Dust Collection Equipment
- Equipment Introduction
- Basic Components
- Dust Collectors
- Trashcan Separators
- Cyclone Separators
- Air Cleaners
- Filter Stack and Clean Out
- Dust Bins
- Detailed Components
- Tool Detail
- Dust Collector Detail
- Cyclone Detail
- Cyclone Design
- Commercial Cyclones
- Small Shop (Hobbyist) Cyclones
- Small Shop Cyclone Compromises
- Fine Dust Cyclones
- Filter Detail
- Fans and Fan Types
- HVAC Blowers
- Centrifugal Pressure Blowers
- Impeller Types
- Impeller Sizes
- Dust Collection Blowers
- Air Starved Blowers
- Blower Housing
- Blower Cautions
- Blower Motors
- Blower Frequently Asked Questions (FAQs)
- Equipment Introduction
Most of the time we should not be filtering but instead blowing our cyclones outside. We should only filter when forced to do so by either local laws, very hot temperatures in air conditioned shops, or extreme cold where a radiant heater cannot keep up with air losses.
Building to flow the air from the outside to the middle takes a sealed filter box. A filter box makes deep cleaning of the filters easier and reduces noise, but at a cost of making normal cleaning harder and having to build a filter box.
With most filters now equally coated and effective when you blow from either the inside or outside, you can build your filter setup either way. It is easiest to build a filter stack with no box. We can either stack the filters or put them side by side and blow the air into the center of the filters. Side by side requires a wye feeding the filters. At the bottom of the filters we make a container to capture the fine dust when it falls off the filters, plus equip this catch basin with a blast gate connected to a standard sized port. This enables us to clean out our filters without having to take all apart or get a "dust bath" while cleaning. Use a long hose attached to that port that goes well away from your shop and downwind, turn on your cyclone, and then tap and lightly blow compressed air (Caution! More than 40 lbs. pressure will hurt your filters!). Thank Dizzy for one of the most elegant implementations of this idea!
Most small shop users have limited money to spend on their cyclones.
A good blower and motor is expensive to buy and takes considerable power making it expensive to operate.
Most small shop cyclones have to fit under a standard eight-foot high ceiling.
With the rates to ship larger items by truck up to five times higher than through the post office, FedEx and UPS, most small shop cyclones end up being compromised in both size and weight to conform to the lower cost shipping restrictions.
Copying an outdoor commercial cyclone with high built in turbulence is just plain dumb as that high turbulence is only there to break the fine from heavier dust. In an indoor small shop system we want to separate off that fine dust, not pay extra for the horsepower to break it loose then pump it into and quickly ruin our filters.
Sizing motors and blowers based on dust collector experience fails to provide the horsepower to power the cyclone leaving these units badly air starved. Air starved means the blower has an impeller that is too small to overcome the resistance it is working against, so it just loafs along moving a minimum of air. It takes about 1 hp larger motor and roughly 2” larger diameter blower impellers to get a big enough “bite” of air for a cyclone to move the same air volumes as a dust collector. As of late 2005 not one mainstream small shop cyclone vendor offered a 2 hp or smaller cyclone that moved the air needed for good “chip collection”. Most sized with 3 hp motors do not move the air needed for good fine dust collection at our larger tools.
It was not until 2005 that these vendors finally started selling cyclones with larger motors, but most only stepped up to 3 hp motors that are too small to support the cyclone and overhead found in average sized shops. Worse, they continue to strongly sell their smaller motor powered cyclones that do not move enough air for even good “chip collection”.
These vendors continue to make short, often round cyclone inlets to reduce manufacturing cost. These inlets create significant internal cyclone turbulence greatly increasing resistance and reducing separation efficiency. Worse, in order to use the same cyclone with a variety of blower sizes, they use very large inlets on these cyclones. When used with a smaller motor unless controlled by smaller ducting, these large openings allow the cyclone to move so much air the motor will quickly burn up. This makes for some incredible maximum airflow test results as reported in the woodworking magazines, but in testing with an amp meter at these airflows most of these units will burn up their motors within minutes.
To fit a cyclone with its blower and dust drum under an 8' ceiling vendors build very short cyclone cones. These short cones add resistance and reduce fine dust separation. Worse, most cones are not properly sized. Sadly, almost every cyclone being sold to small shop workers as of late 2005 had improperly sized cones that either suck the fine dust out of the collection bins or cause the cyclones to clog near their dust chutes forcing all to blow right into the filters.
These less powerful blowers require small shop cyclones to be made with large diameters to minimize the resistance. These larger diameters kill the fine dust separation efficiency putting even more stuff into the filters.
The small shop vendors took their outdoor cyclone copies, made their units with indoor parts, and put finer filters on these units so they can call them fine dust collectors. Putting a fine filter on an outdoor cyclone design built to push almost all the airborne dust outside quickly loads up even a large filter. The higher pressures in cyclone based system even more quickly ruins our fine filters. The increased pressures as these filters get dirty pushes the fine dust through the filters tearing open the pores. Likewise cleaning wear and holes from being hit by sharp chips also quickly open up these filters, so even fine filters soon freely pass most of the finest unhealthiest dust. Some vendors avoid and ignore this concern by simply supplying wide open filters that pass most of the airborne dust right through.
An impeller spins as fast as a saw blade and is a whole lot larger, so can be much more dangerous! A large item slamming into an impeller can cause the impeller to break, warp, or bend out of balance. Even just a little warped or out of balance condition can ruin motor bearings and potentially cause an impeller to explode. That is why I strongly recommend buying a professionally made and carefully balanced impeller.
Next, the clearance between the impeller and it shroud remains small in spite of our being able to pick up much larger blocks and other things. An impeller jam not only ruins the impeller, it often ruins the motor by slamming it into such an abrupt stop that the internals slide on the shaft. Even cyclone blowers that move mostly clean air through the impeller can create jams. If we let our collection bin get too full (about once a month for me), or you have a bad air leak, all shoots right through the cyclone into the blower.
Finally, all impellers should regularly be inspected and cleaned. Unlike the material movement impellers used in dust collectors, caged impellers require far more regular inspection and cleaning to ensure there is no buildup of wood strings, shavings, and other stuff that will clog them and put them out of balance. Too much buildup of sawdust and resin deposits can create an out of balance condition that can ruin motor bearings and cause lightweight impellers to fail and possibly fly apart.
I've been looking for a blower for ages and still can't find a 1.5 to 2 HP motor blower combination anywhere. What should I do?
Why would you want one of these? You really need a 3 hp blower for a dust collector or 5 hp blower for a cyclone. Everyone else is also looking for the same single phase 3 or 5 HP motor-blowers. I've seen used ones sell on EBAY for far more than new ones cost.
You have me so confused I don't know which end is up. Are you saying I need to get a blower to run 1000 CFM at every tool in my shop?
No I am saying make all down drops and mains the same size. Unless you install a commercial dust collection system that is ample to power all ducting runs at once, then any smaller run or down drop will kill the airflow needed to keep the mains from plugging and building up dust piles that pose a fire hazard and can ruin impellers, motor bearings, and filters. After the blast gate you can split a 6” down drop between two tools with a wye. Although only one tool may be running, both end up with airflow. I do this with a couple of tools next to each other. The total duct area for all the splits still needs to be very close to your main that matches the down drop.
Ok, what do the airfoil impellers look like and where do I buy one?
Airfoil impellers move far more air for the same horsepower with less noise. They are designed to work with open systems where they will have a constant supply of air and not too much resistance. With either too much resistance or too little air, the airfoil blades stall creating destructive vibrations and all kinds of other serious problems. If you don't use very efficient ducting that is 5" or larger, open large filters with minimum back pressure, and keep one blast gate open all the time, an airfoil is not a good solution for you. I found four industrial suppliers carry them. Chicago Fan, Cincinnati Fan, and Continental Fan responded with quotes. In 1999 the prices were prohibitive. Each wanted over $250 for quantity one plus a hefty shipping and handling charge. Continental quoted a price of $140 plus shipping and handling for their plastic impeller, but plastic impellers are totally unsuited for cyclones because a full dust bin shoves the heavy stuff right through the impeller guaranteeing eventual failure. My son has some excellent steel impellers with the right compression arbors. For a while Sheldon's Engineering made a special airfoil impeller that does not stall so easily. They offered it a 2 hp and a 3 hp version, but no longer offer these units. I used one of these airfoils for over a year, but eventually upgraded to a larger material handling impeller that did not stall or require me to constantly monitor and clean it. My airfoil had incredible suction and the stalling was not a problem until I added a full set of ducting in my shop. Then even with my all 6" ducting and either an air trap or extra blast gate open it began to stall. The air trap is actually called a barometric dampener and is just a weighted door that opens when the air pressure goes over 7”.
I want a bigger impeller for my 1.5 HP Jet dust collector blower that I am using to power a Wood Magazine design cyclone. I understand that the upgrade will help quite a bit. I have a welder, should I just weld tabs on my current impeller or buy a bigger one? Where?
