White Paper on Deswirl Devices ("air straighteners") used with Cyclones

[allenmck]

Hello. I recently discovered this forum while researching my own DC system. Greatly admire Mr. Thien for his invention, for sharing it, and for maintaining such a high-caliber forum.

Having read several of the threads on this forum regarding what everyone calls "air straighteners", I just wanted to offer a link to some interesting research. The paper offers both experimental validation and design guidelines for the use of Deswirl Devices in Cyclonic DCs. I am in no way connected with the researchers; I just wanted to add to the knowledge base this site offers, as well as offer kudos to whoever brought the subject of vortex straightening to the forum. Lots of smart people here!

***

Well, I tried to post a link to the PDF file of the research paper above, but the forum software said "no thanks." If a moderator wants to correct it, here's the workaround version: www.thaiscience.info/journals/Article/Application%20of%20deswirl%20device%20in%20cyclone%20dust%20separator.pdf

The two main points I took away from the paper are: 1) a three-vaned device provides the most improvement (think peace-sign-shaped cross section) and 2) a short device is quite effective (in other words, the device doesn't have to extend very far into the outlet duct).

If this paper, or similar research has already been cited here, I apologize for the repetition. If not, I hope it is useful.

Thanks again for the information and discussion this forum offers.

[phil (admin)]

Well, I tried to post a link to the PDF file of the research paper above, but the forum software said "no thanks." If a moderator wants to correct it, here's the workaround version: www.thaiscience.info/journals/Article/Application%20of%20deswirl%20device%20in%20cyclone%20dust%20separator.pdf

Sorry, that is a method to stop spammers, no links by first-time posters.  I edited the link in your post and it should work now.

The # of spammers is just unbelievable.  I spend all my time editing-out carp posts and removing spamming accounts.

[allenmck]

Thanks for fixing the link. I figured it was an anti-spam technique.

And thanks again for all the hard work it takes to maintain this site.

BTW, I forgot to mention that the research paper also experimentally validates the "bellmouth" outlet modification (referred to in the paper as "streamlined entry").

Both modifications seem to already be accepted here. But I figured it was still pretty interesting that such a specialized topic had actually been the subject of published scientific research.

[revwarguy]

Very interesting paper, thanks for posting it.

One question I have concerns having a top hat before the DC.  If the direction of the swirl or vortex coming out of the top hat is the same as the direction of the impeller in the DC, isn't that a good thing?

There is no question (at least, in my mind  ;) ) that if the direction of the impeller is opposite of that of the vortex coming from the top hat, then energy must be wasted reversing this flow.  However, if the vortex is straightened out, instead of matching the impeller, doesn't more energy have to be expended by the impeller to move it through the DC?

It seems to me that if you match the direction of the air swirl from the top hat to the DC, a straightener in between would only cause more energy to be wasted.

[retired2]

Very interesting paper, thanks for posting it.

One question I have concerns having a top hat before the DC.  If the direction of the swirl or vortex coming out of the top hat is the same as the direction of the impeller in the DC, isn't that a good thing?

There is no question (at least, in my mind  ;) ) that if the direction of the impeller is opposite of that of the vortex coming from the top hat, then energy must be wasted reversing this flow.  However, if the vortex is straightened out, instead of matching the impeller, doesn't more energy have to be expended by the impeller to move it through the DC?

It seems to me that if you match the direction of the air swirl from the top hat to the DC, a straightener in between would only cause more energy to be wasted.

The short answer is a pre-rotation of air entering a fan adversely affects performance regardless of the direction of rotation.  I've posted this information many times, but here it is again.  It is from a Cincinnati Fan Engineering Manual.

Duct Inlet Spin
"A major cause of reduced fan performance is an inlet duct connection that produces a spin or pre-rotation of the air entering the fan inlet.  Inlet spin in the same direction of the fan wheel will reduce air volume and pressure ratings.  Inlet spin in the opposite direction of the fan rotation will substantially increase the motor horsepower requirements.  An ideal inlet condition is one which allows the air to enter the fan axially and evenly without spin in either direction."

[revwarguy]

Thanks, retired2.  I have read it before as well, but I do not understand why "Inlet spin in the same direction of the fan wheel will reduce air volume and pressure ratings." 

I can believe that the air pressure between the straightener and the impeller is lower without the straightener is lower than with it, but it seems like the pressure difference between the other sides of the impeller and the straightener would be the same with or without it.  It seems to me that ultimately the mass/energy calculations here would show energy being expended by the air against the straightener (lost energy) and then more energy being expended by the impeller against the air, all to get the air moving in the same direction again. 

I understand this is all theoretical, but if it has been experimentally determined that a straightener helps even when the direction is the same to any reasonable degree, then I'll add one just to see.

[retired2]

Thanks, retired2.  I have read it before as well, but I do not understand why "Inlet spin in the same direction of the fan wheel will reduce air volume and pressure ratings." 

I can believe that the air pressure between the straightener and the impeller is lower without the straightener is lower than with it, but it seems like the pressure difference between the other sides of the impeller and the straightener would be the same with or without it.  It seems to me that ultimately the mass/energy calculations here would show energy being expended by the air against the straightener (lost energy) and then more energy being expended by the impeller against the air, all to get the air moving in the same direction again. 

I understand this is all theoretical, but if it has been experimentally determined that a straightener helps even when the direction is the same to any reasonable degree, then I'll add one just to see.

Unfortunately, none of the engineering documents I've seen on the subject of pre-rotation do much to explain the "why" of the phenomenon, but there are plenty of sources available on this subject and they all say the same thing, i.e. pre-rotation in either direction has an adverse effect on fan performance.

