Why Use a Neutral Vane or Air Ramp in a Cyclone

[Bob Summers]

After my last post on cyclones, I read some papers that have raised questions in my mind about whether using air ramps and neutral vanes in cyclones is a good idea. The acutal quotes are below.

It seems like an inlet-vane or air ramp hurt the dust collection efficiency quite a bit. Using either of these would mean that the final filter needs to be cleaned about 3 times as often. And presumably replaced 3 times as often.

Can anyone come up with a reason to use an air ramp in light of this data?

Reference [1] has this to say about neutral vanes, which it calls inlet vanes.

"Several modificati "A cyclone with an inlet helix was tried next. ... This cyclone with the inlet helix performed simalarly to the one having a flat inlet vane. Both the pressure drop and efficiency were reduced when the inlet helix was used. About two and one-half to three times as much material was lost from the cyclone with the inlet helix than from the unmodified cyclone."

Reference [2] is a preliminary report of a computer simulation of a cyclone.

[2] says: "In the plan view of the cyclone streamlines air makes one revolution in the barrel of the cyclone, then makes an abrupt change in direction near the entrance (where it collides with incoming air). This forces the dust-laden air to make a tight turn inward and downward. The fact that the air is leaving the dust behind at that point appears to be more important to cyclone efficiency than the fact that the dust is reintroduced into incoming air. This alternate approach to understanding cyclone dynamics was thought to possibly explain the results of an earlier experiment attempting to prevent such reintroduction". It then goes on to quote the material that I quoted above.

[1] Baker and Stedronsky. July 1967. "Gin Trash Collection Efficiency of Small-Diameter Cyclones", ARS 42-133; USDA Argicultural Reseach Service

[2] "Dust Cyclone Modeling and Validation" a presentation made by Funk and Hughs at the 2000 ASAE Annual International Meeting ASAE Paper no. 004026

Bob S

[Jim Wilson]

Bob Summers provided some more food for thought on cyclone DC's...

Bob, just in case no one else responds...please don't be discouraged from posting on the topic.

I, for one, have been carefully devouring all your posts on cyclones. It is my very next shop project, and the information from you (and Bill Pentz (!)) have been immensely helpful to me.

If you don't stimulate a lot of discussion, it's only because so few other folks have anything to add! (G)

Thanks,

Jim

[Bob Summers]

Thanks Jim, I'm glad someone is finding these posts useful.

I did think of one reason for using either an air ramp or a neutral vane for us hobby shops ... Many of us are frugal :-) and tend to skimp on the size of the blower (because they are expensive) and/or the duct work (hey, dollar is a dollar). There may not be enough pressure drop (power) to drive an efficient cyclone and maintain significant air flow.

One alternative for reducing the pressure drop without screwing up the air flow inside the barrel would be to use a larger diameter cyclone. Within reasonable limits, that will reduce the pressure drop consumed by the cyclone and won't have much effect on separation efficiency; it might even improve it.

One paper, (Funk, Hughs & Holt. Engineering and Ginning: Entrance Velocity Optimization for Modified Dust Cyclones. Journal of Cotton Science 4:178-192 (2000) ) found that modified 1D3D cyclone got the best separation performance with an input velocity of around 2,600 feet per minute. That was also the lowest input velocity tested, though looking at their graph, it looks like the best collection efficiency is around 2,600 ft/min. The collecion efficiency peak is very broad.

The pressure drop was 4+ inches of water at 3,500 ft/ min but only about 2.5 inches of water at 2,600 ft/min. That pressure drop reduction is in the same league as an air ramp.

BTW, a modified 1D3D cyclone has an inlet that is D/2 x D/4, the vortex finder has a diameter of D/2, the vortex finder is D/2 or a little longer and the dust bin outlet has a diameter of D/3.

The drawback to using a bigger cyclone is that it is taller. Many of us have limited height. Most of us can handle a footprint that is 2 or three inches bigger but 3 inches wider will add at least a foot to the height.

Bob S

[Jim Wilson]

Is that paper available online? I have read at least one Funk, et al, paper, but I don't remember the info you are citing here.

A transport velocity of 2600 fpm seems too low to prevent settling in the main trunk, even for horizontal runs. That probably explains why they didn't test at lower speeds.

Jim

[Steve Knight]

well I have been testing Bill Pentz's design and I know it works very well. I get about 2 to 3 weeks between cleaning the filter. I only loose a little airflow. but i make so much fine dust and that's what a cyclone misses. I get a couple of coups of very fine dust out. the stuff will float all over my shop when I clean the filter. more like smoke then dust. I have more airflow with his design then I did with my regular bag setup and a huge felt bag.

[Jim Wilson]

Yeah, and that's the bad stuff. I wondered if this might be a problem and after some thought, came up with an idea.

What if we mount the (cartridge) filter vertically, with a removable solid canister attached to the bottom end? The can would only need to handle a liter or two of dust at the most. Clean the filter periodically by spraying the outside with compressed air. The fine dust should fall into the can, which would be removed and emptied "after the dust settles" (G).

IF this arrangement works, I do see a couple of drawbacks. A big one is that it takes more potentially otherwise usable shop space to mount the filter vertically, as opposed to horizontally near the ceiling. Plus, it requires access to all sides of the filter, which exacerbates the space problem. It also adds another 90-degree turn to the ducting. At first blush, the potential advantages would seem to outweigh the obvious drawbacks, at least to me.

