbows, there are no 1.5R or 2R radius 45-degree D2729/SDR35 elbows. Correct?
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23rd February 2021, 08:45 AM #30Senior Member
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The issue is that the only 45s available are .5R/D. According to the Cincinnati Fan chart you showed, two of them should have the same loss as a single 90, although Bob's data shows it to be less. The SMACNA/ASHRAE shows a .5 R/D 90 to have a loss coefficient of .71 (that is, per Cinc., each 45 is .71/2 and two of them are twice that), while a .833 interpolates to .28. That's a big difference.
Bend Loss.pngDave
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