Proposed Spiral Build - why - and a few of my observations

[Clark Savage Jr]

The image is the drawing of the build I have been gathering materials and refining since I joined the forum. Although I am considerably attempted to jump directly to the in-line separator build I have commented on in Phil's post, I feel I need to stick with some semblance of order in my goal. So I am throwing this out for a look and possible comments. Perhaps it will inspire someone......or not  ???

So this would be a true spiral separator, along with a secondary section that is a traditional Thien top-hat separator but all-in-one.

Some build dimensions for perspective:

   
• minimum dust collection bin of 20", with a split down the center to divide the two separation sections This may be the hardest part since I would need to build it.
• overall width as it sits at widest point ~ 26"
• inlet diameter is 6", guided into a 10"h x ~3"t rectangular transition
• Outlet is 6" and would be a top hat style with the fan assembly on the outlet
• Appears eBay has sheets [48" x 96"] of rolled up HIPs plastic that should be stiff enough and slick enough but still very flexible to bend in one direction - it comes in a roll - to form the walls/spiral of the can - ~ $29.  HIPs can be bent and glued - so the running length of ~149" of can wall can be two layered - offset where one sheet meets the next to form a single smooth and very slick can wall.
• Plan to CNC the slots in the tops and bottom to hold the HIPs can sheet in place.
• Wood 1/2" top and bottom plate - top removable - not depicting the offsets/actual fasteners in the top and bottom - but flush outside not in airflow

Why? This is based on previous experiments I have completed including the offset increasing area exhaust tube - off set from the bottom. The goal being to flow only as much air as is coming in to avoid simply drawing all the airflow immediately to the outlet - dust included - and into the fan thus to the filter. The bottom outlet extension allows me to control the height of the suction and amount [due to the cutout height around the perimeter of the tube] and therefor WHEN it occurs. I found in a build that had a solid tube with such a cutout that as you only draw some air off "slowly" but "in increasing amounts about the can FINAL interior" that you tend to drop dust out to the bottom of the flow by gradually slowing the air speed/pressure front - allowing the fines to propagate outward and not have as much a tendency to simply get trapped fighting against the next incoming airflow. As they drop they are now positioned below the exhaust [the bottom outlet ring] and will continue to circulate due to speed and the next time they emerge into the incoming airflow they are positioned low and below the air outlet position and tend to be "held" in the low position by the incoming majority higher airflow that wished to move down in order to get tot he slot in the outlet tube - and air is stripped off the top/inner airflow. FYI - final height of the upper outlet tube will be less area than the inlet. Starved if flowing just air - but able to flow all air out of a mixed stream as the dust and chips settled out to the bin below.

There are issues. by using areas and not including frictional components plus other consideration I have had to experiment with the idea. Seems, as a simple solution but not exact, if you consider area for flow and then remember that you are NOT flowing just air. Again, If your flow is 1% dust, then you do not flow the same area if it leaves the stream. Therefore inlet and outlet are not the same, outlet is LESS if you consider the dust must drop out of the stream before exiting. So, using percentages of airflow I find around 5% dust capacity allows me to drop the outlet area dimension so as to draw more fine dust produced by my CNC and drum sander. Again, Perhaps the "adjustable feature" of the outlet area will enable me to fine tune for what equipment I use or just dial it in better. But, most likely I will leave it wherever the best fines collection occurs. If your flow contains dust you actually flow more than the reduced area of the outlet. It may seem strange, but the dust is riding the airflow, not part of it in this scenario and it appears to work, for me.

There is a lot more I could talk about, but I am not writing a thesis. Basically, wondering if anyone else has comments about their experiences with this builds parameters that I have found to be advantageous - just have not put them all into one build before this proposed [proposed because although I have been gathering materials, well, plans change]:

   
• taller narrower can and stream - ~10"h and 2.8"w
• longer spiral paths to exit - over 149" of outward centrifugal force experienced.
• note: inlet positioned at the top of can - allowing flow to expand DOWN towards the bottom to encourage dropping of fines/chips
• two independent chambers both sealed to allow separated balance of suction - no return from one section to the other. This was not spoke of earlier - but the two slots are separate and NOT connected - do not wish any vacuuming from one slot to the next. So hopefully the larger [and fines] particles will drop out BEFORE they leave the ring and airflow towards the4 outlet tube is encountered - thus allowing them to remain forced to the outer wall of the spiral and allowed to fall out in the first slot.
• calculated outlet size - limiting flow out based on experiment and areas/percentage of flow is air versus debris.
• increasing by expanding area outlet cutout - helps the air swirl stay established "around" the can rather than "heading to the exit" and limiting outlet. I know everyone sees their dust through a window traveling around the can but I have had success with this method. Most important is the raised outlet bottom piece - match areas inlet/outlet and you may see some benefit in your existing can. You can just add a bottom ring and see what happens and take it out if you don't like it.
• riser off the bottom of the can for the outlet to keep fines in the lower pressure airflow at the bottom of the can to encourage complete scavenging - they are "trapped and can not propagate to the outlet [under it]
• note there is no way to use a bellmouth in this build, unless you can place two opposing - but I have not seen much improvement although the bottom bellmounth [facing up] seems to keep debris along the bottom of the swirl I think it is counterproductive on the "coming down from the upper position air that MAY still have suspended fines and promote moving to/into the outlet.

