Cyclone Modifications

Almost all small shop cyclones started off and continue to be efficient “chip collectors” meaning they separate off the everyday sawdust and chips that you would otherwise sweep up with a broom and just blow the airborne away outside. Airborne dust particles are typically under 30-microns in size, about one third the thickness of a human hair. Medical studies show the more dust we take in the more chemicals that we are going to get from that dust. The chemicals in wood and associated with wood can be poisonous, strong irritants, sensitizers meaning cause us to build worsening allergic reactions, and carcinogenic meaning increase our risk of cancer. In addition to these dangers the under 10-micron sized finest airborne dust particles which are invisible without magnification go right past our natural defenses to lodge deep in our sinuses and lungs to cause long term serious problems. Every exposure to this finest dust causes some measurable loss in respiratory function and some of this loss becomes permanent. The fine sharp particles in wood dust create fibrosis similar to asbestos and fiberglass. Our bodies have a difficult time clearing these particles. Over time almost everyone eventually develops health problems, some serious. OSHA regulations went into effect in 1989 to protect woodworkers in large commercial facilities, but current medical testing shows almost all exposed to dust levels below the OSHA limits will develop health problems that will force about one in fourteen into an early retirement. This should be a serious wakeup call to hobbyist and small shop woodworkers because OSHA testing of our shops shows we consistently have much two to five times higher than OSHA allowed maximum airborne fine dust levels. The reason is of course that we use tools that spray the fine dust all over, dust collection equipment with blowers too small to move the air needed to collect the dust, and filters that pass the finest dust right through. But what takes these issues from bad to dangerous, is our venting our dust collection equipment inside where we trap this fine invisible dust that lasts months to years allowing our shop airflows to keep recycling this previously made finest dust airborne over and over.

The evolution to fine filters and cyclones continues to go poorly and as of 2008 most commercial cyclones come with filters that freely pass the unhealthiest invisible 10-micron and smaller dust particles. The issue is simple. Our small shop vendors that most of us buy our tools from are mostly ignorant of what is required for good fine dust collection and they are not subject to any oversight except what we exercise with our buying choices. Most simply sell outdoor filter material in their dust collector and cyclone bag and cartridge filters. Outdoor filter material is rated after it has built up a cake of dust in the filter pores that does not come out with normal machine shaking type cleaning. When new these filters pass ten to twenty times larger particles than these filters will pass after they become fully seasoned. Unfortunately, to become fully seasoned can take as many as ten to twelve cleaning cycles. With cyclone vendors advertising three to six months between filter cleanings, we end up using our lungs to do the fine dust filtering. Worse, any too careful cleaning on our parts so reduces the filtering level that we end up having restart the seasoning process. The solution is to buy filters that are rated for indoor use when clean and new as required by ASHRAE who sets indoor filter rating specifications.

Many fine dust bag and cartridge filters move more air because of the finer filtering strands made the filters more open, but these filters freely pass much of the finest dust right into our shops. When used on our dust collectors these filters got quickly filled with larger holes from being hit with sharp chips. Worse, as these filters season with a cake of dust, the pressure in them builds up enough to force the fine particles right through. Cleaning our filters also tears open the pores but even faster. Whether we clean too much or let our filters get too dirty, we soon end up with filters that that behave like sieves pushing the finest unhealthiest dust right through. The only solution is to provide enough filter area, and for fine filter material that gets so costly that vendors choose not to do so. Plus, by selling us undersized filters they end up with people constantly needing to replace filters.

Recognizing we had to protect these fine filters, many turned to cyclones. Instead of doing our homework, we mostly went with either building our own or buying one of the vendor supplied typical outdoor cyclones downscaled to fit in our shops. In theory these would be great additions, but most soon found that these outdoor units were designed to put near 100% of the fine dust out the top, meaning instead of the outside air like commercial facilities, right into our filters, so our filters die almost as quickly with current cyclone designs as with dust collectors. Instead of fixing the cyclone designs, our vendors mostly supply more open filters that do not plug so quickly. The few who tried to supply fine filters ended up with dismal airflow as the filters seasoned and rapid filter deterioration from over cleaning and the pressure pushing the fine dust to tear its way through the filters.

