6" PVC Ducting for extraction system

UKworkshop.co.uk

Help Support UKworkshop.co.uk:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Tugalis

Established Member
Joined
7 Feb 2015
Messages
50
Reaction score
10
Location
Cambridge
Hi,

Im just about to purchase my PVC ducting for the new workshop but wanted to run somthing past you guys before I do.

Ideally I would use metal however due to costs I am going to stick with the 6" PVC ducting. Ive been reading Bill Pences website for the last week or so and I dont really want to drop below 6" if it can be helped at all!

Where I have the drops I would normally like to use the below...

http://www.ductstore.co.uk/acatalog/150 ... tml#aTB150

however you cant get this for PVC so I had been thinking about using this https://www.tlc-direct.co.uk/Products/FD44970.html and then using 2 45 bends to bring it back to a horizontal.

Do you think I will loose much suction by doing this? I know its not ideal but I don't want to use a 90 degree T section as that really will hurt the cfm.

Its 6" from the extractor (Felder AF22) all the way to the machines where it reduces down to 4"

Any help is greatly appreciated.

:lol:
 
I would be cautious about using any metal ducting designed for HVAC installations. There are plenty of Internet discussions describing how easy the thin tubing collapses when used for dust extraction systems.

However, based on the Duckstore offerings, I recommend these fittings for the equipment branch connections:

45-degree Stainless Steel Junction

45-degree Stainless Steel Bend

You can use the 45-degree junction with a 100mm branch, and fit a 100mm 45-degree bend to complete the 90-degree split. I'm doing something similar in my shop, but am using 120mm Schuko tubing made specifically for dust extraction systems.
 
Make sure you earth the piping, there can be a big static build up on large bore pipes.

Avoid any thing that reduces diameter and then opens it up again.
 
It is much easier and better value in the long term to use metal piping made and sold for the purpose. You avoid static problems and get much better fire resistance. Whatever you chose to use the drop pipes should not enter the range pipe through the bottom or chippings from distant machines will drop down and fill up down pipes with closed blast gates. My preference is for a 150mm range pipe with100mm down pipes.with individual metal blast gates. Flexible piping connecting to machines is kept as short as possible to reduce friction losses.
 
So I've had a look and the steel spiral I linked to is 0.5-1mm which should be thick enough unless someone more knowledgeable can point me to some reading?

In terms of the drops, would I better to keep these as 6" I'd possible or would it be easier to drop it to 4"?

That's a good comment re joining the base of the main pipe. It hadn't occurred to me that chips etc would drop down. Thanks for that!

I will have another look at the metal ducting, money is tight as.not only have we just paid out for a lease, a new Laser machine and other tools, a lorry drove into my car a couple of weeks ago and wrote it off so we don't have a huge amount. That being said I don't want to be buying twice.

Out of interest, are there any issues using mixture or metal and pvc pipe/fittings or am I asked for trouble?
 
100mm drop pipes easily Cope with the output from my planer which is the largest amount of chippings made in most workshops. 100 mm blast gates are cheaper and more common in the average workshop and most machines in one person operations are fitted with 100mm outlets.
 
Except for my saw, which has a 120mm port, all of the other equipment has 100mm ports. I am branching off of the 120mm main duct and using a 120mm to 100mm reducer, a 100mm blast gate, and 100mm flexible hose to the equipment. The height of my basement shop is less than 2 meters, so with the hanging main pipe, length of the split and reducers, the flexible hose won't be much more than 2 meters. The flexible hose I'm using has a smooth interior.

One of my friends is making his DC distribution using gray DIN 110 residential drain PVC pipe available at nearly any hardware store. There is no shortage of 45-degree junctions, fittings, and hangers. He will not need clamps where the pipes and fittings join, but he will have to make his own blast gates and reducers to fit his equipment. He will wrap copper wire in a spiral around the length of the pipe and ground it.

The Schuko tubing I'm using is 1mm thick and in 1-meter sections. I don't know what the thinnest thickness should be to prevent collapse.
 