In general your cyclone is the same cyclone that most small shop vendors sell, plus or minus some of the modifications suggested on my Cyclone Modifications web pages. If you have a current set of Wood Magazine plans you will see most of these changes incorporated because I upgraded their design for them. You should start by adding a “neutral vane” described on these web pages. That addition alone saves most between 1/2 to 3/4 hp, so is a must when powering a cyclone of this type with a smaller motor. Next, because a cyclone adds so much more resistance, we can often run a much larger diameter impeller without over stressing our blower motors. Not every blower can be safely upgraded. You still need room in the blower housing for the impeller size you pick with enough clearance that stuff does not get jammed between the impeller blades and housing. You also need enough motor to power the larger impeller. Although the fan tables help, making impeller upgrades is often educated guesswork. Make sure the result does not end up pulling too many amps from your motor. Also, be careful that you buy an impeller that is compatible with the motor shaft size and direction of rotation for your blower. Your Jet will only work with impellers that turn counter clockwise when looking at the blade side of the impeller with the motor behind.
Your Jet DC-1100 impeller when attached to your sized cyclone with "neutral vane" upgrade will only move about 600 CFM with minimal ducting. Bumping up to a bigger impeller which is how we overcome the higher resistance of a cyclone will help lots, but still leave you shy of the airflow needed to provide a real 1000 CFM at your larger tools in an average two-car garage sized shop with ducting. This sized cyclone really needs a good quality 5 hp motor turning a 15" diameter impeller to support good fine dust collection from a single lager tool at a time running an average sized shop with ducting. If you have a small shop with minimal ducting, you can make do with an impeller upgrade, but most find this solution works poorly so eventually end up buying a bigger motor, impeller, and blower housing.
Should you decided to upgrade, I know of two approaches that work fairly well with the Jet DC-1100 dust collector blower. Fortunately, Jet uses the same sized blower housing for the DC-1100 and larger 2 hp DC-1200, so putting in a little bigger impeller does not cause clearance problems. You can either weld tabs onto the ends of the impeller blades or buy a bigger impeller. Adding on to my own Jet DC-1100 1.5 hp impeller was more of a welding and balancing challenge than I wanted to do with my MIG welder, but is not too difficult for a machinist with a good TIG welder. TIG can do the welding without adding any metal so keeps all in balance. As an alternative to welding, I ordered the 12" diameter Jet DC-1200 impeller and installed it on my Jet DC-1100 motor. The part number for my 12" was AB411059 and AB430006. I was told the second part number was incorrect, but is what my dealer used to obtain my larger DC-1200 impeller. I later found when helping a friend make an identical upgrade that using that first number and buying directly from Jet Customer service was less expensive and much faster. As of late 2006 the upgrade cost is $81 including shipping.
When I upgraded the changeover was easy. Although the motors had different sized shafts, both impellers had identical arbor mounting holes. That let me simply unscrew the impeller from the motor shaft and from the arbor, then use a pulley puller to remove my impeller. I then swapped the motor arbor mounts from the smaller to the larger impeller. Since the arbor bolts were secured with LockTite, this took some work, but went quickly once I realized that it took using a bigger breaker bar.
After sharing this many wrote that changeover was not so easy. It seems that Jet went through some different motor shaft and arbor sizes on the DC-1100 and DC-1200 dust collectors between 1999 and 2005. John Hardwick wrote advising me that as of 2005 Jet now uses motors with the same sized 19 mm shafts on both their DC-1100 and the DC- 1200. This makes the impellers interchangeable without having to change arbors. If you have an older model you want to change over, you might be able to just swap impellers, change over arbors as I did, or may have to get help from a machinist. One fellow I helped required a machinist to drill out and sleeve his Jet DC-1200 12” impeller arbor to make it fit with the DC-1100 motor.
Don’t try and follow these same instructions for changing impellers in other types of blowers. The Jet uses the same sized blower housing for both the DC-1100 and DC-1200. If you use too small of a blower housing the noise can become unbearable and you might not have the clearances needed to pass material. An undersized or oversized blower can also cause inefficient airflow. Likewise, Jet uses a motor shaft with a threaded bolt hole in the end to ensure securing the impeller. Most motors do not have these kinds of shafts, so require use of a compression arbor to keep the impeller from sliding off the shaft when the motor is mounted vertically. The set screw type setups just cannot safely hold the weight and stress of a heavy impeller hanging below the motor. It costs enough today for the needed machining that most find it less expensive to simply buy an impeller with the right sized arbor. Contact me if you need an impeller.
My concern is the offset of the impeller in the housing, I know it is not centered, but where does it go?
All blower impeller housings should be an expanding spiral also known as a volute that starts from the center of the impeller. Many small shop blowers instead use circles and other shapes that significantly reduce blower performance. Proper design of the spiral depends upon the type of impeller. We can put at much as one third of the impeller into the straight outlet portion of our airflow, so the outer dimensions are not too critical. Generally, a bigger spiral makes for less resistance and better airflow. The closest point on the blower housing, often the gore point occurs where the outlet straight meets the spiral. Caged and airfoil blowers have very tight clearances at this gore point while material movement impellers have relatively loose clearances to keep larger pieces of wood from jamming between the impeller blade and the blower housing.
My plans use different gore point clearances depending upon blower type. For an airfoil blower my plans leave 1/8" clearance between the impeller and the edge of the outlet. My Cincinnati material handling blower leaves a clearance of about 1.3" from that same closest point, sometimes known as Blowers use a tangential outlet for the air. As a rule of thumb, the airfoils go right next to the point in the blower housing while material movement impellers should go no closer than their diameter divided by ten to the impeller side or you get a siren and can have material jams. In terms of noise, you are better off with material movement impellers to make that spacing as much as 50% more.
You used to have links to fellows who made their own blowers and impellers, both wood and metal. I don’t see those links anymore. Why did you pull them down?
The few who recommended making your own blower impeller have pulled down their web pages from too many problems. The wooden impellers even at 1725 RPM kept blowing up so were abandoned as just too dangerous. The couple making impellers from aluminum and steel stock bolted onto a flat plate also kept having balancing problems with the lighter impellers blowing up. A blade letting go from a 3450 RPM motor can generate in excess of twenty tons of force for an instant. I know because one of the test units I was evaluating blew and exploded right through the 18 gauge steel blower sidewall. I also had bad experiences with a few other professionally made aluminum and plastic impellers letting go not quite so dramatically. Although you can easily build a good blower housing (see my blower and airfoil blower pages) I now only recommend buying a well made impeller from a reputable company. For those who are braver, there is an excellent inexpensive book, How to Design & Build Centrifugal Fans by David Gingery on making and balancing your own blower. If you go to the publisher's web page it runs about $4 less expensive than buying a copy from eBay.
Dust collection can be done with anything from a broom and dustpan to an elegant commercial dust collection system. Before deciding on what equipment you want, you should make a decision. Do you want to protect yourself from the very fine unhealthy dust? Without protection the odds are close to 100% that you and even those close to you will eventually develop some fine wood dust related health problems. If you do not care, then ignore all the rest and go buy at least a 1.5 hp dust collector or larger depending upon your shop size and ducting. I believe all small shop woodworkers should protect ourselves and those close to us from fine dust. I know from personal experience that over time fine wood dust is very unhealthy and that small shop woodworkers get far more fine dust exposure than full time large facility woodworkers. Worse, we often use much more toxic woods. Look at the Dust Collection Introduction followed by Medical Risks and Doc’s Orders for more information.
Government standards left hobbyists, the six out of seven small shop woodworkers, and small shop vendors that sell us our tools and dust collection with no standards or oversight except what we force with our purchases. With most small shop woodworkers and small shop vendors knowing little to nothing about fine dust collection, small shop tool and dust collection vendors stayed stuck in “chip collection” technology. Chip collection picks up the same heavy sawdust and chips we would otherwise sweep up with a broom. Government requirements forced large commercial woodworking facilities to leave “chip collection” behind in favor of fine dust collection in the late eighties. With no similar requirements and no pressure from small shop woodworkers, small shop vendors continue to sell equipment that keeps the chips and heavier sawdust off the floor, but lacks either the needed airflow or filters to capture and control fine dust. As a result, much of the advice from fellow small shop workers and even small shop vendors is geared to collect the same stuff that you would get with a broom. This traditional approach makes for a very nice looking shop, but exposes you and yours to dangerously unhealthy levels of fine dust. Most small shop tools spray fine dust all over unless extensively modified. Most small shop dust collectors and cyclones do not move enough air to amply collect the fine dust from our larger and dustier tools even after they are repaired with better hoods. Most of our fine filter bags and cartridges provide a false sense of security because they make our shops look cleaner while freely passing the finest dust shown by medical research to case the worst long term health problems. Keeping our dust collection equipment inside traps this fine dust allowing it to build our airborne fine dust concentrations to dangerous. Unfortunately, making repair becomes our problem because even buying the best recommended solutions leave you just where I was, getting far too much fine dust exposure. If you don't learn what you are doing starting with what the equipment is and how it works, the odds are you are actually going to make your dust problem far worse than if you did nothing.