Good fine dust collection requires far better blowers, cyclones, and filters than we are going to get with our dust collectors and traditional cyclones. In fact, the magazine rated “best” cyclone I installed with an upgraded top quality fine filter is what landed me in the hospital and left my shop testing at more than double the OSHA allowed airborne dust level. The particle counts in my shop soared over 12,000 times higher than found in commercial shops that comply with government regulations. I found myself in a hospital bed sucking on an oxygen hose. Thoroughly pissed, I decided to find out what went wrong and fix it so I could return to my shop. What I learned was:

1. I had to fix my tool hoods and ports to protect the fine dust so it did not get blown away by my blades, bits, cutters, belts, etc. Fortunately how to do that was shared by professional engineers who build the dust equipment used in large regulated commercial facilities. Since our larger hobbyist tools are identical to their smaller commercial tools, I could use their work.

2. I had to upgrade my blower to ensure moving the 800 CFM these same air engineers established through careful testing as needed to gather the fine dust from my larger tools. I also had to ensure my blower and ducting supported that volume of air with enough airspeed to avoid potentially explosive dust piles in my ducting, meaning design my system to provide 4000 FPM airspeed.

3. I then had to get rid of that dust. Ideally just sending the dusty air outside from my cyclone would be best, but zoning problems, neighbors, yard plants, heating and cooling issues, and sucking deadly carbon monoxide from my water heater and HVAC furnace required me to instead filter the air. I found the medical folks wanted me to use filters that are certified by an ASHRAE approved independent testing facility as providing at least 99.9% at 0.5-micons. I also needed to provide at least one square foot of filter area for every two CFM of air movement to avoid the cleaning problems and plugging problem that forces the fine dust right through and quickly ruins our fine filters.

Sadly since I first shared this information back in 1999 little has changed as of mid 2008. We still have these same vendors selling the same basic outdoor cyclone design for use indoors with finer filters and a few of the changes from these web pages like the neutral vane that improves overall airflow. These small shop cyclone vendors still sell what is a great false sense of security. Their cyclones fail to move enough air to pull in the finest unhealthiest invisible dust and they provide filters that pull off the visible dust and freely pass this unhealthiest invisible dust. This leaves us with clean looking shops that have dangerously high dust levels. I bought an expensive air quality meter and tested the air quality in twenty nine shops during late 2005 and early 2006. That testing tested every widely advertised small shop cyclone and most of the 1.5 hp and larger dust collectors. The only shops that passed with even OSHA air quality had:

1. Upgraded their ducting to provide 6” directly to each larger stationary small shop tool. On the two port tools these 6" down drops split into either a 5" and a 4" or a pair of 4" ducts.

2. Upgraded their tool port and hood connections, plus any internal machine ducting to match their ducting sizes.

3. Upgraded their tool hoods to block the fast moving airflows, control the fine dust, and deliver that fine dust for collection.

4. Used real 3 hp or larger dust collectors or 3.5 hp or larger cyclones to move enough air to capture the fine dust as it is made.

5. Vented outside except for a few of the cyclones of my design that vented inside into very fine filters. All other cyclone designs when vented inside failed with a combination of two problems. First, they separated so poorly that they quickly clogged the filters killing the airflow needed for good fine dust collection. Or much worse, the vendor supplied filters advertised as 1-micron and better freely passed in many cases particles sized up to 20-microns. Our small shop vendors inored the ASHRAE requirements to rate their fitlers for indoor use when clean and new and instead chose to advertize fitler ratings based on very dirty fitlers which will not permit moving enough air for good fine dust collection.

Now with that introduction, here are a few things that you can do to make your existing cyclone more efficient. Those who have made just the simple "neutral vane" inlet generally see about 1" improvement in static pressure. That makes a huge difference in total airflow. Those who make all of these modifications end up with cyclones that only have about 3.5" of static pressure. This can save nearly a full horsepower on the cyclone motor. I suggest you start by making sure you have the right size ducting and hoses then do the easy cyclone modifications:

1. So, I have a 24g wood cyclone already made up with a 6" inlet & outlet and the neutral vane. I powered it with my Jet 1.5 hp DC-1100 with an 11” impeller. I would like to know what you would advise at this point for the least possible expense?