1. What size motor (either in watts or horsepower) do you intend to drive the fan?

You will need something enormous for 6" tubing. The amount of dust or chippings the system can carry per cubic foot or cubic metre of moving air depends on the velocity of the air, not the diameter of the pipe. Consider: a sea breeze moves enormous volumes of air, but hardly lifts anything more than a few crisp packets. On the other hand, a tornado in Wyoming really does lift vehicles and even buildings, even though it's far more localised. The significant difference is air velocity.

One man using one machine at a time is quite unlikely to generate enough chips quickly enough to need 6" piping. OK, if that machine is a spindle moulder or possibly a P/T taking thick passes you might get to the point where it's advantageous, occasionally. And if you have several people and machines in constant use, it's a different matter. But those scenarios would need a different class of system, with quite expensive hardware powering it and collecting the "output".

At a maximum I would consider 4" and probably smaller still. If you have a wide-bore, underpowered system (emphasis on 'underpowered' for the diameter), the heavier stuff will just drop out at points where the air movement is slowest (eddies on corners or whatever), and stay there. This may or may not clog things up, depending where the build up occurs.

2. Static build up: you need to earth the INSIDE of the tubing as well as the outside. The outside will otherwise attract airborne dust because of the capacitative effect. Sparks jumping between metalwork pieces inside the tubing or off rounded surfaces inside will potentially cause an explosion. Metal tubing is safe. Insulating plastic tubing -- some of it is high resistance but coducting, which is a lot safer -- would be dangerous. Even my little Makita handheld belt sander has an earthing strap on theextraction outlet (it's about 3/4" diameter!). It's another reason why you need high air velocity - the scouring effect which will stop a film of fine insulating dust building up on the inside of the piping.

I've read Bil Pense, admittedly ages ago. Some of it doesn't ring true, because of simple physics. And when you visit commercial workshops, you quite often see long runs of smaller piping (4" or 6"), but rarely long runs of bigger pipes, even though the final trunk to the cyclone extractor is often quite wide (but relatively short!). I've seen pipe runs that would be very, very awkward to clean out if they clogged (e.g. 100ft of horizontal run 20ft up near the roof), but they have a stonking great fan at the far end, and crucially, high velocity air moving through, so they don't clog.

If you're intending this for a commercial workshop, for goodness' sake get some professional advice before committing to something that may be quite a nuisance to put right later.

Simple 1st-order calculation of relative air velocities for pipe diameters, compared to a 2" pipe:
2": 1
2.5": 0.64
3": 0.4444
4": 0.25
6": 0.1111 (i.e. only slightly more than 10% of the 2" pipe)

This assumes the pipe is frictionless, which is far from the case. Turbulence in the smaller pipes will mean the numbers are slightly better for the bigger piping, but not by a lot. You get the general idea: the velocity drops by the square of the relative difference in diameter.

I note in passing that this is the main reason why dust collecting hoods for things like mitre saws are relatively useless. At the outer end of the hood or chute, the air is moving a lot slower than at the "nozzle end". But the sawdust is moving fast and has the kinetic energy to easily escape the pull of the air gently moving towards the extraction hose. It works on a tablesaw to have a chute at the bottom, because gravity aids you - the dust has to end up on some horizontal surface somewhere, and letting it slide into the suction pipe works. But it won't get the dust to shift off the mechanics and crevices of the saw for the same reason - the air simply isn't moving fast enough over those parts of the saw.

Hope that makes it a bit clearer.

E.

PS: the springy metal spiral often found in plastic concertina piping won't help with static if it's moulded inside the plastic! I have some 4" tubing like that, BUT the plastic itself is conductive. It also helps if that metal spiral is exposed on the inside surface of the pipe. The idea isn't to connect n earth between fittings along the pipe, but to discharge the static build up on the surfaces inside - that's a lot harder if those surfaces are not conductive in the first place
 
I would use these for the drops http://www.ductstore.co.uk/acatalog/150 ... 5_deg.html

Stainless is over kill.

Most of the ducting in our workshop came from Duct Store.

I have no problem with it being too thin, our extractors are pulling over 4000m/hr of air, and even with the gates closed it does not collapse.

