Thread: Air flow calibration measurements

I should be working on something more urgent but I dropped into Pipe-on-line this morning to pick up a few junctions for some mods I'm doing to my DC system, and also picked up some 100 mm bends to test.

This test was primarily done to test the "2 x 45º are better than 1 x 90º bend bends" hypothesis. I did measure this back in 2008 or thereabouts and because I could not detect any difference (using a fairly ordinary air flow meter I borrowed from work) I assumed it was just my technique. Since then a few things have changed, the main one is I tested this out using a very clogged 1HP DC with a sort length of 100 mm test duct. Now I have proper test duct so I can perform more reliable tests, and the new TESTO meter enables me to quickly collect a heap more data and place it straight into a spreadsheet for calculation.

So the setup was similar to the flexy testing using the big (240 mm diameter) test duct.
On the end of that I attached two pieces of 100 mm duct connected by a straight coupler and measured the flow.
Then in between those two pieces I inserted a bend and measure the flow again.
Everything was measured several to ensure some sort of reproducibility.

Three types of 100 mm 90º bends were tested
1 was a standard Female - Female 90º bend This had a midline radius of curvature of 1.4R (1.4x the internal radius of the pipe) = 90FF on the graph
2. was a Male- Female 90º bend with a 1.8R curvature = 90MF on the graph
3 was 2 x 45º bends joint with minimal amount of 100 ducting. = 45-45 on the graph.

Age graph below shows the percentage flow loss for each bend when compered to straight duct flow.
The high the bar the worse the performance of the bend.
Screen Shot 2017-05-06 at 3.29.33 PM.png

The little bars on top of the blues bars are the measurement uncertainty and does not include the uncertainty of the flow in the straight duct.

The first thing that stands out is the 2 x 45º bends don't appear to be doing that well.

Although there's not much in it there also appears to be difference between the FF and MF bends probably due to their slightly different radii of curvature with the MF bench having the larger radii and then smaller losses.

Please note that this applies only to these 100 mm ducts on my system - results on your system may vary but I would be surprised if the order varied. The slower the flow the less difference there will be

I will test some flexy bends next.

Thanks for these ongoing tests Bob.

What about 2 x 45 with around 600mm between them compared to a single 90? I wonder if the 100mm results are indicative of 150mm 90 V extended 45's?

Originally Posted by MandJ

Thanks for these ongoing tests Bob.

What about 2 x 45 with around 600mm between them compared to a single 90? I wonder if the 100mm results are indicative of 150mm 90 V extended 45's?

OK - will give it a go.

Here's a few more

Experiment performed as in post #32
This is all compared to an equivalent length of 100 mm ducting

Screen Shot 2017-05-07 at 3.51.15 PM.png

45-45 no spacer = are two 45 degree bends with minimal ducting in between
45-45 spacer = are two 45 degree bends with about 250 mm of 100 mm ducting in between
45 bend only = a single 45º bend , interesting that the loss is less than half of 2 x 45º ?
90 deg CT flex = a minimal (i.e. tight) 90 degree bend made out of CT flex
90 deg TC flex = a minimal (i.e. tight) 90 degree bend made out of TC flex
90 deg bed = 90 degree MF bend same as tested in post 32.

Because it was so stiff it was very difficult to force the TC flex into a tight radius so the radius for the RC would have been slightly(~20%) larger than for the CT flex.

[PS]
A couple of observations I forgot to mention yesterday,
- on touching the CT flexy bend I notice it was vibrating quite significantly, this is a strong indicator that it is not working efficiently. The TC flexy bend also vibrated but nowhere near as much probably because it is a much stiffer flex.
- the 45º bends are 1R bends which probably explains why 2 of them are not as efficient as the wider (1.8R) Radius 90º bend.

If 150mm results are similar, it would appear that the effort put in eliminating the dreaded 90 bend with variations of 45 bends is a wasted effort, and actually results in more loss?

It's almost like once the air has started to change direction through a bend, that any straight transition in a bend (as in 2 x 45) is seen as another change in direction and results in even more loss. As a single smooth 90 has only one direction change it has less loss, with the exception of a single 45 bend, again only one smaller change in direction.

Originally Posted by MandJ

If 150mm results are similar, it would appear that the effort put in eliminating the dreaded 90 bend with variations of 45 bends is a wasted effort, and actually results in more loss?

Correct, but before anyone goes ripping their150 mm 45º bends out let me test the 150's so see how significant the effect is with them.
I have one spare 150 mm 45º bend under the house and I hope there's a spare at the men's shed.
Also I want to test the 150 mm 90º bends as well.

It's almost like once the air has started to change direction through a bend, that any straight transition in a bend (as in 2 x 45) is seen as another change in direction and results in even more loss. As a single smooth 90 has only one direction change it has less loss, with the exception of a single 45 bend, again only one smaller change in direction.

Sounds like an explanation, and couple that with all the stormwater AND DVW 45º bends that I have are 1R bends.

A Y and single 45º probably should be OK.

Thanks again Bob, yes the 45 Y and single 45 look good, I actually hope the results for 100 x 90 bends are similar for 150mm, it sure make it easy in some places, like my Router table and the Lathe pickup - both are 150mm 90's.

Mike.

