Understanding Dust Collection
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Understanding Dust Collection

1.      Understanding Dust Collection

He said most of our tools use older designs that need considerable modification to keep from spraying fine dust all over. When that fine dust escapes, it becomes near impossible to capture and it takes many hours for typical air cleaners and exhaust fans to bring the dust levels down safe enough to remove our masks. He said air engineers long ago learned they had to modify our tools, often very extensively to protect, control and capture the fine dust at the source right as it is made at each tool. They also learned they had to move far more air to capture this fine dust with our older tool designs than is required to just capture the heavier sawdust and chips. Unfortunately, most hobbyist dust collectors move less than half the air needed for good fine dust collection. Worse, in spite of inflated advertising claims most hobbyist dust collector, cyclone, shop vacuum, and even air cleaner filters freely pass most of the finest unhealthiest dust right through turning these units into what my doctor calls “dust pumps”. The finer hobbyist filters clean up the way our shops look creating a false sense of security because they also mostly pass the finest unhealthiest dust right through. This near invisible dust gets trapped in our shops where it build to incredibly unhealthy levels. Almost any air movement from our dust collection equipment, tools and air compressors launches this finest dust back airborne. Over many years of too much exposure to this finest dust most develop health problems, many serious. He believes the fine filters on dust collector, cyclone, and air cleaner were so bad they led to my being hospitalized with fine dust related health problems. He said I would have been better off without any of those units as they just stirred up and kept the fine most dangerous dust airborne and trapped inside my shop. He also said forget relying on vendor help, we each must do the work ourselves to assemble ample protections to minimize the fine dust exposures. He shared the health risks of wood dust exposure and gave me a list of immediate to dos (see “Health Risks and Doc’s Orders”), and pointed me in some directions to start my research. I immediately took care of the small stuff he suggested then got busy. My research quickly verified my doctor’s comment that fixing my dust collection would be a challenge! Almost all of the available hobbyist information was contradictory, with much of this information just plain bad.

2.      Bad Information

Most small shop woodworkers depend upon many sources for their information and all often give bad dust collection information.

  1. Most woodworking is a relatively private hobby, so much of our information comes from books and more recently videos and the Internet. Almost all of that information was produced well before the awareness that fine wood dust is so dangerous. As a result, most tables, tests, product comparisons, magazine ratings, on and on are based on chip collection, not fine dust collection. Sadly, the magazine and book editors trust their authors, expert testers, and technical staff to do homework that mostly does not get done. At least three of the most popular magazines have recently shared dust collection articles recommending use of HVAC pipe to make complex ducting solutions with multiple down drop sizes. These are chip collector solutions at least ten years out of date from being dust collectors. Worse, in the last few years there have been at least three major magazine articles rating dust collectors and cyclones. Each sited vendor flagrantly exaggerated maximum airflow and filtering claims without verifying. Worse, the dust collector test allowed one winning vendor to supply a test unit with an oversized impeller larger than they ship on that model tested. That makes for great performance but will quickly burn up a motor. That same test killed the performance on the best dust collectors by choking their airflow with too small a test pipe. This same magazine then did the same in reverse on their cyclone tests. They used oversized pipe to give marginal performing units good tests and normal sized test pipe leaving the best performers looking bad. I decided to never rely upon magazine testing for their information when those magazines depend upon the same vendors advertising for their livelihoods;
  2. Many small shop woodworkers rely upon their intuition and experiences with a lifetime of using vacuum cleaners to configure their dust collection systems. Most of us think about our dust collectors as a giant powerful vacuum cleaner that will pull air around obstructions, through small openings, and through small pipes. A real vacuum operates at the pressures to do just this. At typical dust collection pressures air will hardly compress at all, so we need to think about airflow being far more like water. Any tiny pipe or obstruction such as a partially open valve will kill flow;
  3. Many also rely upon communications with their friends, whether through the Internet or in person. Even “old salts” meaning long time woodworkers don’t have a clue as to what is really needed for fine dust collection without doing a bunch of homework. I have to admit giving bad advice myself for far too many years before this whole fine dust issue reached up and bit me in the tail hard. Although this advice from friends is often sincere and meant well, it just cannot be trusted; and,
  4. Finally, a few of us are fortunate enough to go to classes and teaching centers, then share our experiences. As wonderful as those experiences often are, frequently the fine dust collection there is either missing or terrible. That leaves a strong false impression that there really is no concern.

3.      InflatedAdvertising

With no government oversight and uninformed buyers, hobbyist vendors have long sold “chip collectors” that do a great job gathering chips, but do little to protect our health from fine dust. In the late eighties commercial woodworking concerns had to make change to meet government air quality standards, but our hobbyist market failed to follow. Hobbyist vendors continued to push out low cost Pacific Rim imported copies of older tools known to be terrible dust generators. Enough direct sales firms market these mostly copied “tools by the pound” to make the competition for entry-level small shop woodworker business beyond fierce. To stay competitive many firms engage in an ugly advertising war. With almost no oversight, many claim whatever they want. Sadly, even with oversight, our “truth in advertising” laws provide no protection letting vendors legally claim anything they can demonstrate. With the appropriate “tricks” and testing, unscrupulous vendors bombard small shop owners with useless “maximum” performance claims. Credible vendors find themselves forced to either similarly compete or watch their market share rapidly decline in favor of supposedly better performing lower cost products. Here are a few of the techniques that vendors use to make their outrageous claims.