Also we have our systems LEV tested yearly, as we have too by law, it has never been mentioned and has always passed.
 
tomatwark":1ix5eyhv said:
I would use these for the drops http://www.ductstore.co.uk/acatalog/150 ... 5_deg.html

Stainless is over kill.

Stainless steel might be overkill, but the 150mm junction in zinc-plated mild steel is £6.30 more expensive than the stainless steel version. However, the SS tubing is more expensive per meter than the zinc plated steel tubing.
 
Thanks for all of the feedback guys.

I think I will go with the duct store as I've respecced the ducting and I can get it within budget now. From what I can see the guage will Def be thick enough, I'll just make sure I've not got all of the blast gates closed at once lol.

Im using a felder AF22 with the Polyester 0.2micron filter which has a figure of 3100m3 per hour so not small. See below for a link. Cheers for the long reply Viking. From what I've read, the AF22 will be fine with 6" duct, I might change the drops to 4" though if that's the case.

http://uk.feldershop.com/en/Extraction/ ... tml#rating
 
Be careful - the specification says it's only 3HP (2.2kW). IIRC, that's the same as my Delta, and I wouldn't attempt to use 6" duct with that. It may be very efficient, but you really do need that air velocity otherwise you'll forever be cursing the thing and poking in the pipes to shift stuff.
 
Earlier this year I upgraded my own dust extraction from 4" to 6" as the port on my thicknesser is 5" & reducing down to 4" meant the waste wasn't being removed from the machine. Although it's only an old Axminster ADE2200 1.5kw it's worked a treat, no waste in the thicknesser & the best extraction I've managed from the bandsaw.

I bought all my gear from https://www.dustspares.co.uk & was impressed by them.

If you do end up with pvc sticking a length of aluminium tape (foil backed insulation joint tape) along the outside of the pipe run & connecting it to earth will remove static build up, I've just done this with a mate on his plastic ducting & have been amazed by the results. He used to have a thick build up of dust on the outside of the pipes & also the surfaces close to the pipe due to static but the tapes been on about 3 months & there's still no build up of dust.
 
By my maths, Felder's specifications are a little over-optimistic. If the maximum static pressure is 2,150Pa, then the fan should be moving around 600m3/hr/HP - so being a little generous, the flow rate of that extractor (assuming no losses) is about 2/3 the stated 3,100m3/h. Add in the pressure drop of a real system and the flow will reduce further - let's say it's quite well designed and you end up with 75% of the zero-resistance rate, so now we're down to about 1,500m3/hr. I would design your ducting to cope with this.

At a flow rate of 1,500m3/hr, with 6" ducting I might expect you to have a pressure drop of maybe 45Pa/m. Reduce this to 4" ducting, and the pressure drop would go up to ~350Pa/m assuming the flow rate stays the same. Going up to 110mm soil pipe would get you a pressure drop of ~210Pa/m. Small increases in diameter give you big returns.

If your flow rate drops down to 1,000m3/h, then even the 100mm pipe would give you quite acceptable flow losses of ~150Pa/m for moderate length runs and you could safely use this for drops. I would use bigger bore ducting for any lengthy runs back to the extractor though. Bear in mind that at this flow rate, your 6" ducting flow velocity is down to 16m/s - I wouldn't want to go much lower than this without risking having your transported shavings dropping out of suspension in the flow and ending up getting deposited in the ducts. This shows how such a large bore system is ideally suited to large static machines, but would be pretty useless at extracting from smaller tools. A one size fits all design is hard going.

You don't say how long your pipe runs are, but my advice would be that for the your machines that need every last bit of air flow (planers and spindles with >100mm outlets) then the extra expense is worth it and I would limit my use of 100mm ducting to short (up to ~1m) runs. For the others, you can use 100mm for longer runs without too much worry.

Collapse of ductwork is mainly a problem associated with using "vacuum" type HPLV extractors - these often generate static pressures of around 20kPa, vs the 2kPa pressure your Felder extractor can generate. I doubt you will have a problem with this. A quick calculation for elastic collapse shows that even with a 150mm diameter 0.5mm wall steel pipe, the elastic collapse pressure is about 8 times what your extractor can produce.
 

Latest posts

Back
Top