EDIT: What I was trying to say is - it would be nice to know that I'm not really loosing as much flow as I initially thought by using one or two 150mm 90 deg connections

Originally Posted by BobL

- the 45º bends are 1R bends which probably explains why 2 of them are not as efficient as the wider (1.8R) Radius 90º bend.

That's quite a big difference in radius. I agree that is going to explain at least part of the increased losses measured for the two 45s.

OK did the 150 mm tests this morning and here are the results once again relative to a straight piece of 150mm duct.

90deg2RTx =represents a DWV 90deg bend with a radius of curvature of 2 (actually 2.2), Tx = test #, 90deg2RAve is the average of the 5 separate tests
IMG_2167.jpg

45Tx are tests x done for for a single 45deg bend - two tests done on this bend.

45-45Tx are for two 45º bends joined by minimal duct. 3 tests done on this one

90deg1RTx, are the tests done on a stormwater 90º bend with a 1R radius of curvature. 3 tests done on this as well.

As you can see its a MUCH tighter radius of curvature compared to the other 90º bend.
IMG_2166.jpg

I'm showing most of the data so you see how tricky the results are.

Observations
1) These are amongst the trickiest measurements I've made and I had to go back and increase the sample size and number of repeat trials to get enough data to see if there is any difference between these bends. This sort of test really pushes the whole measurement system/process.

2) Apart from the 1R 90º bend, the differences in flow rates from the straight duct are are around 2% or less, so in practice it doesn't really matter what you use.

3) Looking just at the averages, the 90º 2R bend and the single 45 are indistinguishable @ around 1% +/- 0.4%.

4) The 45-45º bend is around 2.3% +/- 0.3% so reasonable well resolved as being slightly worse than the 2R 90º bend but being only 2.3% its no biggie if you have them already installed.

4) The worst performer by far is the 90º 1R stormwater bend at around 7.4 +/-0.8 % loss - so avoid using these unless you have insufficient room to manoeuvre.



150mmbends2.jpg


So 2 x 45º is better than the 1R 90º but just worse than the 90º 2R bend - I don't reckon its worth changing any 2x45º bends for the 2R.
probably worth changing any 1Rs if you have them.

I hope this is of some use.

Well worth knowing, Mannum Men's Shed is completing the 150mm ducting, there are a few more bends to go so we will make good use of this information.

Cheers Barry

Bob, I know it's been said before, but another big thank you for the time and effort you put into this.

It feels to me like the last of the guess work has finally been removed when it comes to ducting our home workshops. Combine all the work and testing you did with bell mouth intakes, machine ports, DC modifications and limitations, filter media, room ventilation, fine dust warnings, and more that I have likely left out. Those of us interested in our heath and our enjoyment of WW now have a greater understanding and a better chance of installing and modifying a DE system that is more affordable and efficient than would have been possible without the effort and information you have supplied.

Mike.

Originally Posted by MandJ

Bob, I know it's been said before, but another big thank you for the time and effort you put into this.

It feels to me like the last of the guess work has finally been removed when it comes to ducting our home workshops. Combine all the work and testing you did with bell mouth intakes, machine ports, DC modifications and limitations, filter media, room ventilation, fine dust warnings, and more that I have likely left out. Those of use interested in our heath and our enjoyment of WW now have a greater understanding and a better chance of installing and modifying a DE system that is more affordable and efficient than would have been possible without the effort and information you have supplied..

Thanks Mike. I enjoy the challenge of these types of measurements especially determining the measurement uncertainty or tolerance side of things. This is something I received a lot of training in as a student and used every day at work. When undertaking research I spent a lot of time measuring small difference between quantities (often at the <0.1% level) so I had to spend much more time on the uncertainties than the actual quantity being measured. I ended up teaching undergraduate experimental physics for 20 odd years where the methods for teaching uncertainties etc are hammered into students. It was interesting to see new students coming it with a very limited understanding of uncertainties and by the time they left most of them had a really good grounding in the area. Several decades later a former student said to me at a reunion "uncertainty management" was one of the most useful things he had learned at uni.

Something I forgot to mention in the 150 mm bend test post was the results I obtained (the actual percentages) apply only to my system and your system may have different percentages but the order (e.g. this is better or worse than this) should still apply.

This is reflected in the % differences in flow seen with the 100mm bends compared to the 150 mm bends. The greater %differences measured in the 100 mm bends is because the same static pressure pulling the air into a duct will cause proportionately greater resistance and hence loss of flow in a smaller compared to a larger diameter bend. If I was to do the same tests on 225mm bends on my system I doubt I would see any differences in the flow loss of the bends.
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Originally Posted by BobL

Something I forgot to mention in the 150 mm bend test post was the results I obtained (the actual percentages) apply only to my system and your system may have different percentages but the order (e.g. this is better or worse than this) should still apply.

This is reflected in the % differences in flow seen with the 100mm bends compared to the 150 mm bends. The greater %differences measured in the 100 mm bends is because the same static pressure pulling the air into a duct will cause proportionately greater resistance and hence loss of flow in a smaller compared to a larger diameter bend. If I was to do the same tests on 225mm bends on my system I doubt I would see any differences in the flow loss of the bends.
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Yes I noticed that trend in other tests you have carried out, it's why I found the 100mm and 150mm tests results so valuable as these are the two most often used sizes in home workshop extraction, along with some of the flex you tested the results are really helpful.

Cheers.