Fortunately, the basic equipment needed for good fine dust collection is not that complex. We start with our tool or machine that creates the dust. That machine has a hood that needs to be placed and shaped just right to control then capture the dust as it is made. That hood needs a dust port big enough to support the required volume of air. The dust port then connects to either ducting or a flex hose that is large enough in diameter to move the needed volume of air. The flex hose or ducting then connects to a blower. Most of us use a blower built into either a dust collector or cyclone based dust collection system. Often we add a trashcan separator to our dust collectors as shown. Older style trashcan and most small shop cyclones are designed to save us from having to clean our filters so often, and to keep blocks of wood, sharp chips, etc. from clogging and punching holes in our filters. Separators also protect our blower fan wheel, known as an impeller, from getting hurt from hitting wood blocks and other heavy materials. We then either blow the dirty air outside or run that air through a filter before it is returned to the shop. Running the dirty air directly outside is best for our health, but is illegal in many areas and unless we provide a source of make-up air can be dangerous as it can suck the carbon monoxide fumes from gas water heaters, heaters, stoves, fireplaces, and other appliances. It also can be expensive to blow our heated or cooled shop air outdoors, so most use filters to return cleaned air to the shop. The blower connects either through a muffler or directly to the filters with a cleanout below to return clean air to your shop or the blower vents outside. The following provides more information on each of these basic dust collection equipment components.
The first step toward catching the fine dust at its source is to use tools with good quality dust collection built in. This means their designs carefully protect the fine dust from any air currents that could blow it away, route that fine dust to the tool hood for capture, and then use good hoods that effectively capture the fine dust and direct it for capture. Festool and a few others have shown that if we can totally capture the airflow all around the working area of a tool that even an oversized vacuum will provide good fine dust collection. Unfortunately, few tools come with good fine dust collection engineered in from the ground up. Most of us instead use small shop and older tool designs that need significant work to keep from spraying the fine dust all over before it can be captured. Air engineers fortunately already did most of the work to show us both what we need to do to modify our tools for good fine dust collection and how much air these modified tools will need to collect that dust. Because most of these tool designs use very large open areas that need a high volume of air for good collection, good fine dust collection often takes double the airflow that it does to just collect the chips.
Ducting includes the pipes, fittings and flexible hoses that carry air and sawdust. It also includes the connection ports on our machines and the dust collection hoods that actually capture the dust. The size and type of duct you choose will often define the amount of airflow that you can get to your machines. Air engineers say you want to use the largest possible ducting that your blower will support with an internal air speed of about 4000 FPM. Dust collectors with 1/2HP to 1 HP motors only develop enough pressure to support 4" diameter ducting without creating clogs. Those with 1 HP to 2 HP will support 5" diameter ducting. And, those with 2 to 5 HP motors will support 6" diameter ducting. Far more information on ducting can be found on my Ducting page.
I did not realize how much duct size and layout impacts dust collection! I learned that in spite of many confusing terms and more than a little misleading vendor information, we each understand airflow far better than you may realize! As a result there is far too much unnecessary controversy over ducting.
For lots more information visit my Ducting Page.
Most small shop woodworkers start their dust collection with a shop vacuum. A vacuum moves a small volume of air at a relatively high pressure. Vacuums do an excellent job for cleaning up small areas. Vacuums do not move enough air volume to be good for capturing dust from machines that emit dust over a larger area. Most vacuum filters are too small and too open, so spray the finest unhealthiest dust throughout our shops. Unless you replace the stock filter on your vacuum with a top quality fine filter, almost all vacuum filters freely pass up to 50-micron sized particles making them “dust pumps” that keep the most dangerous fine dust circulating. My doctor strongly recommends always upgrading our vacuums to finer HEPA filters. Fortunately, most shop vacuum brands can be reconfigured with these somewhat pricey fine cartridge filters. My doctor told me with my respiratory problems to go one step further and replace my big shop vacuum with a large Fein or Festool vacuum with upgraded HEPA filter because of their superior pressure and air volumes. In spite of hurting the pocketbook, I went with a large Fein liking it being so quiet.
Mechanically vacuums are fairly simple systems. A vacuum consists of a motor, a blower, a separator collection tank, and a filter. They use universal motors turning at very high speeds, typically 18,000 rotations per minute and faster. The blower is generally built right into the motor and consists of a blower housing and a high pressure fan wheel attached directly to the motor shaft. Vacuum fans are called caged impellers because they put the blades between a top and bottom plate that keeps the air from escaping while turning. Caution, unlike dust collectors, the air most vacuum impellers move also provides the motor cooling, so a plugged filter, full collection tank, or blocked hose will kill motor cooling can if not corrected can quickly burn up our motors. Air trapped by the fast moving impeller slings out the sides out creating a very high suction in the center that keeps drawing more air in. The vacuum motor blower assembly normally is sealed on top of the collection tank, often with ball valve to shut off the vacuum if the bin becomes full or falls over. The sucked air generally enters the side of the collection tank and is often directed to swirl around inside the tank causing a little cyclonic action that minimizes the amount of dust that goes to the filter.
Vacuums use small impellers that generate very high pressures but only move a little air volume. Their high pressure makes vacuums excellent for cleaning up small areas, but the low air volumes make them poor dust collectors for larger machines. Unlike a dust collector that can only generate a little pressure, roughly able to lift a column of water (w.c) about 7”, vacuums can lift a column of water 60" to 110". Likewise, unlike our dust collectors that can move 1000 cubic feet of air per minute (CFM) or more, the small impellers on our vacuums only move 30 to 90 CFM. Vacuums only provide good dust collection for tools engineered from the ground up with good fine dust collection built in, meaning the tool protects, directs, and delivers all the fine dust right to the vacuum connection before it can be blown away by airstreams from our blades, bits, cutters, belts, motors, etc. With almost all small shop tools using older tool designs that have little if no fine dust collection built in, we need to modify our tools far more than is practical to get good fine dust collection using vacuums. Careful testing by air engineers shows that making reasonable modifications to our tools and their collection hoods, we still need closer to 1000 CFM for effective fine dust collection at our larger and dustier tools. A vacuum moves about one tenth this airflow needed to collect the fine dust from our tools before it escapes.
Vacuums show us an important difference between blowing and sucking air. When we connect our vacuum hose to the vacuum outlet it will blow a strong stream of air that we can use for cleanup. The quickly moving blown air stream hangs together for a long distance until dispersed by friction. Sucked air instead comes from all directions at once. As a result the airspeed for sucked air falls off very quickly. We all know this from using our vacuums and seeing that they will not suck up unless we get the nozzle right next to what needs picked up. It turns out that the airspeed generated by sucking air falls at about the same rate as the area of a sphere grows, roughly 4 times Pi times the distance squared. This explains why our shop vacuums that on blow will stir and move dust all over our shops can only vacuum up dust over about a 3.5" circular area.
Most of us continue to need a good shop vacuum even after getting a big dust collector because a vacuum is the only way we can get enough pressure to pull the air we need for good fine dust collection through our smaller machine ports and restrictive internal machine ducting. Air at the low pressures generated by our dust collectors is virtually incompressible, so just like a water valve, any small opening acts like a partially closed valve and kills the airflow we need for good fine dust collection. This leaves our dust collectors pretty much worthless with any port connection smaller than about 3” because we get no air movement. To get the needed air movement for good chip collection at our smaller tool ports, we need to use a vacuum. We often need to use our vacuum working with both a portable dust collection hood and downdraft table connected to a dust collector together to effectively collect the fine dust from many power and hand tool operations..
Dust collectors consist of a special material handling blower, motor, bag tree with collection bag, and filter. A material handling blower uses a material handling impeller. These tough steel impellers can take hits from blocks of wood and other debris without getting hurt explaining their name “material handling” impellers. They are also made to be fairly self cleaning letting chips and strings slide off instead of getting trapped. Although this works well, letting stuff slip off the impeller blades also lets the air also slip off hurting blower efficiency. The impeller turns in a spiral shaped blower housing that needs to be very smooth to minimize the disruption of airflow. The blower housing outlet then directs the collected air and chips into a bag tree where it spins in a center separation ring. In theory this ring makes the heavier blocks and chips fall permitting only the lightest dust to go up into the upper filter. In practice the airflow inside a dust collector is so violently turbulent that chips and even small blocks constantly crash into and frequently poke holes into our filters. In commercial units placed outdoors where the fine dust that escapes gets blown away into the outside air these open filters work well, but when brought indoors these units build up dangerously high levels of airborne dust.
