Cyclone and Dust Collection Research

Welcome to the updated Cyclone and Dust Collection Research web pages.
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Cyclone Modifications

  1. Introduction

    Bill I've had my cyclone for a few years and at first it seemed to work well, but now it does not suck well enough to do a good job of "chip collection" and I find myself having to constantly clean its fine dust filter. What is going on and what can I do to fix my cyclone?

    Sadly I get similar questions all of the time. Just about the time that the "newness" of our cyclone wears off, suddenly instead of working well we find ourselves spending a lot of time cleaning filters and airflow dropping to so poor we no longer get good collection of the heavier sawdust and chips. Those with particle counters find fine dust collection gets worse and worse. The problem almost always comes down to bad air flow caused by the same few issues. Either you messed up picking a poorly made or sized cyclone, have challenging ducting and hoods, or your cyclone has far too small of a fine filter for the air volume that the cyclone moves. Blower technology is mature meaning the same type, size and speed blower from any of the reputable blower makers will move almost exactly the same air. Small shop blowers are mostly poorly engineered so rarely move as much air as good commercial blowers. This poor blower performance combines with most small shop vendors providing too small of a filter and frequently creates our delayed problems. A clean new filter will move far more air than a clean but well used filter. As filters age they get filled with dust that does not come out with normal cleaning. This is called seasoning. It takes over a year for most small shop filters to fully season, meaning they carry as much dust stuck in their filters as they will carry through a normal cleaning session. A fully seasoned filter gets so blocked it will block up to 80% of our airflow if not properly sized. Compound this reduced airflow with any mistakes we make in not properly upgrading out tool hoods and ports or bad ducting mistakes plus using not that good of a blower and most small shop cyclones and dust collectors end up a year or so later working far worse than when new. Hoods and ducting are far more important than most realize and are well covered on my ducting page. What is not well covered are the cyclone and filter sizing issues, and what you can do to improve your existing cyclone that becomes a marginal performer. This page addresses the what you can do to make your cyclone more efficient and how to address filter sizing issues to get back the airflow.

    The bottom line still ends up the same, you can make things better, but if you don't have the right sized blower and cyclone you will still have fine dust collection issues. We really need at least a 3 hp dust collector or a 5 hp cyclone vented outside to be assured of moving enough air for good fine dust collection. If we are going to filter, then we really need to upgrade to ensure our unit has enough filter. The major filter makers all recommend for air pre-cleaned by a cyclone we need at least one square feet of blended filter area for every two CFM or one square feet of all polyester filter area for every four CFM of airflow. Otherwise you will have poor collection, constantly have to clean filters, and way too often need to replace your fine filters. The following shares the detail why.

  2. Cyclone Sizing

    Properly sizing a cyclone requires us to know how much air we need to move and to also know both the minimum and maximum resistance our cyclone must work against. How much air we need to move is simply 350 CFM at most larger stationary tools if we want good "chip collection" which picks up the same stuff we would otherwise sweep up with a broom. To collect the fine dust we need 1000 CFM at the same machines that only need 350 CFM for good chip collection. To keep our vertical ducting runs from plugging we must have about 4000 FPM airspeed. The air formula FPM=CFM/Area to let us calculate that we need at least 4" diameter duct for good chip collection and at least 7" duct for good fine dust collection. The diameter of that main duct times a fixed number, normally three then gives the diameter of our cyclone. There are ratios that we multiply the cyclone diameter by to compute all other cyclone dimensions. Good chip collection requires at least a 12" diameter cyclone and good fine dust requires at least a 21" diameter cyclone. Standard ratios and dust bin end up making a 21" diameter cyclone too tall to fit under the normal 8' tall shop ceiling, plus 7" duct is difficult to find and expensive to buy. If we use an oversized blower impeller and motor we can generate more pressure and force more air through a smaller duct. My cyclone design uses an oversized impeller to let us use 6" diameter duct which lets us use an 18" diameter cyclone.

    That still leaves us to size our blower impeller, motor and housing. To size these things we need to know how much air we want to move and both the minimum and maximum resistance levels in our system known as static pressure. Resistance is measured in water column inches (W.C.) measured by how high a blower must push or pull a colored column of water in a looped tube to overcome the resistance. To estimate static pressure we can either add up the individual resistance measures of every part in each ducting run, or use a static calculator to make an estimate. The maximum resistance occurs with the longest, smallest, roughest ducting run with the dirtiest filters. Minimum resistance occurs with the shortest, most open, smoothest ducting run with cleanest filters.