  1. Blowers configured with special hyperbolic inlets and with no filters or ducting move just about double the air that they move under real working conditions. Vendors advertise these maximums forgetting to tell their customers that the performance is a curve with real maximum in use performance about half the advertised volume. As a result most hobbyist dust collectors, cyclones, vacuums, and air filters provide less than half the advertised airflow and nowhere near the airflow needed for good fine dust protection;
  2. Motors for an instant while starting draw four or more times their maximum working amperage. By measuring this highest amperage and converting it to horsepower a vendor can advertise a real 1.5 hp motor as having 6 hp. Standard 15-amp 120-volt household circuits will safely run up to a real 1.5 hp motor. With a circuit breaker upgrade of these circuits can run a real 2 hp motor, but most recommend instead using 220 volts for this sized motor and larger. Only those who buy from reputable vendors are going to get home with a motor that actually delivers its promised advertised horsepower;
  3. Filters can simply build up enough dust cake to prove any level of filtering desired as long as they don’t also have to pass any air. The key is never follow standard engineering practices and say what airflow comes with any level of filtering. Sadly, with far too little filter area many hobbyist filters in our dust collectors, vacuums, and even air cleaners quickly clog and stop moving the air we need for good protection. Even with clean filters, at working airflow levels many hobbyist filters pass dust twenty to thirty times larger than advertised turning our blowers and air cleaners into dust pumps that recirculate these unhealthy fine particles;
  4. Tools are often advertised and come with ports and such that make them look like they have good fine dust collection built in. When we get these tools home they instead blast dust everywhere. Sadly, any dust port or dust bag added to a tool lets the seller legally advertise the tool as having dust collection built in. Few vendors mind stretching that fiction further by also saying their tool has good fine dust collection built in. With no standards or legal requirements many tools are designed with just enough dust collection to make the claim without actually working.

Using the rules by which this industry operates, advertising claims are about as useful as my telling you that my car for sale gets 92 miles per gallon. Moreover, I can prove it coasting down a mountain and reading the MPG gauge. Shoot, I can even sell you an old clunker saying it has a brand new motor and tires by buying a new electric toy car with new tires and tossing it in the glove box. Although perhaps provable, the information provided fails to give the facts needed to make an informed decision.

4.      Bad Tools

I learned that we need to capture the fine dust at its source and get rid of it, but our hobbyist tools and dust collection equipment does not do either well. Our tools often have tiny dust ports far too small for our dust collectors and often even too small for our vacuums. Our tools mostly lack designs and hoods that will control the fine dust before it is captured so spray fine dust everywhere driven by the air from our blades, bits, cutters, belts, motor cooling fans, etc. We need to replace our tools with ones that control that dust or modify our existing tools with hoods, ports, etc. that control, protect, and direct the fine dust for collection. Add hobbyist dust collectors and cyclones that only provide half as much airflow needed to collect the fine dust, plus filters that pass most of the fine dust right through and hobbyist dust collection is a dangerous mess that is hurting people.

5.      Bad Support

Finally, most hobbyist woodworking stores and tool vendors reinforce these poor ducting designs by mostly only carrying inappropriate ducting and dust collection equipment. They mostly only sell 4″ diameter sized ducting with sharp ninety-degree bends, Ts, and very rough interior walled flexible hose. This stuff does fine job keeping the shop floor clean and looks sharp, but it cannot move enough air to capture most of the fine dust from our larger machines. With the pressures available from most hobbyist sized blowers anything less than 6″ hoses, flex hose, duct, fittings, and ports will choke the airflow quickly below the 800 CFM we need. Far more information is available on my Equipment and Ducting pages.

B.     Basic Information

Here are the basics we need to understand for effective fine dust collection.

1.      Airflow:

Dust collection manufacturers provide dust collection design firms with both Air Volume Requirements tables and Airspeed Requirements tables so these firms can design commercial air systems that will carry materials without plugging or building up piles in the ducting. Air engineers who work for these manufacturers have done considerable work as have many of the universities to build careful tables that show just how much air needs to be moved and at what speed for each different type of material.

a.      Airspeed

For wood dust and chips, careful testing shows airspeed of between 3700 to 3800 FPM is needed to pull in the chips and heavier sawdust from our machine hoods. We then need about 2500 FPM to keep our horizontal ducting runs clear and about 3700 FPM to keep our vertical ducting runs from plugging. Wanting to ensure good dust collection without buying and paying for running too large of a blower, most air engineers recommend we design our dust collection systems to maintain a duct speed of 4000 FPM in our mains to keep our ducting clear and keep the chips and sawdust entrained, meaning airborne. This 4000 FPM has become an industry standard that is well tested and proven solidly to work to move the dust, but it does not collect the dust.

b.      Air Volume

We also need to know how big of a volume of air is needed to be moved at each machine to collect the dust. Knowing that FPM = CFM/Area where Area is the area to be collected from in square feet, we can calculate this volume. A little algebra shows CFM = FPM*Area. We can then measure the area of each machine that needs collection, convert to square feet and then multiply by FPM to get the required CFM. This theory approach works fairly well and shows most large hobbyist stationary tools need between 350 to 450 CFM of air volume for good chip collection. Verifying these calculations with testing is expensive and takes lots of work. We need to test each different type and size of woodworking machine working a variety of materials. Fortunately, almost all of this work was done for us and published ages ago. Since larger hobbyist vendor tools are the same as smaller commercial tools, we can use these same test results. Good chip collection on almost all hobbyist larger stationary tools requires between 350 to 450 CFM just as calculated.

