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

[Phil Thien]

If it weakens the vortex, separation is negatively affected.

It doesn't weaken the vortex, if anything it modifies the shape, intensifying it.

[Tom Lange]

We are getting to the guts of it.

Phil, i think you you are right and your references are helpful that the vanes are there to improve fan performance. That matches my own industrial experiences with large fans (300 HP), we always wanted a long straight inlet to the fan or straightening vanes. Since our fans were always separate from our cyclones (also big 10' diam and 30' tall) we did not have this problem that the compact design seems to create. Our outlet tubes (we called them tubular guards) were always free of any internal obstructions. That is why I was surprised when I got this thing taken apart for transport.

If the INNER vortex...not the outer one that runs along the large diam wall and cone... Is some how interfered with down near where the outer vortex inverts to the inner one-- then separation efficiency for the very small particles will be hurt. These particles are the ones where the forces from spinning in the outer vortex are not strong enough to get the particles through the air to contact the outer wall slowing them down. In my experience ... the smaller diameter inner vortex determines the separation efficiency...it spins faster...e.g. Tighter, and can push the particle into the outer vortex (folklore had it this happened down in the cone) and improve it chance to make it to the wall versus get carried up the air exit tube.

Doing a CFD calc ( something that itself is probably more costly than 3 brand new systems! ) would probably settle it, and may have to include the fan wheel which would be very complicated. I wonder if Jet or whoever designed it?.. ever did that? The rest of the cyclone's proportions seem well matched to the air ratings.

Good discussion though thanks for the input.
Tom

[Aleks Hunter]

WHen you say you always wanted a long straight inlet, How long do you mean, is there a rule of thumb for inlet diameter vs. length of straight run?

Thanks

[Tom Lange]

Aleks,
My memory was you need at least 4 inlet diameters after any disturbance like an elbow etc. So, if you have an 8" inlet, you would need at least 32" of a straight run before the fan inlet. If I had my old Fan Engineering manual ( it was small and black like a bible), I could look it up but I heard it was out of print and I don't have it anymore. With CFD so prevalent no one uses The old rules of Thumb any more...probably a good thing.

Tom

[Michael W. Clark]

Phil,
I don't disagree with your point about system effect on the fan. Like the reference states, inlet vane dampers or "cat's eye" dampers use this theory to reduce the fan flow and conserve HP. However, given the small size of the cyclone and fan (approximately 1/15th of the one in the TCF bulletin), any improvement in airflow due to the straightening vanes is going to minimal, if at all measurable. There are many other factors that are going to affect the flow in a much greater way. We generally do large cyclones as well. We are starting up one of our cyclone systems next month that has a quad arrangement (4 cyclones in parrallel), each cyclone is 50 feet tall and the system fan is 400HP. No turning vanes in the outlet pipes or flow straighteners, guaranteed performance of the cyclones. We used an inlet box on the fan inlet so the fan vendor would account for this system effect. Inlet boxes have a known system affect along with inlet elbows, outlet elbows, and inlet/outlet ductwork. If you have system effect factors on pre-spin air, I would like to see them as they may be useful. We have supplied some smaller cyclones in the past with vertical mount fans, no straightening vanes, and no performance issues. I would say that TCF probably supplied a large number of these fans and they design vertical mount fans for some of our scrubbers. Again, no straightening vanes in those either and they have cyclonic separators.

IMO, the straightening vanes were added for marketing. This is a retail, off-the-shelf cyclone system. They can test it without inlet duct or filters and rate it higher if there is any gain in volume. Thus giving them a marketing edge over their competitor. Since this is a light commercial / home-hobby unit, abrasion is not a consideration. An industrial cyclone on wood dust must be constructed to resist abrasion and is often constructed of abrasion resistant materials or uses linings in wear prone areas. The outlet pipe is one of the major wear areas. Anything you put inside that is required for performance, will be a problem later and could result in loss of product or non-compliance with environmental requirements. This, the added pressure drop, and expense is the primary reasons that straigheners are usually avoided if at all possible.

Mike

[Michael W. Clark]

It doesn't weaken the vortex, if anything it modifies the shape, intensifying it.

Intensifying the vortex requires a higher velocity in the outlet pipe. A measurable efficiency gain requires a significant increase in velocity and therefore substantially more energy (pressure drop). If the flow increased by 10-15% as claimed in the TCF bulletin, you would not be able to measure the increased efficiency due to the slightly stronger vortex. The vanes in the outlet pipe only have the potential to disrupt the natural flow patterns of the vortex. If you had the 10-15% flow increase, you would see more efficiency attributed to the increased centrifugal force being applied in the body and cone of the cyclone, not due to a stronger vortex. This is simply not enough of a velocity increase in the outlet to have a significant, measurable effect.

I have run efficiency calcs that significantly increase the pressure drop of the cyclone through intensifying the vortex, only to yeild efficiency gains in the fractions of a percent. Of course this is cyclone and application dependant. The OP's cyclone is relatively small collecting wood dust which is considered coarse by most of the particulate dealt with in industry. The small cyclone on a small flow with coarse material is going to be very efficient. Most of the larger particles are easily collected. Since there are very few small particles, additional efficiency gains require considerably more effort. Pressure drop and centrifugal force play less of a role in fine particulate colletion. You need to allow more time in the cyclone for the fine particulate to move to the outside wall where it can be collected.

I think the biggest downfall of the home system setups is that there is no space below the cone for the dust to disengage from the vortex. The collection barrel is fine until the dust level gets too close to the bottn of the cone, then it is re-entrained and sent over to the filters. This is always happening on some level as the barrel fills, but becomes quite evident when the fills to the bottom of the cone and beyond. The finer the particulate, the more likely it will be re-entrained.

