Volume below the baffle

[WayTooLate]

This may be a topic for Phil, but perhaps others have had experience with this... 

Does the amount of volume below the baffle matter?  If not, what about shape? 

I have a wide belt sander that exhausts out the top (as most do).  My large DC does a pretty job job exhausting the dust out of the machine - we have minimal fines inside or around the machine after extensive use. 

However, I want to prevent as much dust as possible from going to the bags.  I also don't want to climb on top of the machine to empty trashcans.  My idea is to place the separator inline  from the machine's exhaust and our overhead pipe to the DC.  Then, under the baffle place a tapered cone to a pipe that drops to a trash can on the floor. 

Has anyone else tried something as unusual as this?  Does it work?

I will look forward to any replies!
Jim

[alan m]

i was thinking of doing this when i started building my 6" seperater. i wanted to make a really large diameter seperater and put a funnel on the bottom to narrow it in to 24 " (the dia of the barrel)
my plan was to make the seperater around 40" in diameter and put it up in the attic and use the funnel to drop it down the 4 feet to the barrel

i didnt d o it in the end but might down the line.

i was going to continue the same diameter as the seperater down about 5" bellow the baffel so that the angled side of the funnel wouldnt affect anything.

i dont see why it wouldnt work.
i would keep the sides of the funnel fairly vertical . it willl take more height but no material can sit on the wall of the funnel
if anything it might help isolate the already seperated out dust from the spining air mass

[retired2]

Volume should not matter as long as you keep it air tight.  However, shape could affect separation if you do something crazy near the baffle.  There have been a number of photo's posted that show a swirl pattern in the fines that collect in the bottom of the waste container. 

This swirl pattern tends to confirm what you would expect, i.e. the swirling air in the separation chamber imparts a similar spin on the air in the waste chamber.  However, the speed of the spin in the waste chamber is most likely much slower.  If you disrupt that pattern near the baffle you might adversely effect the separation.

If it were me, I would start the sloping sides of the cone a few inches below the baffle rather than immediately below.  If the slope of your cone is very steep, i.e. converging slowly, then it might not have much impact on the separation even if the slope begins immediately below the baffle.

[phil (admin)]

Volume should not matter as long as you keep it air tight.  However, shape could affect separation if you do something crazy near the baffle.  There have been a number of photo's posted that show a swirl pattern in the fines that collect in the bottom of the waste container. 

This swirl pattern tends to confirm what you would expect, i.e. the swirling air in the separation chamber imparts a similar spin on the air in the waste chamber.  However, the speed of the spin in the waste chamber is most likely much slower.  If you disrupt that pattern near the baffle you might adversely effect the separation.

If it were me, I would start the sloping sides of the cone a few inches below the baffle rather than immediately below.  If the slope of your cone is very steep, i.e. converging slowly, then it might not have much impact on the separation even if the slope begins immediately below the baffle.

I couldn't have said it better.

[alan m]

iv noticed a lot of swirl paters in the dust in my barrel. there is a high spot below the start of the slot. 
i would agree with retired2 that it could efect seperation but as with all these hypothetical situations. no body really knows until its tried

try it and report back. even a complete failure could unearth a gem of an idea

[WayTooLate]

We are pretty well committed to improvising a separator over the wide belt sander.  I will include pictures when it is completed. 

As I lay-out the design, I'd like some experienced input of overall diameter...
Separator #1 is on a Planer - generating healthier chips and very few fines.  It has a 6" duct.  A stand alone DC attached to it works rather well.  The only mod we have made is to add a row of long bristle brushes across the from to keep chip 'spray' from exiting the front of the machine.  I expect my slot to be 1.25 - 1.63" wide to allow curly chips to fall through.  I am also planning on making the separator about 10" tall for a 6" duct.  It will sit on the floor adjacent to the planer and be a conventional top-hat-over-a-barrel design.

Separator #2 is on the Sander.  It does only finish sanding - so everything is fines.  It has a hood fir each belt and a vent for the interior cabinet that goes to a 8" duct.  Since it will all be overhead, I can make the separator any practical vertical height.  With an 8" duct, is there any advantage to a three-story 'Top Hat' design?  This will have a 8-12" high ring under the baffle and then a cone tapering to a hose to drop into a barrel on the floor.

So... my question remains, what is the optimum diameter for these separators?  I don't worry too much about loading the DC, I have a 12" trunk line and plenty of horsepower.  (I will have to add a third port to the trunk line to keep the 6" and 8" lines from starving the airflow down the 12" trunk)  I am looking for optimal separation to prevent as much material as possible from reaching the bag-house. 

I will welcome any recommendations and the ensuing discussion!
- Jim

[alan m]

seperator 1 sounds perfect.


the larger the internal height of the seperater the better.
having more wall bellow the baffle wont affect seperation. all thet is done above the baffle
but that section below the baffle will help stop the dust getting pulled back up

diameter wise i would be aiming for 24"  for the planer seperater and 36" for the sander one

f you have a 12" line  there. i would drop with 12"  and reduce down on the tool side of the seperater. the more airflow the better

[WayTooLate]

Alan -
The diameters you are suggesting seem counter-intuitive (but I am often wrong in my assumptions...).

The sander is 'fines only' - so I would assume that it would be the smaller diameter.  Making tighter "spins" to force the dust particles centrifugally against the outside wall.   

The planer creates the larger chips - with greater mass that requires less 'turning' to remove it from the airstream.     

I don't think I want to use the separator as the transition point for the ducting.  If I starve the 12" line with an 8" intake, I will reduce the airstream velocity too much.  Currently, I have just over 5500fps through my 12" line = >4300cfm.  If I choke that with an 8" intake, that same airspeed, is only 1900cfm.  The blower will be able to draw more, but even if it can get 2500cfm through the 8"pipe, once the air gets to the 12" line, the air speed is less than 3200fps - not high enough to keep the debris in the airstream.   I will accumulate dust inside the duct and that is unacceptable. 

