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Because improperly designed ducting can cause fires, fail to meet local codes, etc., these drawings, plans, procedures and words are for information only. Ducting consists of all the hoods, tool ports, flex hoses, duct, wyes, blast gates and other piping components needed for good "chip collection" and good fine dust collection.
Before staring we need to first decide on a suitable level of dust collection because our level of protection defines what we need to do. All other levels of dust collection try to meet an air quality standard, but air quality standards are a mess. As a biomedical engineer, long time university engineering instructor, and one who got badly blindsided after I trusted the "best" small shop cyclone system to protect my health, I strongly recommend all small shop workers pay very serious attention to fine dust. I am not too worried about explosion, a little concerned about fire, and a lot more concerned about building up dangerously unhealthy amounts of fine dust in our shops. Starting in the early sixties insurance data and medical studies showed a strong causal relationship between airborne wood dust exposure and health problems with woodworkers. Second, the high silica (glass) content in wood creates sharp jagged fine invisible wood dust particles that cause even more damage to our skin and respiratory tissues. Facilities that fail to meet the air quality standards can be cited, fined and even closed down. Fortunately, the better air engineering firms share most of what they learned on the Internet covering what we need to do to capture the fine dust before it can escape. They did considerable experimenting and testing, followed by decades of refinement to establish the CFM requirements tables similar to the one linked from AAF that show what airflow measured in cubic feet per minute (CFM) we need to collect at each type and size of woodworking tool before it can escape. Air engineers also came up with the formulas and tables to ensure we have the right ducting and blower.
This engineering information shares most of the minimums we need for each type of dust collection.
Because air will compress, we assume we will get nearly the same flow on both sides of a small obstruction because the air will speed up to get around a small obstruction. Air at the low pressures we use in dust collection, air is more like water, and does not compress.
Because air is near incompressible at the low pressures we use in dust collection, we end up with the pipe diameter controlling our air volume.
Ducting Diameter affects the amount of airflow just like the size of the pipe and water valve size affects water flow. For instance a 1.5 hp dust collector that can move a maximum of 1100 CFM moves far less air than that maximum depending upon what sized ducting we use. Ducting layout is not that difficult, but the details must be done right to ensure a good working system. A good working system must move the amounts of air we want for good collection and ensure the air speed in the ducts keeps the dust entrained, meaning kept airborne in both the mains and the down drops. Our smaller blowers also require us to use properly sized duct with long sweeping big radius curves or 90-degree joints broken up with a straight run between two 45-degree joints.
Always use the largest diameter duct that your blower can use with the least number of restrictions. Unlike big industrial sites, most hobbyists should run the same sized ducting, fittings and hose right up to their machines. The next most important aspect of building a good ducting system is which type of ducting material you will use. Although I am a fan of and personally use the higher end steel snap lock type ducting that is very pricey, I started off using HVAC pipe which worked well, then shifted to use of the thinner PVC pipe that is readily available in my area due to all the agricultural irrigation.
Ideally we need the smoothest pipe we can get to minimize resistance and still ensure the level of safety in our shops. There are lots of ducting choices and often we slip into a mode where we think if something costs less it is not as good. It is best to buy your dust collection pipe, fittings and flex hose from either a firm with free shipping or from a local supplier, otherwise you can easily pay nearly as much for shipping as for the cost of the ducting. Making your dust collection connections can be done with glue, screws, special duct sealant, and many other things, but most find that slipping all together works best. Although the Kanaduct shared above in Transitions works well for joining different materials and diameters, this can be a little pricey and requires mail ordering. I found at an electronics outlet some nice screw-in mounts designed to be used in electronic cabinets. The traditional approach to have hanging ducts from a high ceiling simply uses heavy galvanized steel wire or plumbers strapping screwed to the ceiling or looped around a pipe. Another solution for hanging duct is to use stranded metal cable (called wire rope) with special clamps that let you adjust the length. A far more elegant and expensive solution for hanging duct is to use wire rope with special adjustable fittings that allow you to quickly do the installs and set the pipe height. Although most prefer to hang their ducting from the ceiling, there are also lots of ways to put your dust collection pipe under your floor. For those who have a shop that sits on a heavy subfloor, it can be very convenient to run your ducting under the floor.
