Top Hat Design Guidelines

[Tacuabe]

I thought so, too, but I didn't say as much.

Anyhow, that's what the sketch shows. See the Section drawing and read note iii)

I'm still baffled.

[retired2]

Your sketch suggests inlets should be round.  There is an argument to be made that suggests a rectangular inlet is better.

[Tacuabe]

Yes, I'm aware of that. Remember this is a sketch to provide a start point to would-be builders. Refinements are supposedly incorporated later on.

However, I can add a note to explain there should be a transition from the round inlet pipe to a rectangular inlet port in order to increase efficiency.

[phil (admin)]

No, I meant that the outlet tube should be D from the baffle.  Not 1/2D, but D.

In the past, I've told people to keep the outlet tube 1/2D from the top.  ALL REFERENCES HAVE BEEN FROM THE TOP, never from the baffle.  For a 2-1/2" pipe, that would be 1-1/4".  That would probably be a little more than 1/3rd the distance between the top and the baffle.

For a 4" pipe, that would be 2".  Again, probably a bit more than 1/3rd the distance between the two.

For a 6" pipe, that would be 3".  Once again, probably a bit more than 1/3rd.

In all those cases, approx. 2/3rds would be left between the bottom of the outlet tube and the baffle.

Now, some people are asking about making taller separators for lower resistance and better separation of fines.

But if we go with 1/2D from the baffle as a general rule, we're placing the outlet tube closer to the baffle than we have before.  We don't want to increase resistance or cause what I call baffle sweeping.  So 1/2D to the baffle is just too close to the baffle, as a general rule.

In some cases, the taller separator may not be tall enough to put the outlet a full D from the baffle, in that case maybe we would try 1/2D.

I know some of you want to make guidelines that will take into account every single permutation.  It is hard.  We have to be careful not to provide a compromised set of guidelines because many of the builds are unique, and about balance of one thing in favor of another.

[alan m]

phil these last couple of posts have me completely confused.

in a single height situation you were saying 1/2 the diameter of the outlet . so if it was a 4"outlet then the end of the outlet would be 2" from the top of the seperater but also 2"from the baffle.
this assumes that the seperater is the height of the diameter of the outlet(and inlet if it was round
i dont understand where the 1/3 is comming from.


if the seperater is a double height unit. then  if it is a 4" outlet

if we use the 1/2 diameter of outlet  then you will still have 2" from the baffle to the outlet but have 6" from the outlet end to the top of the seperater
surly going from 1/2 diameter to 1 full diameter of the outlet will allow more bypass



you are going on the diameter of the outlet.
surly it should be the size of the inlet

in an all round pipe system the inlet will be the same as the outlet but rectangular inlets are showing better performance
so if a 6 "pipe is used with an 8"x 5" transition for the inlet.
a simgle height seperater will be 8" from the baffle to the top.
so the outlet 6"round pipe will be 1/2 the inlet height (in this case 4") from the top and also from the bottom

in a double height seperater then the total internal  height will be 16" . the outlet will be 1/2 the inlet height so 4 " from the baffle and 12" from the top
this si my understanding of everything on this site exept what you said in the last few posts.

can you expalin where this 1/3 is coming from

[phil (admin)]

The height of the unit is often (typically?) larger than the D of the pipe.  There is often an inch or two above/below the inlet, so for a separator using 4" pipes, you might have 5" to 6" of height.  So 4" outlet extending 2" below the top does not extend to 2" above the baffle, it is further from the top of the baffle.

Again, much of this depends on an individual build.

[Tacuabe]

Phil:

It is now clear what you said before. However, in most units pictured in this forum the chamber height is usually the inlet pipe diameter (with nothing to spare, above or below). So the "rule of thirds" would be difficult to sustain.

An exception would be those that use a transition to a rectangular inlet port which MAY be higher than the pipe diameter.

There is also the case of a "two-story separator" (Howard in Toronto) where it is specifically recommended to place the inlet as far up as possible and extend the outlet below the inlet. The final figures were 6" pipes, 12" high chamber, outlet extending 9" into the chamber and 3" from baffle. This is for a case of "mainly fines".

However, if the "rule of thirds" MUST be applied, then minimum chamber height would be h = 1 1/2 d. This will allow 1/2 d projection of outlet pipe into chamber and will leave its end at d from the baffle.

Ideally, for a newbie, the best initial suggestion would be what should be/not be done. Then let him face the construction as he sees fit.

Although the mechanics and physics of the whole thing may be involved, the construction is comparatively simple. Usually, improvements and modifications are not difficult to incorporate. And there's room enough for experimenting in a safe way.

[phil (admin)]

However, in most units pictured in this forum the chamber height is usually the inlet pipe diameter (with nothing to spare, above or below).

Use Google images to search for "thien separator" and you will see that most have some space above and below the inlet.  that is because most use an pipe through a hole, and the board has to have some material above and below the hole to maintain strength.

