I've been reading all the excellent articles on dust extraction but I have been left with one area I have not seen info on. Most of the flexible hose that I've seen being used and tested is with the spiral wire embedded in the tube wall leaving a ridged wall to the tube. I have seen some flexible tube that had a smooth inner wall (usually used for food grade tubes) but I have never seen a suction comparison between the 2 types of flex tubes. Anyone seen any info regarding the efficiency of the 2 types of tubbing?
Dust extraction - flex hose. Smooth vs normal
- Thread starter Welsh Brian
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Everything I've read on the topic will tell you that ribbed tubes are less efficient and should never be used for long runs. They are fine for connecting to machines where they dampen vibrations. My instincts would be to use steel tubes for bulk of the extraction as they can be earthed to prevent static. Has anyone used those transparent plastic pipes sold by Axminster ?
Inspector
Nyuck, Nyuck, Nyuck!
Generally the smoother the surface the better. I have never seen a comparison like you want, I suppose because there are so many definitions of what a smooth inner wall would be. I have seen heavy walled water suction hoses that just are pretty much smooth inside but it isn't very flexible. I did play with an online duct loss calculator as it has a number of duct material choices and hose with exposed wires and hose with wires covered are among them. Duct Friction Loss The calculator is a great way to play what if scenarios of duct layout to see what would be better, including elbow radii etc. Results show pressure losses and velocity. Imperial only so you'll have to do the conversions to get metric results.
Only selecting 6" round at 1,000CFM and leaving the other parameters as it was when opening including the 100' length.
Galvanized ducting had 6.805 inches of water loss.
PVC had 5.637 inches of water loss.
Hose with exposed wires had 16.207 inches of water loss.
Hose with covered wires had 10.702 inches of water loss.
That shows that the claims that hose has up to three times the drag as smooth wall ducting is true and is shows that wire covered hoses are better that those with exposed wire.
If you can get a smoother walled hose that has enough flexibility for your circumstances then by all means get it.
Pete
Only selecting 6" round at 1,000CFM and leaving the other parameters as it was when opening including the 100' length.
Galvanized ducting had 6.805 inches of water loss.
PVC had 5.637 inches of water loss.
Hose with exposed wires had 16.207 inches of water loss.
Hose with covered wires had 10.702 inches of water loss.
That shows that the claims that hose has up to three times the drag as smooth wall ducting is true and is shows that wire covered hoses are better that those with exposed wire.
If you can get a smoother walled hose that has enough flexibility for your circumstances then by all means get it.
Pete
eribaMotters
Established Member
Is the Maths as simple as that. I have no denying the smoother the better, but the drag is only on the hose walls, what goes down the rest would be unaffected. I'm thinking a scenario of a river, where the water slows down along the banks but still runs with full force out in the centre.
Colin
Colin
Pete's using the calculator to make a simple apples to apples (6" ducting only) comparison to try and answer the question.
@eribaMotters , you're right that the physics are more complex and the drag from the boundary layer falls off as you get nearer the centre of the pipe.
If you use a bigger diameter, the effect is less.
If you use smaller diameter hoses than the 6" one Pete chose for comparison, the difference between the types of inner wall have a greater effect on total flow.
Very small hoses like shop vac extraction, the effect should be especially large but shop vacs make so much suction that I imagine the difference between a bad hose and the latest super smooth Festool thing makes little practical difference in use.
For HVLP dust collectors, these generate only low pressure drops and the selection of pipe, diameters, fittings, hoses and cyclones, which all introduce pressure drops, has a big impact on the final airflow.
@eribaMotters , you're right that the physics are more complex and the drag from the boundary layer falls off as you get nearer the centre of the pipe.
If you use a bigger diameter, the effect is less.
If you use smaller diameter hoses than the 6" one Pete chose for comparison, the difference between the types of inner wall have a greater effect on total flow.
Very small hoses like shop vac extraction, the effect should be especially large but shop vacs make so much suction that I imagine the difference between a bad hose and the latest super smooth Festool thing makes little practical difference in use.
For HVLP dust collectors, these generate only low pressure drops and the selection of pipe, diameters, fittings, hoses and cyclones, which all introduce pressure drops, has a big impact on the final airflow.
Inspector
Nyuck, Nyuck, Nyuck!
Colin the surface drag extends out from the walls more than you would think, affecting the flow overall. One of the reasons dust collector flow claims are up to double of what they actually can move. They take the reading in the centre of the duct where the flow is the highest. Take readings across the duct and the average brings it down. Even the addition of a bell mouth entry can improve the airflow in a duct by several percent just because it smooths out the airflow at the inlet of the duct. Without it the sharp entry has a turbulent zone around the edge that reduces the flow into the duct. Think of it like the wing of an airplane. The smooth flow over the wing generates the lift to allow the aircraft to fly. Cover the wing with frost and the drag from those ice particles are enough that the aircraft can't fly or even get airborne. It all adds up especially in a small system where there is little difference between good collection and failing.
Pete
Edited to make the last line clear.
Pete
Edited to make the last line clear.
Last edited:
Inspector
Nyuck, Nyuck, Nyuck!
If you want to the calculator will work with any size duct. Try it with 4" and see what the results look like. Just change the flow to 400CFM because that is about the limits of a 4" hose with any HVLP dusty.
Pete
Pete
Thanks everyone. It all has a solid logic to what you have all said. How can we entice one of the youtubers to run a test for us? Same 4" sucker and say a 4m length of pvc pipe, 4m standard flexy pipe and the same flexy length of smooth bore pipe. I am tempted with the smooth bore pipe even though it is quite a bit more expensive but it would be good to know how much more "suck" I would get for the extra bucks!
Jameshow
Established Member
Hope I'm not thread jacking too much...