There are two common types of separators used dust collection, "trashcan separators" and "cyclones" The trashcan separators use a special lid that fits tightly on a trashcan or other large drum. Trashcan separators then use some combination of three different techniques to separate off the heavier dust and chips. They force dirty air to turn a sharp corner forcing the heavier particles to be slung off for collection. This is called centripetal separation. They use cyclonic separation to spin the air inside the trashcan causing the heavier particles to go to the can walls where it will eventually drop. They also use drop box separation. A drop box simply runs a dirty air stream into a large enough volume that the airspeed drops below what can keep the larger particles suspended airborne or entrained as dust collection engineers say. A properly sized trashcan separator will separate off roughly 85% of the dust by weight. Since average wood dust is about 85% heavier sawdust and chips with about 15% by weight made up of light airborne dust, makers of trashcan separator lids often claim close to 100% chip separation meaning almost all heavier dust and chips. Unfortunately, trashcan separators do not scale up well. Typical small shop trashcan separators work well until you have a system that moves more than about 450 CFM. That means any dust collector over about 1.0 HP will have problems with a trashcan separator unless the airflow to that collector is strangled by using far too small 4" ducting that limits the airflow. Using a 1 HP blower with 5” or larger ducting increases the airflow and keeps the dust from separating. Too much airflow simply scours the trash can empty of all but large blocks. My 1.5 hp system with a trashcan separator worked fine until I upgraded to 6" ducting after realizing my 4" ducting was strangling dust collection airflow. After upgrading to 6" ducting the increased airflow left my trashcan scoured clean. We could build a much bigger trashcan separator that would work, but for the 1000 CFM we need for good fine dust collection, we end up needing a 5’2” wide trashcan that sits ceiling high. Without trashcan separators we are left with a fairly expensive and difficult to clean multistage filtering system or using a cyclone to provide ample separation. Most choose to use a cyclone separator.
In spite of what some would like us to believe dust collection cyclones are simple tanks with no moving parts and have been used for wood dust separation for over seventy years. As shown in this animated graphic, cyclones are made up of just a few parts, an air inlet, an outer cylinder, an air outlet, and cone with dust chute that connects to a collection bin. Dirty air comes in through the inlet. Almost all large commercial woodworking facility cyclones are agricultural cyclone designs that cause this incoming air to crash hard into the air already spinning inside the cyclone. The resulting very high turbulence breaks the heavier and lighter materials apart. The spinning air throws the heavier materials outward to the cyclone walls. Airflow on the cyclone walls is slowed by friction. Heavier particles get trapped in the slower moving air and gravity slowly pulls these heavier particles down. The cone on the bottom of the cyclone is angled just right to keep the airspeed constant to keep the heavier particles pressed tightly to the cyclone walls. These heavier particles continue to slide downward and eventually exit out a dust chute into the collection bin. The dust chute is sealed tightly to the bottom of the cyclone with no air leaks to stir up the collected dust. A full dust bin or bad air leak causes the cyclone to pump all right through with little or no separation. Near the bottom of the cone is a reversal point where the spinning air without these heavier particles reverses direction. That cleaned air then spirals up through the center of the cyclone then exits through the cyclone outlet.
My respiratory doctor recommends installing a good quality ceiling mount air cleaner to help keep the shop air cleaner. He said the ceiling units are worthless for protection while you work, but they do help to clean up the air so every time you go back, you don't start with the same problems. I asked which one and he said that the magazines rated Jet, Delta, JDS, and Penn State all pretty close, but he bought a Jet for himself because he likes Jet tools. In looking at many woodworker posts and some magazine reviews, it appears that he did pick one of the best.
In following up I decided that if I caught the dust at its source and just let my cyclone run, an air cleaner would not be needed. An air engineer friend said that AAF had done extensive testing and found that was not accurate. An air cleaner is setup to stir the whole volume of air in a room. Without that stirring, much of the fine dust will be missed. Unless you setup your cyclone system so the air coming out of the filters blows in a directed stream at close to ceiling height, what happens is the air creates a narrow racetrack between whatever gate is open and the cyclone filters without doing a good job of cleaning the rest of the air. Moreover, a good air cleaner uses a small motor, meaning my dust collector would use more power and cost far more to run, plus is noisy.
My bottom line with any dust collection system is does it capture and get rid of the fine, most unhealthy dust. Commercial woodworking firms and air engineers have long known the best and least expensive way to get rid of the fine dust is to separate off the heavier sawdust and chips with a cyclone and then blow the remaining fine airborne dust outdoors where it quickly dissipates and settles. Exhausting outside requires opening a door or window to provide makeup air to avoid pulling deadly carbon monoxide backward through our vents and flues for our heaters and fired appliances. Even in the coldest below freezing weather my radiant dish heater keeps me warm when exhausting outside. I hear radiant heaters cannot keep up as the temperatures drop well below freezing. I know my air conditioner cannot keep up at all when exhausting my shop air during our very hot summer months. Since it is illegal to exhaust outside in my area I must use filters. If I did not have a legal requirement to not blow outside, I would setup a wye with blast gates that would let me blow the air from my cyclone directly outside without filtering most of the time. By changing the gates I could when needed instead filter the air.
Sadly, we cannot trust vendor filter claims to protect our health. I trusted what I believed to be reputable small shop vendors to provide a cyclone and filters that worked as they advertised. After the first set of equipment failed to move even half the advertised airflow, I landed in the hospital. Not wanting that to ever happen again I stupidly threw money at the problem letting the highest rated cyclone vendor configure my shop with their top of the line cyclone, filters, and ducting. That unit had even more airflow problems than my first, plus the filter soon self destructed as it needed cleaned constantly. At my vendor’s advice I replaced their garbage internal cyclone filter with an even bigger and finer top quality filter than they offered. Within a month I was back in the hospital and nearly died because that vendor also lied badly. I don't ever want my family or me again exposed to the under 30-micron airborne fine wood dust that causes maladies from allergies to cancer.
Because small shop filters are not subject to Federal oversight or testing, most small shop firms rate their own filters. Sadly, to improve sales many disreputable vendors play filter rating games similar to the games they play in rating their blower performance. Frankly, they are playing with our health. Engineering practices rate filter performance when clean and new and rate filter resistance when the filter builds up a semi permanent cake of fine dust that lodges in the filter strands. This cake of dust does not normally go away with normal filter cleaning, so a new filter that starts will end up building about five times its initial airflow resistance. For instance a typical so called 1-micron bag filter starts off with a resistance of about 0.5” or less then as it becomes “fully seasoned” meaning builds up its permanent dust cake, the resistance will typically be about 2.5” after every cleaning. Because filter material makers also give the filtering level on fully seasoned filters, many vendors claim that as their filtering level. There is roughly a twenty to thirty fold difference in how fine a filter will strain when “fully seasoned” versus new. The less honest vendors make things worse by making up their own filtering claims and forgetting to include the needed airflow information. They know that if they plug a filter enough they can “prove” any level of filtering they want. Sadly, my personal testing and testing by a number of other university professors shows most small shop vendor fine filter bags and cartridge filters freely passed between ten to twenty times the sized particles claimed and stop passing the air we need for dust collection long before reaching a third of their claimed filtering levels. Add most vendors making their fine filter bags the same size as their open filters and the problems just get worse. Fine filters need far more area, so they plug more quickly. This plugging kills our needed airflow, plus drives the pressure up enough that the fine silica (glass) particles that trees use for strength end up getting pushed to cut and tear their way through turning our fine filters quickly into wide open sieves that pass most of the finest unhealthiest dust.
I now only trust filter ratings provided by an American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) certified independent testing laboratory. ASHRAE is not a government organization, but instead a private, non-profit group of professional engineers that set the standards for their industry. The ASHRAE standards require a filter when new to capture 99% of the particle sizes claimed at full airflow without any cake. Because filter testing is very expensive, most vendors pass on the ratings of the filter material they buy and do not independently test each of their filters except for HEPA filters that require each filter to be independently tested and contain a unique certification test number. Unable to find any small shop certified dust collector bags and knowing that most bags have so little surface area that they need constant cleaning which exposes me to the very dust I must avoid, I personally use industrial 0.3-micron certified dust collection cartridge filters with a cyclone to protect those filters. My doctor says those without my allergies should use 0.5-micron certified cartridges.
Early dust collection systems used bag filters where the dirty air was blown into the bags and the bags were kept clean with a shaker that turned off the airflow for a bit and shook the bags out. As vendors moved over to using more cartridge filters they were forced by early filter designs to flow the air from the outside in. As cartridge filter technology matured, that stopped becoming necessary allowing blowing the dirty air either from the outside or from the inside. Since the larger chips and blocks destroy cartridge filters, most vendors put in place a cyclone or other type of separator to protect their filters from material hits. This works great in large commercial systems where automatic bin emptying systems keep the cyclone bins clear. In small shops cyclones provide a false sense of security in terms of filter protection. With a good cyclone almost nothing goes into our filters most of the time. Unfortunately, when a cyclone’s dust bin gets full, all including pretty large blocks go right through the impeller then slam into the filters. Knowing what a mess it is when an impeller blows, I moan when I see vendors offering light aluminum and even plastic impellers. Even if the impeller survives, the heavier chips and blocks punch cartridge filters full of holes and can ruin them in seconds. This is why I recommend if you want to maximum filter life to either constantly check your dust bin level or buy an automatic detector that lets us known when it is time to empty our bins.
Setting up our filters does require a little special attention. If we blast an incoming stream of dusty air on a filter surface, that filter will soon become history at least where the air first hits. We also must have enough filter area to handle the volume of dust to keep the filter from too quickly plugging or failing. We also must ensure we have ample filter area so that the filter does not add a lot of resistance which kills overall airflow.
