Dust Collection Basics

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 3" 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.

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 small shop 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.