Because I included both a sink and small shower, I ran water and drains in dug trenches, plus another trench to bring in power.
I also tapered each of my trenches with just a little grade and put drains at the end of each trench. We are in an interesting time right now where the computer world is changing so fast that this leaves an opportunity for woodworkers. There are many nice accessories that you can use with your dust collection, but realize that even with these accessories a dust collection system does some functions poorly.
My first accessory mistake was buying a transition that mated my 4" dust collection hoses to the tiny 1.25" port on my band saw.
My dust collector came with an upper and lower filter bag that worked poorly and required constant emptying because I made lots of dust from preparing rough stock. My next mistake was replacing the dust collector bags with an easy to empty lower plastic bag and fine oversized fine upper filter bag. With Instructables you can share what you make with the world, and tap into an ever-growing community of creative experts. Now I know that anyone can go to freewebs (ahem, webs) and get a free website, but that requires no skill. Making simple websites like those are really easy but the end produce would be just a rather simple website. To learn more about dust collection solutions visit the Dust Collection Section in the WOOD Store.
Although I'm a writer by education and profession, I have a not-so-secret DIY and craft (of almost any kind) addiction. This page shares what the experts who guarantee customer air quality advise us to do to fix our tools, tool hoods, tool ports, ducting, and ducting layout to provide good fine dust collection. All hobbyists and six out of seven professional woodworkers work in small shops that are not government inspected or covered by any standards except what we choose to put in place for ourselves. To get good "chip collection" we need hoods that block and control the fast moving airstreams from our blades, bits and cutters.
That is why I think all dust collection equipment should be placed or at least vented outside.
Most small shops trap the fine dust inside where it builds to such high levels that a typical small shop woodworker gets more fine dust exposure in a few hours of woodworking than a commercial worker receives in months of full time work. Trees produce strong toxic chemicals to protect themselves from predators plus many woods are further contaminated with other toxic chemicals from insecticides, herbicides, preservatives, etc. The air engineering firms who guarantee customer air quality prepared for years to meet the new standards. Medical studies and insurance records already showed most workers in OSHA compliant facilities eventually developed dust related medical problems. They established in tables like the Cincinnati Fan Engineering Data the air speeds measured in feet per minute (FPM) we need to collect various sized chips and transport those chips through ducting without plugging or building up dust piles.
They also found these CFM tables are worthless unless we also modify our older tool designs that spray fine dust all over with better hoods, ports, and internal ducting to protect, control, and deliver the fine dust for collection as it is made. They came up with simple formulas and approaches to help design our ducting to move the needed air with ample volume and speed at each tool.
Roughly 350 CFM proved ample air volume to do good "chip collection at most of our larger small shop stationary tools and dustier operations. Based on that knowledge, I assumed that we could minimize the length of that obstruction by using tapered adapters right at our machines and still use their built in 4" ports. Any obstruction, small pipe, or tight turn will kill our airflow dramatically just like closing a water valve.
To test this for yourself, try a simple experiment I learned in engineering classes long ago. Unlike a shop vacuum that generates roughly ten times the pressure, air from our blowers will not do a good job of squeezing around obstructions or through small openings. This typical small shop dust collector blower only generates 4" to 6" of pressure when working. We constantly trade off our ducting size to move the right air volume at ample speed with minimum resistance.
A leak between the collection barrel and cyclone will quickly clog your filters just like when the barrel gets too full killing any ability to collect the dust. It takes very little airflow to move really fine dust, yet we need far more airflow to capture that same fine dust than we need to pick up the same dust we get with a broom.
Because air is compressible to double the amount of air you move measured in cubic feet per minute (CFM) you must double the fan speed or double the surface area of the fan. Unfortunately, most of the popular woodworking sites, dust collection vendors, and even dust collection books recommend large shop traditional graduated ducting designs that will not work safely with typical small shop dust collectors and cyclone systems.
At dust collection pressures air is more like water and will barely compress at all so any restriction or smaller duct acts like a water valve and kills our flow. Note this system uses a 3+ hp blower needed to support an 8" diameter main with only 6" down drops. If you have a unit where material can directly hit the impeller, then rocks and pieces of metal can hit that impeller, cause small sparks and those sparks fall into your sawdust and slowly grow into a disaster, often many hours after you leave your shop. If your ducting is too small, then it instead of your blower defines the CFM that your system can provide at your machines to pick up the dust.
