Dust extractor flow in pipes

[Hornbeam]

I am using 4 inch flexi for extraction, with the extractor positioned so I dont need more than~ 2m of hoze to any machine
How much flow is lost through a flexi compared with a smooth bore as I am trying to work out the gains from a smother bore against the losses of having to have a longer route to some machines
Ian

 

[DBT85]

Someone will come along with numbers shortly, but the answer is enough that you want smooth bore for as much of the run as possible. Ideally you run the main duct to a gate right next to the machine you want to use and then just run a minimal a mount of hose to it.

 

[Inspector]

Two to three times more than smooth pipe. So your two metres of hose is equal to four to six metres of pipe. The stiffer the hose and smoother the inside surface the better, as is a straight hose rather than one coiled.

To illustrate I played with an On-Line Duct Friction Loss Calculator. You'll have to convert to metric if you are more comfortable with it.

Calculating 6" pipe, at 1,250 CFM (maximum flow you can get from 6"), 7' long (2 metres), no elbows etc. The total duct loss in inches of water are.
Using Galvanized Metal was .743".
Using PVC was .602".
Using Flexible duct (wires exposed) was 1.746".
Using Flexible duct (wires covered) was 1.159".

Using 4" and 450CFM (maximum flow you can get from 4") and keeping the rest the same.
Galvanized was .804".
PVC was .659".
Flex (exposed) was 2.015".
Flex (covered) was 1.297".

Pete

 

[Hornbeam]

Thanks Pete, but struggling to understand the units and what it means in practical terms.
Staying with the 2 meter length of coiled hoze and assuming that my extractor is pulling 450cfm at the connection.
What am I actually pulling at the end of a 2m hoze
What is the suction pressure for 450cfm ie at the exit end of the extractor and at the end of a 2 metre hoze
Thanks again Ian

 

[twodoctors]

Short answer is as above. Smooth is probably better as it laminar flow rather than turbulent flow.

For the layman (and I'm no expert) the Hagen–Poiseuille equation tells you that you want shortest and widest circuit you can make.

A

 

[Inspector]

Each DC will have a pressure curve. Unfortunately few manufactures provide them for hobby machines. The highest static pressure will be with the duct blocked off. So zero flow with the most static pressure. At the other end of the spectrum it will have the lowest static pressure when the inlet to the DC is wide open and the air is flowing at maximum speed. At the points along the curve are the different combinations of flow to static pressure. So for the sake of argument if your DC has a static pressure of 5", wide open using 2 metres of uncovered wire hose you will lose 1.297" of that flow so you have 3.703" of static pressure, with a corresponding loss of flow. Now the machine you hook it to will have a static pressure loss in it that is hard to quantify without a bucketload of calculations and tests that I am not qualified to do. Lets pretend it is 1.5" because of the turbulence and restrictions etc inside it. Table saw slots and holes for instance. That means there is now 2.203" of static pressure to pull air with. With tight turns of the hose and reductions in the diameter of the connections in order to get the hose to fit you have a little more reduction in static pressure with the corresponding loss of flow. The more that is done that reduces the static pressure takes away from the amount of air that the DC will move, which reduces what it can capture. While it may still pull most of the visible sawdust and chips it won't capture the suff you can't see floating around the shop. You'll need to wear a good respirator all the time.

Until you are in the vacuum cleaner static pressure ranges (up to 140") you can't get anymore the 450 CFM through a 4" pipe with hoses less. A dust collector will have somewhere between 5" and 16" of static pressure. The 16" only on 5hp machines with 16" impellers. It has something to do with area in the duct and the amount of surface drag of the duct walls. When you have 6" duct you can flow up to 1200 CFM because the area to wall surface drag is better. The reason I preach for bigger DC's and larger diameter ducting.

