Cyclone Versus Single-Stage Dust Collectors

      An in-depth comparison of cyclone and single-stage dust-collector technology, with some details about fan blade characteristics and some thoughts on filter cleaning. August 14, 2007

Question
I am building a new shop and am looking for a new dust collector. I see the cyclones have lower CFM ratings than their standard counterparts. Why? Are the cyclones more efficient? For instance, cyclone A has CFM rating of 2000 cfm at 10" static... From the same manufacturer with the same hp rating, the standard type (bags) have 3900 cfm at 8" static. I know that on the standards, you can basically cut the number in half for true performance ratings, but what about the cyclone?

Forum Responses
(Dust Collection Forum)
From contributor B:
The difference is in the resistance losses of the cyclone's pre-separation stage. A single-stage dc is more efficient in terms of lower power consumption per delivered CFM. Place the exact same blower on both DCs, and the single-stage will easily outperform the cyclone.

When you say that the actual flow numbers can be cut in half... well, this may be true for the consumer model dc's that use free-air ratings (no pipe, no filters, etc). These ratings are often expressed as "Max CFM" with no accompanying pressure rating. When you see an airflow rating accompanied by @8.9"sp, that's an actual CFM rating at a very specific resistance.

Whenever you select a DC, you have to assess your shop's air-requirement first. Size your ducts to provide optimum velocity for that volume, yet at workable resistance levels. Once you've determined that you need 1200 CFM, for example, and a 7" pipe, you'll have a velocity of 4492 fpm. Plenty of velocity for vertical runs, and even headroom for suction pressure losses as you filter a load. In a 7" smooth-walled pipe, you need to be flowing 1069 CFM for 4000 fpm in your vertical runs, or 935 CFM for 3500 fpm in your horizontal runs.

Once you've determined CFM, the next step is to look at the static pressure losses along the entire branch. That includes hood-entry losses, pipe-length friction losses, and losses from fittings such as wyes and elbows.

A very rough example of a required suction level for 4000 fpm in a 100 foot long 7" smooth-walled pipe, would be 1069 cfm @ 5.525" WC. You'd be well-advised to consider a higher pressure to account for filter loading losses.

Remember, fittings such as wyes and elbows must be converted to an equivalent length of straight pipe to calculate losses. You may find that the same airflow I just mentioned would only serve a 65 foot pipe, with three 90 degree elbows.

Most if not all of the consumer model cyclone brands use less efficient radial-blade material-handling fan-wheels. DCs such as the Torit-Donaldsons go with back-inclined fan-wheels. This type of fan-wheel offers far more efficiency, more CFM at lower horsepower and power consumption. Back-inclined fan-wheels would be inappropriate in a dirty airstream, and require periodic attention on a cyclone.



From contributor U:
Would you elaborate on this a little more:

"Most if not all of the consumer model cyclone brands use less efficient radial-blade material-handling fan-wheels. DCs such as the Torit-Donaldsons go with back-inclined fan-wheels. This type of fan-wheel offers far more efficiency, more CFM at lower horsepower and power consumption. Back-inclined fan-wheels would be inappropriate in a dirty airstream, and require periodic attention on a cyclone.

I always learn something here.



From contributor B:
On a typical single-stage dc, you'll usually see a 6 blade radial material handling fan-wheel. These fan-wheels are designed to be used directly in a dirty airstream. Cyclones by their very nature are designed to pre-separate most of the waste, and this gives you an opportunity to use a more efficient fan-wheel.

An airfoil fan-wheel would be very efficient; however, even with a slightly dirty airstream, this type of fan-wheel will require frequent cleaning of the foils, and are therefore better suited for use in ventilation blowers. A caged back-inclined fan-wheel offers close to the same efficiency as the air-foil, but is less likely to load with dust. This type of fanwheel has flat blades. The airfoil, when viewed in cross section, resembles an airplane's wing.



From contributor N:
Wondering why a cyclone cartridge couldn't be cleaned by connecting an inlet hose of the dust collector to the cleanout port under the cartridges. This would be done with the collector running and looks like the turbulence created by the recirculating air would tend to knock the dust loose from the cartridge. I realize this dust would simply be recirculating, but perhaps some of it would eventually end up in the cyclone collector instead of the cartridge.