Mike

[Phil Thien]

I don't disagree with your point about system effect on the fan. Like the reference states, inlet vane dampers or "cat's eye" dampers use this theory to reduce the fan flow and conserve HP. However, given the small size of the cyclone and fan (approximately 1/15th of the one in the TCF bulletin), any improvement in airflow due to the straightening vanes is going to minimal, if at all measurable.

The improvements are proportional. That is, making the changes on a small (home hobbyist) system should yield the same percentage gain as on a larger industrial cyclone.

Yes, the #'s are smaller. But the home hobbyist is the guy that doesn't have any CFM to spare. Making a change to a cyclone that provides a 10 to 15% improvement in FPM may result in meaningful improvements, especially when it is already operating at the margins of what is acceptable for fine dust collection.

And it can allow the motor to work less hard, consuming less energy. And it costs nearly nothing to implement.

I don't agree w/ your assertion that it was added for marketing. As much as it pains me to say anything positive about Jet or WMH, LOL, it is a good design feature and I think they should be commended for it.

[David Kumm]

Michael. if a company ( I'm thinking Clearvue because of the ability to mold PTEG ) offered an add on vortex breaker for those with the ceiling room, could that improve efficiency with fines or would each unit need to be specifically designed for each cyclone. With sanders becoming more and more a home shop machine, dust in the cartridges is becoming more of a problem. Steel cyclones are expensive to modify for fine dust but a " plastic add on vortex breaker " should be affordable if it actually accomplishes something. Dave

[Phil Thien]

Intensifying the vortex requires a higher velocity in the outlet pipe. A measurable efficiency gain requires a significant increase in velocity and therefore substantially more energy (pressure drop). If the flow increased by 10-15% as claimed in the TCF bulletin, you would not be able to measure the increased efficiency due to the slightly stronger vortex. The vanes in the outlet pipe only have the potential to disrupt the natural flow patterns of the vortex. If you had the 10-15% flow increase, you would see more efficiency attributed to the increased centrifugal force being applied in the body and cone of the cyclone, not due to a stronger vortex. This is simply not enough of a velocity increase in the outlet to have a significant, measurable effect.

Respectfully disagree.

I want the vortex BELOW the tube, not IN the tube.

And I'm not saying the improvement in separation is going to be "significant." I'm saying these changes can and only will increase separation rates, they won't decrease them.

That is, adding straighteners doesn't make small improvements to airflow at the expense of separation. Straighteners yield small improvements in airflow, and if anything, improvements in separation as well.

[Phil Thien]

I forgot to mention, BTW, that straighteners also reduce noise.

All this stuff I'm reporting, BTW, was first brought to MY attention by a gentleman that goes by the handle of retired2 at my forum. He is a retired engineer that found straighteners added to this Thien-based separator increased FPM (approx. 10%, up to 15% when used in conjunction with a bell mouth). The straighteners also reduced motor draw and system noise, as well. My own experimentation (and reading, of course) has confirmed this.

Separation rates are tricky to measure with the equipment I have. But they certainly haven't worsened.

[Michael W. Clark]

Michael. if a company ( I'm thinking Clearvue because of the ability to mold PTEG ) offered an add on vortex breaker for those with the ceiling room, could that improve efficiency with fines or would each unit need to be specifically designed for each cyclone. With sanders becoming more and more a home shop machine, dust in the cartridges is becoming more of a problem. Steel cyclones are expensive to modify for fine dust but a " plastic add on vortex breaker " should be affordable if it actually accomplishes something. Dave

Yes, but the issue is with ceiling height. If you blow the nominal 8' mark, your market is more limited. You can accomplish the same thing by not filling the bin to the bottom of the cone.

[Michael W. Clark]

The improvements are proportional. That is, making the changes on a small (home hobbyist) system should yield the same percentage gain as on a larger industrial cyclone.

Yes, the #'s are smaller. But the home hobbyist is the guy that doesn't have any CFM to spare. Making a change to a cyclone that provides a 10 to 15% improvement in FPM may result in meaningful improvements, especially when it is already operating at the margins of what is acceptable for fine dust collection.

And it can allow the motor to work less hard, consuming less energy. And it costs nearly nothing to implement.

I don't agree w/ your assertion that it was added for marketing. As much as it pains me to say anything positive about Jet or WMH, LOL, it is a good design feature and I think they should be commended for it.

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.

[David Kumm]

Yes, but the issue is with ceiling height. If you blow the nominal 8' mark, your market is more limited. You can accomplish the same thing by not filling the bin to the bottom of the cone.

So if you have a 2' length of flex from the cone to the drum, you are effectively creating your own vortex breaker and it doesn't really need to expand the diameter of the cone again? I run a long length of clear flex and the dust and air is spinning like crazy inside it. Dave

[Michael W. Clark]

Respectfully disagree.

I want the vortex BELOW the tube, not IN the tube.

And I'm not saying the improvement in separation is going to be "significant." I'm saying these changes can and only will increase separation rates, they won't decrease them.

That is, adding straighteners doesn't make small improvements to airflow at the expense of separation. Straighteners yield small improvements in airflow, and if anything, improvements in separation as well.

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.

[Michael W. Clark]

So if you have a 2' length of flex from the cone to the drum, you are effectively creating your own vortex breaker and it doesn't really need to expand the diameter of the cone again? I run a long length of clear flex and the dust and air is spinning like crazy inside it. Dave

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.