    Blower technology is mature meaning the same type, size and speed blower from any of the reputable blower makers will move almost exactly the same air. This consistency lets us check a fan curve from any of the major name blower makers to size our blower. Sadly, most small shop blowers are not nearly as well made so the best we can hope for is what is normal for commercial blowers. We start by sizing our impeller. Go to a good fan table for material handling blowers, locate the column that has the same maximum static pressure as estimated for our system then pick the first impeller size that will move as much air as we want, generally 350 CFM for chip collection or 1000 CFM for fine dust collection. The lower the resistance the more air a blower moves and the higher the horse power needed. To size the motor we stay on the same line where we found our impeller and check to see how many horse power will be drawn at our lowest resistance level. Blower motors move the most air and do the most work when they have the least resistance. We pick from our table the blower housing size or our selected impeller that uses the same or next larger size opening as our main duct.

    All of this information leads to some pretty bad news for most small shop dust collector and cyclone owners. We need at least a 1,5 hp dust collector or 2 hp cyclone for good chip collection in a typical one-car garage sized small shop, but it takes at least a 3 hp DC or cyclone to give good fine dust collection in this same sized shop. Most small shops are typical two-car garage sized and typical shops need at least a 2 hp DC or 3 hp cyclone to get good chip collection and a full 3 hp DC or 5 hp cyclone to get good fine dust collection. A typical three-car garage sized larger shop needs even bigger equipment. If your cyclone and blower are smaller, then know that you are not going to move enough air to get good fine dust collection. For every twenty pounds of sawdust we make, we are also making enough fine dust to cause 15,141 two-car garage sized shops to fail an EPA air quality test, so every 1% missed in collection dumps enough fine dust to cause 151 two-car garage typical sized shops to fail their EPA air quality tests.

  3. Filter Sizing

    Many small shop vendors would like us to think sizing our filter is rocket science because most have serious problems with using fine filters. The commercial dust collectors and cyclones that our small shop vendors copy were never engineered to use fine filters. This equipment was engineered to use open filter or cyclones that separate off the heavier material then blow the airborne dust away outside where it quickly dissipates with no visible trace. If we use these units with fine filters we end up putting one pound of filter clogging airborne dust into our filters with every twenty pounds of dust we create. One pound of airborne dust will clog a 100 square foot fine filter to point it will barely pass air which is why we often must clean our fine filters. Unfortunately, cleaning fine filters rapidly ruins them. This is why most commercial shops that use fine filters must replace their expensive fine filters quarterly. Small shop vendors do not want their customers upset with having to constantly clean or replace filters, so most cheat and sell more open filters that they claim are fine filters. The more open the filter the less airborne dust gets trapped, the longer between cleanings, longer the filter will last, smaller the filter can be, and less the filter will cost. The fine invisible dust these filters pass damages our health so slowly that unless you are a serious athlete you would not detect any changes as they occur.

    This nonsense went on for nearly two decades until individuals started getting access to affordable accurate fine particle counters like the 0.5-micron Dylos Pro counter. Strangely the particle counters made matters worse instead of better. Rather than get embarrassed by being shown to sell too open filters, many cyclone and dust collector vendors started selling much finer filters that are too small. These work well initially but as they season they eventually seriously block the airflow we need to actually collect the finest unhealthiest invisible dust. If we do not collect the fine dust, then even the finest filter in the world will not protect us. The result is my inbox continues to be filled with emails from people upset because their systems start off working great, but over the course of about one year the airflow drops so low they no longer even get good chip collection and the meters consistently show poor worsening fine dust collection.

    The filter material makers provide charts and tables that tell us how much filter are we need for a given air volume. Two different types of fine filters are used with woodworking, blended polyester cellulose filters and the roughly twice as expensive all polyester filters. The minimums are one square foot of all polyester filter material for every eight CFM of airflow and one square foot of blended filter for every four CFM of airflow for the blended filters. However, to minimize plugging and maximize wear all the major filter makers recommend twice as much filter area. In short we should use one square foot of filter area for every four CFM for all polyester filters and one square foot of blended filter area for each two CFM of airflow.

  4. Filter Fixes:

    Wynn Environmental Filter Pan Now with that introduction, the easiest way to determine if the worsening performance is filter and airflow related is to simply increase the size of your filter. Until recently this required you to spend the money to buy new filters. In many cases our particle meters tell us we don't need to replace filters, so paying for an expensive upgrade to get more surface area is foolish because there is a better option. Today a much better option is to increase your filter surface area by adding more filter area. Wynn Environmental now offers their filter pans that will add 30 more square feet of very high quality fine filter area and a nice pan to capture the fine dust when you clean your filters. Adding one of these to the standard 95 to 120 square feet filter sizes that too many vendors sell will drop filter overhead by a huge amount and greatly increase your airflow. This roughly one third increase in total filter area will immediately let you know if the issue is poorly sized filters. Moreover, when it comes time to upgrade filters you can simply release the clips and move this filter pan to your new filter.