Many still wrongly say that we must blow from the outside into our cartridge filters, but that is mostly a hold over from the days when cartridge filter manufacture required this approach. Blowing out to in requires a filter box to route the air. Building a filter box is fairly easy. We need to ensure no airflow directly hits any filter material and provide clearance around the filter material of roughly the filter diameter divided by four. That’s about 3” for most cyclone filters. It is best to orient our filters vertically so the filters wear more evenly. With that all said is a filter box what I recommend? For most I think not.
Collect and store dust in metal cans to prevent fire danger. Yes, lots of people collect into cardboard and plastic drums, but we are going to be smarter than that! Be aware that a collection can that is too short or too close to a dust port will often end up with enough air currents inside that the fine dust will just blow out and into the filters.
Most small shop dust hoods are poor quality, use too small of an inlet port, and will have to be rebuilt to protect, control, and deliver the dust. You can look at the dust hood examples (click here) for more information on building dust hoods that will pick up the dust at the source. Also, the various Internet woodworking forums have many excellent homemade dust collection hoods in their archives. Be careful of the advice you get on these forums, much of what you will hear is from people who are still on their first round of dust collectors and so enamored that they will defend their relatively ineffective designs and systems nearly to the death. There have been some very funny, in a sad sort of way, wars on the forums as people get more concerned about addressing the fine dust.
The connection from your machine to your ducting is often a problem with small shop equipment. In looking at the CFM requirements table it is clear that larger small shop machines need 6" dust port connections or a pair of connectors typically a 5” and a 3.5”. Because the small shop industry remains in the "dark ages" in terms of only going after the same dust we would get with a broom, most small shop workers will have to upgrade their machine dust ports themselves. It is a real shame to buy a top notch piece of equipment and suddenly have to make a big 6" hole in it. For more information visit my Ducting Ports information.
Dust collectors used in large commercial woodworking concerns are designed to sit outside and move enough air to collect from every woodworking machine running at once. In most commercial dust collectors the dirty air after the bigger chips and blocks are removed by the separation ring goes up into a large bag filter. These filters are often made of very open weaved fabric or felt that lets almost all of the finest airborne dust sized about 30-microns, roughly a third the thickness of a human hair blow right through into the outside air. The rest of the material either falls into the lower collection bag or drum. Most commercial dust collectors have multiple bags arranged into what is known as a bag tree. Because the filters on these units so quickly clog with a thick cake of dust most larger commercial dust collectors come with an automated cleaning system that either shakes the filter bags clear or uses blasts of air to regularly clean the filter bags. Fire codes and building codes generally require commercial dust collectors to be placed not only outside, but in strong containment because fine wood dust when mixed with the right amount of air not only burns, it can explode violently.
Small shop dust collectors, often called hobbyist dust collectors, are downscaled versions of outdoor commercial dust collectors designed to move just barely enough air to collect sawdust and chips from one machine at a time. As a result, small shops move a flex hose between machines or install ducting with blast gates that shut off the airflow to all but the desired machine. Small shop dust collectors work just like commercial dust collectors. They use a material handling pressure blower to pull in dirty air from that one machine at a time and then run that air into a bag tree. The bag tree also uses a separation ring with the air then going into a filter.
There are some significant differences between small shop dust collectors and commercial dust collectors. Unlike commercial units, most small shop dust collectors use a lower bag that is also a filter to help make up for the filters being so small in area. The smaller the filter area, the more frequently the filters need to be manually cleaned. Unlike our commercial units that adjust airflow with pulleys, most small shop dust collectors bolt the motor shaft from a fixed speed 3450 RPM induction motor directly to a material handling impeller. These fixed speed motors make it near impossible to adjust airflow, so most vendors build their dust collection blowers as a compromise. To keep their small shop dust collectors from moving more air than their motors were built to handle, vendors carefully size the inlets and outlets to restrict the airflow so an open air condition does not cause the motor to burn up. When confronted with the added overhead resistance of our tools, hoods, ducting, and filters, these restrictions leave these dust collectors running at less than half their maximum rated capacity.
Another important difference is small shop dust collectors move a large amount of air at a relatively low pressure. This makes them good for picking up chips over a larger area but lacking the pressure to be good for vacuuming. Because these blowers generate so little pressure, they will barely compress air at all. This means unlike commercial shops where the ducting carries the air for all machines running at once, the ducting in small shops should stay the same size as the blower inlet all the way to each machine and the machine ports need to be sized the same. Otherwise, any small ducting runs or restrictions will get severely kill the airflow just like partially opening a water valve. It also means that you should never reduce down a dust collector pipe to fit a small machine, but instead should use a shop vacuum on smaller machine ports because only a shop vacuum will have the pressure needed to pull ample air through the smaller port openings.
The most important difference between a small shop dust collector and a commercial unit is usage. Most small shop woodworkers ignore fire and building codes and put our dust collectors inside. This is terrible news as most small shop dust collectors come with the very same wide open filtering material designed to pass almost all of the fine airborne dust right through. As awareness of the dangers of long term fine dust exposure increased, many vendors of this equipment began offering finer bag and cartridge filters. These filters almost always start off doing a much better job and their much smaller fibers allow more airflow, but soon these filters fail. The finer filters clog far more quickly so need a much greater filter area or some way to separate off much of the dust before it can reach and clog the filters. The cost to add an automated cleaning system is prohibitive and manual cleaning quickly wears these filters out. Worse, as these filters get caked with dust, the pressure increases enough to push the finest dust right through tearing open the filter pores. Between frequent cleaning, holes punched by flying chips, and pushing the fine dust right through, even the better cartridge and bag filters rapidly open to pass most airborne dust. Our using the outdoor units inside traps this fine dust inside where it lingers for six months or longer before dissipating. During this time airflows from our tools, dust collection equipment and air compressors launches this growing volume of previously made dust explaining why almost all small shops test with dangerously unhealthy airborne dust levels.
Most small shop woodworkers, me included, tend to buy a severely undersized dust collector after finding our shop vacuums are just not big enough. In spite of most moving half the air needed for good fine dust collection, these units work so much better than our shop vacuums that few of us realize we have a problem until someone gets ill or we buy a bigger tool that the small collector cannot keep up with. Moving about half the air needed for good fine dust collection lets far too much of the finest unhealthy dust escape capture also contributing to most small shops testing with dangerously unhealthy airborne dust levels.
Cyclones are well understood with many computer modeling programs available to help optimize their designs to separate two different weights of materials. Cyclones are well researched and broken into seven basic cyclone types plus some hybrids optimized to separate different materials or gasses. The smaller the cyclone diameter the larger the blower needed to overcome the resistance and the better the fine dust separation. It takes more work to push air in smaller circles, but tighter circles create better separation. Cyclones with longer cones provide better fine separation and more resistance because the air has to run more revolutions. The larger the cyclone diameter the less resistance and smaller blower we can use. A cone too long ends up plugging because the airflow keeps the dust from dropping. A cone too short causes the airflow to dip into the collection bin and suck the dust right out. A cone with walls either too wide or too narrow creates a wandering vortex like the bottom of a tornado that sucks the separated dust off the walls then pushes it right into our filters. The cyclone theory shows our ideal cyclone sizing for maximum fine dust separation in a typical small shop that only collects from one machine at a time turns out to be a 13” diameter cyclone powered by a 7.5 hp motor where the unit ends up standing over nine feet tall. Many high end commercial cyclones used for indoor fine dust separation are exactly this size, but none of the small shop vendors offer anything even close. If you want to find out more about cyclones and cyclone design do a Google search on cyclone design followed by a search on swirl tubes because cyclones are a special class of what researchers call swirl tubes.
The huge cyclones we see outside of almost every large commercial woodworking facility are agricultural cyclones because dust collection firms have for over fifty years relied upon the extensive agricultural cyclone research to size and configure their cyclones. For nearly a century the various types of agricultural cyclones have been used to separate things like grain from husks or dirt and sand from cotton fiber. The different types of agricultural cyclones are well studied and we even have engineering spreadsheets that when fed the various parameters and come up with the operational efficiency of each type of cyclone. One of the nicer things about all that work is these cyclones can be tuned, meaning designed to very effectively separate almost anything. They are ideal for commercial woodworking shops when tuned to separate and drop the heavier sawdust and chips into a dust bin while blowing the airborne dust out the cyclone top outdoors. These cyclones maximize internal turbulence to break the finest dust away from the heavier dust. On average woodworking produces dust that is about 15% by weight fine airborne dust with the rest heavier, so these types of cyclones have long been “tuned” to provide about 85% dust separation by weight. This means they drop close to 100% of the heavier dust and chips into a collection bin and blow near 100% of the airborne dust away outside.
