In the previous post, I told you about how I thought my dust collection system was good enough and kept workstations clean, until I noticed a layer of fine dust all over the dust collector and the surrounding area. This made me realize that while the system is picking up fine dust, it’s also spraying it all over the shop.
Fine dust stays suspended in the air and moves around at even low air speeds so for sure it’s making its way back to my nose.
I decided to dig even deeper into my own system, and make it more like systems I’ve designed for larger shops. There is a balance of cost and performance, but if you make the right choices you can get a kick-butt system for relatively low cost, and upgrade in the future as your funds allow.
It’s not often you think about your dust collector filter, but without a good one you aren’t accomplishing what your dust collection system is entirely meant for: dust extraction and containment.
A filter that’s too porous won’t contain the dust, it’ll spray it back out at you. That’s kinda dumb. If you have an undersized filter, you won’t get enough airflow (cubic feet per minute, or CFM) at the tool and it won’t pick up the dust in the first place.
My Old Crap
My old system was basically an “upgraded” plain felt filter bag, supposedly filtering down to 5 microns, and a lower “breather bag” made of woven polyester which let 30 micron particles through. The filter surface area was also way too low, causing a higher pressure drop than it should. The inside felt surface wasn’t properly treated to hold the proper dust cake, so the material overloaded with fine dust which further increased my pressure drop.
I also found the top filter bag spring clamp device too hard to use (it would often pop open), and the lower collection bag has a ring that fits on the inside diameter of the barrel ring. This sometimes springs back and coats me with dust when I empty it. Totally makes me want to wait until the last minute to unload it.
Now the 1.5 hp Jet DC-1100A itself is pretty good; I get about 567 cfm at my planer even with the restrictive filters, rough flex hose, and undersized ductwork. I’ve had no problems with the impeller, motor, or housing. I’ll probably keep it for now, and upgrade to a 3 hp system later on. I ran out of room on my panel to add another 220V breaker, so I have to upgrade that too.
Goals to Aim For
I decided I wanted my new filter to do certain things, like:
- Filter most fine dust particles down to 1 micron
- Have low static pressure loss to my system
- Increase the CFM to each tool
- Easy unloading/ emptying
- Easy maintenance/ cleaning
To do this I decided on a singed felt type filter bag rather than a cartridge type filter, from American Fabric Filter Company. The 16 oz. felt has a clean filter air permeability of around 18-35 CFM/sq. ft., so AFF recommended sizing them for greater capacity than that for when the dust cake is fully developed. These bags are sized at 14 CFM/ sq. ft., assuming around 800 CFM. Upon upgrading to a more powerful collector I’ll just get a tow-barrel collector and order a second filter bag so the total filter surface area is enough for the increased CFM.
Unpacking the New Bag Filter
Weeeeeeeeee! I love when new stuff shows up. Buying things online makes it like Christmas on random days.
Ok. I ordered a top filter bag, a bottom non-breathable (sealed) cotton-duck canvas collector bag, and some band clamps to hold them in place.
I asked for loops at the top of the filter bag so I could make a ring and post out of metal conduit to help maintain the shape of the bag when the dust collector is off. I don’t want it to collapse and break up the dust cake until its fully seasoned. Plus it looks cooler.
The bottom bag is an airtight duck canvas. The reason I ordered this piece is that I don’t want a 5-6 mil solid poly bag (for looks mostly, and durability too). I don’t want the bottom bag to “breathe” because it would have to be the same singed felt material as the top bag. AFF doesn’t recommend using felt as the collector bag because it’s not meant to support the weight of all those chips and dust, especially when full.
Plus, you don’t want to count on the lower bag as part of your filter media square footage. When the bag gets full, then you’ve just cut your filter area, which will increase the static pressure and lower the CFM you get at the tool. So all of my filtration is designed to be from the top bag.
These 1/2” band clamps have a spring-loaded screw that functions as a quick release for convenience. My previous bag attachments (described above) really sucked. On AFF’s website, they say that for critical situations they recommend using 1-1/4” wide by ¼” thick high-density closed cell foam gasket tape, which I might do just to make sure I don’t have any blow-by.
When I unpacked the box, I was impressed with the quality of stitching, the fabric itself (including the singed inside surface), and the packing job. I ordered 2 band clamps, and they gave one extra (very cool I think). The overall turnaround time was pretty quick, and they’ve answered all of my questions thus far so the customer service has been really awesome.
