Building a large Thien baffle - Dust collection

[MattRoberts]

As part of my workshop refurb, I needed to build a new Thien baffle. I'm installing 125mm ducting, and my 100mm Thien baffle would just choke the airflow too much. In addition, the container I was using was too small in terms of diameter - you really need a large diameter container in order to maximise the airflow.

I'm using a plastic dustbin this time.

First things first is to cut out four pieces of ply. I'm using cheap 18mm ply here, but you can use anything with a decent thickness.

I'm using these 3mm pins to help locate the centres - it's dead easy to drill a 3mm hole, pop a pin in and then attach another piece to it.

I made a trammel for my router a while ago, and it's just the thing for this. Normally I'd use a circle cutting jig on the bandsaw, but the height of this baffle will be too tall for my rubbish bandsaw, so the router it is.

I then built a box to surround the air intake. It's a 125mm round to rectangular piece.

It's simple glue & brads construction, and intentionally left long for the time being.

Next I start to route out the circles. I'm using a 5mm spiral upcut bit, and taking shallow passes so I don't stress it out. I have two pieces of ply connected together via a 3mm pin, and I route completely through one of them and score the surface of the second.

Once the second piece is scored, I separate the two of them and then rough cut the second piece with the jigsaw, so as to save my router cutter some work.

It's then a simple task just to clean up using a flush cut bit. This thing is a complete beast of a cutter.

I can then do the layout for the air intake box and the inner circles.

More to follow...

 

[skipdiver]

Watching with interest as i want to do the same during winter when i am less busy.

 

[Steve Maskery]

Excellent, Matt. I have a cyclone clone and a Thein separator. Both work well.
Where did you get your rectangular-to-round plastic adapter?

S
PS I thought it was Adaptor but my spell-checker has complained and written Adapter. Am I losing it? I have a BB* coming up, so it's not beyond the realms of possibility.

*Big Birthday...

 

[morturn]

Thanks, good read that, looking forward to the next instalment.

 

[MattRoberts]

Then it's rinse & repeat with the circle cutting - these are the two inner circles. The inner circles are what the plastic walls will attach to, and the diameter needs to be close to the diameter of the bin in order for the baffle slot to be close to the inner edge of the bin. All will become clear.

A quick dry fit shows how the four circles fit together and allows me to see what needs to be cut on the intake housing.

By clamping the inner circle to the housing, I can trace the curve that needs to be cut from the housing on each side.

Joining the lines. I'll cut through the 'base' of the housing using the tablesaw, and then the curve of the sides with the jigsaw. Again, normally you can do this using the circle cutting jig on the bandsaw and cut it neatly in one go, but the housing height is just too large.

It was about this point in time that my only wood blade snapped, so I ended up finishing the cutting with a metal cutting blade. That was some tough work.

In addition to cutting the curve to match the inner circles, a curve is also cut to provide a channel for the plastic walls to sit in and meet the edge of the. It's not exactly pretty, but it's functional and will be hidden from sight anyway. I also cut a recess along the wall using the multi tool - this will allow the plastic wall to sit in it and prevent there being a lip that could impede incoming airflow or collect dust.

Time to route the channel on the underside to accept the rim of the bin.

The fit is so snug that I can lift the bin by the wooden circle (not particularly easy when trying to also take a photo).

Another dry fit, and you can see it starting to take shape now. I'm using the off cut from the intake housing as a support for the opposite side. I'll trim it later.

 

[MattRoberts]

Excellent, Matt. I have a cyclone clone and a Thein separator. Both work well.
Where did you get your rectangular-to-round plastic adapter?

S
PS I thought it was Adaptor but my spell-checker has complained and written Adapter. Am I losing it? I have a BB* coming up, so it's not beyond the realms of possibility.

*Big Birthday...

I got the adapter from ebay - a search for 100mm or 125mm ducting brings up plenty of listings with all of the components you'd need.

I also thought it was adaptor - adapter seems somehow wrong! And congrats in advance on your 30th

 

[Steve Maskery]

You charmer, you.

S

 

[MattRoberts]

I cut the plastic sheet to the right height and attach it using a few screws with flat steel washers. Later I will remove this and put a bead of sealant underneath then re-screw. My plastic sheets aren't big enough to have one single strip (which is preferable), so instead I have to do it with two, and have them meet on the opposite side's support post.

Here I am completing the circle with the second half of plastic wall.