Whatever airflow I can't get through the 8" line I will have to supplement with an additional port(s) to keep the airspeed in the trunk line over 4000fps.    I realize that is an uncommon situation for most readers of this website - most of us are  starved for every cfm we can squeeze through their system.  But we can only allow dust in three places: 1) on the floor or worktable; 2) in the bottom of the separator; 3) in the final bag/filter.   Anywhere else presents a hazard that is costly in fines or dangerous in the workplace. 

I will look forward replies about the separator diameter!
Thanks.
Jim

[alan m]

the reason i was sugesting 24" for the planer was that it would be around the same as the barrel that you are putting it on. this would benifit from being a larger radius but i thought that overall you could get away with it on that machine because there was very little fines.

having a larger diameter for the sander will create less resistance on the system  and allow higher speeds. the higher the speed the better the seperation



i wasnt sugesting that you use the seperater to reduce down .  you will still have to have a 12" inlet  and then split off to the sander but allowing air in so that your not starving the system like your sugesting

if it is posable , i would open up the dc ports so that all the 12" is going through the sander. if not then let in the rest of the air as close to the sander as posible. that way it will help get the dust away as fast as posible

i had to do this with my planer. the 4" port was starving the dc system and i was getting no extraction. i put another 4" dc port on a y fitting beside the planer. now its great.

[WayTooLate]

Alan -
Thanks again for your prompt, thoughtful insights and assistance. 

Your comments and suggestions have posed a provocative question:  Do I want to make the overall diameter as large or small as practical? 

• When you swing a weight on a string, the longer the string, the greater the linear velocity around the larger circle.  So bigger would seem to mean faster (better?) 
• The larger the space, the greater the volume of air and the greater force needed to get that air mass rotating.  Can it get so big that the air flow moves directly from the peripheral intake to the center output? (without creating a circular motion)
• Smaller diameters force greater angular changes in the air flow.  Does this redirection make the particulates hit the outside perimeter and then drop out?
• Conventional cyclones are quite small compared to their duct diameters.  This seems true for large 10,000cfm Torit/Donaldson units as well as Dust Deputy units for shop vacuums.  Why is this?
• If I have a large airspace, do I need as much centrifugal force for separation?  (Imagine if I had a sealed box as big as my garage.  In one corner, I had a 5" duct coming from my table saw.  In the opposite corner, I had another 5" duct going to me DC.  How much dust would make it through the 4000cu ft to get caught in my final filter?  Bizarre thought, I know - but an interesting exercise... ) 

I will look forward to the comments and answers.  If possible, I may build both a 2.5D and a 4D unit.  Depending on results, I can simply alter the In/Out fittings to change to the better performing design.  I don't know if I can justify extra testing attempts to my boss, but we may just build multiple heights at 1.25, 1.75 and 2.5D heights...  If I design the parts cleverly, I can just swap out different outer bands at alternate heights...

I will look forward to everyone's comments and posting my results...
- Jim

[retired2]

Alan -
Thanks again for your prompt, thoughtful insights and assistance. 

Your comments and suggestions have posed a provocative question:  Do I want to make the overall diameter as large or small as practical? 

• When you swing a weight on a string, the longer the string, the greater the linear velocity around the larger circle.  So bigger would seem to mean faster (better?) 
• The larger the space, the greater the volume of air and the greater force needed to get that air mass rotating.  Can it get so big that the air flow moves directly from the peripheral intake to the center output? (without creating a circular motion)
• Smaller diameters force greater angular changes in the air flow.  Does this redirection make the particulates hit the outside perimeter and then drop out?
• Conventional cyclones are quite small compared to their duct diameters.  This seems true for large 10,000cfm Torit/Donaldson units as well as Dust Deputy units for shop vacuums.  Why is this?
• If I have a large airspace, do I need as much centrifugal force for separation?  (Imagine if I had a sealed box as big as my garage.  In one corner, I had a 5" duct coming from my table saw.  In the opposite corner, I had another 5" duct going to me DC.  How much dust would make it through the 4000cu ft to get caught in my final filter?  Bizarre thought, I know - but an interesting exercise... ) 

I will look forward to the comments and answers.  If possible, I may build both a 2.5D and a 4D unit.  Depending on results, I can simply alter the In/Out fittings to change to the better performing design.  I don't know if I can justify extra testing attempts to my boss, but we may just build multiple heights at 1.25, 1.75 and 2.5D heights...  If I design the parts cleverly, I can just swap out different outer bands at alternate heights...

I will look forward to everyone's comments and posting my results...
- Jim

If you swing a rope with a weight attached, it is true that the linear velocity at the weight will be greater than any point on the rope closer to your hand, but the angular velocity is the same at any point along the rope.  I submit that in the case of a very large diameter separator, the angular velocity will slow due to the larger volume and therefore the perimeter velocity will be lower than the entrance velocity.  So, bigger to an extreme is not better, but bigger is better than too small!  Now don't ask how big is too big or how small is too small.  It seems most builds are successful and the diameter that is correct is the one that just fits the waste barrel.

Your second burger dot is exactly what will happen if the separator is oversized, and it is the reason why the assumption in the first burger dot is wrong.

I'm not sure about your question about angular change promoting waste separation.  For sure smaller diameters cause higher sp losses and that is undesirable.

Cyclones are sized the way they are because someone a lot smarter than any of us has engineered every last ounce of performance out of them.

The answer to how much dust would make it to the filter after passing through a 4000 cu.ft. space, is probably very little.  With a garage sized bump in the line the air speed would slow so much that most of the dirt would settle out on the floor.