Even a short run of duct that is too small for a blower will cut the airflow down to the highest speed that pipe can sustain. Don't do like many and run a 6" or 8" main trunk line then come off with smaller duct or flex hose.
His mixed combination of Imperial and Metric sized ports on his tools required lots of different reducers and expanders to work with his 6" mains. Today with the price of oil driving PVC pipe costs crazy, if cost was a major concern I would use HVAC pipe.
To minimize resistance, the interior of the pipe needs to be as smooth as possible and you need long smooth sweeping smooth joints on all your fittings. The smoothest walls that make for the least resistance come with using plastic coated pipe and fittings such as PVC pipe or plastic coated metal or wood duct.
John Koster reminded me that standard plumbing neoprene "no-hub" connectors provide a good less expensive locally available solution for those who need to connect dissimilar pipe materials. Just about all HVAC supply stores and ducting supply firms offer one or more of these types of systems.
If you are fortunate enough to be building your shop from scratch, you can build in some trenches. Living right on the edge of an industrial area, I was fortunate enough to be able to bring in three-phase power. The mistake I made with those drains was sharing them with the sewer drain without a water trap to kill any odors. That let me cover up the trench and not create problems moving around my equipment and tables that were on rollers. There are huge quantities of surplus very well made raised computer floor that provides a raised floor of 8" to 12" to permit running electrical, communications, cables, etc. So little air was moved I went back to using my powerful shop vacuum on all tools with small ports. It moved less air than my dust collector, just about half of what the vendor claimed in the advertising.
When using them I wear my mask, open up the side door and garage doors and then run a big fan in the side door to keep that dust blowing outside. Just because a tool comes with a 4" dust port doesn't mean you should use 4" main ducts or drops. Corrugated tubing creates three times more static-pressure resistance than the same length of smooth pipe. A single duct of 30' or more that wraps around more than two walls of a shop reduces air velocity and increases the risk of dust buildup. HIRE A PROFESSIONAL ENGINEER to design, specify, test, and certify performance of any industrial 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. If you have not already read the earlier pages, I recommend you do so as good ducting is not required for good fine dust protection. Department of Labor, Office of Safety and Health Administration (OSHA) is charged with protecting the health and safety of workers in large commercial facilities.

We need about 350 cubic feet per minute (CFM) air volume with an airspeed of about 4000 feet per minute (FPM) to pick up and transport the dust and lighter chips.
Woodworking shops located in commercial buildings subject to fire marshal inspection must comply with local codes which mostly follow the National Fire Protection Association (NFPA) recommendations.
Every fine dust exposure causes some measurable loss of respiratory function, some of this loss becomes permanent, and continued exposure over time builds debilitating health problems.
These toxic chemicals can cause irritation, cause poisoning, cause neurological damage, increase our risk of certain cancers, and cause us to rapidly build dangerous allergic reactions. They did considerable study, engineering and testing and now have over two decades of experience with most reaching the same basic conclusions with similar solutions. The Australian Ministry of Health says this exposure level eventually leaves all ill with roughly one in fourteen forced into an early medical retirement due to dust related health problems.
Plugging and dust piles pose a potential fire hazard and when these piles break loose over time they will destroy blower impellers, bearings and filters.
Also with permission below shares some of the vendor recommended hood and collection designs for most major tools. They built static calculators like the one linked here that estimate the resistance created by our ducting, flex hose, duct fittings, hoods, filters, cyclones, separators, etc. I then received an email that asked me to further clarify, because in many instances this will not work. This means any obstruction, small port, undersized hood, restrictive internal air pathway in a tool, small section of hose, or restrictive duct fitting will act just like a water valve and seriously reduce flow. With the added overhead of our filter and minimum ducting, that pressure is only ample to move about 800 CFM when hooked up with a short piece of 6" flex hose.
Likewise, many small leaks in the ducting quickly add up to the equivalent of having an extra open blast gate that will severely cut the airflow at your machines killing performance and potentially leading to plugged ducting. To make sense of this on my other pages I share a simple experimental game moving a balloon and two straws. This will require a four-fold increase in static pressure that will cost you a NINE-fold increase in horsepower!