Time to cut the slot in the base. This is one of the reasons why the 3mm pins are so useful - everything can reference off the same centre point. I cut the inner and outer lines roughly using the jigsaw (with the metal blade still!), and then trimmed the radii neatly using the router. The base is upside down in this view, and you can see the recess for the bin rim and how close the slot for the dust to fall down is - right up against it ensuring a maximum radius.

Flipping it the right way up, I added a hefty chamfer to the outer radius to give the circling dust an easier route to fall down the slot.

 

[MattRoberts]

A couple of clamps held the plastic taut while I screwed it in.

I could then go around the perimeter adding screws - the thickness of the 18mm ply provides a decent height to fit a screw in.

Then disaster strikes. I grab the sealant and snip the top off, only to realise it's white and not transparent. Now everyone is going to see my horrible sealant beading! Oh well. I seal up the intake housing.

And then around the walls. This time i use my fugi tool to salvage some dignity and get a consistent 5mm bead.

A jigsaw and a rasp make simple work of the main intake hole.

I added 20mm oak dowels around the perimeter to add support, and did a quick dry fit with the extractor on.

More to come!

 

[MattRoberts]

I knocked up a quick base for it to keep the baffle in the correct position when changing the bin. The height of this is actually about 40mm taller than the top of the bin. You can just about see a small wedge at the back that allows the bin to sit tight into the recess

Another wedge at the front is used to bring the front tight into the recess simply by kicking it in. I later trimmed this so it doesn't protrude quite as much.

Now on to the testing.

I take an initial reading using an anemometer rigged up to a 125mm spigot. This allows me to keep the anemometer in exactly the same place each time and get consistent readings. I'm not too concerned about what the true reading is, I just want to be able to compare before & after figures as I make adjustments and through trial and error find what makes a difference.

The initial reading is 11.7 metres / sec. Let's see what we can do to improve this.

First I want to find out what difference the protrusion of the extractor intake makes in the centre of the circle. Here are the results

Large protrusion: 11.7 m/s
Small protrusion: 11.5 m/s

Large protrusion it is then. I can then seal the extractor intake in place.

Sealed intake: 11.9 m/s

Next is to seal the ducting with tape - it's basically like electrical tape but much wider, and gives a completely airtight seal. Up until this point, I had my ducting loosely connected to allow for adjustments.

Sealed ducting: 12.8 m/s

Quite a difference! The lesson learned here is to ensure you have everything completely sealed, regardless of how tight you think the push fit might be.

For reference, the air speed for the extractor with nothing connected to the exhaust and only a 2 metre straight pipe connected to the intake was about 24 m/s. With the Thien baffle and all of the ducting in place, it cuts the air flow almost in half, although realistically you'd never be able to reach this airflow anyway, as you'd need something on the exhaust and any reasonable amount of ducting would also reduce this figure.

I also attached my previously built 4 inch thien baffle, and the airflow was restricted down to 9.2 m/s, so making a 5 inch baffle is definitely recommended if you have a 5 inch extractor port.

In conclusion, the very best thing you can do to keep maximum airflow is to have no dust separation in place and exhaust the air outside straight into some sort of container. Obviously this isn't practical for the vast majority of people, as it will be throwing clouds of dust and chips all over the place, and the neighbours are unlikely to be too happy.

Introducing a separator such as the thien does reduce the airflow quite a bit, however the benefit of it separating the dust so effectively and the ability to so easily change the bin is a good compromise in my book. To give an example about its efficiency, I have nothing on the exhaust yet because I'm going to drill through the wall and exhaust it outside. It's currently exhausting against the clamps that sit behind it, and after a few days use like this, I can't see any dust build up at all.

You can see here in this test the amount of dust spinning inside the baffle and nothing visible coming out of the exhaust in the top left. Obviously there's still harmful invisible stuff coming out for sure, hence me looking to exhaust outside, but it means that I can exhaust straight into the air without fear of a pile of visible sawdust building up.

If anyone has questions or suggestions on how to improve the airflow, please let me know!

 

[siggy_7]

Nice job you've made of that. I'd be interested in your critical dimensions (separator diameter and height) and what blower you're using, preferably with power and impeller dimensions. I make that an airflow of 565m3/hr. Does your anemometer also give you the ability to measure local static pressure? You could measure the pressure drop caused by your baffle then, by taking measurements at the inlet and outlet.