From contributor B:
I suppose you could do that, but air returning into the filter from the cyclone would make that a very inefficient process. There'd be virtually no airflow into the filter from the outside/in to dislodge impacted waste.

You're probably better off shutting down the cyclone, and using compressed air. With no air entering the filter from the cyclone, there are no forces to drive the fine dust into the filter. It will simply fall into the clean-out trap.



From contributor N:
Perhaps we could build a mini-cyclone inside the filter also to drop out the dust. Seriously, I was thinking that the dust-laden air being able to pass through the filter cartridge in an axial rather than radial direction would change things drastically. It would be like cleaning a filter cartridge by blowing air down through, realizing that this isn't as effective as blowing compressed air from outside in. (But it would be a much larger volume of air than out of a compressor line.) Just seems like one ought to be able to take advantage of that huge volume of air for filter maintenance. Some commercial dust collectors, as you may know, use "reverse-pulse" filter cleaning, which is a sudden reversal of air flow. Would enjoy hearing any more thoughts.


From contributor B:
In a pulse-jet, or flow-reversal system, air flow would reverse through the filter media. In order for you to duplicate that with your setup, you'd have to dismount the filter from the cyclone outlet, and cap one end of the filter, while drawing from the opposite end. Unless you had a spare filter to replace the one you removed, you'd most likely re-circulate all that fine dust right through the cyclone again, and out the exhaust port.

As far as cleaning the filter goes, you also have to remember that air-speed through the filter fabric is very low. If you try to suck the dust out of the filter with 1000 cfm, and your cartridge filter is 100 sq.ft., the airspeed through the filter is only 10 ft per minute. That's less than 1/8 of one mph.

Also, think of what would be required in regard to dc alterations. You'd need a dual outlet on your cyclone, in order to clean the filter on outlet A. You'd also need a damper to close off airflow to filter A. Most of that removed dust would simply flow through the cyclone once again, only to find itself in filter B.

The reason it went to the filter in the first place is the lightweight density. A lot easier to simply blow it out with a blowgun and compressor. Not too close though, as you can easily damage the filter. Good luck with your new cyclone.



From contributor N:
Thanks for your insight. I can see now that you have a wealth of experience in the area of dust collection. This is an interesting discussion and hope you don't mind me carrying it a bit further.

Going back to my original thought of attaching an intake hose to the cleanout gate at the bottom of the cartridge hopper with the unit in its otherwise normal configuration: I'm guesstimating that the airflow down through the cartridges would be in the area of 50 mph. Looks like this would knock at least some of the dust loose from the filter pleats and carry it back through the cyclone.

How much of the dust in the cartridge hopper being constantly re-circulated through the system would eventually end up in the cyclone hopper? I realize that this fine dust ended up in the cartridges in the first place because of its fine nature. When I say "being constantly re-circulated," I'm thinking in terms of, say, a 10 minute cleaning cycle.



From contributor B:
It's really quite impossible to say what would happen. Just too many variables involved. Waste density, air velocity through cyclone, cone design, waste chute design, uptake tube sizing, etc. You'd have to determine why the dust made it to the filter in the first place, then correct the airflow with your cleaning cycle.

Regarding the air velocity through the cartridge, you will get that 50 mph through the port at the bottom of the cartridge, but at the filter surface, it'll be much lower. CFM divided by filter area in sq.ft.

1000CFM / 100 sq. ft. filter = 10 fpm (roughly 0.09mph)



From contributor N:
The air would not be passing through the filter pleats but rather past them in an axial direction down through the cartridge. I estimated the inside diameter of the cartridge (outside diameter minus twice the pleat thickness) at approximately 8 inches, which is approximately 1/3 of a square foot. Using 1500 CFM equates to approximately 4500 FPM velocity or approximately 50 mph. There would in theory be no air passing through the filter pleats, as it would be a closed loop system during the cleaning cycle.


From contributor B:
Okay, I see what you're saying now. Give 'er a go, I say. You never know. May work out great. I don't think you'll get a deep cleaning of the filter, but you may dislodge some of the waste in the pleats.


From contributor N:
Thanks for letting me think outside the box on this one. It was important for me to get the idea across even if it isn't feasible. This will be my first experience with a cyclone so looking forward to it.

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