Small shop dust cyclones, often called hobbyist cyclones are almost all downscaled versions of the same outdoor commercial cyclones that are inappropriate for indoor use. Most small shop woodworkers wrongly believe that these vendors are subject to government oversight and controls. Because small shops are also not subject to regulation or testing, a large group of vendors have made their careers by providing inexpensive copies of commercial equipment. Sadly their dust collectors and cyclones are so bad most will not let our shops pass even easy air quality tests. The problem with cyclones is not just making their normally less than good quality copies. With cyclones their lack of knowledge of either airflow or cyclone design has created a mess of more than a dozen vendors who keep copying each other all producing the same two inappropriate commercial outdoor designs. These designs don’t even do a good job of chip collection. In addition to these being the wrong cyclones to copy, the vendors have made these cyclones work even more poorly by adding additional compromises to address a few small shop realities:
Between addressing these issues, their lack of knowledge, and trying to get market share small shop vendors pretty much ruined their cyclone offerings with too many compromises. Most of these vendors made no upgrades until after I pointed out the problems shared my suggested solutions on my Cyclone Modifications web pages. Some of the compromises follow.
In spite of mostly being sold as fine dust collectors, almost all small shop cyclones provide small improvement over our single stage dust collectors with bag and cartridge filters that just cannot filter our air. Although most of the small shop vendors have adopted changes from my web pages to reduce the internal turbulence and increase separation efficiency, they still have not fixed the filtering problems. Most continue to pump too much dust into too open filters which quickly ruins these filters. It also causes our shops to build up dangerously unhealthy dust levels. I only recommend using these types of cyclones if you buy at least a 3 hp unit turning no less than a 14” diameter impeller, and you toss the filters then exhaust the air directly outside.
Although just about any cyclone design will separate the heavier particles needed for outdoor use, it takes considerable engineering skill and work to build an efficient indoor fine dust separating cyclone design that will work with the normal challenges found in small woodshops. My friend Jim Halbert came up with his neutral vane that helps traditional outdoor cyclones to be a little more efficient. I also added quite a few additional improvements to these types of designs detailed on my “Cyclone Modifications” page that many vendors are now copying, but even with all these enhancements those remain outdoor units in my opinion only suitable for blowing the fine dust away outdoors with no filtering and no return of the air into our shops. Finding these units too rapidly ruin filters and knowing I cannot blow the air outside, I went back to the physics and engineering to build a better cyclone.
Designing a good fine dust separation into a cyclone compatible with indoor use with fine filters requires a whole different design and considerable work. The smaller the cyclone the better the fine dust separation, but it takes a much bigger and more powerful blower to push air in smaller circles. Longer cyclone cones also provide better fine dust separation, but the cone needs to be the right proportions to keep the airflow from either sucking collected material from the bins or holding the dust too high where it eventually causes plugging. We can compute a near ideal cyclone proportions and cone length for maximum fine dust separation efficiency based upon air speed, inlet, particle size, particle weight, and cyclone diameter. Doing so ends up with a cyclone too tall to fit under an eight foot ceiling. Likewise, larger cyclone diameters and outlets provide less resistance but worse separation because their size drops the air speed needed for separating off the particles. Basically, I had to start over from scratch. My biggest challenge was reducing the high internal turbulence, laying a smooth airflow right on the inside wall of the cyclone, and then redoing the whole cyclone geometry to fit under an eight foot ceiling and still target the finer particles because almost any internal turbulence kills separation efficiency. Once I finished the calculations and theory my wife began making fun of Mount Cyclone that grew as I spent the next two years refining that theory into a more practical solution. Interestingly, I was not alone in this quest as Larry Adcock announced his WoodSucker design about the same time as I shared my cyclone plans. He did a great job both on his design and special more efficient blower.
Although it took considerable work with much iteration I finally came up with a workable cyclone design that fit under a standard 8’ ceiling while fixing the high horsepower losses and terrible separation. My design uses a minimum of power to efficiently separate off most of the finest particles so they don't clog our filters. It minimizes the air turbulence from the parts to dramatically reduce resistance and motor horsepower requirements while significantly increasing fine particle separation efficiency. It took designing a whole new type of inlet to carefully use a laminar flow to stabilize and start the air stream right on the side of the cyclone. It also required a new cyclone interior design geometry. I also had to engineer my own blower then test various motor and impeller combinations to power my cyclone and move enough air to ensure moving the air needed at our larger tools for good fine dust collection. Many said making my design to use a smaller motor was a waste of time because fine filters only need cleaned a couple of times a year with existing cyclone designs. I mostly only smiled knowing the reason those filters need so little cleaning is they freely pass almost 100% of the airborne dust. I knew from upgrading to finer filters that these cyclones packed the filters near solid with dust if not cleaned every twenty minutes when I used my big sander or routed MDF.
Filters in dust collection were originally used in outdoor systems to capture chips and larger sawdust particles that don’t just blow away into the outside air. Fine airborne dust is mostly made up of particles sized about 30-microns and smaller. As a result, most outdoor filters were made to freely pass particles 30-microns and smaller. With fire and building codes requiring placement of most dust collection outside, these filters simply let the fine dust blow away into the outside air. The problem is when brought indoors these types of filters which are still the standard on most small shop equipment are “dust pumps” that fill our air with huge amounts of fine dust. This fine dust poses an immediate hazard that we can pretty easily address by wearing a good NIOSH approved mask. I personally like the 3M 7500 model cartridge mask of the right size to properly fit my face.
Unfortunately, this fine dust problem does not just go away after we finish our woodworking, but instead builds to often very dangerously unhealthy levels. It takes six months or more for bacteria, mold, mildews, and fungi to break this fine dust down and for normal airflow to dissipate this dust. During this time if we keep adding more dust with more woodworking, the fine dust levels just keep growing and growing. Every time we work the airflow from our tools, dust collection, vacuum, fans, and air compressors launches this fine dust over and over. With fine dust spreading rapidly in any shared air and so easily carried on our clothes, skin, and hair we tend to contaminate all areas we visit while woodworking including our homes and vehicles. Soon, instead of the fine dust being a shop problem, it becomes a twenty four hour a day seven day a week problem for us, all close to us and our pets. Knowing it is near impossible to capture all the fine dust as it is made we need to regularly blow out our shops. Still, with a little work ensuring we have ample airflow, have fixed our tool hoods, upgraded to large enough tool ports, and have ample sized ducting we can capture most of the fine dust. Once it is captured, I recommend using a cyclone separator just like the large commercial woodworking facilities then blowing the fine dust away outside. Sadly, some like me live in areas where blowing that dust outside is illegal, or live in climates far too extreme to allow blowing outside. For us, we have to clean our dusty air before returning it into our shops.
There are a few ignorant people pushing bag type filters for indoor dust collectors and cyclones. The heavy all poly felt material is great stuff and exactly what I recommend in a 30-micron material for outside use. Indoors, using this kind of material even in fine filter bags is not very wise. Almost all small shop dust collectors and cyclones come with a standard bag filter unless we pay for an upgrade. Most stock filter bags that come with this equipment are made from open filtering material that freely passes airborne particles up to 30-microns in diameter all day long turning our indoor dust collection equipment into dangerous “dust pumps” that recycle the most dangerous dust in our shops as long as they run.
As more and more small shop workers become aware of the need to improve their fine dust collection, many vendors are now selling what they call fine filter bags. Most small shop “fine” dust collector bags pass between twenty to thirty times larger particles than small shop vendors advertise until so clogged they barely pass air. There are a few reputable filter bag makers, including AAF, Highland Hardware, and others, but not many that admit their real filtering levels. The reputable filter bag makers admit that their fine bag filters will provide us with little to no protection from the finest 2.5-micron and smaller unhealthiest dust that medical testing shows leaves close to 100% of all woodworkers eventually developing long term health problems, some serious including sinus and respiratory damage, cancer, polyps, etc. As we increase the fineness of our filters we must increase the filter surface area, but most filter bags are made roughly the same size so most quickly clog requiring frequent cleaning to stay effective. Worse, that clogging increases the pressure which forces the fine silica (glass) particles that make wood strong to cut and tear their way through the fine filter strands soon leaving even a really good filter a useless sieve. The cleaning we need to do to protect our filters not only gives regular "dust baths" in the very dust we need to avoid, but also quickly wears out filters. In short, if you use small shop vendor filter bags, your dust collection system needs to be outside.
Dust collectors blow a very fast moving stream of air into their filters. Heavy chips in that air stream punch holes in the filters eventually making them too open. Likewise, most dust collector bags and cartridge filters also have too little surface area, so frequently clog killing the airflow needed for fine dust collection.
For maximum health protection we need to use cartridge filters rated by a certified lab with ample surface area. Cartridge filters provide far more surface area in a smaller space to greatly improve airflow. Their disadvantages are they cannot take direct material hits and will clog quickly unless you use a pre-filter or separator.
Although many small shop vendors have come out with their own cartridge filtered dust collectors and cyclones, most small shop cartridge filters have serious problems from not having ample surface area and being made with filtering material that is so open it freely passes most of the finest unhealthiest dust. Although many now choose the convenience of using one of the new cartridge filtered dust collectors, many woodworking operations will quickly clog the filter pleats making for continual maintenance problems and the cartridges wearing out fairly quickly.
To amply protect our health, we need to use independently certified cartridge filters that provide ample fine dust filtering and sufficient area to do a good job filtering the fine dust.