To comply with 1989 government standards to also provide good fine dust collection, these same large equipment vendors and university staff went back to work. Their testing showed that airspeed of at least 50 FPM effectively moves airborne dust. This makes sense because we all know from watching dust particles in a beam of sunlight that it takes very little air to move the airborne particles. With tools designed and built from the ground up to protect and control that fine dust for collection, the prior “chip collection” air volumes work well. Festool and a few other brands have shown with their special tools engineered from the ground up with good fine dust collection built in that totally controls the airflow around where the wood is being machined actually get good fine dust collection with an oversized shop vacuum. Unfortunately, most of us use tools with minimal or no “chip collection” built in. Our tools and their open cutting areas allow the airstreams from our blades, bits, cutters, belts, motors, etc. to blow the fine dust away before it can be collected. Almost all air engineers say the only way to provide good fine dust collection for most hobbyist and older tool designs requires us to replace the hoods, sometimes remake the tools and provide nearly double the air volume to collect the finest dust as it did to collect the heavier chips and sawdust. The calculations show the same thing because collecting the finest dust at most current tool designs requires delivering our airflow over a much larger area than just at the port at the end of a hood. Both the math and testing show we need to provide close to 800 CFM air volume movement to get good fine dust collection.

At first these larger air volumes do not make sense because we know it takes 50 FPM to move the fine dust and 4000 FPM to move the heavier sawdust and chips. We already know from experience why it takes more air. When we use our shop vacuums they only pickup up right next to the end of the hose. The reason is unlike blown air that holds together for quite a distance, air being pulled or sucked by a vacuum comes from all directions at once. This means that the area being pulled from is roughly a sphere. It also means that our airspeed will fall off at roughly the same rate as that sphere area grows. Airspeed for sucked air falls off at roughly the same rate as the area of a sphere expands given by the formula Area=4*Pi*r^2. Most air engineers target for a duct speed of roughly 4000 FPM because this is what we need to pickup most woodworking dust and keep our vertical ducts from plugging. If we use that 4000 FPM in our air formula where FPM=CFM/Area we can compute how much air is moved in different sized pipes. We can then divide those airflows by 50 FPM to see how big of an area each will cover, translate into square inches then convert to the surface of a sphere. Although the math is fun, the bottom line is airspeed drops below the 50 FPM we need for good fine dust collection very quickly. Our 2” duct at 4000 FPM only supports 87 CFM and that turns into less than 50 FPM at only 4.47 inches from the center of our duct. Our 2.5” standard vacuum hose only supports 136 CFM at 4000 FPM which turns into only 50 FPM at 5.59” from the center of the hose. This explains why we see almost no pickup just 2” inches from the end of our vacuum hoses. A 4” duct at 4000 FPM airspeed only supports 196 CFM which turns into less than 50 FPM at about 6.71” from the center of our hose. A 4” duct that only supports 349 CFM at 4000 FPM only gets 50 FPM about 8.94” from the center of our duct. Many round this to 9” and use this as the standard for 4” duct. Our 5” duct that supports 545 CFM at 4000 FPM only supports 50 FPM out to about 11.18 inches. Our 6” that supports 785 CFM at 4000 FPM only supports 50 FPM out to about 13.42”. And, our 7” duct which supports 1069 CFM at 4000 FPM only gives our needed 50 FPM out to about 15.65”. In short we need to move a lot of air to ensure capturing the fine dust.

This unfortunately creates some controversy that is very important to small shop woodworkers. One group of hobbyist vendors would like for us to believe that we can get great fine dust collection by just fixing our tools to control and protect that fine dust. They are 100% correct, but also 100% dead wrong because testing of older tools shows nothing short of starting over with a total rebuild engineering fine dust collection in from the ground up will fix our existing tools. Air engineering testing shows that even with the best recommended changes, we really do have to move this additional air if we are going to have successful fine dust collection. Otherwise, we remain stuck with good “chip collection”. I believe as we move toward the far more stringent medical air quality standards already adopted by the European community, the only way for commercial firms to meet these standards will be to buy new tools with good fine dust collection built in. Meanwhile, I continue to agree with and support the standards from those who want to replace the hoods and move more air because my pocketbook is not up to replacing my tools. Frankly, the price of the newer tools with dust collection built in remains so ridiculously high they have not yet earned my interest. It takes a lot of work on our part to get good fine dust collection with our current tools. Each of us must modify our tools to keep the fine dust protected from being blown away, controlled until it can be captured and then directed for delivery right into our dust collection hoods. We also must use a blower large enough to ensure moving ample air at our tools.