[Schreck]

If the blower impeller spins at 1800 RPM and the air entering the blower inlet is also spinning at 1800 RPM and in the same direction, then the blower would move no air.  If the air is spinning at 900 RPM the blower will move some air, but not as much as when the air entering the blower is not spinning at all.  Air entering the blower that is spinning in the direction opposite that of the impeller is bad, as you would expect.

Adding a straightener will introduce some pressure drop, but it is far less significant than the benefit from increasing fan output.

Try searching Google Images for "inlet guide vane" and you will see many examples of devices that manipulate the pre-rotation of air entering fans.

Thaiscience.info seems to be overloaded at the moment   :(

There was a thread regarding CW vs. CCW last year that covered this issue.

[allenmck]

retired2 is right. I think he is the one who introduced the subject to this forum in the first place(?), which was a really insightful modification. I was gonna ask if he was an engineer by training, or just a smart civilian. :) It was also impressive that he did his own experimental testing and shared the results. IMHO advice is worth ten times as much when it comes from first-hand observation rather than "I read somewhere."

Regarding why the vortex spinning in the same direction also causes a performance hit, I read somewhere ;D (and this is a rough paraphrase) that it is due to the impellers not being able to impart as much force to the fluid. I kind of pictured it like this: when you throw a baseball TO a batter, he can crack the h--- out of it. But if you imagine a stream of baseballs coming from behind a batter, and the batter trying to hit one hard enough to speed it up, it seems like the timing of the swing would be impossible to get right. A lot of whiffs vs. a good, solid connect.

That might be total BS, but at least I could stop wondering and use the information, because the results were pretty impressive. 19% improvement (don't know if it is a 1:1 improvement in collection, or a 19% improvement in an aspect of the process). But the two mods are such easy things to do, why not give it a shot?

I'm always open to being proved wrong (kind of the point of experimenting, after all), but the explanation about the impellers not being able to move the air at all if it is spinning in the same direction doesn't seem right. That would kind of stall the whole system, right?

Anyway, Retired2 and many others have brought in a lot of useful, creative, information. And that seems to be a big motivating factor here: building your own system that is equal to, or even better than, commercial offerings. Wringing every bit of efficiency out of the system can only help. It does make you wonder how much research the commercial manufacturers have done. Or whether other considerations get priority over what seems like cheap and simple ways to push efficiency as high as possible?

When I get a chance I want to start a thread about how the super high-end DC systems (like Felder's top line) take collection to the current highest level. I'm pretty sure I know how they do it. But there are a couple of practical aspects I'm betting guys like Retired2 can figure out no problem.  :)

[retired2]

allenmck,

I once replied to one of "vodkaman's" posts with the following response.  It describes who I am.

"Vodkaman, your comments make me chuckle because they put you among a group of people in this world, me included, who have a compulsive need to understand how everything works whether we truly need to know or not, and we are always on a quest to make them work better whether they need to or not!

I don't know how many times in my life a new aquaintence has ask me "Are you an engineer?"  Most of the time it is asked in a way that is not necessarily intended to be a compliment.  It is more like "If you are an engineer that would explain your nutty obsessive complusive behavior and your need to fix things that ain't broke!"

[phil (admin)]

I had theorized the reason straight air was always preferable was that the individual vanes on the blower wheel would always be evenly loaded.  I had imagined the key was that each vane was allowed an opportunity to carry its burden on each rotation.

Air spinning in the same direction as the wheel probably would try to load one area of the blower more than others, and air spinning the opposite direction would probably do the same and also cause the wheel to have to overcome the inertia.

But I'll admit that I always sort of hoped that Retired2 would just tell us why.

;D

[retired2]

I had theorized the reason straight air was always preferable was that the individual vanes on the blower wheel would always be evenly loaded.  I had imagined the key was that each vane was allowed an opportunity to carry its burden on each rotation.

Air spinning in the same direction as the wheel probably would try to load one area of the blower more than others, and air spinning the opposite direction would probably do the same and also cause the wheel to have to overcome the inertia.

But I'll admit that I always sort of hoped that Retired2 would just tell us why.

;D

Phil, do you know what a SWAG is?  If not, it's an engineering acronym for an answer that lacks absolute certainty.  So, here's my SWAG.

I believe the reason pre-rotation effects fan performance is simply because it changes the angle of air impingement on the impeller blades, and consequently the flow resistance.  In one case that resistance is lower than the optimum fan design, and in the other case it is higher.  The fan is designed and optimized for incoming air which has no spin, so any deviation from that ideal will be detrimental.

So, what do you think, Phil?   Uh, before you answer, give me a break, I haven't been a practicing engineer for nearly 30 years!    :)

[Jack]

So, absent a deswirler, is there a better side of the hat to make the inlet?  Clockwise or counter clockwise?

[retired2]

So, absent a deswirler, is there a better side of the hat to make the inlet?  Clockwise or counter clockwise?

Maybe, but I can only speak to my experience with counter-rotation.  My test data shows the air-straightener provided approximately 50 cfm more at a reduced amperage.  Normally amperage drops when less air is being moved, but in my case I got more for less, so clearly the air straightener was justified.

I have no way of knowing what the penalty might have been if my pre-rotation had been in the same direction as the fan, but that is the direction that the experts seem to suggest is worse. Their comment is always that this pre-rotation moves the fan curve down and to the left.  Simply put, that means lower cfm and less SP.  They don't say by how much probably because there are too many variables.

[bpotts]

Okay, and the commentary on the use of turning vanes at the entry to the Thien chamber, especially w/ regard to the top-entry situations?  These are worth a 50% reduction in SP drop at elbows.

Likewise, what about the use of stream vortex inducers just inside the feed duct where it dumps into the Thien chamber?  Tuning of this twisted jet of air would help compress it and thereby accelerate it; and it's rotation could be oriented to get it to hug the wall of the chamber at the bottom where the dust needs to be deposited?

--Cheers, Bradley