    Using a particle meter will tell you quickly if you need to replace your filters. Many woodworking clubs and individuals have purchased their own meters and are kind enough to either do the testing for you or let you borrow their meter. The best way to test is to start with a clean shop and filter. Open all up and while wearing your respirator mask use a leaf blower or large air compressor to blow all out thoroughly while a strong fan blows out a side or back door. Make sure your thoroughly blow down the outside of your filter as well. Our filters are made of all polyester plastic resin or polyester resin mixed with cellulose fiber. In either case the air going through the filter builds a large static charge that will collect airborne dust on the outside of the filter. When you next turn on your cyclone these dust coated filters fill our shops with that static collected fine dust, so we do this clean out and external filter cleaning to make sure it is not there. Then let your fan continue to run with the main door cracked for at least a half hour after all is blown out. It is a good idea to let the air in your shop settle for a whole day. Then do a meter reading of the outside air, the air in your shop before it gets stirred up, and another reading after you turn on your cyclone for five minutes. If the meter reads nearly the same then all is well. IF the meter starts climbing after you turn on the cyclone and just keeps getting worse then your filter is shot because the fine dust is working right through it back into your shop air. If all is behaving, then you need to make some serious dust. I cut 54 linear feet of 3/4" particle board on my table saw. If you have really good dust collection and your shop air remains clean then no filter problems. Most will have their meters all but peg due to the poor collection of their cyclones. If the meter pegs reads real high then you need to leave the cyclone running and see if your shop air quickly recovers. This is a dilution problem that is at best ugly. Every time our system removes a full shop of air we only get about half of the airborne dust. Every 5.66 ounces of fine dust generates enough dust to cause 15,141 typical two-car garage sized shops to fail an EPA air quality test. It can take a lot of full air changes each reducing by half to pull the air under EPA limits. If your filters are bad, then the particle counts will at first fall rapidly then kind of stick at an abnormally high level.

    The other method used in industry to know when a filter needs replaced is to use a pressure gauge and measure the static pressure of your system with the same longest run open after every filter cleaning. What will happen is the static pressure will build as the filter seasons then hold fairly steady for a while. As soon as the pressure drops two water column inches below the fully seasoned pressure level it is time to toss and replace your filters.

    If you have the all polyester filters, then instead of replacing your filter when you see that two inch pressure drop, what you do instead is thoroughly wash your filter in accord with the filter maker instructions. Washing redistributes the filter pores so the filter works almost as good as it did originally. After three to five washings the polyester filters are also shot and need replaced. Most in private industry find that with the all poly filters costing double the blended filters that by the time they factor in the down time and labor to do the washing, that it is easier and less costly to use the blended filters. For very cost conscious small shop operators who don't count their time, then the all poly is a less expensive alternative.

  5. Cyclone Fixes:

    Now with that introduction, here are a few things that you can do to make your existing cyclone work better. I suggest you start by making sure you have a big enough blower on your cyclone. Then ensure you have the right size ducting, hoses, tool hoods and tool ports. Then add a "neutral vane" unless your cyclone already has one. Then install the right size and type of filter for your cyclone or vent it outside. All of the other repairs will take a lot of time and money, but will not make all that much difference. 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.

    1. Install a "neutral vane" into your cyclone. As near as I can tell the term "neutral vane" took a common HVAC air director term and misused it to make up for the basic design flaw in many early small shop cyclones. These cyclones stop the inlet right at the surface of the outer cylinder. The result is the air whips around once then crashes right into the incoming air creating all kinds of turbulence and poor separation. Small shop cyclones are copies of agricultural cyclones designed to severely beat up cotton to separate it from sand and dirt. It is also used to separate grain from chaff. As a result these cyclones send almost all of the fine dust right into the filters. Extending that inlet by adding an extension that goes to the center edge of the cyclone generates about 1/3 less turbulence improving airflow, but does little to reduce the amount of fine dust going into our filters. You can easily make this extension by using a piece of HVAC snap lock pipe from a home center. To find the "exact" optimum performance place, use an amp meter on your motor and move the pipe in and out. The ideal place is when the amps are at a maximum, as that is when the impeller is pushing the most air. That is generally with the pipe ending very close to the center edge of the cyclone (a perpendicular line at the end of your inlet pipe would go right through the center of the cyclone looking from above). When the pipe is in the right position, the amperage will be at a maximum. Just either screw or pop-rivet that pipe and you have the biggest improvement that you can easily make to your existing cyclone.