Although we all enjoy having bragging rights and a shop full of all different sizes of ducting looks incredible, for these impressive looking ducting designs to work in our small shops we would need monster sized blowers able to collect from all our tools working at the same time. This means duct sizes must be much larger for a good fine dust collection system because these systems must move lots more air. We always must avoid any sharp turns, harsh reductions and undersized duct, undersized flanges, small tool ports, etc. If you have this kind of set up, you should never clean your shop floor or other areas that might have metal or stone bits with your dust collector. If your ducting is too large you might not maintain the airspeed needed to keep sawdust and chips from building up and blocking your ducting. The smaller pipe will kill the airflow needed to keep the air in the mains moving fast enough to avoid plugging and building up dust piles. He found a very elegant solution that gives far better airflow than we get with even the nice tapered reducers shown above. I suspect most of my pipe and fittings would come from one of the large box stores with my wyes and longer radius bends coming from a local ducting supplier. The next best is smooth walled laser welded steel pipe followed by a tossup between top quality metal spiral dust collection ducting and fittings and HVAC metal ducting, followed at a far distance by corrugated metal pipe and flex hose.
Also notice that in spite of some vendor claims, these values show that PVC moves more air with less friction than even equal sized spiral ducting. They only cost a little more and make tight joints that normally don't need anything more to hold them in place. I snapped a chalk line then put in one of these every other stud held in place with a 2" deck screw.
The simplest are the "8" shaped metal ferrules that you squeeze onto the cable with a crimping tool. One of the better known that many woodworkers have used successfully is the Gripple System. Both of my two larger shops had nice sub floors and I ran most of the ducting under those floors.
I've helped build one of these type shops and did most right, but also learned a few lessons the hard way. The floor I built was put over those utilities on well-tamped ground covered in 4" of crushed rock then a vapor barrier. If I were to do this again today, I would make the trench sized with a lip to use the surplus aluminum raised computer room floor tiles. Had I used that raised computer room flooring my trench would have been big enough to also include lines for my shop vacuum, drainage, and possibly water if permitted by my building codes.
Those cracks made for a moisture problem that was not compatible with my galvanized ducting.
I also bought the 5" Delta replacement port for my band saw lower port, plus used Lockline adjustable pick up duct with hood pick up above by the table attached to my strong shop vacuum. Every filter cleaning left me and my shop covered in the fine dust I bought that fine bag to avoid.
That cyclone killed the airflow below about 450 CFM, plus did not move enough air to keep the ducting clear. For quick cleanup after using these dusty tools I wear my mask, open all up with the fan running, and use my big compressor or leaf blower to quickly blow out the whole shop then turn on and leave on my ceiling mounted air cleaner. How about seeng if a Festool Plug-It cord is immune to the Saw Stop blade.Jon Jon: Almost done with my Stumpy marathon! Neither I (Bill Pentz), American Air Filter, nor any other references or links on these pages will accept any liability for the applicability of this information to your specific situation or any damages or injury caused to people or property from the use of this information or from any associated links. With most of these facilities already blowing their fine dust away outside facility owners complained strongly against the proposed OSHA standards saying the new requirements were unnecessary and cost prohibitive. In spite of badly inflated advertizing claims where many vendors claim their dust collectors have over triple this airflow, only two of the best performing 1.5 hp dust collectors actually provide 350 CFM in real use when connected to standard 4" diameter ducts and tool ports. There are many NFPA codes that our small shop vendors tend to misstate for their own convenience. The medical insurance data show roughly one in eight woodworkers who work in facilities that vent outside develop such serious wood dust health problems that they are forced into early retirement. In fact, fine dust known by researchers as particle material (PM) is so extensively studied that a Google search on "PM health risks" will turn up over forty-seven million references. Roughly one third of all woodworkers eventually develop some allergic reaction to wood dust and about seven in thousand develop such strong allergies they must stop woodworking.
They found fine dust spreads so rapidly even a big air cleaner or exhaust fan needs hours to bring the dust level down enough to be safe.
The American Conference of Industrial Hygienists (ACGIH) responded recommending five times lower airborne dust levels and these recommendations were supported by the Environmental Protection Agency (EPA).