c.       CFM Requirements Table

The following CFM requirements table gives the airflows required at each size and type of stationary woodworking tool to meet different dust collection standards. These tables are not provided by the standards organizations, but instead like tool hood designs are closely guarded industry secrets as the hood designs and these table values are what permit a professional dust collection firm to ensure their systems will meet a particular standard. When I built these web pages our small shop vendors were throwing a wide range of numbers around with little to no clue as to what those numbers meant and they badly confused the different airflows needed to provide good fine dust collection. Specifically, the top magazine rated small shop dust collector provider who continues to pretend an authority role in dust collection which is not borne out by either their information or actual performance of their products said the CFM numbers to meet “chip collection” standards were more than enough to pull in the fine dust. They assumed without checking their facts that because it takes so little airflow to move airborne dust, that the much higher airspeeds required for “chip collection” were more than ample to also pull in the fine dust. They were dead wrong, but so few knew anything about dust collection that we ended up with most of the magazines and even the two books on small shop dust collection following that same wrong lead leaving a mess today where most end up confused. For what it is worth, after reading over my web pages that same firm now has copied my information without permission and sadly has had the audacity to blame me for creating some of this confusion that they caused. American Air Filter (AAF) was one of the leading firms who built fine dust collection systems and filters to meet government air quality requirements. They were kind enough to allow me to share their proprietary airflow tables to counter some of the massive confusion over airflow requirements for small shop woodworkers. Remember this below table is useless unless you start by upgrading your hoods.

The above CFM requirements table only provides the airflow required for good fine dust collection at our larger stationary tools. This table intentionally does not address the airflow needed to capture the fine dust at smaller and hand held tools. There is a huge difference in collection requirements for smaller tools. For instance, my 5” fine sander gets excellent dust collection when hooked up to a small shop vacuum, but the 5” aggressive grinder/sander made by the same vendor that looks near identical can only be controlled when used in a containment room as it will spray dust and chips twenty feet or more. Getting good fine dust collection on our hand held tools is often a matter of using good judgment. If your smaller tools spray fine dust all over like my electric and air powered sanders, then you need to provide better dust controls and often move far more air to capture the fine dust particles. The only way to get good fine dust from some of my hand and stationary power tools with built in tiny ports, is to use a vacuum hooked to the small port plus dust collector hooked to a portable hood that draws the air from right next to the where working. Often I also need to use my downdraft table with sideboards up. Whenever I find myself needing the downdraft table to control fine dust, I try to put on my mask and work outside, or wear my mask with the exhaust fan running, then leave when done work and stay away until the air clears!

If you look closely at this table, you will see that we really need about 350 CFM at most of our larger stationary tools to get good “chip collection”, meaning picking up the same sawdust and chips that we would otherwise sweep up with a broom. Capturing the fine dust ample to meet OSHA standards requires far more airflow, roughly 800 CFM at our larger tools. Because too many people still get ill at OSHA air quality levels the American Conference of Governmental Industrial Hygienists (ACGIH) recommends a five times tougher standard which needs about 12.5% more dust collection airflow. The medical community has been pushing hard for nearly twenty years for a fifty times tougher than OSHA air quality standard and that only takes moving about 25% more airflow. The European Union has already adopted this medical recommendation as its own standard and it is what I recommend for most small shop woodworkers. CFM Caution Please realize that this table shows the airflows needed for the different levels of collection for larger tools. For smaller shop tools with smaller ports we often need far less airflow, but at much higher pressures than can be delivered by dust collection systems. For these smaller tools we often need a powerful shop vacuum with fine filter, sometimes a down draft table, and often a portable hood connected to our main dust collection that we can move to where we are making sawdust and chips. You also should notice from this table than many tools such as the table saw requirements are given with upper and lower pickup requirements. If you don’t use two pickup hoods on your same tools as shown in this table, you need to follow the song’s advice and change your evil ways!

Moving enough air is not enough. To capture the fine dust at the source the air engineering firms who deliver systems guaranteed to meet air quality standards found most tool hoods need upgraded. The reason is simple. The tips of our saw blades and cutters are launching dust at over 100 miles per hour and a typical dust collection system has the air moving at less than sixty miles an hour. This difference in speed means if we don’t have hoods that either catch the dust or block it from getting launched it will escape. Looking at your table saw you need both an upper blade guard hood that moves 350 CFM and a lower cabinet hood which moves at least 440 CFM to meet OSHA standards.

CFM Experiment At this point many are not quite ready to believe that we need to move more air volume to collect the finest dust. Here is a simple game experiment to help you be more comfortable.

Airflow requirements for good dust collection seem a paradox. 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. To make sense of this on my other pages I share a simple game that tries to use air and two straws to move a balloon. One person is only allowed to blow and the other to only suck. The one who blows always wins because they move a directed stream of air that can push that balloon all over. That directed stream of air goes quite a distance before

friction will slow down the air. Sucking pulls air from all directions, so airspeed drops off at the same rate as the area of a sphere. That formula is 4 times pi times the radius squared, so it takes moving a huge volume of air make any effect even a tiny distance away. Most of us already know this from using our shop vacuums that will only vacuum up right next to the hose nozzle, but on blow will send dust everywhere. The same thing happens with our fine dust collection. Any fine dust that does not get protected by a well designed tool with a good dust hood and then vacuumed up gets launched by almost any airflow from our blades, bits, belts, cutters, motors, etc. The only way to prevent this is to ensure our tools keep the fine dust controlled then move enough volume of air to capture it before it gets launched.