   1. Try it as is?
      This setup works well for chip collection, but you need at least a 3.5 hp motor and 14" diameter impeller to make it move enough airflow to meet the OSHA air quality standards. To meet EPA and medical air quality recommendations you really should use a 5 hp motor turning a 15" diameter impeller. Your original design called for using a 1 to 1.5 hp motor running an 11" impeller, yet that design needs roughly 3/4 hp to just power the cyclone and an 11" impeller is too small to even use the available hp. A better solution is to shop for a 5 hp pump motor, rebuild it yourself and then buy a 15” impeller. It took me nearly two years of looking before I found a 15" impeller on eBay because there are too many of us all looking for the same thing. I instead recommend buying a new impeller from Clear Vue Cyclones and then scrounging for your motor if money is really an issue. Before leaving this, I do have to point out that my oxygen generator, nebulizer, inhalants, and daily medications cost a whole lot more every month than the cost of a good cyclone with top quality blower and filters. Remember to put your priorities in the right place.

   2. Have a new cone made up that's the length you recommended?
      This would help very little and is not worth doing. The Wood Magazine design cone that is used by so many other small shop vendors is a weird length arbitrarily chosen by the makers of the cyclone that was copied to permit fitting these just barely under an 8’ ceiling. Like the PSI, this design makes the air "turn the corner" a little early causing the larger planner chips to hang suspended and clog the cone. Just keep an eye on it when planning as that seems to be the only time there is a problem.

   3. Have a new tall 3X cone, a 9" outlet made and a 9" round inlet?
      In theory the 3X cone really helps with the finest dust separation, but in actually testing this length on a traditional cyclone design is not nearly as efficient as a 1.64 times the diameter length on my design. You will get some improvement with that 3X cone length, but by the time you make this and your other changes it would have been far easier to simply build my design.
      
      You will also need to change the inlet, outlet, and upper cylinder meaning basically rebuild a whole new cyclone. A 9” round inlet on our small shop cyclones is a joke and sadly how vendors get such huge maximum airflow numbers which are meaningless in real use for either airflow efficiency or dust separation but somehow give top ratings in the magazine tests. The optimum dimensions for an 18” cyclone would be a 5’ long rectangular 9”x4.5” inlet. Using a 9" cyclone outlet is important, but it also needs to be the right length. The large diameter of this outlet minimizes exiting airspeed to reduce turbulence and keep from pulling the fine dust off the cyclone walls. The length of this outlet likewise has to be just right to avoid the air turning the corner instead of spiraling down the cyclone and high enough that it does not suck the fine particles off the cyclone sides. The closer the cyclone outlet to 1/2 the size of the outer diameter the slower the airflow on exit so the better the particle separation. As long as you put in a "neutral vane" the 6" inlet is fine as is.

   4. Should I just scrap it and start over (I paid a sheet metal guy to do this so it will cost me another $120)?
      If you are going to keep the small motor and impeller I’d add the neutral vane and quit. If you are going to remake all the things you suggested just build my design and build a bigger blower.

2. I am building a push through cyclone. My plans for the blower are to use my current 2 hp DC blower in a push through configuration to see if it works OK. Before using it I will have a local machine shop weld 1” tabs onto each blade and balance the result. That will give me a 14” impeller that will just barely fit in my existing blower housing. Is this a good way to go? If not, why? What would you recommend? How much will it cost me to weld on the tabs on my impeller and get it balanced?
   I always recommend against either making your own impeller or modifying a current dust collector. With tons of force involved any mistake can create a deadly impeller explosion. The push through design increases your risk by not protecting the impeller from material hits. Worse, the results almost always sound like sirens because the blades get too close to the blower shroud generating lots of extra noise. In fact, speed up the impeller and you now know how to make an air raid siren. I always recommend selling your DC complete unless you have a 3 hp unit with 14” impeller or larger. Take that money and buy a good impeller and motor then make or buy the right sized blower housing for your impeller. If you go this way you should use a heavy steel material movement impeller as found on most dust collectors. Since you have to cube the horsepower to double the CFM, anything you can do to make things more efficient is usually a good idea. Regardless, I paid $60 for welding and $80 to have an impeller modified and dynamic balanced. I got away with this because the blower housing I had was actually made to handle either the 11” impeller supplied or the 12” impeller offered on the next larger model. My maker used one size blower for a whole bunch of impeller sizes. In my fairly large metropolitan area there is one and only one shop with the tools to do this custom balancing. They charged more than the cost of a new impeller. If you can find someone in your area with these tools, they get to name their own price. Alternatively, just about any 1900 CFM or larger dust collector motor/blower with 14” diameter impeller should work fine for you.