Adaptor vs adapter - they are essentially interchangeable, although in British English (so English!) my understanding is that adaptor is preferred for objects which adapter is used more commonly for people - i.e. one who adapts. https://www.theguardian.com/guardian-ob ... le-guide-a

 

[siggy_7]

Nice job you've made of that. I'd be interested in your critical dimensions (separator diameter and height) and what blower you're using, preferably with power and impeller dimensions. I make that an airflow of 565m3/hr. Does your anemometer also give you the ability to measure local static pressure? You could measure the pressure drop caused by your baffle then, by taking measurements at the inlet and outlet.

Adaptor vs adapter - they are essentially interchangeable, although in British English (so English!) my understanding is that adaptor is preferred for objects which adapter is used more commonly for people - i.e. one who adapts. https://www.theguardian.com/guardian-ob ... le-guide-a

 

[MattRoberts]

Nice job you've made of that. I'd be interested in your critical dimensions (separator diameter and height) and what blower you're using, preferably with power and impeller dimensions. I make that an airflow of 565m3/hr. Does your anemometer also give you the ability to measure local static pressure? You could measure the pressure drop caused by your baffle then, by taking measurements at the inlet and outlet.

Adaptor vs adapter - they are essentially interchangeable, although in British English (so English!) my understanding is that adaptor is preferred for objects which adapter is used more commonly for people - i.e. one who adapts. https://www.theguardian.com/guardian-ob ... le-guide-a

The separation chamber is 52cm diameter and 21cm high. The extractor is a Sealey 2hp. The impeller is 24cm diameter of I recall correctly.

The anenemoter only gives air speed readings unfortunately

 

[gregmcateer]

Nice job, Matt.
Thanks for a very informative WIP.

 

[will1983]

Lovely job Matt.

Funnily enough I've just finished my Thein separator this weekend.
I also went for a large (tall) unit sitting on top of a 70l dustbin however mine uses 110mm soil pipe. I made my own round to rectangular transition out of 1/8" ply (lots of little pieces glued together) and used a 1hp Rexon blower unit. The chamber itself is also 1/8" ply strips glued together and then judicious use of the sander to make it nice and smooth inside.

I tested it yesterday, first impressions are good with no evidence of bypass however like you I intend to exhaust mine outside just in case. I'd recommend this design to anyone considering adding a separator to their dust collection system.

 

[MattRoberts]

Wow, I'm impressed that you made your own transition! That must have taken a lot of patience

100 / 110mm pipe is the right diameter for 1hp,so that's a good choice. Nice one

 

[will1983]

Thanks Matt.. Yes it was a little tedious but I think it was worth the effort.

I made a jig to hold the piece of round pipe and a rectangular former in the correct alignment whilst I fitted the ply pieces. Once the assembly was strong enough to hold itself I removed the jig and completed the other side of the transition.
It isn't quite as smooth as I would have liked inside but if I make another in the future I now have the method nailed down.

How wide did you make your drop slot? Mine is 40mm wide with a heavy chamfer to the underside. I wanted to make this suitable for sucking up planning shavings as well as sanding dust hence the extra width and it seems to work ok.

 

[MattRoberts]

Mine is 30mm. Funnily enough, I used my planer with it for the first time at the weekend and the shavings were getting stuck, so I'll need to increase the size. It will be interesting to see if it has any effect on the airflow, but I doubt it will. I'll test before and after.

 

[will1983]

Cheers Matt, I'll be interested to see the result of increasing the width.
My testing has not been anymore scientific than just sucking stuff up and seeing if anything came out of the exhaust.

 

[siggy_7]

I expect it's the same or similar blower to this then: http://www.sealey.co.uk/PLPageBuilder.a ... uctid=8088 (250mm diameter). Their stated maximum flow rate of 618cfm seems believable, at about 2/3rds the theoretical maximum for an impeller of that diameter and power. So based on those specs, your separator is reducing the air flow to about 50-60% of the unrestricted maximum. I think you'll struggle to do much better than that with the blower you have.

If you're happy with the separation efficiency then I would leave further tweaks alone. As your blower has a small diameter impeller, it's optimised for high airflow and very low suction - my one suggestion for a change would be to use a blower with a larger diameter impeller to give you more pressure drop to play with. As that's the expensive bit, I wouldn't suggest buying new equipment unless you're unhappy with performance.