Maybe a dumb question?

[ryan.s]

Does the "drum" the Thien separator sits on top of have to be circular in shape or can it be a rectangular box with a circular opening on top to fit the baffle? I think I understand the concept of how the separator works, basically circulates the air and chips so they are not sucked in through the center outlet but instead fall through the baffle. Once the chips fall through the baffle does it matter that the catch "drum" isn't circular? Maybe makes it less efficient? The reason I'm asking is it's hard to find the correct size drum for the space I'm trying to use for the separator. It would be much easier for me to build a rectangular box to custom fit the space. Thanks!

[Vodkaman]

I don't think it is a dumb question at all.

My interpretation of the Thien cyclone, is that the slotted baffle serves two main purposes. It keeps the volume of the vortex tight and thus keeps the speed high, which in turn keeps the particles away from the centre and from being drawn into the outlet. Secondly, by confining the airflow, the contents of the bin are not disturbed. The bin is subject to pressure changes, but not airflow.

I believe that you can change the shape of the bin below the baffle to suit your application, without affecting the function. This is just my opinion, but makes sense to me. The bin will still be subject to collapsing pressure forces, should the inlet pipe become blocked. If their is a realistic chance of this happening, then you have to design for the occasion, by strengthening the box or including a vacuum relief valve of some sort.

I am sure more opinions will be forthcomming for you, hopefully one with first hand experience of your design requirement.

Dave

[Bulldog8]

In my completely unscientific opinion the container shape does not matter. The cyclonic scrubbing occurs at the baffle not below it. Therefore, the shape of the container does not matter. In fact I would think that a square or rectangular vessel could possibly provide even more protection from dust coming back up through the drop slot when compared to a round container. In a round container configuration the drop slot is aligned with the edge of the container so it is possible that dust could be drawn back up through the slot.

[jeth]

First up, hello all, my first post here.  Hope its ok to have dragged up an old thread but I was thinking about this aspect of my separator construction and after endless keyword combinations my search pulled this up, after a couple of similar questions that never recieved a reply..

I have spent a few days, almost solid, reading and digesting a portion of the information here.  Interesting to see the tweaks being made to Phils great concept and some really excellent engineering going on. Thanks to all who have shared their ideas and results.

I will post some details of the build I started today at some point.  In the process of knocking it together I have come up with this same query.  Does the wall of the container below the slot need to aligned to the the inner wall of the separator chamber?
The above answers are interesting, and suggest some offset here is ok, but both are stated to be opinions. it would be interesting to hear Phils thoughts, though I wonder if the fact he hadn't previously replied means he hasn't tested this design factor?

My thoughts are that either a) as suggested above the wider cavity below the baffle might help the dust to settle more easily and stop it rising back up the wall into the separator, or, b) the sudden change of volume and lack of the wall extension below the slot could affect the airflow in the separator in a negative way.  I simply don't have a good enough understanding of the physics to be able to  say which way it would go.

a) would suit me but I'd sure like to be a bit more certain of the outcome :) Any thoughts or anyone built a separator  with an offset between the drop slot outer edge and the wall of the collection container?

[phil (admin)]

The diameter of the drum can be greater than the diameter of the separator.

It is important that the area below the drop slow is unobstructed.  Some people have built units that have a small ledge below the drop slot, and that really isn't ideal as it restricts debris from dropping down into the drum.

[retired2]

a) would suit me but I'd sure like to be a bit more certain of the outcome :) Any thoughts or anyone built a separator  with an offset between the drop slot outer edge and the wall of the collection container?

Most of the top hat designs I've seen have an offset.  In fact, it is pretty hard to avoid an offset if you use a dado to receive and seal the top lip of the waste drum.  My offset happens to be 3/8".  For a sketch see "Reply #3" to the thread I started titled "5", Rectangulare Inlet, Bellmouth Outlet......"

When you look at how waste accumulates in the bottom of the drum, it is obvious there are some air currents inside the waste drum, but I believe (another opinion) they are insignificant, and that the separation process is almost entirely dependent on the dynamics of the air flow above the baffle rather than below.  Now having said that I wouldn't use some crazy design that is likely to introduce chaotic air flows (turbulance) near the drop slot - above or below!

I have been doing a lot of testing in the past few weeks, and since my separator has clear plexiglass walls, I have a good view of what is going on inside the chamber.  I have not filled my waste drum all the way to the drop slot, but I have never seen any indication that dust or debris is being sucked back into the chamber from the waste drum.

p.s.  I'm assuming you are talking about a round waste container.  You aren't by chance thinking about a square or rectangular one are you? 

[jeth]

Thanks a lot for your replies....

Now having said that I wouldn't use some crazy design that is likely to introduce chaotic air flows (turbulance) near the drop slot - above or below!

Nothing crazy, but the materials I have to hand have presented me with two options. One would have the separator in a 5 gallon bucket (~11" dia) , dropping directly below the baffle into the existing collection drum of my Ridgid vac, which is "6 gallon" (~15" dia). The offset would be a couple of inches, considerably larger than 3/8". Is this likely to be a problem?

My other option is to increase the diameter of the separator chamber to match the 15" lower drum, aligning the drop slot with the wall of the collection bin and avoiding any ledge above or below the slot. I am concerned the extra chamber volume might reduce separation of fines in this 2.5" hose shop vac system.

Which is the best option or is this really not an issue?

[retired2]

Thanks a lot for your replies....

Nothing crazy, but the materials I have to hand have presented me with two options. One would have the separator in a 5 gallon bucket (~11" dia) , dropping directly below the baffle into the existing collection drum of my Ridgid vac, which is "6 gallon" (~15" dia). The offset would be a couple of inches, considerably larger than 3/8". Is this likely to be a problem?

My other option is to increase the diameter of the separator chamber to match the 15" lower drum, aligning the drop slot with the wall of the collection bin and avoiding any ledge above or below the slot. I am concerned the extra chamber volume might reduce separation of fines in this 2.5" hose shop vac system.

Which is the best option or is this really not an issue?

I didn't realize you were building a small shop vac separator - I don't have any experience with those.  However, there are plenty of people on this forum who have built them.  Maybe one of them will give you a recommendation, but I think your concern about the second option may be valid.  Shop vac's don't move a lot of air and a 15" diameter separator may be approaching the limit of what they can handle.

[jeth]

Ok, thanks again retired.  The shop vac is to have a first run and get me by for at least fine dust collection till I can afford a larger DC which I know will be required for the jointer/planer etc.

My Ridgid vac is the dumpy little 6 gallon model, and the filter was just a couple of inches off the bottom of the drum. As i want to stack the system I decided to mount the vac unit on to a 5 gallon bucket to sit above the separator,  making a taller narrower chamber around the vacs filter canister and more space between it and the outlet pipe from the separator below. 
I will use the vacs squat 1" diameter container below the baffle.
I have the option of a scrap piece of large diameter steel tube to make the separator the same size as the vac container or i could simply sit the 5 gallon bucket that will be acting as the vacs new container into another bucket, which seems to seal nicely and creates a 4" cavity at the bottom , a pretty good size it seems for the separator chamber. The base of the bucket is well reinforced and i think i can cut the drop slot directly into it taking care to sand edges smooth to avoid any hang ups.  This option would leave 2" or so of offset between the drop slot and the wall of the collection container. Maybe I should just try it, if it fails I could rig something with an extra 5 gallon bucket at the bottom to get rid of the offset, though i'm liking the vac container as i can keep the casters :)

Probably best if i post further description of my build and pics in a new thread as a general overview here is getting off topic, sorry.