They provide blower fan tables that let us use our airflow requirement and resistance level to look up what sized blower with the right size impeller and motor to move the amounts of air we need.
We can use smaller pipe, but then have to have a much bigger blower to generate extra pressure. This also means our tapered and smaller adapters from our ducting to our tools are all but useless because they also kill our needed airflow. To support the minimum 800 CFM we need for fine dust collection, we need to upgrade those 4" port connections to 6" and the ducting inside our machines to 6". That pressure will only pull about 550 CFM when connected with 5" flex hose and only about 350 CFM when hooked up with 4" flex hose.
Typical dust collection blowers need 7" diameter duct and 7" diameter down drops that split into tool hoods that equal about the same as a 7" diameter circle. This also means with our dust collectors and cyclones only able to collect from one tool at a time, we must design our ducting with all the down drops the same size and horizontal mains of that same size or at most one size larger.
This makes sure that stuff does not build up in down drops with closed gates to create plugging.
You need about 3000 feet per minute (FPM) airspeed to keep light sawdust moving horizontally and about 3800 FPM to move it vertically. You have to keep three 3" ports open at once or two 4" ports to avoid the plugging a 6" main and even more open for larger mains. Kanaflex makes a polypropylene rigid duct hose they call Kanaduct with interlock construction which allows the inside diameter to be changed by twisting the hose. Also, most "official" dust collection pipe uses proprietary connectors and sizes that often limit your expansion to working with just that firm. Use of two 45 degree fittings with a short segment would work in place of these long radius bends almost as well if those are not available locally. The low cost S&D PVC (plastic) pipe (see my PVC site if you want to do "magic" with fitting PVC into your ducting.) is generally one of the best small shop dust collection material choices because it is smooth, far stronger than most HVAC metal pipe or spiral pipe, costs less, and fittings are a fraction of the price. I recommend the PVC ASTM 2729 "Sewer and Drain (S&D)" pipe for most small shop woodworkers. Most agree that not gluing is by far the preferred way to go, as you may change your mind on your shop arrangement, and you will for sure eventually catch something that clogs requiring you to take one or more joints apart. These connectors are easily placed and removed for access, provide a good seal on an adjustable range of diameters, and act as good "dampeners" for vibration and noise transmission.
That screw also secures the open edge of the block and squeezes down on the clamp to hold the clamp in place.
Most boat supply stores sell both the cable and ferrules, or you can get them from a cable supply store. The rule is never hook a dust collector or cyclone to any size pipe less than 3" in diameter.
When I bought a set of air gauges and did some testing, I realized that my trashcan separator was not good news at all. I later learned during the medical air quality testing done on my shop that my so called filter was a sieve that freely passed most of the finest unhealthiest invisible dust. After digging into the engineering behind cyclones, I realized that almost all small shop cyclones were copies of the same original design. Most cyclones have intake ports of 6" to 8", and some two-bag collectors have 5"- or 6"-diameter intakes, so consider at least a 5" main duct. The PVC tee shown at near right adds more airflow resistance than the metal dust-collection pipe's gradual bend. Well, Monica over at Craftynest has re-defined the duct tape wallet into something that's actually CUTE. No claims are expressed or implied as to the safety, usefulness, or accuracy of this information. Studies commissioned by facility owners appeared that make many false claims such as woodworking makes no fine dust, venting outside blows the fine unhealthy away, there are no health risks from wood dust, and because commercial facilities vent outside their workers receive almost no fine dust exposure. For instance one popular cyclone vendor who sells ducting designs and parts strongly puts down the use of PVC duct because it does not meet NFPA standards.
They also formalized this process into something that most can follow without special training or expertise to come up with a good dust collection system. Likewise, if we use bigger pipe, we then have to use a bigger blower to keep the airspeed up enough to avoid our pipes plugging and building up dust piles. Good fine dust collection requires 1000 CFM which requires 7" diameter ducting or using a bigger more powerful blower that generates the pressure to pull more air through a 6" duct.
The only way to use smaller duct is to get higher pressures and we can do that either by speeding up our impeller or using an oversized impeller.