3. You mentioned the rectangular inlet would improve airflow about 30% on the WOOD plan. How would that compare to the WOOD plan with the neutral vane??
   I made that comment before the "neutral vane" became a popular fix. The "neutral vane" fix improves cyclone efficiency between 30% to 40% and so does the use of a rectangular inlet that goes in and creates its own similar “neutral vane” effect. Unless you rebuild the whole rest of the cyclone put in the easy to add neutral vane and call it good enough.

4. Your suggestions about the 12”x4" airfoil blower are interesting but I know nothing about it. Do you have some links where I could find such an animal?? Do you still need the 2-3HP to run this fan?
   I gave up on using airfoil impellers and caged impellers to power a cyclone. These units are not self cleaning like typical material handling dust collector impellers. As a result they can build up strings and shavings that can throw them badly out of balance and quickly ruin our motor bearings. Additionally, even with my special modifications an airfoil impeller tends to stall at about 7” of resistance. Stalling also causes the impeller to vibrate bad enough to soon ruin motor bearings. If you have more than a small one car garage sized shop or you are not willing to regularly check and clean your impeller, then it is not a good idea to use an airfoil or caged impeller. All the major fan/blower makers sell the airfoil impellers for $250-$450 each. Check Cincinnati Fan, New York Blower, Continental Fan, and American Fan web sites. I think that is too much to pay! I see these once in a rare while on the Internet surplus shops selling for about $30 each. Likewise, on EBAY many sell an EBM 1265 CFM HVAC blower that has a plastic airfoil impeller that can be modified to work. Also there is a 1600 CFM plug fan that uses a caged impeller that can work. Remember all of these must go on the "clean air" side of the cyclone. Modification is not easy. And yes, you still should use a 2-3 hp 3450-RPM blower motor.

5. You mention the outlet should extend only 2.25" below the inlet. Does this apply for the STOCK Wood cyclone (with a neutral vane added)?
   Actually, my spreadsheet wants the outlet to extend 1/8 of the outer diameter below the inlet, including neutral vane. The spreadsheet will automatically compute the correct length for each different sized cyclone. Anything less results in the air turning the corner and not separating well. Unfortunately, this is only true of my design. Those with straight inlets need to use different length outlet tubes that both keep the air from “turning the corner” and keep that outlet tube from sucking the fine dust off the cyclone walls.

6. If the cone is too short for the cylinder what about shortening the cylinder? Effects on performance????
   Don't bother, you mess with the cylinder dimensions and you could mess up the separation badly.

7. Many engineered DC's I see about the countryside seem to have very short cylinders and very long (by comparison) cones... and now that you mention it, rectangular inlets also. Why doesn't yours?(
   There are about seven major styles of cyclone design. Each works to varying degrees and for different weights of products. The design on my spreadsheet is optimized for separating fine wood dust with a relatively small motor and blower. The long thin cyclones you see need far bigger motors, do a great job on chip collection separation, and blow the fine dust away outside.

8. I am taking your advice and upgrading to a 9" outlet. That will make the cyclone outlet 9" going into a blower inlet that is only 6". Is it ok to reduce the cyclone outlet right at the top of the cyclone so the blower can be top mounted?
   Yes, the key here is to make sure that it is big enough to not create a fast airflow out and long enough. Otherwise it will cause the incoming air to "turn the corner" and greatly reduce separation efficiency. You can make for less turbulence if you make a taper. I made one from MDF to make a smoother transition. Most of the time, I just take off the face plate of the blower and the 9” cyclone outlet ends up defining a 9” blower inlet.

