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.
"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.
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.
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.
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.
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.
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)