The one who blows always wins because sucking pulls air from all directions so airspeed falls off so much that you cannot affect the balloon unless your straw is almost toughing it. The wrong pipe size, leaks, fittings that change the air to abruptly, overly long runs, an inefficient separator, and a poorly designed cyclone can each cost far more CFM loss.
Instead, our small shop blowers only move enough air to collect from a single machine running at once. At the machine the down drop duct may split into two collection pipes for two collection points so long as the overall cross sectional area is the same or only slightly larger than the down drop, but otherwise our ducting remains very simple. Also, for maximum efficiency, particularly with a cyclone, this layout keeps a straight run that is at least 4' or longer going into the inlet. I used a 30-gallon metal trashcan on one unit and a 40-gallon metal trash can on the other, both with metal lids and metal flex duct going to them from my cyclone. On that same 4" ducting a 1 HP unit that gives 650 CFM maximum rarely will maintain 350 CFM. With special tools required to make junctions, you also end up generally paying for a number of custom made parts.
The 2729 PVC pipe and fittings cost far less than metal or standard schedule 40 PVC, are much stronger, don't leak like metal pipe, and have a much lower coefficient of friction than even spiral metal ductwork. The best way to seal leaking temporary joints is with standard 2" aluminum duct tape found in home and HVAC stores. I bought a large spool and a pound of ferrules for what it would have cost me to buy just a few pieces from a boat store.
That worked for about two weeks until I bought a few more machines and ended up having to crawl back under the floor and redo all.
It pulled the real 1100 CFM I got from my dust collector through a 6" test pipe to under 450 CFM.
That design was never made to be used inside with fine filters, so I designed my own more efficient cyclone.

Tapered reducers, like the one below, change duct diameters with minimal static-pressure loss. Your actions are your responsibility - VERIFY and CHECK information out before proceeding, and don't attempt anything without the required skills. Although none of these studies were blessed as peer reviewed medical articles, they did cause enough confusion that they delayed and badly watered down the OSHA air quality standard. Fine airborne dust spreads so quickly that good air quality requires collecting the fine dust at the source as it is made before it can spread.
Although that upgrade helps, wood dust has since been classified as a cancer causing agent so medical experts recommended a fifty times higher than OSHA air quality standard which has already been adopted by the European Union. We need the same airspeed to pick up the chips as we do to keep it moving in vertical ducting runs, so collection and vertical transport speed are often interchanged.
Since we use the same size and types of tools as smaller commercial tools, most of what we need to do for good fine dust collection is already laid out in detail. Unfortunately the 350 CFM needed for good "chip collection" falls far short of the airflow needed for good fine dust collection. For machines like my table saw with an interior duct, I had to use a larger port and a larger interior hose.
If made too large it does not maintain the airspeed in the ducting to avoid plugging and a buildup of dust piles. It is hard to find 7" diameter duct and if you find it, it is expensive compared to readily available 6" duct. This is why we must move so much air to pull in dust from all around the working areas of our tools.
With just one ducting run at a time open, we need much simpler ducting designs than the commonly recommended graduated ducting designs that collect from all tools working at the same time. Too little airflow plugs vertical runs and builds piles grow in height then grow longer and longer in horizontal runs. Additionally, duct itself adds resistance that can also seriously hurt our flows, so a good ducting design uses all the right sized ducting components and provides the shortest straightest runs possible with minimal sharp bends or joints.
This keeps the incoming air very smooth making for far better material separation and improved efficiency as turbulence kills efficiency. That makes the dust more awkward to empty, but safer and the metal flex duct is much less expensive.
On one side of the strip is a male rib and on the other a female socket very similar to zip-lock bags. Likewise, HVAC metal ducting thickness and size can vary considerably, but most recommend getting at least 26 gauge pipe.
The 2729 PVC is rated for many times the pressure that even large industrial blowers generate. A good metal cable tie gun will cinch these up tight enough even heavy PVC pipe will not slip down a wall. For my second shop I put a fitting every six feet along the walls and down the center of the shop. You can put in your ducting, power, water, and whatever else you need or want to run under that floor, then have easy access to make changes. It works well to power my downdraft table, collect chips from my router table and larger tools, and pick up the leaves that constantly get blown into my garage shop. Air engineers went through years of testing and decades of refinement to work through the details to meet proposed OSHA air quality regulations.
