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Actual numbers for comparing a long radius 90deg vs 2 45deg elbows

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Forum topic by Perplexed1ne posted 09-25-2020 02:35 PM 445 views 0 times favorited 7 replies Add to Favorites Watch
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Perplexed1ne

25 posts in 186 days


09-25-2020 02:35 PM

Topic tags/keywords: long radius 90degree elbow cfm

My main ducting is all above the 8’ ceiling of my garage workshop. For my drops I have enough room to use a 45 deg wye to the wall, straight duct, and then a 45deg elbow down the wall. I also have room for 90deg long radius elbows.

I keep reading the rule of thumb upside to the dual 45’s is less CFM loss compared to a long radius 90deg at the wall. But, the downside is my low ceiling height and desire to keep the ducting above the ceiling, only loosing wall space to ducting.

The consensus that I keep reading is “don’t over think this, just do something.” That is likely correct.

BUT, can anyone be specific (like with real numbers or percentages) about the CFM consequences of choosing a 45deg wye oriented through the ceiling to a 45deg elbow compared to a long radius 90deg coming off of a parallel to the ceiling 45deg wye down the wall? I am using 6” PVC drain duct.


7 replies so far

View Russell Hayes's profile

Russell Hayes

25 posts in 62 days


#1 posted 09-25-2020 05:15 PM

So if I understand you have either:

1.A parallel to the ceiling and main run wye, then a 90 down (or two 45’s).

2.OR a 45 degree wye off the main pointing down then a 45 to the machine.

Option one is a superior design, even with the added 45, so you don’t get backfill in the down drop when it is not used – then a burst of debris to your impeller when opened.

Of course you can eliminate the last 45 on the drop by running your duct at an angle across the wall. However the loss of wall space may negate that design. In which case do what you have to do.

-- Have a hobby? You should have a business.

View Madmark2's profile

Madmark2

1715 posts in 1472 days


#2 posted 09-25-2020 05:30 PM

You’re loosing more to the high lift than the 90 vs 2×45 will ever cause.

Run low and you’ll only have one lift at the DC.

-- The hump with the stump and the pump!

View AlanWS's profile

AlanWS

86 posts in 4441 days


#3 posted 09-25-2020 05:59 PM

Here’s an online calculator that lets you estimate pressure drop given flow, or flow given pressure drop for various configurations:
http://www.freecalc.com/ductloss.htm

If you prefer equations, look here: https://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html

-- Alan in Wisconsin

View AndyJ1s's profile

AndyJ1s

463 posts in 638 days


#4 posted 09-25-2020 06:11 PM

If I understand choice 1 correctly, it has a total 135 degrees of turn, albeit in two planes, which makes no difference.

Choice 2 has only 90 degrees.

All else held equal (e.g. radius of turns), choice 2 wins.

I did a little experimenting on the freecalc ductloss calculator mentioned above, and found that, if the radius of curvature is the same between one 90 and each of two 45’s, then the total loss is identical. Go with whichever has longer radii.

-- Andy - Arlington TX

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Perplexed1ne

25 posts in 186 days


#5 posted 09-25-2020 08:56 PM

I am figuring out how to use the calculator and appreciate all of the help. I want to get this up and running and before I become an engineer. Since I am in no danger of becoming an engineer, might as well get this up ASAP. But, it costs no more to do it right than to just cobble it together with less optimal results.

In choosing to split the intake at a machine between the machine’s top and bottom, like a miter saw or table saw, would there be a difference between a 6×6x6 wye that had the reducers to 5” and 3” after the wye fitting closer to the machine, OR use a 6×5x3 wye and run that size duct to the top and bottom.

View Madmark2's profile

Madmark2

1715 posts in 1472 days


#6 posted 09-25-2020 09:18 PM

If you are going to neck down, do it at the minifold. Ditto for the blast gates. This will reduce the static volume and thus the static pressure.

Use metal, not plastic, blast gates.

-- The hump with the stump and the pump!

View Redoak49's profile

Redoak49

4921 posts in 2872 days


#7 posted 09-27-2020 12:26 AM

The difference between a long radius 90 and two 45s, is only a couple percent, so use which ever works best.

Note: BobL on the Aussie Woodwork Forum actually tested and compared. The Aussie forum is IMHO the best source for Dust Collector information.

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