[Tacuabe]

Here's an update on my Shop Vac arrangement.

Considering what was commented in this thread, I decided to increase the height of my separator. The original height was 35 mm (slightly more than the 32 mm inlet pipe). I added a new ring on the underside of the chamber. This second tier is 20 mm high, therefore total chamber height is now 55 mm.

The inlet was left unchanged and thus is as high as possible inside the chamber. Following this mod, there was a noticeable improvement in fines separation.

[Tacuabe]

Here are a few photos. You can clearly see the new 20 mm thick ring I added. The baffle is now removable for eventual cleaning or in case of clogging. Please note that the outlet protrusion (16 mm) into the chamber was NOT modified. I failed to point that out in my previous post. Therefore, the outlet is now 39 mm up from the baffle.

The top of the chamber and the upper side of the baffle clearly show how the dust swirls during operation.

[BernardNaish]

Completely fascinated by the interesting ways that Phills' important discovery has been modified as wood workers attempt to probe the science behind it in a very practical way.

1. We are creating a circulating doughnut of air but mismatching a round inlet pipe to a rectangular cross section. That may be why this pipe needs to hug the top. Hence there will be quite a lot of turbulence probably reducing efficiency. If the inlet tube is also made into a rectangle and extended far enough back, to where it morphs into the round collection pipes, to produce a minimally turbulent linear flow we may improve the removal of fine dust.

2. We are relying on the outlet pipe to form the inner layer of our doughnut and know that the height of its end above the baffle is a key factor. I suspect that setting the outlet pipe into a larger diameter cylinder (of wood?) would help reduce turbulence and increase airspeed that we know would probably improve performance.

3. If a fluid moves across the orifice of a tube held at right angles to the fluid flow, then a negative pressure is created inside the tube.  (water pump effect dating to Roman times.) The drop slot is just a wide narrow tube and the vacuum created is what draws our dust and chips into our can.

I suspect that I may be able to get very good dust and chip extraction from this airflow by:

A. Taking my 4" inlet pipe and connect it to a 24 " long rectangular inlet pipe of slightly larger surface area then I will get good untroubled airflow into my separator.

B. Making the inlet section width 1 1/8" ID wide to match the drop slot.

C. If my 5" outlet pipe is set inside a cylinder of wood so that the gap between the two walls is 1 1/8th "

D. Set my outlet tube plus thick collar so its end is > 2 1/2" from the baffle plate.

E. Arrange for the bottom end of the outlet tube to line up with the bottom of the rectangular section input pipe.

F. If I form the end of the outlet tube assembly into an aerodynamic bell mouth.

G. Make the inside diameter quite small, say 12", to get a high circulation velocity and hence a greater vacuum.

G. Listen to the feedback I hope I get from this post and make my Top Hat as smooth and airtight as I can.

If I have stolen your ideas please forgive. Looking forward to your usual honest feedback.

[alan m]

Completely fascinated by the interesting ways that Phills' important discovery has been modified as wood workers attempt to probe the science behind it in a very practical way.

1. We are creating a circulating doughnut of air but mismatching a round inlet pipe to a rectangular cross section. That may be why this pipe needs to hug the top. Hence there will be quite a lot of turbulence probably reducing efficiency. If the inlet tube is also made into a rectangle and extended far enough back, to where it morphs into the round collection pipes, to produce a minimally turbulent linear flow we may improve the removal of fine dust.
true i also think it allows more of the dust to be against the wall

2. We are relying on the outlet pipe to form the inner layer of our doughnut and know that the height of its end above the baffle is a key factor. I suspect that setting the outlet pipe into a larger diameter cylinder (of wood?) would help reduce turbulence and increase airspeed that we know would probably improve performance.
generally speaking you want the air on the outside sping as fast as possible and the air in the middle spining slowly

3. If a fluid moves across the orifice of a tube held at right angles to the fluid flow, then a negative pressure is created inside the tube.  (water pump effect dating to Roman times.) The drop slot is just a wide narrow tube and the vacuum created is what draws our dust and chips into our can.

I suspect that I may be able to get very good dust and chip extraction from this airflow by:

A. Taking my 4" inlet pipe and connect it to a 24 " long rectangular inlet pipe of slightly larger surface area then I will get good untroubled airflow into my separator.
are you saying 24"x whatever makes the corect area or the distance between the rectangular end and the round end
B. Making the inlet section width 1 1/8" ID wide to match the drop slot.
that should work . but you dont want to restrict the air movement either
C. If my 5" outlet pipe is set inside a cylinder of wood so that the gap between the two walls is 1 1/8th "
im not sure if there is anything to be gained by having a larger outlet than inlet. you will reduce airflow by reducing

D. Set my outlet tube plus thick collar so its end is > 2 1/2" from the baffle plate.

E. Arrange for the bottom end of the outlet tube to line up with the bottom of the rectangular section input pipe.
as long as you have 1/2 the diameter of the inlet above the baffle

