Cyclone Design...why are there straightening vanes in the air outlet tube?

[David Kumm]

It probably spins all the way to the drum, correct? The vortex extends much farther down than most people realize. The hose helps and it makes it much harder to pull the dust back up into the cyclone. A properly designed receiver is better and would require less headroom than a hose or pipe providing the same function.

So, would a properly design receiver be something a company could produce as an aftermarket add on that would make sense in conjuction with a drum or widebelt usage on a cyclone?

To PHil's example, what % of increase in cfm or fpm could be attributable to the addition of the bell mouth? My understanding was that a bellmouth inlet added significant enough flow that most units were tested with one for purposes of marketing claims. Dave

[Michael W. Clark]

So, would a properly design receiver be something a company could produce as an aftermarket add on that would make sense in conjuction with a drum or widebelt usage on a cyclone?

To PHil's example, what % of increase in cfm or fpm could be attributable to the addition of the bell mouth? My understanding was that a bellmouth inlet added significant enough flow that most units were tested with one for purposes of marketing claims. Dave

David, Yes, the reciever could be added later provided headroom exists. You would want to keep it installed all the time. No harm in having it on planer, shaper, jointer applications either.

A bellmouth hood entry loss factor is about 0.04VP. A plain end duct hood entry loss factor is about 0.93VP. A duct with a 4000 FPM (VP=1"wg) velocity and having a plain opening (round duct end) will have a hood entry loss of about 1.93"wg (1" of loss is added as an acceleration factor to get the air from 0 FPM up to the duct velocity). The same duct with a bell mouth will have a loss of about 1.04"wg. If the duct is connected to a flat plate (table saw cabinet), you have an entry loss factor of 0.5 or hood static pressrue of 1.5"wg assuming 4000 FPM duct velocity. The bell mouth absolutely affects system losses and therefore fan operating point. Even though you have reduced the losses, you are pulling more CFM and usually more BHP or amp draw.

[Phil Thien]

The motor is working harder, reference your TCF example. The BHP went up. More air is being pulled at a higher pressure, and the fan is more efficient, but it is requireing more BHP/CFM. The increase in DP is more than the increase from flow, there is at least an addition 1" of SP required and one can only assume this is from the vanes. Even though the fan curve has shifted up due to a higher efficiency, they are at an operating point that is higher on the fan curve. The higher up the curve you move, the closer you get to an unstable operating point.

I did not say "always," I said "can." In the case of retired2 (at my site), he found adding straighteners reduced motor draw while also increasing airflow:

"What I've since discovered is that it isn't an aberation at all, but rather it is part of a consistent pattern that shows the addition of an air straightener allows the fan to move more air with fewer amperes. If you go back to my test data and compare the numbers for a given test without the air straightener to the same test with the air straightener, you will see that in every case the fan is moving more air with fewer amps when the air straightener is installed. So, if your DC is marginally powered for your system, as mine is, milking every last ounce of performance out of the fan is important." Reference: My forum, which I cannot link here. PM me if you need links.

[Phil Thien]

I am saying the increased efficiency due to a stronger vortex that results from a 10-15% flow increase will not be measurable. You are talking about efficiencies on the 1/10 of a % scale or less. There will be more efficiency improvement in the body of the cyclone due to the increased centrifugal force than from the increased vortex (notice this also has a pressure drop associated with it). If you are trying to collect fine material by applying brute force (pressure drop), you will be limited on the collection efficiency you can acheive.

I never said the improvement would be dramatic. I merely said separation would be increased, not decreased.

Small increases are still increases.

More is better.

[David Kumm]

I'm interested in knowing what a properly designed receiver would look like. How could it be built? Dave

[Michael W. Clark]

The dimensions are relatively simple but I cant give them out for proprietary reasons. Ive been told they are on the internet, maybe even on our site somewhere.
Mike

[Tom Lange]

It is an interesting catch 22. You work really hard to move as much air away from your woodworking device ... to carry away not just the chips and shreds but the fine dust as well...only to have that dust escape the cyclone. Provided the cartridge filter works (as I said it seemed pretty big... rated for 2 microns and is 10 " diameter and 5' tall) that is a trade off well worth making. However, if that bag below the cartridge becomes filled up with more than a gallon of fines per hour of shop use ... I don't know if I like the trade off. When I picked up the unit there were too many 'larger particles' in that bag for my taste. Does anyone else out there have any experience ? Especially with sanding... 100 to 150 grit? Cutting out the vanes is strongly on my mind, but with everything painted that is something I won't be able to weld back easily if the unit does not function. I am just building my shop and upgrading from a 1HP Penn State DC (4" through a trash can ... essentially a chip collector) that was in my basement shop.

[Wade Lippman]

When I picked up the unit there were too many 'larger particles' in that bag for my taste. Does anyone else out there have any experience ?