I'm going to assume you have a low powered (1 or 1 1/2 hp) DC and it comes with a 4" duct and you don't have a lot of room. If you have to use a hose get one that has the inside wires covered but if you run smooth pipe to the machine with minimal or no flex you will have an improvement to the flow if it is less than 4 metres long. The one caveat to that is that each elbow in a solid duct reduces the suction so if you need to use 2 or 3 elbows for the rigid duct you won't gain as much. If you play with that calculator I linked earlier you can see how much more static pressure loss you'll get with them.

As to the units you will have to find a conversion calculator like the one that comes with a Mac and put the numbers in to get the metric data you are familiar with. 450CFM is 764.555 cubic metres per hour, if I did it right. :roll:

The airflows you DC is supposed to have from the sales brochure is without ducts or filters. Practically speaking it will deliver half the airflow when hooked up to pipes, hose and machines.

I'm likely adding to your confusion but as the others said use as much hard duct as you can. Keep the runs as short as you can. Use the least amount of flex hose as you can. Use bigger ducts if you DC can handle it. If you haven't already bought a DC get as big as you can possibly afford and fit.

Pete

 

[user22161]

Short answer is as above. Smooth is probably better as it laminar flow rather than turbulent flow.

For the layman (and I'm no expert) the Hagen–Poiseuille equation tells you that you want shortest and widest circuit you can make. A

But, when extracting chips and dust, won't "widest you can make" allow material to drop out rather than carried by flow. Surely, the wider the circuit, the lower the speed of airflow?

 

[DBT85]

Short answer is as above. Smooth is probably better as it laminar flow rather than turbulent flow.

For the layman (and I'm no expert) the Hagen–Poiseuille equation tells you that you want shortest and widest circuit you can make. A

But, when extracting chips and dust, won't "widest you can make" allow material to drop out rather than carried by flow. Surely, the wider the circuit, the lower the speed of airflow?

Well yes it has to scale appropriately with the size of the extractor. Using 100mm pipe with a shop vac is going to cause issues, just as using something slightly larger but with 150mm will do.

 

[user22161]

Well yes it has to scale appropriately with the size of the extractor. Using 100mm pipe with a shop vac is going to cause issues, just as using something slightly larger but with 150mm will do.

In common with others, my extractor has a 5" inlet with a Y converter to 2 x 4" which they say can be used simultaneously. Considering hard ducting but not much 5" around at sensible prices as junctions tend to be fabricated to order. Will most likely go for 6" uPVC, reducing via blast gates to short lengths of 4" flexible. Suspect that will be better than 2 x longer lengths of 4".

 

[TheTiddles]

Bigger pipe is larger area for less wall shear, but slower flow. You need a certain speed of flow to move particles of a certain size.

Rough walls and sharp corners increase wall shear.

There are lots of equations you could use for a single phase fluid... throw dust and debris into it, you are no longer single phase. Every corner, expansion, contraction, change in wall geometry or junction will have an effect, place them close to each other and the equation that governs it changes, sometimes reversing it totally.

As for laminar flow... not a chance, it’ll be fully developed turbulence in a matter of a few inches in.

So, you could try and work it all out. Or you could try and keep it at smooth as possible and give it a go, if it works ok the you’re done. I could do the first, I have only ever done the second

Aidan

 

[Doug71]

Don't overthink it.

I have a Jet bag over bag type extractor in my workshop, attached to it is a flexi hose from Rutlands, think 5m long. I swap the hose between machines as needed, extractor never moves. If it's connected to spindle moulder the hose is almost fully stretched but if it's connected to PT which is next to extractor most of the hose is curled up on the floor. It sucks and clears the chips regardless.

 

[mbartlett99]

I know the OP did ask for figures so fair enough but a 2m run? Think you're sweating the details there my man - you'll have 1m no matter what you do just at the machine.

 

[Hornbeam]

Thank you for the answers.

I know the OP did ask for figures so fair enough but a 2m run? Think you're sweating the details there my man - you'll have 1m no ma

With the layout of my workshop and the position of my extractor I can reach the 3 main machines(12 inch panel saw 12 inch planer/thicknesser and 600mm bandsaw with a 2 metre flexi. The only time I use a longer one is for the router table or when sweeping the floor. I also use a smaller vac with hand power tools

 

[mbartlett99]

Thats great - so your comparative loss on such a short run is close to nil. If, and why would you, you installed a fixed system with hard pipe you'd still have some flexi to connect to the machine and would have introduced a longer run and its fittings.