The simple can may still be the best - in the long run - as per retired2's build. But then we would not be having as much fun either  8)
If I change my mind and go that way I think I would still use the raised bottom outlet towards the center of the can drawing off the air from the stream limited to area calculations, with a taller can and narrower transitioned inlet.

That's it. Will advise as/if/when I finish. Also will be looking to do a better job of writing down measurements rather than just observations - will require the installation of several manometers/vacuum gauges to report. I intend to do that no matter what I build next. But I have been intrigued by this since  my initial drawing several months ago. I have been experimenting with the parameters since then. Now I am on iteration #24 and finally satisfied with it and the ability to build it fairly cheap.

[retired2]

Interesting concept for sure.  Unfortunately, the outcome is very hard to predict without investing a lot of time and work, but I'm anxious to hear the results.

My biggest concern with your concept is friction losses.  Consider that a 6" elbow can have losses equivalent to 6' to 24' of straight pipe depending on the bend radius.  Looking at your sketch, you have what looks like approximately 8 ninety degree bends.  In a best case that would add up to losses equivalent to 50' of straight pipe, or a loss of about 2" of SP.  In a worst case, it could be 8" or 9" of SP.  That's as much as some blowers have with no load. 

Admittedly, my analysis is quick and dirty, and probably not terribly accurate,, but it is good enough to show there is a potential for losses to be high enough to render your build unworkable.  Much will depend on the performance of your fan.  I think you are going to need a very good one.

[nucww]

I agree with retired2 that the internal walls are an additional source of friction.  The dust is slowed by the friction of the walls and hence the collection by the outer walls is quite effective with the Thien concepts. The proposed design has almost two revolutions of dust on the wall without a collection hole. Its not clear what is accomplished with these walls.   As you noted, you have two sections in the waste can to prevent cross flow in the can. If the holes did exist on all the spiral walls there isn't much to prevent the flow from going down into the can and back up again.  The cross flow and the pressure drop were reasons for me not to look at this type of design.  Also, it seems like there will be a dead spot on the bottom plate near the outlet tube that may collect dust and require periodic cleaning.  This spiral design does seem to allow more time for the fine dust to get the the walls so as retired2 says if you have the hp to drive it go for it.

[Clark Savage Jr]

I may have an issue with my post, and am going to clarify a sidepost, that came up today as I was responding in a thread by retired2 to keep everything together.

It had to be done - a new 4" hole in my bandsaw!

Basically, yes, I understand the restrictive nature of friction. But I have done a few real world seat-of-the-pants builds leading me to this proposal. Per my other "side post in the other thread":

"......my spiral with eleven feet internal with less than 10' of run to any of my equipment equates to less than........ I would add the internal of the spiral is smooth walled and area compensated - not enough to be a "free flow" air column calculation but enough to outflow the restriction in a vacuum situation versus my fan abilities - and larger area than my fan inlet. Yes, keeping up speed is not as important in that [should have been "this"] situation.................."

In my case the DC is close to the equipment - in my case less than 10' to everything - bandsaw, CNC, spindle sander, drum sander +, with the CNC and drum sander the closest.

The internal frictional component and airspeed is less based on a slightly larger area than the fan intake and in a top hat application there is vacuum present rather than pressure. Semantics - less pressure - as there is always pressure/relative to ATM. The path is longer with less speed, yet still building centrifugal force on the particulate. The purpose being to allow it to drop and not remain air entrained in a pressure/velocity front. I am not attempting to convince anyone, the proof as they say is in the pudding. It will need to be proved. I will need to finish it. It will take me some time due to a few other nagging everyday life obstructions.

Friction is present always, but there may be some trade off between "free air column flow" versus restricted flows - how and how much they are restricted as in quantify and qualify. Anyone with HVAC ducting or other experience will understand the free air moving ability versus.......and we all know runs to the equipment and distances are just as important in the scheme of things. All things equal, speaking for me I do not have an ideal situation either by any means. I have only my seat-of-the-pants and cleaning schedule to advise me.

In the past [and now] I have placed glass plates with and without the the thinnest of Vaseline around my work area. They appear clear/transparent when clean. Observing them may not reflect the ability of my dust collector as it relates to speed and such, but one of my builds left a lot more dust in the air - the kind I did not see with my eyes until it built up on the glass enough to "fog" it after three hours of CNC work. Very telling fines that were invisible to my eyes. I did not see the dust in the room - but could feel something in my nasal area and coughed a little. I found out, although it was collecting dust, either placement of the collection or the DC were at fault. Turns out the DC was passing more fines to the filter area - I ran my bag inside my filter as I do during testing and found a large cake of fines in the bag slowing my airflow dramatically thus not collecting properly. So changed the design, cleaned the bag, problem solved. This was due to my "increasing air tube" flowing too much combined with a too small can. In that design I found too much airflow to the filter which changed in the larger can or scaled design - the fines in the first iteration were too close to the exhaust and could not drop out before simply exiting to the filter area, so I passed more to the filter and clogged my bag.

So yes, throwing this out there for thought too. Sometimes there are other issues that are not discussed but perhaps should be when we post results - such as distance and plumbing to equipment - as a "system" and not as a "unit". I hope this comes across as helpful and not as arrogant. Some of these issues may help us to understand why some of the same type builds for others sometimes work and sometimes don't . Me included.