Meeting these higher air quality standards requires the same hood, port, and tool upgrades, but more airflow. We can get by with a little less airspeed to keep the dust moving in horizontal runs because horizontal runs do not have gravity working against us. Each of us can go through this process to determine our needs, pick a blower large and strong enough, configure our ducting, separate off the heavier dust, and either exhaust the fine stuff outside or appropriately filter that dust. They found that meeting OSHA air quality requires just over 795 CFM and larger better hoods.
This makes sense as a garden hose would empty a city water tank far slower than a 6" diameter pipe because the garden hose is too small. To get our 1000 CFM that larger tools need for good fine dust collection through a 6" duct or hose we need about 5,100 FPM and that takes at least a 15" diameter impeller when turned by a standard 3450 RPM direct drive motor. Any fine dust that does not get trapped by the dust hood gets launched by almost any airflow from our tools, belts, cutters, motors, etc.
Between use of the very inefficient flex hose, too small of a diameter pipe, and too many sharp angled fittings in their ducting, most hobbyist woodworkers severely kill the potential airflow needed for good dust collection.
These piles and plugs pose a serious hazard as any spark can quickly get blown into a ducting fire. If your dust collection system is properly sized it can go anywhere in your shop, but most use smaller dust collectors and cyclones, so these should be placed as closes as possible to your tools that require the most CFM for good dust collection. With a two-stage unit using a separator or a cyclone before the blower this becomes less critical. Even reducing ducting size right at the machine for the shortest possible distance to a small 4" port will still kill system performance.
Depending upon direction of twist this duct either expands or contracts in diameter up to about 50%.
Most "snap lock" ducting in the larger home centers comes in light 30-gauge or heavier 26 gauge galvanized steel. I've seen no flex with my PVC pipe even when doing testing on 10 hp blowers driving a 16" impeller. Permanent joints are better sealed with either duct sealing compound or polyurethane caulk. Each had a plug and was covered by a piece of tile, so when I needed to make a change, I just had to lift the tile, remove the plug and put in a flex hose to the machine.
Large facility owners successfully lobbied amply to get this standard removed before it was fully implemented. Also, most small shops never create the right mix of airborne dust to create an explosion, and our systems do not generate enough static to ignite an explosion.
These firms found the most existing tools came with no dust collection or at best poor "chip collection" built in, so worked poorly for good fine dust collection. These material handling tables show we must maintain airspeed inside our ducts of at least 3800 FPM to collect (and vertical transport) most normal sawdust but larger chips need up to 4500 FPM airspeed for collection. Instead of just having to cover a 4" diameter area they found for good fine dust collection our hoods needed to cover roughly an 18" diameter collection area.
Much of that hose is poorly made with ribs sticking into the airflow adding up to nine times the resistance of smooth pipe. We control water flow by opening and closing our faucet, meaning add just one constriction in the line and it can kill our flow.
Without an oversized blower, if your duct is smaller than 7" to your larger machines, then it will not move enough air to capture the fine, most unhealthy dust.
The only way to prevent this is to have good hoods that keep the fine dust controlled and move enough volume of air to capture it before it gets launched. These practical suggestions can help you to first address the inefficiencies before having to pay for expensive motors, bigger blowers, and more electricity. Also, when plugs and piles break loose they slam down our duct so hard they blow apart our joints, run through our separators, ruin our blowers, and destroy our expensive filters.
The smaller ducting, flex hose, and small ports limit the maximum airflow just like we proved in that little air test sucking experiment! This results in very smooth clean transitions without all the nonsense of expensive transitions. Unfortunately, if you make your duct pipes or cyclone out of the thinner metal, you could collapse your cyclone and long metal duct runs if you have a large blower and should leave all the blast gates closed. Alternatively, you can use either the more expensive but less efficient heavier spiral pipe or the 26 gauge snap lock HVAC pipe. Do not use a dust collector as a vacuum because any steel screw or nail picked up hits the impeller and can put a spark in the collection bin. Meanwhile, the American Council of Governmental Industrial Hygienists (ACGIH) recommend a five times tougher than OSHA standard.
This same vendor then justifies their sale of cyclones that fail to comply with the NFPA rule that all indoor cyclones must be fire and explosion proof because there has never been a reported incident of a small shop dust triggered explosion.