Most hobbyist vendors offer dust collectors that move too little air volume to provide the needed airflow to keep our ducts clear with a hobbyist system designed to only run one machine at a time. Trying to collect from more than one machine at once or having air leaks in your ducting can make this already poor airflow worse. If the airspeed drops too low we get plugging. Although a few shops get duct plugging, a far more frequent problem occurs when the airspeed drops below what will keep the dust airborne. This causes our ducting to build up internal dust piles. These piles grow in height until they restrict the airflow enough that the speed climbs back up where it will pull the dust along. This causes these piles to get “topped” and simply grow longer and longer. They also tend to fill any down drop between the pile and blower with a closed blast gate. Dust in these piles and filled down drops pose a potential explosion hazard and a serious fire hazard. When the airflow is restored from opening a larger gate the pile breaks loose and surges all at once down the ducting. The high dust concentrations in these piles can explode if ignited. Should there be some metal in that pile that strikes a steel impeller or steel blower housing with the air dust mixture close to critical, the resulting explosion can level a shop or garage. Likewise, any spark that lands in one of these piles can quickly get blown into a duct fire. This is why I strongly recommend against installing plastic ducting in systems without strong enough airflow to keep the ducting clear. Additionally, when these piles break loose they go slamming into our blowers and filters, eventually ruining motor bearings, impellers, and our expensive filters.

1.      Ducting Size

The hobbyist tool industry is stuck on the older 4” diameter ducting that was only ample to collect the chips at the old 450 CFM standard. Although many mistakenly think of their dust collectors as giant vacuums, they really are not. Depending on size and brand shop vacuums generate pressures that will suck a water column (w.c.) between 40 and 110 inches high. A shop vacuum has the pressure to suck air around many obstructions and through smaller openings. Dust collector blowers generate about one tenth the pressure leaving the air more like water, hardly compressible at all. Any restriction, small machine port, obstruction, or even sharp bend kills airflow just like opening a water faucet a little. The resistance of our pipes can kill the airflow we need for good fine dust collection because a small pipe diameter acts just like a water valve. The resistance of our ducting pipe and hoses is so high that a 1 hp dust collector will only give us a real 195 CFM with a 3” pipe, 350 CFM with a 4” pipe, and 550 CFM with a 5” pipe. Dividing CFM/FPM gives ducting area in square feet which converted to a diameter shows we need 6” pipe to get our needed 800 CFM airflow and 4000 FPM duct speed.

Unfortunately, bigger is not always better because once we reach that 800 CFM we need to be very careful to not use such a large pipe that the airspeed in our ducts drops too low and causes plugging or dust piles. Using traditional ducting designs that were meant for shops that ran all runs open at once does not work for small shops that only have blowers big enough to just run one ducting run open at a time. Making our ducting look pretty by using all different sized down drops kills the airflow in our mains causing them to build up dangerous dust piles. Most small shops need to use just 6” duct throughout their dust collection system.

2.      Resistance Calculation

The overall resistance in our dust collection system, known as static pressure, defines how big of a blower we need to power our system and move the desired air. Our overall static pressure is a sum of the resistance of our ducting plus all other resistance in our system. Most hobbyist vendors never mention overall resistance because their blowers generally lack the power to move the needed air and overcome the resistance for a small shop, so in spite of advertising claims do a poor job collecting the fine dust. We do a resistance calculation to get a reasonable approximation of the overall resistance of our system. We measure this resistance in terms of water column (w.c.) inches a blower must overcome to deliver the desired airflow. There are static pressure calculators that will compute the total resistance for our dust collection ducting and other components. An excellent one is provided on my web pages, but after going through the work to do this calculation, most end up with the same results. After helping hundreds configure the ducting for their shops and doing these calculations, almost all small shops need only 6” ducting and fall into one of four categories. Those with no ducting who move a 10’ flexible hose between machines end up with about 4” w.c. resistance for their systems. Those with shops sized about the same as a one-car garage with ducting end up with about 6” of resistance for their ducting. Most hobbyists have shops sized about the same as a two-car garage and end up with about 8” of resistance. Those with three-car garage sized shops have about 10” of ducting resistance. Those with larger shops must do the detailed calculations themselves. Most small shop owners should go through using that calculator to verify their shop needs. When using this calculator note that it allows you to add in other system overhead that affects our airflow and blower needs:

      1. Trashcan Separator Resistance

Trashcan separators will typically add about 4.5″ of static pressure if used.

      1. Cyclone Separator Resistance

Cyclones will add 2.25” for one of my design or up to 4.5″ for other brands. Adding a neutral vane some of these reduces the static pressure to about 0.5″. Modifying the inlet and adding an air ramp can reduce that pressure even more.

      1. Muffler Resistance

Mufflers will add resistance depending upon type. A straight through “glass pack” similar to the design shown on my muffler page adds about 0.15″ of static pressure. A baffle type muffler typically used on cars, large air compressors, etc. can add up to 2.5” of static pressure.

3.      Blower Size

Blower sizing is something we can easily look up once we know the overall resistance of our system and the airflow requirement of our largest tool. Almost all hobbyist blowers are standard material movement pressure blowers and almost all are turned by fixed speed 3450 rotation per minute (RPM) motors, so all have very similar efficiency. Although blower housing, impeller height, and blade angle do come into play a little, the primary performance factor is impeller diameter. This means we can use just about any material movement blower fan table to pick the smallest blower that will meet our needs. The following table will give you a pretty good idea of what you need in terms impeller diameter and motor horsepower to move a given volume of air at different static pressures. To use this table we look down the column with the resistance calculated for our shop until we find an entry that meets our required airflow of generally 800 CFM. The table then shows the impeller diameter, required horsepower,and gives the ideal blower opening. A fan table is critical in helping us size both our blower impeller and motor for an effective dust collection system.