      Those who add a "neutral vane" generally see about 1" improvement in static pressure. That makes a huge difference in total airflow.

    2. Next replace the cyclone bag or under sized cartridge filters with big enough fine cartridge filters to save your lungs and get rid of the extra back pressure. Regular bags and cartridge filters add about 2" of resistance when clean and new and up to 5" of resistance when fully seasoned. They can block airflow dead when dirty. Properly sized fine cartridge filter adds about 0.25" when clean and new and only about .5 when fully seasoned. Because the bags have so little surface area, poorly sized fine filters quickly plug building up a huge amount of resistance.

      I bought two Donaldson-Torit 0.2-micron through flow cartridge filters, each with only 225 square feet of area. These were very pricey filters, should have been at least 500 instead of just just 450 square feet, and arrived with shipping damage. That firm was just not set up to deal with small shop purchase volumes. I finally found and recommend Wynn Environmental. Dick and Rick Wynn have been very helpful both with information, supplying top quality filters for good prices, and top quality flex hose and hose clamps for excellent prices as well. They also carry the much bigger Farr compatible 300 square foot filters. Although they offer the 0.5-micron filters, I instead strongly recommend their MERV-15 rated "Nano" filters instead. they will leave about twenty times less fine unhealthy invisible dust particles in your shop air. I recommend a pair of these Nano filters to maximize air filtering and minimize the time and trouble to clean filters. Many of those who have written are most pleased with Wynn. Others have used other brands including a few who get used large truck cartridge air filters. In any case, make sure you get the ones open on both ends! I suggest you shop carefully and watch shipping costs closely because these huge units can be prohibitively expensive to ship. Picking up locally is often less expensive.

    3. Make as long and straight of run as you can from your shop ducting to your cyclone. Having a straight last 5' going into your cyclone is most critical to minimize turbulence. This 5' distance is the minimum needed to smooth the airflow and result in far better fine particle separation and more dust pickup at your machines.

    4. Finally, take a serious look at your blower. Air at dust collection pressures is more like water so any restriction, bad rough ducting, etc. will seriously hurt your airflow. Unless you use an oversized impeller you need at least a 7" blower inlet and outlet and at least a 14" diameter blower impeller. If your blower has a 6" diameter inlet or outlet then you need at least a 15" or larger impeller and at least a 5 hp motor, you will end up with a system that is not going to collect the fine dust. Those with 2 and 3 hp motors are the ones who invariably find their systems work well at first then later work poorly. They also find when they get particle meters that their systems do a poor job of good fine dust collection because they just do not move enough air. With 3 hp and smaller motors your most open closest run to your cyclone can push your motor over its maximum amperage. This is why I chose to spend the extra few dollars and go with a real 5 hp Leeson motor. That bigger motor also will support a 15" or even 16" diameter impeller that gives a real 1000 CFM through 6" duct. If you have ample motor and impeller, but blower with 6" inlet and 6" outlet, upgrading your ducts to all 7" will help considerably.

  6. More Serious Work:

    And if you really want to do it right, toss that cyclone and go build a new one from my web pages. The following things will also help.

    1. The first and most important serious modification is to go with 6" ducting and 6" flex hose right to your machines and change all the machine ports to full 6" inlets. It is a fact that the 4" ducting we used for chip collection will not support much more than about 450 CFM when powered by a typical hobbyist sized blower. That diameter pipe, flex hose, and machine ports are just too small to support the 800 CFM needed for fine dust collection. Without that airflow you are not going to gather the very fine most dangerous dust. At the typical volumes and pressures used in dust collection we need 6" duct to move 800 CFM. Air engineers design for that 800 CFM at each larger machine and they also configure their systems with enough blower to make sure the air speed stays around 4000 feet per minute (FPM) which is needed to keep vertical duct from plugging.

    2. At the typical volumes and low pressures used in dust collection air is virtually incompressible. Air will not speed up much at all to get around a short restriction, so any reduction will just plain kill your airflow well below the 800 CFM required at each larger woodworking machine. For the same reason flex hose adds from three to nine times the resistance of smooth walled pipe. Make sure you use a minimum of flex hose and that any you use has smooth interior walls as that only has about three times the resistance of smooth pipe. Anything else will greatly cost airflow.