For those unable to afford the time or money to invest in a cyclone-based system with good cartridge filters, I recommend buying one of these cartridge filtered based dust collectors, or better yet, modifying your own dust collector to use a better cartridge filter (see Adding a cartridge filter to a dust collector). Using a trash can like separator to try and get that dust before it gets to the cartridges is a problem. These separators that work wonderfully with 4" ducting hardly work at all with the higher airflows and 6" ducting needed to collect the finest, most unhealthy dust at the source. Almost all go with use of a cyclone separator as the only other really viable option other than buying a commercial dust collection system. Still, if space or money are significant constraints, buying a good quality Jet cartridge unit or even better yet, add your own cartridge to the dust collector, as a good fine cartridge will afford far better protection than even the units with so called fine filter bags. For more information see the Jet Cartridge dust collector write up on the Cyclone & Dust Collector Review page.
A fan uses a motor to turn a blade that pulls air in then pushes it out at a higher speed. How well a fan performs depends upon many factors, including type and number of blades, size, blade speed, motor horsepower, air density, air temperature, etc. Fans are rated based upon their performance measured in terms of how much air gets moved, at what speed, and at what pressure. Most dust collection fan performance is measured in Imperial units (feet, inches, and pounds). Airspeed is measured in feet per minute (FPM). Air movement volumes are measured in cubic feet per minute (CFM). And, pressure is measured in terms of suction pressure, known as static pressure. We measure static pressure by recording how many inches up a tube a fan can suck a water column (w.c.).
Fans move air by turning different kinds and shapes of blades ranging from flat like a ceiling fan to complex spiral and airfoil shapes. Most fans look like common table, box and ceiling fans that use fan blades to push air through the blades. Although small shop woodworkers often need exhaust and cooling fans, most fans are not suitable for dusty use or able to power a cyclone or dust collector. Most do not move enough air volume, and even if they do move enough air they don’t generate enough pressure to overcome the resistance of our ducting, filters, and separators. Many are also unsuitable for dust operations because they have their motor in the air stream for cooling. Dusty air builds up a cake of dust that will overheat the motor resulting in eventual motor failure and possibly a fire. Almost all dust collectors and cyclones use a special fan known as a centrifugal pressure blower. Rather than cover every type of fan, this only touches on the most common and those most used by woodworkers.
Many small shop workers need to use a good exhaust fan in their shops to quickly blow out high concentrations of dust. An effective exhaust fan should move enough CFM to change out all the air in your shop at least once every three minutes. To compute the cubic feet of air in your shop multiply shop length times width times height. Divide that by 3 to get the minimum sized exhaust fan, but more is better. Even with this roughly twenty air changes an hour, it still takes up to two hours to bring really dusty shop air back to safe. During this time you should either stay out of your shop or wear a good cartridge filter dust mask.
Many also use a cooling fan to blow air on them when they work to keep away the fine dust as it is made. It only takes a little airflow for these to work well. For years I used an inexpensive box fan placed in one door of my shop and opened the other door. Now I use the same fan I use for exhausting my shop just turning it around. These fans will blow the dust back in your face unless you put them behind and to your side.
Inline fans are another type of HVAC fan. These fans sit inside ducting and push the air through. They are often used in very long duct runs to boost airflow. Although inline fans can generate plenty of airflow and often enough pressure for good dust collection, they still make poor dust collection blowers. They depend upon the air stream to keep their fan motor cool so pose fire hazards when used with dusty air.
Blowers are a special type of fan. Blowers use a motor that drives a fan known as an impeller inside a case (shroud) that directs the airflow. Dust collectors and cyclones use a centrifugal blower to move a large volume of air at a lower pressure than a vacuum. A small shop blower will provide between 4" and about 15" static pressure. That's about 1/20th of a vacuum. The advantage of a blower is it moves a huge amount of air ranging from about 450 cubic feet per minute (CFM) for a 1 hp blower to around 2300 CFM for a 5 hp unit. Air is sucked into the side of a blower and hits the impeller. The impeller is a spinning large flat disk with vanes. These vanes and centrifugal force sling the air out sideways through the blower exhaust port creating a vacuum that draws more air into the blower inlet. Blowers do the most work with an open unlimited amount of airflow and the least work when all is closed off and there is no airflow. That makes sense because the more air a blower's impeller pushes against, the harder the motor has to work. When there is no airflow, the motor only has to keep the same air turning in a circle, so the motor does the least work. Most dust collector makers use an impeller that they size small enough to prevent a blower from doing so much work that it would cause the motor to overheat. A few instead put some form of restriction on either the blower inlet or outlet to limit how much air can pass. One seriously wrong myth about blowers is you need to turn them off when the airflow is closed. Leaving them running with no airflow lets them loaf without doing hardly any work. What can happen is a powerful blower will collapse thin HVAC duct and thin cyclones if all the gates get closed!
Many write asking if they can use Heating Ventilation and Air Conditioning (HVAC) blowers for dust collection because these blowers are so common and inexpensive. HVAC blowers typically use a squirrel cage type blower. Squirrel cage blowers have a round slotted cylinder that sucks the air in from the sides into the center then pushes it out a blower outlet. Because HVAC Blowers can move plenty of air in terms of volume they make good air cleaners. They make terrible dust collector blowers because they cannot generate the pressures needed to overcome the resistance of our hoods, ports, ducting, separators, and filters.
Most dust collectors and cyclones use a special type of centrifugal pressure blower for dust collection. Centrifugal blowers move a large volume of air at ample pressure. They use a special kind of fan known as an impeller. An impeller is a flat disk with vanes or blades attached to one side. The spinning blades sling the air off sideways with considerable force (note arrows in picture show impeller rotation direction). The spiral shaped blower shroud directs that air to an outlet. The exiting air creates a vacuum that pulls more air through the blower casing or shroud known as the blower housing inlet.
Centrifugal Blowers create considerable noise. They make lots of low frequency noise that is tough to control. Just like a big subwoofer it goes right through walls and ceilings (I know thinking about my daughter as a teen with a big subwoofer in her room and later her car). They also generate unbelievable amounts of high frequency noise. In fact air raid sirens that can be heard for miles are centrifugal blowers with tight clearances that turn at high speeds.
Impellers are the fan inside pressure blowers that push the air. Dust collection impellers known as either material movement or radial impellers are flat round plates with fins. They work by slinging the air off the blades creating a vacuum that pulls more air into the center of the blower. Air engineers name blowers based upon the type of impeller blade used. Blower impellers are chosen both to address the noise issues and how dirty the air they need to move. Dust collector makers use very tough but inefficient heavy steel or aluminum impellers built to take hits from blocks of wood and other shop debris that might get sucked up. These designs known as either material movement or material handling impellers limit the buildup of wood strings, shavings, and other material that could cause the impeller to become unbalanced.
Although tough, the material handling impellers made with straight vanes going out from the center creates what are called radial blowers. Radial blowers are not much used on dust collectors because they make too much noise. This noise is difficult to control. Just like a subwoofer the low frequency portion of this noise goes right through walls and ceilings (I know as my teen daughter had a big subwoofer). Tilting the vanes away from the direction of rotation creates what are called backward inclined (BI) blowers. A BI impeller reduces the noise but is not very efficient. Designers use more or bigger vanes to make up for efficiency losses. Tilting and curving those blades away from the direction of rotation creates what are called backward curved (BC) blowers. A BC impeller reduces the noise more without quite so much loss in efficiency. Curving and tilting the vanes away from the direction of rotation reduces the noise but requires the maker to use more vanes and larger motors to make up for efficiency losses. Most better dust collector and cyclone blowers use backward curved impellers while the less expensive and poorer quality use radial impellers.
If you have a cyclone before the blower that protects the impeller, then in theory you can use a more efficient caged impeller. A caged impeller has a ring on top of the blades that keeps the air from spilling off the top of the blades allowing us to get more pressure and efficiency. High-pressure blowers, for instance our vacuum cleaners, use caged impellers often turning at a very high speed. In practice a cyclone with a big air leak, open dust bin, or full dust bin ends up pumping everything right through the blower. Allowing this kind of debris to hit a caged impeller is only asking for trouble. Almost any buildup of materials immediately throws our impellers out of balance and that can quickly ruin motors and motor bearings, plus lead to our impellers exploding. Most vendors do not want to build a system with this much risk, so few will use a caged impeller. The WoodSucker II cyclone that is no longer made used a caged impeller that is so efficient it let that 2 hp cyclone system perform like 3 hp units.
When challenged with no additional 240 volt power in my shop, I chose to use an even more efficient caged impeller I had custom made. Normal airfoil impellers replace the flat vanes with airfoil shaped vanes. These hollow vanes would quickly wear out if constantly hit with sawdust and larger pieces of wood, so airfoil impellers should be placed after the filter so they only handle clean air. In addition to the normal problems with buildup of strings and shavings, airfoil impellers (AI) also have a serious problem with stalling if they don't have enough airflow or the pressure gets too high. That stalling causes buffeting that can destroy a motor, motor bearings and even our airfoil impeller quickly. To use and airfoil based blower you have to have a system with a very low overall resistance and also use a custom designed airfoil. The one I helped with is much heavier in weight, and a special modification to let it handle more pressure before it begins to stall. Even so I still had to make sure I always ran my airfoil with at least one 6" blast gate open. Eventually I added a weighted trap door that opened when the pressure got too high to keep my airfoil blower from stalling. Many don't want to mess with this overhead and just use the less efficient material movement impellers. I personally went with an airfoil unit to get the flow I wanted and still be able to use a smaller motor that would work on the power available then in my shop.
