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Dust Collector Filters

1.      Filter Basics

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

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

a.      Filter Types

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

b.      Filter Life

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

c.       Filtering Level

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

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

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

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

d.      Filtering Resistance

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

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

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

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

e.       Filter Sizing

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

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

With typical 30-micron all polyester filters appropriate only for use on outdoor dust collectors and cyclones that return no air into our shops, we need about one square foot of filter area for every 25 CFM of dirty air. With “chip collection” only moving about 400 CFM these filters only need about 15 square feet of filter area. Boosting the airflow to the 800 CFM we need for good fine dust collection pushes the size of these open outdoor filters to needing at least 30 square feet of filter area.

With typical 0.5-micron all polyester filters used for indoor air filtering we need about one square foot of filter area for every 4 CFM of dirty air. This means our 800 CFM needed for good fine dust collection at our larger tools requires 200 square feet of fine all poly filtering material and at least 400 square feet of the blended filtering material. Fortunately, both materials come folded and made into cartridges that enable us to get this much filter area in a small space. An open filter that freely passes 30-micron sized dust only needs about one square foot of filter area for every 50 CFM of dusty air run through the paper blended filters and about one square foot of filter area for every 25 CFM run through the all spun bond felt filter material.

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


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