If you look closely this table also shares the minimum sized blower inlet because the blower inlet should be the same size as the ducting size. If you just blow the air outside without adding a separator and filters, that inlet size is often the ideal ducting diameter for optimum efficiency and minimum resistance. If we have filters, a trashcan separator, or a cyclone we must use a larger blower to overcome that extra resistance and that opening may not work for sizing our ducting. Since most small shop systems use filters and a separator, almost all dust collection systems are configured just big enough to move the desired air at the higher resistance level. When our oversized impellers with small motors are run without that resistance, they try to move too much air, overstress and will soon burn up their motors. Hobbyist vendors and magazines test our blowers running without cyclones and filters or ducting. This means these blowers are moving at close to maximum airflow at minimum resistance. Since almost all motors are designed to handle four or more times their running loads while starting, vendors can get away with this testing for a short while before the motor overheats and fails. This makes for a mess for small shop woodworkers trying to make an informed purchasing decision. Those test results often show air volumes that will quickly burn up our motors from trying to move too much air. The bottom line here is any hobbyist fan table that does not also include amperage draw at each resistance level is not to be trusted because the blower more likely than not is running the motor well over its rated maximum amperage! I redid the testing done by two hobbyist magazines on dust collectors and cyclones. I got the same airflow results, but also found every single cyclone and all but the Jet and Delta dust collectors drew more than maximum motor rated amps when tested the same way because each let in too much air. The sad part of this is the better quality Jet and Delta units that actually move more air, tested below the “best” rated units that were burning up their motors. If someone tells you that they are getting more CFM with a smaller motor, you should ask at what static pressure and what motor amperage. Based on the below fan table, most small shop woodworkers that use a cyclone separator with fine large cartridge filters need at least a 13” impeller turned by a 3 hp motor. Since 13” impellers are very difficult to find, we mostly end up buying 14” or larger impellers turned by a motor smaller than could handle an unrestricted airflow, or just use a larger more appropriate motor as I recommend and get even better fine dust collection.

4.      Blower Fan Table

5.      Ducting Designs

In spite of a need for good ducting designs, small shops are not subject to government oversight, standards, or testing so many vendors offer designs and products based upon articles, books, and even experts on small shop woodworking that recommend older chip collection standards that move half the air needed for fine dust collection. Worse, most also recommend plumbing our shops with large mains and smaller down drops sized just right for each machine. These designs sell well because they look pretty. They may collect chips but often make the fine dust levels worse and are potentially deadly. Unlike commercial shops that have a huge blower sized to run all ducts open at once hobbyist blowers are mostly only big enough to collect from one machine at a time. Opening more than one duct requires more airflow than our blowers can deliver, so fails to collect the dust. At typical dust collection pressures air is like water and will barely compress at all. Opening just one small pipe limits the airflow far below what we need to keep the mains clear. The mains build up dust piles that pose a fire hazard and when airflow is restored these piles slam into and can ruin our impellers and filters. Most single station small shop dust collection systems need to all use the same sized pipe, tool ports, and flex hose to prevent these problems;

6.      Ducting Type

Small shop woodworkers are known to make their ducting from anything that carries dust. Hobbyist vendors are not subject to fire and building code standards, so typically sell plastic ducting. Most sell their dust collectors with a length of plastic flex hose. This hose is so small and rough inside that it instantly kills half or more of the dust collector’s airflow. Vendors then offer the same diameter ducting and much more expensive smooth interior walled pipe, but the result still works terribly. Most, including me, then turn to the experts who mostly use HVAC pipe and fittings because they are cheap, available, and will work for chip collection. HVAC ducting leaks badly, is all engineered for the air to flow the other direction so the joints collect shavings that lead to plugs, plus all the fittings were designed for much lower pressures so kill airflow at typical dust collection volumes and pressures. Worse, the cheapest 30-gauge HVAC piping is too thin and will collapse if you have a larger dust collector and all the gates get closed at once;

7.      Filter Basics

My study showed the best way to deal with fine dust is blow it outside and not let that fine dust return back into the shop. Almost all large commercial woodworking shops that get regular air quality testing do blow their fine dust away outside to stay in compliance with the 1989 OSHA maximum airborne dust requirements. Blowing outside is illegal in my residential area, so I had to filter my air. A little study showed that shops that must pass regular air quality inspections and filter their air almost all use 0.5-micron filters. My respiratory doctor says the medical recommendation is those with existing problems always wear a good NIOSH approved mask during dusty operations and use 0.2-micron filters.

The indoor filtering issues are a little more tricky made worse by vendors either in ignorance or intentionally providing bad information. Most dust collectors and cyclones come with the standard outdoor felt filter material certified at 30-microns meaning these filters freely pass almost all airborne dust. Most small shop vendors that advertise “fine” filters only provide a false sense of security. Most small shop vendors provide dust collectors and cyclones with either inappropriate wide open outdoor filters, filters with outdoor ratings that freely pass twenty times larger particles, and or filters sized far too small so they soon self destruct. The result is the same, most of these filters turn our dust collectors and cyclones into “dust pumps” totally inappropriate for indoor use. Just knowing what level of filtering we want is not enough. Our testing found in spite of advertising claims every small shop dust collector and cyclone we tested had filtering setup for “chip collection”, not fine dust collection. We need to understand some basics about filters and that there are different standards when it comes to rating filters.