With costs to replace existing tools prohibitive, air engineers found they could get good fine dust collection at most older tools by moving enough air and upgrading tool hoods to better contain, control, and deliver the fine dust for collection. The same simple ducting formula feeing in our 50 FPM needed to collect the fine dust over that 18" sphere shows we need.
Always buy and use a minimum of smooth interior walled flex hose as it that only adds about three times the resistance of smooth pipe. Bumping up to 6" pipe causes problems for the under 1.5 HP units because the air speed (FPM) can fall too far and make dust block the pipes, but with this bigger pipe, the bigger units end up going to 800 and 900 CFM.
The retail price per meter is abot $24 which is cheaper than having to buy metal reducers and flexible hose.
The fittings even in the light metal have enough support they are not at risk of collapsing.
Also, most dust collectors make poor vacuums because the airspeed is too low for good pick up. The ACGIH standard is what most larger commercial firms follow to avoid personal injury lawsuits. Strangely, this same vendor totally ignores the NFPA guidelines and sells cardboard dust bins that are a bad fire risk, plus they only use aluminum impellers that the NFPA abandoned years ago. They developed new CFM requirement tables for each size and type of tool that with hood upgrades moved enough air to ensure capturing the fine dust. Air engineers established most woodworking operations that do not make heavy big chips are well covered if they target their blowers and ducting to move 4000 FPM airspeed. Rounding that to 800 CFM and keeping our 4000 FPM airspeed in our ducts to keep them clear says we need 6" diameter duct to move the 800 CFM needed to meet OSHA air quality standards. You cannot because our lungs cannot create the pressure needed to overcome the high resistance of that small pipe.
Getting a larger central storage tank has little effect unless you put it higher in the air where it can generate more pressure. That's why knowledgeable woodworkers use oversized blowers and 6" ducts and 6" flex or 7" ducts and 7" flex when using most cyclones and dust collectors. As a result, you need to use at least 5" duct for any hobbyist blower rated up to 1100 CFM and 6" or larger duct for blowers rated 1100 CFM to 1800 CFM. Another inexpensive option is building square or rectangular ducts from Melamine coated particleboard.
My cyclone moved more air and also separated off the material before it got to the impeller so worked well with my new floor sweep. The medical recommendations, European Union and Environmental Protection Agency (EPA) agree on a standard that is fifty times tougher than the overturned OSHA standard.
NFPA strongly recommends against use of aluminum impellers because when hit by a piece of metal such as a screw or staple aluminum launches sparks similar to sparklers. They found the only effective way to get good airborne dust control at each tool source is to upgrade tool hoods to contain and control the fine dust, upgrade tool ports to not restrict airflow, move far more air to ensure collecting over a large area to capture the fine dust before it can escape, use larger diameter ducting able to carry ample airflow, separate off the heavier sawdust and chips, then get rid of the fine dust. Their testing found we need about 900 CFM at our larger tools to cover a little more area and roughly 1000 CFM to meet medical air quality recommendations that are now the European standard. As with water, getting a bigger blower with more horsepower does little good if the airflow is too restricted by the size of the ducting, tool ports, hoods, and duct openings. Unfortunately, the more air a blower pushes the higher the amp draw, so if you open things up too much with a dust collector smaller than 3 hp or cyclone smaller than 5 hp with a cyclone you can burn up your motors. I hot melt glued in big neodymium super magnets on the front of that sweep to collect nails and screws before they get sucked up. The same formulas show we need to use 7" diameter ducting to move the air for both the ACGIH and medical standards already adopted in Europe. Unfortunately, many alternative ducting materials are flammable so are totally inappropriate for use anywhere except in plain sight. When climate or local laws preclude exhausting outside, far more work and cost is required to amply filter the air for returning it safely into our shops. If we use too small of ports, duct, fittings, or outlets, we kill the airflow needed for fine dust collection. Putting a flammable duct material in a ceiling area or enclosed attic can create an unacceptable risk of fire.
In 1989 when OSHA finally declared wood dust a nuisance and set air quality standards, most large commercial shops were already running OSHA compliant dust collection. I used the thinner 22 ga outer shell to make the cone and some ducting.Frank Spillman: I love my Wixey gage too, it was a good investment!

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