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duct transition change in elevation, or why parallel lines will never meet without an angle

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Forum topic by Perplexed1ne posted 09-21-2020 07:49 PM 826 views 0 times favorited 13 replies Add to Favorites Watch
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Perplexed1ne

25 posts in 185 days


09-21-2020 07:49 PM

Topic tags/keywords: dust duct design

My cyclone is in a big closet off of the garage. The ducting will be above the ceiling. I need to go from the 6” inlet from my cyclone up and through the ceiling of the closet where the ducting will run across the joists and drop down through the ceiling at the machines. The inlet to the cyclone has a bit of an up angle but not enough to make the angle I need across from it.

The inlet is 24” down from the ceiling and I have 4.5’ to the wall/ceiling juncture of the opposite side of the closet. That is about 20 something degrees sez Pythagoras. I can think of a couple options but don’t know how to think about the consequences to CFM and SP. There is also the issue of having a nice straight shot to the cyclone to improve separation. The least turbulence close to the cyclone, I read, is better.

One way is to use just enough flex ducting on each side of a straight duct to make up for the difference, with the least of the difference at the cyclone and the most of the difference at the ceiling juncture. The other option is to use those adjustable metal elbows that “screw” to increase or decrease the angle.

Is one of these options better from a CFM/SP perspective?

Any other ideas would be most welcome.


13 replies so far

View Aj2's profile

Aj2

3414 posts in 2681 days


#1 posted 09-21-2020 08:11 PM

I think your over engineering just hook it up the best you can and use it. Keep as much 6 inch in the system as you can. And use as little 4 flex hose as possible.
Good Luck

-- Aj

View Fred Hargis's profile

Fred Hargis

6502 posts in 3376 days


#2 posted 09-21-2020 08:28 PM

Along the lines of what AJ said, with DC ducting we often have to do things that aren’t perfect to get everything hooked together. Just do it and do it lose sleep over it.

-- Our village hasn't lost it's idiot, he was elected to congress.

View ibewjon's profile

ibewjon

2078 posts in 3676 days


#3 posted 09-21-2020 08:43 PM

Use the adjustable 90’s if you can, then seal the joints with tape or paint on duct sealer. Smoother,and less loss than the rough interior of flex.

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Perplexed1ne

25 posts in 185 days


#4 posted 09-21-2020 10:54 PM

I’m mostly 6” pvc duct in this system. I found a flexible pvc “rubber” connector that comes in 6”. It’s only 8” long but I’m gonna get a couple and see how much wiggle room I can achieve. My hope is for it to at least cover the bottom gap that canting the inlet duct up 20degrees will introduce. I’m thinking the swivel elbow will manage the top leveling. There is a paint-on duct sealant that I can use on the inside of the flexible elbow to seal and smooth the elbow.

But, I agree that it’s important to just get it up and working. I’ve got a 5hp motor/blower with 1850CFM/5”SP perched on top of the cyclone. I’m not gonna fret.

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Aj2

3414 posts in 2681 days


#5 posted 09-22-2020 12:36 AM

I’ve thought about using pvc but was concerned about static. I really don’t want to get shocked every time I’m near a machine that’s running.
Mostly glad I didn’t because I’ve changed my ducts at least a half a dozen times since I’ve had my cyclone.
Penn state 3hp with 14 inch impeller. It pulls tons of air on my jointer it sucks the boards down to the table nicely.
So you’ll be doing much better.

Good Luck Always

-- Aj

View DaveMills's profile

DaveMills

48 posts in 282 days


#6 posted 09-22-2020 01:42 PM

I haven’t done the math, but presumably one option to make the angles work would be to move the cyclone back a bit, so that the angle becomes acceptable? I actually did that for mine – based the position of the DC on the angle the pipe came down from the ceiling.

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Perplexed1ne

25 posts in 185 days


#7 posted 09-22-2020 01:50 PM

My back is against the wall…

The closet is 6’x8’ and while I thought about orienting the duct along the long side it would come out into the house rather than the garage. It would mean longer ducts to the drops. But, yes, wish I had more room. Thanks.

View DaveMills's profile

DaveMills

48 posts in 282 days


#8 posted 09-22-2020 03:34 PM

Have you thought about heating and bending the pvc?

View Perplexed1ne's profile

Perplexed1ne

25 posts in 185 days


#9 posted 09-24-2020 02:24 AM

I did think about it but in my mental experiments 6” PVC kinks or collapses. I have heated/bent 2 inch PVC pipe to good effect. Maybe I will experiment and have thrown in that towel prematurely.

There is a “rule of thumb” that 3x the diameter of the pipe before or after a joint settles down the turbulence introduced by air hitting the back wall of an elbow. So, with 6” pipe that is 18”. I am thinking that I can have an 18” straight run into the cyclone and start introducing 22.5degree elbows and play with 18” sections and both 45 and 22.5 degree elbows and see how far the gets me to overcoming the 2’ rise from the inlet to the joists. Its a closet so I have some room to play with by angling the inlet and placing the cyclone along the long wall to give me more that 4.5’ in which to make the rise.

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DaveMills

48 posts in 282 days


#10 posted 09-24-2020 02:54 AM

When I used to bend aluminum pipe for auto stuff, we filled it with sand to keep it from kinking. I wonder if the same idea would work for pvc.

View ibewjon's profile

ibewjon

2078 posts in 3676 days


#11 posted 09-24-2020 03:15 AM

Tape the ends shut before heating. The heated air expands and helps keep the pipe from collapsing. Or push on caps, but those cost more. We used expanding plugs when bending conduit. I have made thousands of bends in pvc up to 6”. You can also keep shaping the pipe back to round as it cools.

View Perplexed1ne's profile

Perplexed1ne

25 posts in 185 days


#12 posted 09-24-2020 12:51 PM

This morning I was thinking about all the ways I could bend PVC, cobble elbows and straight duct, or use flex hose. Then it struck me that I was tying my little brain in knots to deal with a half inch of sheet rock ceiling between me and a straight inlet duct.

I am going to blow out the ceiling at the end of the closet by a couple feet across and frame out a section that will allow my motor/blower the height for the inlet duct to be parallel to the joists. This will cost less than one PVC joint in scrap 2×4’s and sheet rock.

Thank y’all for allowing me to over and now under engineer.

Cheers!

View tomsteve's profile

tomsteve

1077 posts in 2102 days


#13 posted 09-24-2020 01:10 PM



This morning I was thinking about all the ways I could bend PVC, cobble elbows and straight duct, or use flex hose. Then it struck me that I was tying my little brain in knots to deal with a half inch of sheet rock ceiling between me and a straight inlet duct.

- Perplexed1ne

Aj2 was right!!!LOLOL
been here did this with my dc system. figgered all that hurt my brain too much and just did it. system works great!

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