a.      Filter Types

We mostly use two types of filter material in dust collection, all spun bond man made felt filter material used on most dust collector bags and the same stuff blended with cellulose (paper) fibers. Filter thickness and internal strand sizing defines initial filtering ability. The blended filters costs about half as much to make so are less expensive, but require just about double the filter surface area to provide the same level of filtering as the all spun bond. When the spun bond filters begin passing too much dust they can be thoroughly washed to restore their original filtering ability. This can be done four or five times greatly extending the filter life of a spun bond filter over a blended filter that needs replaced when it begins to pass too much fine dust. To get more filter area in a given space, vendors fold the filter material into pleats and put the results into a cartridge form. The poly-cellulose (paper) blended material is used mostly in filter cartridges. Because the spun bond filtering material is quite a bit thicker, the thicker material requires twice as many cartridges to equal the same area as cartridges made from the thinner less expensive blended material. we typically only get half as much filter surface area in the same sized filter cartridges.

b.      Filter Life

A number of things reduce filter life. As filters plug they kill the airflow needed for good collection, but cleaning rapidly breaks down the filter pores and so does clogging. Clogging increases the

air pressure enough to force the fine dust to tear its way through the filter pores, so the more undersized the filler, the more quickly it self destructs. Although clogging and cleaning are bad, what often first kills most small shop fine filters are sharp chips. Dust collectors and cyclones with full collection bins blow these sharp chips into our filters punching the filter material full of larger holes. As a result, most small shop fine filters end up turning our dust collector and cyclones into “dust pumps” that fill our air with dangerously unhealthy dust levels. To protect fine filters and address the clogging, you can either buy an expensive graduated filter with automatic cleaning system, or use a cyclone separator. For most to get best filter life today we need cyclone separators to amply protect our fine dust filters.

a.      Filtering Level

All filters start new able to effectively filter some fixed size of dust particle at a given airflow. As the filter ages many fine particles get embedded in the filter fibers creating a “dust cake”. Every time we clean our filters by blowing them down or shaking them we leave some of this cake embedded in the filter fibers. After about three full cleaning cycles a filter will provide about 50% better fine filtering because this cake helps the filtering. This is known as filter “seasoning”. After about nine cleaning cycles most fine filters are at their maximum dust cake and provide their best filtering. As a result most filters provide a range of filtering providing their worst when new or after cleaning and cycling to their best filtering as they plug.

This gives rise to two different industry standards used to rate filters. Filters used indoors are rated based on their worst case filtering when brand new with no dust cake. This assures the filters will provide through each loading cycle a given level of filtering. This loading cycle for fine filters results in roughly a twenty fold difference in filtering between a new filter and one that is fully “seasoned”. Each cleaning cycle on a “seasoned” filter still results in roughly a ten fold difference in fine filtering ability. Filters used outdoors where the fine dust that escapes the filters just blows away harmlessly outside are rated by the filter material makers based upon reaching their maximum seasoning. Because of this range of performance, we need independent filter evaluations. The American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) certify independent labs to test filtering material. ASHRAE is not a government organization, but instead a private, non-profit group of professional engineers that set the standards for their industry. ASHRAE sets the standards for indoor air quality testing, filtering, and airflow. They approve and oversee independent testing labs to provide filter testing and performance certification. Because this testing is very expensive and most filter makers buy their filtering material in bulk from a material maker, most vendors only provide the certification from the material maker except for HEPA filters that are each individually tested and certified. ASHRAE labs test new clean filters at their rated airflow. This certification says that filter will stop particles of a particular size and larger 99.9% of the time.

The manufacturers also share the filter testing as this filtering material builds up a thick cake of dust in the filter fibers. As this dust cake builds it can take up to twelve cleaning cycles before the filter fully “seasons” achieving its maximum filtering. The actual filter performance will then vary between the indoor rating at worst and a fully loaded outdoor rating at best. We need this loaded airflow for sizing our filters to ensure they have ample surface area to handle the airflow and dust loading. Careful testing by a number of my professor friends showed most small shop vendor “fine” filter bags advertised with clogged fully loaded filtering level as their filter rating. Our testing found many vendors who claimed to sell 0.5-micron, 1-micron, and 2-micron filters sold filters that freely passed 10, 20 and 40 micron particles through filters showing the same roughly 20 fold difference between the best and worst filtering levels. Clearly these vendors were selling filters based upon outdoor fully “seasoned” ratings. Although these claims may be accurate and appropriate for outdoor equipment, that information is inappropriate for indoor filter rating. Those ratings turn our dust collectors and cyclones into “dust pumps” when used indoors constantly spewing out fine dust depending upon how clean the filter. They freely pass the finest unhealthiest 2.5-micron and smaller dust particles. This leaves us with poor health protection and left breathing the fine particles through much of each filter’s performance cycle. These units should be only be used outside without any air returned to our shops.