F. If I form the end of the outlet tube assembly into an aerodynamic bell mouth.
the bell mouth shows some good signs of increase eficiency
G. Make the inside diameter quite small, say 12", to get a high circulation velocity and hence a greater vacuum.

you will create more resistance on the system and loose more airflow with a smaller diameter seperater

G. Listen to the feedback I hope I get from this post and make my Top Hat as smooth and airtight as I can.

definetly. sealing everything is a mustIf I have stolen your ideas please forgive. Looking forward to your usual honest feedback.
its very hard to say whos ideas are theres. other than phils baffle  everything is a combination of ideas cathered here and there from many scources. retired 2 has introduced a few ideas he has seen in his day job. the beauty of phils design is that as long as the result is the same it doesnt matter how is built. iv seen maybe 10 different ways to build  it

[BernardNaish]

Thanks for your response alanm.

Ref: My point A. I am suggesting a 24" long rectangular tube with a section of 1 1/8th" by 5 5/8" inside dimension giving a cross sectional area slightly greater than that of our systems 4" tube.  The transition between the round pipe and the rectangular tube would need to be as long as possible and at least twice the diameter of the round tube. This should get us a turbulent free input system as we can achieve.

Ref: My point C. I am suggesting that the gap between the outer wall and the outlet tube be reduced to 1 1/8th" regardless of the outside diameter of the outlet pipe. Hence the outlet pipes wall would need to be increased by setting it into a cylindrical block of wood to make a "thicker" wall. The purpose of this would be to maintain the greatest possible turbulent free air flow circulating around the outer walls while still having a separator wide enough to have a reasonably good air flow. With a bit of luck the air bleeding over into the bell mouth of the outlet pipe will tend to push the dust and shavings down towards the baffle and through its slot.

I am stuck with a 4" metal collection system as our Club has just installed it to collect from all our wood shredding machines. As the fan inlet to our new HVDC is 5" I connected a simple drop can with a tangential inlet pipe between it and the 4" collection system. This had worked well when we were just using some 2" hose and a shop vacuum cleaner (LVHP) but not with the new 4" metal system and the much greater volume of air moved. Some chips now get scrubbed out of it and the fine filter we have fitted gets quite a lot of dust on it after a short time. Hence we need a cyclone or a Thien separator. I would rather not use a cyclone because of the air movement loss cause by the cyclone and by the long pipes needed to get up and down from the top. Hence my prolonged study (several sleepless nights) of the very interesting posts on this site.

I am trying to tie down some details so I can make one and at the same time hopefully add something to what is known about making one so I can share it with you all.

I am also trying not to revert to my former life as a research technologist as I really only want to work wood now. My experience suggests that a series of experiments with a standardised collection source and standardised material being moved needs to be done to test various elements of the Thien separator. This would allow us to tease out the critical point which is how much of the fine dust is caught by the separator and how is this affected by various configurations. Phil has done some of this I know and if I still worked in a university it would be easy to get this done as a student project.

Disposing of the waste collected needs to be done with great care as tipping dangerous fine dust into a plastic trash bag or venting it outside is not just irresponsible it is potentially dangerous to the operator and to others. Damping this down with water is not an option as there would be a danger of it igniting spontaneously. Ours gets given to the chickens that one of our member's keeps who are kept happy playing with it and who mix it with their droppings making it good for composting while the birds keep cleaner.

Regards Bernard (sometimes known as prof amongst other less positive names though I have never been one.)

[alan m]

i thin the only way to see if the 1.125 " width all around would work is to build it and see.
something in me is torn apart. half thinks it wont half thinks it might.
you should build the seperater normally and add a piece to create that situation and see what diference it makes .


i too would like to seea proper sccientific test done so that each element can be teased out to see how it performs in all its different configurations


its a shame im not still in collage. this would make a great thesis. with all the cnc machines ect we had i could have made up loads of different configurations and concepts and done some simple testing on all the machines we had . like feed x no of feet through the planer,drum sander,spindle moulder,6 cutter,beam saw,cncs etc. all those machines have a known feed rate and are consistant in their dust production

[BernardNaish]

I need to build a Thien top hat separator so there is no reason why I should not include a large removable flange in the top plate so I can swap in different outlet pipe configurations. Building it so that I can have the baffle height adjustable is somewhat more difficult but I might be able to do it. I will scratch my head for a bit.

Building different widths of top hat is further than I want to go so I will leave that to other people.

I agree that feeding from different machines in a standard way that mimics typical workshop operations would be a very interesting test. However to keep it a bit simpler I will make up a large quantity of different wastes and mix them thoroughly to create a standard mix that I can keep by me for testing. I will then feed them to the system in a standard way. I will weigh how much material is fed in and how much is contained in the Thien collection can and the difference will be a reasonably accurate measure of efficiency.

Much as the past draws me to want to measure pressures and air flows I am not going to do that as I want to stick primarily to hand woodworking.

Happy wood smoothing.