For the 10th time, if you are getting a gallon of fines per hour and any large particles, you have a serious problem.
Almost certainly a leak, but if you have absolutely ruled that out, I suppose it might be worth a try to cut the vanes out. I doubt they are the problem, but they probably don't help much.

[Tom Lange]

One of the things we did with our cyclones to prevent re-entrainment ... something we used to have a lot of issues with build-up in the 'throat' was to put something we used to call a 'Chinese hat'. Not the most politically correct in todays world... but it was almost exactly the angle of the cone. Is this what people call a 'vortex breaker' today'? I know those cyclones are still in use today after 30 plus years. The cone 'reflected' the inner vortex back up the tube keeping the area at the throat discharge 'quiet' from turbulence and allowed material that slid down the wall and dropped below that hat to be separated. We used those is really short cyclones where we could not put in the really tall ones. It worked surprisingly well. I have not seen that in any hobby cyclones? Come to think of it... if there is still a vortex in the collection drum because of a really short discharge pipe before the drum... you could add it to the lid of a 55 gal drum or fiber drum. Tom

[Tom Lange]

I have a question on % efficiency you guys are talking about. Are you guys talking mass % of the total stream or a particle count efficiency? In other words, # of 10 micron particles separated divided by the # of 10 micron particles in the incoming air stream. Is that what you guys are saying will only improve by 1/10 of a %? Or are you talking lbs. of dust separated (all particle sizes) divided by the total lbs. of dust in the incoming air stream? I completely agree that if you have only fractions of a % change on a particle basis, then the 'more airflow' from 'straightening vanes' is a MUCH better tradeoff. On the other hand...on the lbs of dust basis... I just don't know it is the single digit micron dust that we are really trying to get out. Without straightening vanes...I know that increased pressure drop will occur across the cyclone as a square of the inlet airflow anyway and the separation efficiency should also go up for the really small particles because the inner vortex will be spinning faster. I would expect the pressure drop through the cyclone with straighten vanes to be higher, at the same airflow, then without them? Is that wrong? Or is it that the vanes 'stop the vortex' at the same airflow and reduce the pressure drop. Tom

[Tom Lange]

I would want the vortex below the tube as well... but it seems hard to imagine that you could have no vortex and then 1 inch later have a fully developed one. I am worried that it kills the inner vortex all the way to bottom. We used to see material contact the inside of the outlet tube so I know it was a wall that contacted dust and slowed particles down ... if they had enough gravity to work their way down that tube wall and back into the bigger diameter you could still capture them. If you just fed in fines from a sander and nothing else you could get a sense of that. Maybe wood dust is so light sp. gr. that it changes this thinking. We used to believe that the other larger particles help the efficiency which is why efficiency experiments are probably hard to do. Koch and Licht published a good paper on efficiency curves, where they used pretty good methods IMO, in the late '70's, but that reveals how long it has been since I have been in this game. tom

[Michael W. Clark]

Hi Tom
I've heard of the china hats you mention but Im not familiar with their application. What I was referring to is another chamber below the cone to allow the dust to disengage and the clean gas to turn and go back up the center of the cyclone.

The efficiency I was referring to is % weight. The small cyclone is inherently very efficient on the coarse dust, 20-30 micron. The smaller single digit micron dust takes a lot of energy or a larger cyclone to see a noticable efficiency improvement.

My thinking is in line with yours on the inner vortex formation. If you eliminate it in the outlet tube, there would likely be an effect below the outlet tube.

I think Wade is likely correct on the leaking. While I don't think the vanes help, I don't think they would disrupt the efficiency as much as you are experiencing. Also make sure there are no interior ledges when you put the body back together. It should be lined up properly.

[Phil Thien]

I would want the vortex below the tube as well... but it seems hard to imagine that you could have no vortex and then 1 inch later have a fully developed one. I am worried that it kills the inner vortex all the way to bottom. We used to see material contact the inside of the outlet tube so I know it was a wall that contacted dust and slowed particles down ... if they had enough gravity to work their way down that tube wall and back into the bigger diameter you could still capture them.

My experience has been that once something is entrained in the air mass in the finder, it is nothing getting out.

BTW, how were you seeing stuff contact the inner walls of the vortex finder? Were you working with separators made from transparent material?

If you just fed in fines from a sander and nothing else you could get a sense of that. Maybe wood dust is so light sp. gr. that it changes this thinking. We used to believe that the other larger particles help the efficiency which is why efficiency experiments are probably hard to do. Koch and Licht published a good paper on efficiency curves, where they used pretty good methods IMO, in the late '70's, but that reveals how long it has been since I have been in this game. tom

[Tom Lange]

It left smear marks ... We were working with a material that was not a liquid, but when it contacted a surface at high speed it would leave part of itself behind. We called it 'make up' but it would accumulate to a certain thickness and the abraid away.
tom