Some vendors went one step further claiming filtering levels that all but kill airflow before that filtering level can be achieved. Our testing found a few vendors 50-micron filter bags they advertised as 1-micron filters. Sadly, the “truth in advertising” laws let any vendor claim any level of filtering that they can demonstrate. They can demonstrate any filtering level as long as they do not also share the airflow at that filtering level. They simply let the filters get dirty enough so they no longer pass any sized particle they want to claim. Just because at some of these claimed filtering levels filters pass almost no air does not bother these vendors a bit. There are a few reputable filter bag makers (AAF, Highland Hardware, possibly a few other exceptions, but not many). Our careful filter testing left me now only trusting filter ratings provided by an ASHRAE certified independent testing laboratory. Unable to find any certified dust collector bags and knowing that most bags have so little surface area that then need constant cleaning which exposes me to the very dust I must avoid, I personally use certified cartridge filters with a cyclone to protect those filters. I recommend all others do the same.

b.      Filtering Resistance

The maximum dust cake is important in sizing our filters because this is when a filter has the most resistance. Filter resistance changes as the filters get dirty. That resistance is at a minimum when a filter is new and clean. This resistance climbs until it builds a dust cake that provides maximum filtering with good airflow, known as when a filter becomes fully “seasoned”. As the filter continues to clog, the airflow drops until the airflow all but stops and the filter is at maximum resistance. Air engineers use this fully caked resistance level to size our filters large enough to flow the volume of air we want to move. This maximum resistance is also very close to when we need to deep clean our felt filters in a washing machine and start all over building up a whole new cake of dust.

This resistance has two major concerns in dust collection. As the filter plugs resistance rises and we lose the airflow needed for good fine dust collection. Additionally, as this resistance rises it causes the pressure inside the filter to increase. The increased pressure pushes the finest particles through the filters tearing open the filter pores as they pass. Eventually, a filter gets so torn open it no longer provides good fine filtering and needs to be replaced. Large commercial filtering setups constantly measure filter resistance with a pressure gauge. A pressure sensor detects when a filter clogs too much and institutes an automatic cleaning cycle. If after cleaning the resistance is too low, then that filter is shot and needs replaced. A typical new filter on a cyclone or dust collector will start with a resistance of anywhere from a low of 0.25” to about 1” of resistance. As the filters clog the resistance will increase until a cleaning cycle starts.

In small shops we become our filter sensors and our performance is far less reliable often not cleaning our filters until the pressure has risen 5” or more which will quickly destroy filters, so we should also monitor our filters. In small shops we also can use a pressure gauge or do similar monitoring with an amp meter. The amp meter will read highest when the filters are new and flow the most air. That is when the motor is doing the most work. As the filter plugs the airflow drops causing the motor to do less work and drop the amp reading. When that reading drops enough we need to clean our filter. Good filters will have about the same meter reading after each cleaning. When a filter becomes worn out and passes too much air the “clean” filter amp readings will go higher telling us it is time to replace our filters.

This resistance moves through a considerable swing. My measurements of my standard 30-micron felt bag type filters when new was about 2.5”, yet after “seasoning” that resistance rose to over 4″ of resistance with a “clean” filter due to that buildup of fine particles embedded in the filter material. Fine bag type filters use thinner filtering strands. These fine strands make them overall more open and able to move more air. This more open condition left my 30 square foot bags with only 0.25” of resistance when brand new, but after seasoning they settled at about 2” of resistance after cleaning. The typical roughly 90 square foot cartridge dust collector filters are fairly open so add about 0.5” of resistance when new that grows to about 1.5” of resistance after cleaning. Use of 300 square foot commercial fine cartridge filters starts with a resistance of only 0.25 when new that grows to about 0.75” of static pressure after seasoning. My recommended 600 square foot of poly-cellulose blended filters stabilized at only 0.25” of resistance.

c.       Filter Sizing

Filter fabric makers provide guidelines on how much surface area is needed for each type of their fully “seasoned” filter material. That area depends upon the size of particles filtered, volume of air moved, and amount of dust being filtered.

Many small shop vendors inappropriately use the wrong information to size their filters. Many use same wide open filter sizing standards for the wide open 30-micron filtering material when sizing their finer filters for indoor use. Many vendors also forget that we are filtering very dirty air and size their filters based on manufacture recommendations for filtering the relatively clean indoor air that only calls for about one square foot of the all poly filters for every ten CFM of air and double that for the paper blended filters.

With typical 30-micron all polyester filters appropriate only for use on outdoor dust collectors and cyclones that return no air into our shops, we need about one square foot of filter area for every 25 CFM of dirty air. With “chip collection” only moving about 400 CFM these filters only need about 15 square feet of filter area. Boosting the airflow to the 800 CFM we need for good fine dust collection pushes the size of these open outdoor filters to needing at least 30 square feet of filter area.
With typical 0.5-micron all polyester filters used for indoor air filtering we need about one square foot of filter area for every 4 CFM of dirty air. This means our 800 CFM needed for good fine dust collection at our larger tools requires 200 square feet of fine all poly filtering material and at least 400 square feet of the blended filtering material. Fortunately, both materials come folded and made into cartridges that enable us to get this much filter area in a small space. An open filter that freely passes 30-micron sized dust only needs about one square foot of filter area for every 50 CFM of dusty air run through the paper blended filters and about one square foot of filter area for every 25 CFM run through the all spun bond felt filter material.

We can get by with smaller filter sizes, but doing so causes the filters to wear out and fail far quicker. A filter sized half the manufacturer’s recommendation will only last a quarter as long. Although most small shop vendors provide a minimum of filter size because filtering material is expensive, this is exactly the opposite of what we should do. Most makers of large dust collection equipment for shops that get regular air quality inspections make their units with twice the manufacture recommended minimum filter area. Doubling the filter area cuts the resistance four fold, cuts cleaning to one fourth as often and extends filter life four fold. This is why I strongly recommend using a pair of the 300 square foot poly blended filters on my cyclone design when we could get by with just one.