Anybody have any thoughts on this? Have you seen it before -- or even better -- have you tried it?

Jim

[Bob Summers]

If you send an email to the right person at the ARS, that's the USDA's Agricultural Research Service, they'll send you a copy. A Google search on the title should turn up the right page to order from. I don't remember where the right page is right now.

Searching for "cyclone" at gives you abstracts and summaries but it doesn't allow reading or requesting the papers. One of the summaries turned up is

Dust Collection Efficiency of over-Sized Cotton Gin Cyclones that looks interesting.

The 2,600 ft/min is the inlet velocity of the cyclone not the velocity in the duct work. For example, going from a 6 inch round pipe ( 28 sq inches) to a 4.5 x 9 inlet (40 sq inches, which is about right for an 18 inch, modified 1D3D) will give about a 30% reduction in velocity. As you've noted, dropping the velocity too fast or too far will likely lead to choking. Reducing the velocity too fast will probably cause unneeded pressure drop too.

Bob S

[Larry C in Auburn, WA]

I can't tell if you're being serious or joking because many (most?) systems are just as you describe such as:

[Bob Summers]

I'm glad to hear that you're happy with the cyclone.

BTW, your postings have convinced me that cannister filters are so much better that it's not worth looking at fabric filters.

If the numbers from the paper I mentioned above are correct, then without the air ramp and neutral vane you'd only have to clean the filter every six to nine weeks instead of every two to three weeks. To me that would be a worthwhile improvement!

OTOH, if your airflow dropped low enough you'd just not collect the dust in the first place and you'd never have to clean the filter. :-)

How big a blower do you have? And how do you decide that your filter needs cleaning? Did you build a manometer? Or do you just notice the airflow is low?

Bob S

[Bob Summers]

So, are you're talking about a cross between canister filter and shaker bags?

IIRC, some of the Oneida units have the filter in that sort of arrangement.

Of course the Jet canister filters do essentially the same thing except they use a flapper of some sort to scrape the dust off the inside of the filter.

I wouldn't worry too much about another 90 degree bend after the blower; it can be a gentle bend in bigger pipe than used for the input duct. And it's hard to do much better anyway. Whether it uses more space or less space depends on the shop. For my cyclone, I've got the filters mounted vertically above head height.

Bob S

[Steve Knight]

this is about what I have I have a 8" lid on the bottom. I hit the filters with my air house and dump the lid. I get one full lid and a couple of lesser. not much escapes anymore but a little does. the way to stop it would be to blow then wait a few minutes and dump it. the best way is to put a hose or pipe on it run it outside turn on your system and blow out the cartridges. one fellow has done this. but I don't have access to the outside.

[Steve Knight]

they work so well two of the right ones will not restrict air flow at all. I can take the endcap off of mine and little air comes out the end.

I don't think mine has them. just the extended air inlet and the pipe going down the middle. the newer design. but remember I make far more fine dust then most people do. so for most they would go far longer between cleaning. mine does a fantastic job of separation. but when working tropicals like I do and like padouk I really make a lot of very fine dust.

(G) it does not drop much but even a little bit will let some of the fine dust escape that I don't want out. it does not really need cleaned yet.

right now I am using the one from my jet 2hp DC 12" I want to upgrade it to a

14" My piping got a bit weird when I changed from the jet to the cyclone. Bill Pentz made mine and sent it to me. I know it need cleaning when I start smelling fine dust from cuts. I usually would get a 55 gallon can of fine dust and planer shavings between cleaning. no bulky shavings for me (G) all fine stuff.

[Jim Wilson]

No, not joking, just ignorant. Believe it or not, I haven't seen one configured like that until your link. I feel a little sheepish, but also encouraged -- at least the idea is confirmed. Thanks for the reference.

Cheers,

Jim

[Bob Summers]

That sounds like a neutral vane. That's where the inside of the air inlet extends to the vortex finder (that's the pipe in the middle). A neutral vane has about the same impact on the pressure drop and the separation efficiency as an air ramp but it's a lot easier to fabricate. I didn't know that Bill had changed his design. The last time I looked, his design used an air ramp (a spiral ramp from the top of the inlet that goes around once and meets the bottom of the inlet from the side).

It sounds like you might be right on the edge of not having enough air flow.

Those planes you make must be really sharp to make shavings that fine. :-)

Just smell it. That's how you know it needs cleaning. I like those simple solutions. My dust capture isn't good enough for that; I really need an overarm guard.

Bob S

[Steve Knight]

Ok I was not sure what it is called. you have it right. it is a pretty effient design. I get very good airflow.

I have great flow using the blower I have. my pipe is a bit weird as I had it centered in the middle of my tools but then it got moved back into the corner and the pipe is a bit weird now. it just costs too much to change it. it will be cheeper and easier to put a 14" blower on the cyclone. but I bet I have better airflow then most. it's just that I make finer dust then most.

the real challenge is to get the dust caught in the first place. most of my tools needed mods. my delta 14" bandsaw has a 4" hole in the door. my jet 6x48" and disc sander had two 4" hoses on it on on each tool. I had to seal the original ports as they were worthless. my yet 6x89 edge sander has a 6" hose and a 4" hose on the end. my tablesaw has a 4" hose on bottom and a modified overarm guard with a 4" hose. everything has 6" as close to the tool as possible.