9. I got sucked in to buying a 1.5hp DC that was "rated" at 1200cfm (just like its 2hp brother). The dealer told me the fan was the same on both and at the time I didn't know the difference. Help?
   I would not be surprised to find the fan was identical. One very popular supplier of cyclones and blowers for hobbyists sells exactly the same blower impeller with 1.5, 2 and 3 hp motors. Sadly the 1.5 hp blower is air starved, meaning it does not get enough air to even use 1 hp of power. Putting two bigger motors on that same blower that run at exactly the same speed shows they do not have a clue. There is zero additional airflow as they end up just as air starved. I returned the one I bought like this after a long nasty fight with the vendor. By the time I paid shipping charges to return the two larger units they sent as replacements, I could have bought the best commercial blower and motor made. P.S. This vendor has since hired me as a consultant and I worked with them to increase their blower and impeller sizes so they are now well balanced for their dust collector and cyclone products. Sadly, I was not able to convince them to go with large enough units to provide good fine dust collection.

10. Do you think if I picked up that second 10" fan and housing cheap that putting them in series would be as good or better than a 2 or 3hp 12" or 14" fan??
    That could be brilliant or a really bad idea! The more airflow the harder a blower works. If you cut off the airflow the blower loafs along using the least amount of power. If you supercharge the amount of air going in with a series of blowers all the blowers get so much air their motors can quickly burn out. Using two in series can be made to work if the motors can handle the extra load. One of my friends powers his two cyclones with a pair of 12” diameter dust collection blowers but he had to upgrade to more powerful motors. The biggest advantage of this is he now gets well over 1200 CFM through his 6” pipe, but he has to pay for far more power than running a single 5 hp blower with 15” impeller.

11. I built a Wood Magazine design cyclone out of 30 gauge metal as they recommended. I just finished using your resistance calculator and came up with my roughly 3-car garage sized shop and ducting calculating out to 10" of water column inches of resistance not counting the 4.75” for the cyclone. First, is this reasonable, and second when I use the resulting nearly 15" with the blower table I need a 5 hp motor turning a 16" diameter impeller. Is that reasonable, and if it is will it still work with my cyclone?
    The 15" is very reasonable and pretty much normal for most of us with 3-car garage sized shops and one of the early cyclones. It is also typical of those with 2-car garage sized shops with lots of ducting and quite a few Ts and Ls with tight bends. And yes, the 5 hp motor and 16” impeller are both needed to overcome the resistance of your shop. You might be able to get by with a 3 hp and 14" impeller if you really clean up your ducting run and go with all the modifications to your cyclone recommended here, but I think you will be far happier with a 15” impeller and 5 hp motor. And yes, your cyclone will work with a 5 hp motor and 16” impeller that large provided you never make a mistake and turn it on with all the blast gates open or have any short runs with huge hoods. You need to use an amp meter to make sure you don’t have a problem. Problems are easily addressed by putting a blast gate that you partially close until your most open air situation pulls less amps than your motor rating. Now in terms of your cyclone, can it handle this much pressure, only a maybe. One of my friend's kids closed the last open duct on the same design cyclone that did not have the recommended wooden rings to keep all round. His cyclone ended up about 3" thick and his 30-gauge HVAC lightweight straight long ducting runs all crimped. Had he followed my advice and gone with all 6" ducting and 6" hoses, with nice straight smooth runs he could have gotten by with a smaller impeller and less suction, but his cyclone would still have been at risk.

The drawings, procedures and words shared on these pages are for information only. Your actions are your responsibility - VERIFY and CHECK information out before proceeding, and don't attempt anything without the required skills. Although I've taken every care to ensure what I have done and presented is safe, dust collection equipment uses electrical components and blowers that when improperly built, used, or maintained may cause serious injury or even death, so USE THIS INFORMATION AT YOUR OWN RISK! At the same time, unless you as a woodworker provide appropriate protections for the fine wood dust you make, you put your health, the health of those close to you, and even the health of your pets at risk. Long term exposure to fine wood dust eventually harms most woodworkers. Please take the time to protect yourself and those close to you. HIRE A PROFESSIONAL ENGINEER to design, specify, test, and certify performance of any dust collection system if you have a commercial or an industrial application, allergies, other medical problems, people working for you, a large shop, work with hazardous materials, or are subject to regulatory oversight. Neither I (Bill Pentz) nor any other references or links on these pages will accept any liability for any damages or injury caused to people or property from the using of this information or from any associated links. No claims are expressed or implied as to the safety, usefulness, or accuracy of this information.