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9th May 2017, 03:20 PM #46.
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Because the CT flex 90º bend was so inefficient (33% loss) I went back and explored it further.
It turns out the losses relative (to straight duct) for a 90º bend vary between 20 to 35% - this is the benefit of doing multiple trials.
After a bit of fiddling about it was clear that small changes in the radius of curvature of the bend was having an effect but not in the way one might expect.
Remember I only have a fixed length of flex (~300mm) so to generate the close to 1R radius of curvature I had to squeeze the flex together, for ~1.5R I let it relax, and for ~2R radius I wrapped the flex around a 100 mm diam pipe - this stretched the flex and put it under tension
What I found was the the radius of curvature close to 1R was more efficient than around 1.5R, while the 2R was more efficient than both
This result is unusual because it should be more efficient at 1.5R than at 1R.
Perhaps it is because when the flex is tight (compressed or stretched) the flex vibrates less than when it is more relaxed?
One needs to bear in minds that,10 of the 20 to 35 % losses comes from the the fact that the flex itself even when straight is relatively lossy.
This is consistent with the better efficiency of the TC flex which is quite stiff so it doesn't vibrate as much and comparing the flexible CV 150mm ducting with the stiffer 150 HT flex discussed in post #25.
What this tells me is that the more flexible flexy that folks seem to gravitate towards is far from ideal and should be used only in very short lengths.
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9th May 2017, 07:15 PM #47Woodworking mechanic
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I noticed in the test with the 150mm bend tests, the pipes are facing the ground. Would it make sny difference if they where facing to the side or upwards to eliminate any chance of return turbulence from the ground?
I know with certain testing we do (not air flow but sound) we need a minimum of 3m clearance.
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9th May 2017, 07:46 PM #48.
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Good question and I did notice reductions in flow if things got too close even along side the openings.
When testing the lengths of flexy I could not just let it lay on the ground as that did affect the flow by about 15% plus it sucked up a heap of sand and dirt!
I tested the flow through a length of flexy with the opening at 1.5 m above the ground and then ~500 mm above the ground (laying on those chairs in post #25) with the openings elevated slightly upwards and they were the same.
The 2R bend (highest air flow) was tested with the right angle duct straight up/down and sideways and it made no measurable differences.
I would have done all the tests with the duct point upwards but this was tricky because it was raining and the air intake was outside the shed
The bends were all tested with the opening the same height above the ground so that effect is sort of cancelled out.
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5th July 2017, 03:12 PM #49Woodworking mechanic
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Question BobL regarding the Testo.
How long are you finding the batteries last? I wouldn't have done as much measuring as you and I just had to fit the 3rd set!
Cheers
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5th July 2017, 03:36 PM #50.
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