Impeller sizes are carefully set by dust collector makers to keep an "open air" configuration from causing a motor to work beyond its design capacity and burn up. This leaves most dust collector impellers on the small side. That is a big problem because every portion of a dust collection system adds resistance including ducts, dust hoods, filters, separators, and even cyclones (see my Ducting page for a resistance calculator). With too much resistance dust collector blowers become "air-starved", meaning they can't get enough air to operate effectively. This is an interesting balancing act. A vendor that makes a dust collector that works well for a large shop full of ducting will have their dust collectors burning up when used in a shop with minimal to no ducting. Our buying a blower big enough to overcome that resistance ends up with our paying for a much larger motor that barely gets used.
Dust collection blowers come in two flavors, material movement blowers or clean air blowers. Clean air blowers used in dust collection use caged and airfoil impellers. Almost all dust collectors instead use material movement blowers with impellers that only get a maximum of about 45% power efficiency. Caged impellers get closer to 65% efficiency and airfoil blowers get up to 80% efficiency. Efficiency is very important for blowers because we have to cube the horsepower to double the airflow. Regardless, almost all dust collectors and most cyclones use material movement blowers because of the problems with caged impellers discussed previously. Dust collectors and cyclone need a tough blower housing and impeller that can take material hits. They also need “self cleaning” impellers to keep from building up debris and strings on the impeller that can throw the impeller out of balance and quickly ruin motor bearings. If the blower is on the clean side of the filters, a much more efficient clean air caged or airfoil blower can be used.
Those with larger shops or who do not want to go to the bother of constantly checking and cleaning their blower should use a material movement blower. Most material movement blowers used for dust collectors and small shop cyclones attach the motor shaft directly to the impeller to make them less expensive to build. This poses some tradeoffs. In addition to rarely running at more than 40% efficiency, these units typically turn at normal induction motor speed of 3450 rotations per minute (RPM). That prevents them from having their speed adjusted for optimum impeller performance, so we have to adjust impeller diameter to get optimum motor performance.
The only small shop cyclone that used a caged blower was the WoodSucker cyclone that is no longer made. These impellers were a heavy caged backward inclined impeller that moved about the same air volume with its 2 hp motor as do most other cyclone vendors with a 3 hp motor. Even with its heavy steel impeller, it still needs regularly checked and cleaned. My even more efficient airfoil impeller moves about the same air with a 2 hp as other cyclones with a 4 hp motor. Although far more efficient, these designs come with costs. Even when protected by a cyclone our impellers can be hit with heavy blocks and debris whenever we have a bad air leak or full collection drum. To handle these potential material hits, these impellers need to be put on the clean side of our filters or made very heavy. Heavy impellers are hard on motors when coming up to speed. They also must be regularly checked and cleaned, especially after an air leak or overfilling a dust bin. Only those willing to do regular maintenance should consider using a caged or airfoil impeller. I personally built my airfoil blower with clear polycarbonate plastic sides that let me regularly inspect the impeller for build-ups.
Airfoil impellers have one more significant drawback because they stall if the pressure gets too high. When the blades stall they do so unevenly creating severe chatter and destructive vibrations that can quickly ruin motor bearings and potentially cause the impeller to fail explosively. Too much ducting, large cyclone, etc. cause too much pressure. That makes airfoils inappropriate for any shop with considerable ducting, an inefficient cyclone, or plugged filters. I helped address this problem by working with Paul Payton, an air engineer to design a custom airfoil with a special internal baffle that raises the stall pressure from 6.5" of static pressure to just over 8”. To use this special airfoil requires minimum ducting, large all 6” diameter ducting, and a special air valve that opens whenever the pressure exceeds 7". This is the equivalent of always running my unit with one gate open to ensure enough airflow to prevent stalling.
Air-starved blowers due to dirty filters, undersized duct, plugging, closed blast gates, etc. are fixed one of two ways. Industrial blower makers change pulley sizes to speed up the rotation of the impeller. This faster speed increases pressure to compensate for the overhead, but most impellers are not well enough balanced to speed them up very much. Small shop blowers are direct drive with fixed speed motors (normally 3450 RPM with 60 cycle current) so need a larger impeller with sometimes a bigger blower housing and motor. Caution! Often upgrading impellers immediately voids our dust collector warranty because running a big impeller without enough resistance will quickly burn up a motor. Although some vendors use the same impeller with different horsepower motors, the get no better performance because each of their motors still turn at the same 3450 RPM speed. The only time adding horsepower without increasing impeller size is when a motor is too small to start with.
Much of the safety and efficiency of a blower as well as how much noise it makes depends upon the shape and design of its housing. Do not take blower designs lightly because without a good heavy housing, the quickly moving impeller has the potential to launch junk and even explode with over twenty tons of force. Almost any blower shroud will work to move air. For good dust collection and noise control we need a well engineered blower housing. Air coming into the blower is controlled by the size of the blower opening. That opening needs to be carefully sized to allow full airflow without restriction or we lose considerable performance. Additionally, that inlet needs to be small enough that we never get into an open air situation where the blower motor tries to push so much air it quickly burns up. Likewise, the blower outlet needs sized carefully to not also add performance killing resistance. In addition, the blower shape and internal clearances are important. Air pushed off the blades gets channeled by the blower housing to an outlet. The housing needs to be made with a smooth even opening spiral known as a volute. Without a smooth evenly opening curve, you get performance killing pressure waves and bad vibrations that can ruin motor bearings and cause the impeller to fly apart. Finally, the only difference between a pressure blower and an air raid siren is impeller speed and internal clearance between the impeller and housing.
Although dust collection blowers are fairly simple, as many small shop workers have found out, mistreating one or making your own incorrectly can be dangerous! With 20,000 pounds or more of force, making a mistake can result in catastrophe. As we move from “chip” to fine dust collection, the chances of making a bad blower mistake increase greatly. The tiny blocks of wood that our chip collectors could pick up turned into sawdust when they hit our heavy impellers. With more than double the air volume we can now pickup small 2x4s and much larger items for a potentially much bigger problem. Additionally, all impellers can build up heavy deposits of dust and sap, so should regularly be inspected and cleaned because too much of a buildup of sawdust and resin can create an out of balance condition that can ruin motor bearings and cause our impellers, especially the light weight ones to fail and possibly fly apart.
Almost all small shop dust collectors use an electric induction motor with its shaft directly turning an impeller that turns at about 3450 rotations per minute (RPM). Most blower induction motors turn at 3450 rotations per minute (RPM) and use an external cooling fan. Better quality motors are totally enclosed fan cooled (TEFC) motors. TEFC motors not only have sealed bearings, the entire motor is sealed to prevent dust and other contaminates from clogging up the moving parts. Many inexpensive import motors are not sealed and end up failing from dust fouling up their starting circuits.
Most small shop equipment today uses extreme sales numbers to rate their motor at absolute maximum power instead of rating them as to what they can really provide on an ongoing working basis. Running a motor for very long at over its working horsepower rating will cause it to burn up. With wrong information or no information from the vendors, most who build their own cyclones find they have to check with an amp meter to ensure the motor is not pushing so much air that it overheats and fails. Inexpensive (<$30) clip-on amp meters can be purchased at small shop electronic stores, Sears, and many large hardware stores. With the dust collector motor as one of the most heavily used in your shop, getting a poor quality motor does not make good sense. Unfortunately, the cost difference between a top of the line American made Baldor or Leeson motor and an inexpensive dust collector import motor is about double. If you have to go with the less costly motor, get one that is at least 25% bigger than you need to minimize the potential overheating problems.
Although almost all small shop blower motors are single phase, industrial blowers almost always use three-phase power. A three-phase motor has three powered wires. This permits these three phased motors, known as industrial motors, to be made without the complex starting circuits and to run with far less vibration. They tend to be a far better solution for powered tools because of that additional smoothness, longevity, and lower cost. Unlike standard single-phase induction motors, they also easily adapt to speed controllers to make them variable speed. Unfortunately, to make them work, unless you have three-phase industrial power available, you need to buy or build a power converter. Although building a power converter from a large industrial motor is not that difficult, it is way beyond the scope of dust collection. You can always do some Internet searching if you want to understand more about electrical power and the induction motors used with dust collection blowers.
If you want to learn more about motors you might want to visit this excellent military electric motor training, maintenance, and trouble shooting (http://www.usace.army.mil/publications/armytm/tm5-683) web page. It has a full chapter on motor starters and overload protection circuits and methods. Three-phase motors don't have this problem, as they don't need the capacitors to shift the fields for startup. Also make sure you check out Dust Collection Electrical and Safety.