I have been given a load of 5" pipe can I step up from 4" to 5" and then back down to 4" without an issue.
I have 3 x 5" blast gates I can get some reducer cheap enough??
I have been given a load of 5" pipe can I step up from 4" to 5" and then back down to 4" without an issue.
I have 3 x 5" blast gates I can get some reducer cheap enough??
Ollie78
Established Member
If you do it right it could create a velocity stack effect.
Jameshow
Established Member
Does that work in a closed pipe.
I'd imagine a larger bore with be more efficient in moving a volume of air??
Ollie78
Established Member
Certain vacuum manufacturers have pipes of increasing diameter towards the motor end (Sebo is one) which it is claimed aids suction.
A large bore moves more air but given the same sucton a smaller pipe will flow faster, I think.
I am obviously not a scientist.
Ollie
Inspector
Nyuck, Nyuck, Nyuck!
I don't understand why you want some YouBoob doing a test where they all use the fan type anemometers which with DC size ducting give erroneous readings of 50% or more over actual when you could use the calculator I linked and get the comparisons.
4" ducting of 4 meters (13') with 400CFM gives .978" of H2O with PVC, 1.904" of H2O with wires covered hose and 2.957" of H2O with exposed wire hose.
What the heck does that mean????? A little 1hp DC on it's best day only has 3" or 4" of static pressure suction so it will loose 1" to the PVC leaving 2" or 3" of H2O to gather and pull the dust. The uncovered wire hose will take 3" static pressure away from the 3" or 4" static pressure the DC has leaving almost nothing to gather and pull the dust with. A slightly bigger DC (1 1/2hp or 2hp) will fair a little better and a big 5hp DC with 12"+ of static pressure will easily cope with the duct/hose fitting losses as long as they are not miles long. With small systems you should make every effort to have the smoothest airflow you can.
Fan type anemometers are great measuring airflow in the outdoors, through a door opening or when the velocity and airflows are low. In a dust collector duct where you want 4,000 FPM (feet per minute) the air has to speed up to go through and around the fan and housing causing it to give bad readings. They are also influenced by the position in the duct and angle they are presented to the airflow. Hence my first paragraph. They should be at least using a thermal/hot wire anemometer which is about the size of a pencil and taking 8 or so readings across the duct and averaging all those readings. Better still is to use a pitot static anemometer and take the same readings across the duct. None of the YouBoob presenters spewing content do this because they don't know any better and the instruments cost more.
Static pressure is the suction produced when the duct is blocked off. The pressure drops as the duct is opened to full where it is at its lowest and the air is flowing at its maximum. Good dust collectors will come with a fan curve. Few do. It shows the relationship between static pressure and the airflow. If the system resistance of all fittings, ducting and the airflow through the openings of the machine you are collection from is too high all the dust isn't collected and it floats around you to breath.
Pete
Inspector
Nyuck, Nyuck, Nyuck!
When you go from small to large pipe the velocity in the duct will drop and at the other end when it goes back to small it will increase again. The problem is that bigger pipe in between can't hold the dust in suspension and it drops out staying in the duct. In extreme cases the duct will plug up until the cross sectional area is back to the small duct size where it can pick up big clumps and they slam into the impeller unless there is a cyclone in the way.
A 4" duct can move around 400cfm. When you move upon to 5" you can flow about 800CFM so double. Using the calculator like I did above in post 14 the 4" has .978 static pressure loss and a velocity of 4586 ft/min. The dust will stay in suspension. The 5" is .321 static pressure and a flow of 2935 ft/min which is barely enough to keep the dust in suspension so the heavier particles will fall out and lay in the duct.
That is why you shouldn't go from 4" to 5" and back to 4" again.
Pete
Jameshow
Established Member
I was thinking that I would run the system in 5" but attached to a 4" extractor at one end and flexible 4" hoses at the other.
Inspector
Nyuck, Nyuck, Nyuck!
I'm thick today. Does that mean you are going to go ahead with your thinking or give up the idea based on the information I gave you?
Pete
Jameshow
Established Member
No not at all just seeking clarification - if the 5" pipe is connected directly via a 4" reducer to a standard 4" extractor??
@Inspector answered your question for you far better than any youtuber could.
But here are a couple of points to try and help.
A 4" HVLP extractor doesn't make a lot of suction.
Here is a link to a good small one. To their credit they explain exactly how they measure their performance in the "performance" tab and as such their figures are trustworthy in a way that most are not.
https://lagunatools.com/classic/dust-collectors/b-flux/
You can see that the suction produced by the b-flux measures between 3 and 5 inches of water gauge. Measured in the middle of just over a 5 foot long flex hose attached to the extractor. A 64" length of flex hose is about the shortest you'll get away with to connect the extractor to either a machine or whatever fixed ducting you have installed, and the performance is measured with a new filter, so it really is a best case real world scenario for the machine.
Use the calculator mentioned linked from @Inspector's post, with a 12 foot long round flex duct at 400cfm airflow and the difference in pressure drop between the hose with exposed wires and without exposed wires comes out at essentially 1" water gauge.
That difference is between 20 and 30% of the total suction produced by the extractor.
So it is a significant difference.
Inspector
Nyuck, Nyuck, Nyuck!
The airflow in the 5" pipe will be limited to what the 4" DC inlet can pull. About 400cfm and your flow in the 5" will not be enough to keep the dust in suspension. The 5" pipe will fill with dust. Bad idea.
To exaggerate the situation to hopefully make it more clear. If you had the DC outside and stuck that 4" DC hose in the wall of your shop, would it suck up the dust from your saw sitting in the middle of the room? Nope, because there wouldn't be enough airspeed through the shop (a giant duct) to get the dust. If you want to use the free 5" ducting you need a bigger DC that has a 5" inlet port.
Pete