I'm running similar sized machines and have a 1.5kw axminster extractor rated at as I remember 2000cbm per hour through about 6m of flexi and it all works great. I will hopefully be installing fixed ducting at some point as dragging the flexi around is a pain and I'm forever tripping up over it. At which point my drops will be at least 1m to each machine.

 

[Hornbeam]

The reason for the original enquiry was to look at whether installing a fixed system with longer pipes would be better than current as it would avoid me tripping over flexis.
ian

 

[Torx]

I did the BOHS P601 course (to test LEV systems) last year - you learn a bit about design but really that’s P602. There’s so much to it I still have to get the books out as I don’t do it every day (and quite honestly I’m more interested in qualitative than quantitative results; they say more, especially to the lay person, about whether the system works or not).

Suffice to say, flow in the duct is only one element of good design, you can have great flow and still have an ineffective system. I’ve failed a lot like that.

HSG258 is worth a look - I tried posting a link but I’m too new! Google will find it for you.

Note that the chapter on Hood design (the ‘hood’ being whatever is at the end of any duct, not necessarily an open scoop - it could be the port on your machine and the void within) is lengthy and detailed, and the ducting is only a smallish section (though this is more of an overview than a ‘bible’, there are many, many more books on the subject). From my experience testing (and unofficially experimenting with my own designs over the last 15 years), the hood makes an enormous difference, regardless of what is going through the ducts. The hood design will also quite dramatically affect the flow in the duct, so while the numbers in the sales brochure might look good, it might not be the case with your particular machine.

Filtration is generally terrible with hobby machines, so the advice above for RPE and filtration in the room is good.

Flexible ducting - use good quality, keep the length to a minimum and earth it if you’re attaching it to steel ducting. Smooth walled steel ducting is better than spiral wound.

Then get your mate to do some cuts in the dark while you watch with a torch at a 15deg angle! :lol:

Best of luck!

 

[TheTiddles]

Not wishing to turn this into a total geek-fest but as Torx says, transitions are really interesting (no really, they are). A straight step in pipe diameter is better than a too-short taper, seems counterintuitive but it’s quite markedly worse.

Aidan

 

[Inspector]

Thank you Torx for the info about HSG258. I've bookmarked it for further reading and referencing. Much appreciated.

I saw the part about viewing light with a narrow beam of light from 15º off. What I didn't see was how small a particle this method makes visible to the eye. Did I miss that part or is it written somewhere else?

Anymore goodies that can be read online?

Pete

 

[Torx]

Anymore goodies that can be read online?

Pete

Still not sure I can post links, but there is a very good HSE guide to Tyndall illumination - I just found it first hit with a search for * hse 'the dust lamp' *.

You could also look in to particle counters, they’re one of those instruments that can be very cheap or very expensive - I have an HT9600 which is somewhere in between, and will measure particles down to PM2.5 - respirable. But I find they can be difficult to interpret. I’ve seen some interesting pocket sized ones recently, not sure how good they are but I’m fairly sure @peter millard (do tags work on here?) has one as I saw him repairing it on his YouTube channel. Maybe he’s posted here already about it?

 

[Inspector]

You can post links after you have 3 posts as long as you aren't doing it for gain. The rules here are short and pretty straight forward. topic54297.html

I have a particle counter I bought years ago in a group buy on another forum, now long gone. I was interested in the Dust Lamp because everything I have read says we can't see particles under 10 micron with the naked eye. http://www.dylosproducts.com/dcproairqumo.html

An Aussie forum has a very good section devoted to dust collection that you might find interesting. They have played with some of the newer small/inexpensive particle counters and the mod has compared the accuracy to calibrated ones. https://www.woodworkforums.com/f200

Pete