Thread: Flexible ducting advice

Originally Posted by tony_A

. . . . . which would give the option of using some of the leftover it for gentle bends in the main line rather than using pvc elbows.
Is this worth considering or is the friction loss from a metre or so of flexible pipe likely to be greater than that of a 45 deg pvc bend.

It's better to use elbows.
BUT
There are ELBOWS and elbows.

The stormwater elbows have much tighter radius of curvature than the DWV elbows so unless you have no alternative using the DWV elbows will be better and always better than flex.

This is sort of discussed in this thread
Rigid pipe for dust collector

None of the manufacturers of ducting seem to give any friction loss figures for their ducting.

Friction losses cannot be reduced to a single number, as well as the surface roughness it depends on the fluid type and speed so it has to be calculated for every situation.
The roughness of a flexy depends on its internal structure and how much it stretched during installation.
Bill Pentz pressure flow calculator spreadsheet indicates the pressure loss of 6" flexy is 3x more than straight pipe.
3ft of flexy is equivalent to ~2 90º elbows (but he doesn't say what radius of curvature bends he refers to)
These figures are only approximate as the type of flexy and the way it is installed varies greatly.

Some info about radii of curvature is available in the pictures in post #7 of this thread - you should read the whole thread.

I try to restrict the use of flexy to machine connections or where a length of it can be used in lieu of a vacuum cleaner.

If you know bit about fluid dynamics there is a reasonably accurate on-line pressure drop calculator that takes into account pipe/flexy roughness here
Pressure Drop Online-Calculator

However, determining the actual roughness of flexy is VERY tricky.
When flexy bends and stretches the surface roughness is not constant across or along the internal walls - also you have no idea where the max flow is - it may be more along the outside inner curve of the dust but it may not and it may change over time. That's why measuring the actual flow is safer, although also not necessarily that easy.

Thanks Bob
Understand that generalisations are all you can make with flexy but that answers my question. The other side of the equations is that you need the performance curve for the impeller if you are going to accurately calculate the resistance and flow rate in the ducting. Have asked clearview but they haven't replied.
Tony

Originally Posted by tony_A

Thanks Bob
Understand that generalisations are all you can make with flexy but that answers my question. The other side of the equations is that you need the performance curve for the impeller if you are going to accurately calculate the resistance and flow rate in the ducting. Have asked clearview but they haven't replied.

It will be faster to contact Bill Pentz direct.

Originally Posted by tony_A

Thanks Bob
Understand that generalisations are all you can make with flexy but that answers my question. The other side of the equations is that you need the performance curve for the impeller if you are going to accurately calculate the resistance and flow rate in the ducting. Have asked clearview but they haven't replied.
Tony

For the average user the fan curve is not important, it is overkill for the average shed and will support at least two BG's open if not three. if planning a large/many machines installation of course that changes things. I will also presume you are going to turn the fan at 60hz if not then perhaps seek advice.

Bob
What brand of flexi did you used in the mens shed. Is it worth paying twice the price for a duct with higher abrasion resistance or just go for the cheap stuff? Both are recommended for wood working dust.
One brand in my current 100mm flexi has a black plastic liner (like duc tape) which is starting to delaminate and foul up the pipe. I have no idea what make of duct it is but would steer clear of it next time. The local supplier does not keep 225 mm duct in stock and will only order it in after payment.

Is it worth getting anti static ducting? I've never had a shock off the 100mm ducting I currently use but see anti static ducting being recommended for wood dust. I presume the risk of dust explosion is very low in a one man operation like mine don't want to get a shock every time I grab the duct.

Originally Posted by tony_A

Bob
What brand of flexi did you used in the mens shed. Is it worth paying twice the price for a duct with higher abrasion resistance or just go for the cheap stuff? Both are recommended for wood working dust.
One brand in my current 100mm flexi has a black plastic liner (like duc tape) which is starting to delaminate and foul up the pipe. I have no idea what make of duct it is but would steer clear of it next time. The local supplier does not keep 225 mm duct in stock and will only order it in after payment.

I agree about the problems with the abrasion resistant stuff and for one man DIY wood dust operations even the soft food grade stuff will last for many years.
At the mens shed we only used 150 mm to connect to machines?
We got ours from Carbatech but that stuff is pretty soft and tears a bit more than I like and there is better stuff out there.

Why the 225 mm? I thought I managed to talk you out of using it for bends?

Is it worth getting anti static ducting? I've never had a shock off the 100mm ducting I currently use but see anti static ducting being recommended for wood dust. I presume the risk of dust explosion is very low in a one man operation like mine don't want to get a shock every time I grab the duct..

Why are you grabbing the duct? There should be minimal need to touch it

I'm not saying there is ZERO chance of explosion, even though the Physics is against it but the chance of a wood dust explosion is so low that no one on this forum or elsewhere that I know of has been able to has been point to a single explosion caused by wood dust. There have been a few fires in DCs but these are caused by hot objects being sucked in to DCs.
Some folks don't like the electric shock and that's bout the only reason for getting the anti static stuff. Grounding the ducting is only marginally effective anyway.

Whatever you get make sure you try before you buy. The claimed measurements don't always agree with what you get. Some stretches/compresses more or less than claims.

Thanks bob
Some manufacturers recommend anti static duct for wood working situations and I was wondering if this was necessary.
Intend to follow your advice and use pvc bends but want a movable BMH for the wood lathe and will put that on the end of a length of flexy. Also a movable BMH at the work bench.
What is your opinion on a short length (about 200mm) of flexy at the machine attachment point to take any vibration.

Originally Posted by tony_A

Thanks bob
Some manufacturers recommend anti static duct for wood working situations and I was wondering if this was necessary.

Sometime you should ask Bill Pentz what he thinks about most things most manufacturers say about wood dust collection

Intend to follow your advice and use pvc bends but want a movable BMH for the wood lathe and will put that on the end of a length of flexy. Also a movable BMH at the work bench..

If you go with a Clearvue CVmax, that nominally pulls 1800 CFM which makes the nominal air speed through a nominally 225 mm (actually 240 mm) diameter pipe around 3700 FPM.

If you go with a 150 mm duct the duct will only pull about 1250 CFM )or a bit more even with the Max) but the resulting nominal air speed is 6400 FPM

So the 150mm duct collects less air than the 225 mm duct and so will collect less fine dust
BUT
The higher air speed means it will collect a few more chips - it also reduces chip scatter.

For short (<300mm) spindle work you would be better off with the 150 mm BMH as I know it pulls enough air to collect all the fine dust anyway and will collect a few more more chips.
For big bowls or longer spindles go will the large BMH.
You could make up a set of BMHs (6-8-10") and swap them out according to what you are doing - that would be really nice.

Same goes for a BMH at the workbench.

What is your opinion on a short length (about 200mm) of flexy at the machine attachment point to take any vibration.

I've never see vibration as an issue but Sometimes it does make it easier to fit large ducting to a machine because lining up 2 x 150 ports is not always easy
For example I have under floor ducting to my TS and it comes up direct under the machine to a hopper inside the cabinet.
Originally I had a 200 mm length of flexy to do the connect but I worked out a way to connect with a hard PVC bayonet.
I'd show you the junction itself but the underneath of my TS is full of crap.
Maybe I'll try and draw it.