    3. For those who made early designs with only a 4" or 5" cyclone inlet pipe, you really need to open up the inlet port to 6". When you do this you must use an amperage meter and ensure the motor is not getting so much air it overstresses. Likewise you should at the same time install a bigger outlet pipe in your cyclone that is half the cyclone diameter.

    4. If you have a less than 2 HP blower you need to upgrade to a unit that is 3.5 to 5 hp with a 14" or larger diameter impeller. The build your own blower plans on these pages will get you an excellent blower that you can use to power either a cyclone or dust collector. Something that most don't realize is that you can almost make a 55-gallon drum (no offense to those who used one) into a really good cyclone if you add enough power. The air engineer sizing spread sheets always round up to ensure ample capacity. They show that our typical hobbyist woodworking cyclones should use a 5 horsepower motor turning a 15" impeller.

    5. If you already have a larger blower with only a 4" inlet, you might consider cutting that opening bigger if all else does not work. The cut does not have to be that clean, but if you want perfection you can buy an industrial flared round intake covers known as a flange, pay someone to make you one, or adapt a sheet metal faring from your local hardware store. A less expensive option is to make up a new cover out of 3/4" to 1" MDF or plywood to mate to the 9" or 10" cyclone outlet pipe. Use polyurethane caulk to glue this to the top of the cyclone (caution - silicone caulk will eventually let loose on galvanized metal due to a chemical reaction). Remove the existing blower inlet cover and make up a new cover out of 3/4" plywood that matches the plywood on the cyclone and the new inlet diameter. Screw the two plywood sheets together with a weather seal or caulk between them using a piece of plastic between all so they can be taken apart if needed.

    6. You also can also just upgrade the impeller on your unit, but I would not consider doing so until after opening the ports. Most industrial and home built cyclones are pull through units. This protects their impellers as long as the dust bins don't get full from wood knots, cats, and other debris except very fine dust. Shop vacuums put their filters before their impellers to protect them from material hits. With your current blower shroud you can either buy a bigger impeller or have a welder add more blade area to the tips of your impeller blade. This gives more airflow, but if done poorly could cause your impeller to explode sending pieces all over or burn out your motor. I bought a bigger industrial impeller and carefully checked my motor amperage to make sure it stayed in its working range.

    7. If you open the ports or change the blower, you need to test your unit. Dust collectors use impeller blowers that work the hardest when they push the most air. If allowed wide open air access with a big impeller, most blowers would quickly push so much air that the motor would draw too many amps and either cut out if protected from overheating or burn up, so dust collector makers limit port and impeller sizes. They also use less efficient heavy steel impellers that are self-cleaning and can stand some heavy pounding. If you closed the intake and tested with your amp meter, you would find your unit is loafing! Yep, in spite of the noise these things take it easy when they are not working and should be left on instead of being constantly turned on and off. Surprisingly enough, adding a cyclone and lots of ducting reduces the airflow until the motor is barely working. It astounds me that so many when finding that their units are not "sucking" enough, want to run out and buy a bigger motor. If it is barely using the horsepower available what does adding more horsepower do when the bigger motor turns at the same speed except waste your money and electricity? On the other hand, opening all up wide especially with a bigger impeller can draw enough amps to quickly burn up a motor. You need to carefully check your final system with an amp meter while all blast gates are open fully to ensure you do not draw too many amps.

    8. Change the round inlet to a tangentially mounted rectangular box that is as long as the diameter of the cyclone. This will put the air into the cyclone smoothly and right on the surface to minimize turbulence and give up to a 50% reduction in resistance compared to the original design.

    9. Change the cyclone bottom cone dust outlet to 6" in size. This lets the separation action go down further and makes for better separation. It also cures the problem with these cyclones plugging when doing planning. The disadvantage is making this larger opening ends up with more of the fine dust getting blown into the filters instead of dropped into the dust bin. Frankly, cone sizing is a real art where changing one thing screws up others.

    10. Add a circular 360 degree air dam better known as an air ramp with the inlet appropriately tilted. This helps fine particle separation and provides an additional big saving in overall air resistance. Unfortunately, unless your cyclone diameter and cone are just right adding an air ramp actually decreases separation efficiency.

    11. Make the cone length 3 times the diameter. Work by the Cotton researchers found this traditional cyclone gets better separation with a longer cone. Again this is an art where the cone length if too short will suck the fine dust off the cyclone walls into the filter and if too long causes cone plugging when planning.

  7. Frequently Asked Questions (FAQs)
    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 120.php (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.

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