Am I missing something???? - Flex duct vs 90 degree bend vs 2x45 degree bends

[emgeecay]

Can anyone please help me to understand the article on "calculating dust collection needs by the numbers" which has appeared multiple times in Wood Magazine?

For a 6” system, achieving a 90 degree change of direction, using Worksheet 2 and Table 3 from the article:

Option 1 - 1x 90 degree bend = 12’ Equivalent Length of Rigid 6” Duct. This results in SP Loss of 0.540. This looks crazy as it has a lower SP loss than 2 x 45 degree bends (or identical if the 1’ of rigid duct between two 45 degree bends is ignored), yet almost every article on dust collection says that 2 x 45 degree bends is far superior to a single 90 degree bend

Option 2 - 2 x 45 degree bends with 1’ rigid duct in between = 13’ Equivalent Length of Rigid 6” Duct. This results in 0.852 SP Loss. Again this looks crazy. It is the highest SP loss, yet almost every article on dust collection says that 2 x 45 degree bends is the best way to achieve a 90 degree change of direction.


Option 3 - 3’ flexible 6” duct =9” Equivalent Length of Rigid 6” Duct. This results in SP Loss of 0.405. Again, crazy as it is the lowest SP loss, yet almost ever article on dust collection says flexible duct is the worst option.

Perhaps I am misunderstanding how the calculations should be applied and if so I would be grateful for advice on correct interpretation.

[BobL]

Option 1 - 1x 90 degree bend = 12’ Equivalent Length of Rigid 6” Duct. This results in SP Loss of 0.540. This looks crazy as it has a lower SP loss than 2 x 45 degree bends (or identical if the 1’ of rigid duct between two 45 degree bends is ignored), yet almost every article on dust collection says that 2 x 45 degree bends is far superior to a single 90 degree bend

Most books on dust extraction don't take into account all teh other issues associated with bends eg radius of curvature of the 90º bend.

The radius of curvature of available BENDs is usually described in terms of the radius (R) of the pipe or duct.

Worst of all is a 90º bend with a 1R radius of curvature- these are the only 90º bends I have been able to find for stormwater ducting
Just slightly better are teh standard 2x45º bends each with a radius of curvature of 1R
Best of all standard fittings is a 90º bend with a 2R radius of curvature - these are not available for stormwater but are available for DWV and fortunately the 6" DWV fittings also fit stormwater pipe.
You could use 6 x 15º bends but they don't improve things that much.

There are specialised communications PVC 90º bend fittings with 3, 4, 5 and 6R radii of curvature however they don't gain much on the 2R bends.
Another reason for not using these really wide radii of curvature bends is that they occupy a lot of space especially in
The biggest improvement is obtained by just going from 1R to the 2R.

The 2R 90º bend is the only DWV fitting worth buying in the DWV range - for all the others use stormwater.

A 900mm length of flex bent into a 90º bend geomtrically equates to about 4R bend but the actual efficiency will depend on the floppiness for teh flex material
The stiffer the flex the better it is but most soft polyurethane flex will turn that bend into a <2R equivalent.
Really stiff polyethylene flex may be as good or even better than a 2R rigid bend - BUT you will have to put up with the space issue.

Another factor is air speed. if the DC uses a small impeller and/or is connected to a choked machine the air flow will be low and there will not be that much difference between the various bend types to the point where differences cannot be measured.

[Maple71]

Thank you BobL for clarifying this issue.

[emgeecay]

Thank you BobL for your very prompt reply. Your observations about radius of bends and availability in Australia are particularly useful as so much info on the web is US based and not relevant here.

However I would value further insights/data/evidence on the actual calculations from the Wood Magazine article. You’re right that they don’t specify radius of bends but assuming constant radius ie 1R it seems counterintuitive that flex hose has less SP loss than 90 degree bend which in turn has less SP loss than 2x45 degree bends.

[BobL]

You’re right that they don’t specify radius of bends but assuming constant radius ie 1R it seems counterintuitive that flex hose has less SP loss than 90 degree bend which in turn has less SP loss than 2x45 degree bends.

That's an assumption I wouldn't necessarily make.