Installing the Filter Bag
I removed the “old bags” first. While I was at it, I wiped down the collector frame and motor housing to start with a clean slate. While that was drying in the sun, I prepared the new filter bag for installation.
To do the lower dust bag first, I threaded the ½” band clamp through the belt loops. Then I noticed that they only sewed on two of them. I was expecting at least 4. With only two, the top of the bag doesn’t hold its shape and folds in on itself, making it hard to unload the dust into a trash can, and re-install. The canvas is quite thick, but having 4 loops will allow the band clamp to keep the bag opening’s circular shape.
So, I went ahead and sewed on my own loops from nylon straps. Yep, I was sewing. I shouldn’t have to do that; 4 should be the standard.
Now with 4 loops, I wrapped the bag around the barrel. Slightly loose, but cinches down well except where I had to fold the fabric to get rid of the slack. I solved any potential leakage problem by taking a piece of self-adhesive foam weatherstrip tape and placed it on the barrel underneath the folds. This way the voids are filled by the compressed foam tape for a good seal.
The upper bag fit perfectly, with no slack, and it sealed very well. Here they gave 4 loops, and also they gave 4 hanging loops on the top too. All very nicely done. The top diameter is a whopping 40”, so it spanned two truss bays. I used rubber-coated steel hooks with threaded tips to suspend the filter bag. Later I may make a hanger system out of copper pipe that attaches to the collector barrel so the bag’s not connected to the rafters.
Overall, with the 4 loops on the top bag it keeps its shape nicely, which is good for maintaining a good dust cake. It looks like a hot air balloon.
Measuring the New Airflow
As you may know, I bought an anemometer. This way I can actually measure the airflow to see how much is actually being drawn from each machine, and if it’s close enough to the recommended values. I have the ACGIH handbook (1998 edition), where they list the recommended CFM for different tools. The more CFM you have the further out you’ll grab the smaller particles from the collection point.
The ACGIH handbook has a tutorial on the subject of distance from your dust collection inlet and capture of fine dust. To summarize, just put the inlet airflow really close to the actual source, like 6”-12” close. After that distance, the air velocities can’t capture and draw in these tiny but dangerous particles. In the Dust Collection Resource Page, I’ll walk you through what I’ve learned from reading that (yeah, I actually read it).
Part of the reason I replaced my old bag filters with these is because I wanted to decrease my system’s pressure drop. Doing this requires increasing the filter media surface area, plain and simple. If you have lower pressure drop, you have more CFM.
Ok, now to my airflows. Before I took apart my dust collector to take measurements for fitting the new bag, I measured some velocities to get the CFM. For example, I measured 567 CFM at my planer. This was while the small surface area bag was installed, although the lower bag, still a “breather” bag, wasn’t quite full. It’s a woven material at like 30 micron or worse filtration, so the filter side of my system wasn’t overly restrictive to begin with, but not as good as it should be, but it let fine particles out.
There’s a 4” duct branch to my planer coming from a 5” main and 6” duct riser from the collector, and about 5 ft. of 4” flex duct. If I wanted more CFM, I could increase that duct and flex to 5”, the tool connection to 5” (and still keeping at 4000+ fpm velocity, where Velocity = CFM/Duct Area). Or I could get more filter surface area. Or both.
With the new filter, I’m getting 726 CFM at the same measurement point, or 8,325 fpm velocity! I was not expecting that much of an increase. The combination of a fan inlet improvement, plus the huge filter surface area have dropped the losses enough to give me a 21% increase in airflow. The ACGIH recommendation for a 13” planer is 785 CFM, so I’m happy for now. I know if I upsize the branch to 5” I could get more.
But because most tools come with a default 4” connection, I ran the branch at 4”, but I increased the main to 5” to relieve the static pressure a bit. If I take the time to increase the tool outlet, I see an opportunity to upsize the branch too.
I also took a measurement at the jointer (8” blades). With the old filter I was getting barely 300 CFM. Now I’m getting 552 CFM which includes 9 ft. of flex hose. If I take off the 9 ft. of flex, I get 680 CFM. As you can see, adding just 9 ft. of 4” flex hose in this CFM range results in an 18% drop for my collector fan and impeller. The ACGIH recommendation for a jointer greater than 6” is 440 CFM. Even so, I plan to increase this branch drop from 4” to probably 5” because I want more CFM since I notice shavings can get caught up too easily in the hood connection.
WoodChip Tip: If you think about it, if you’re using one tool at a time, the duct size should be the same from the tool opening-to the flex- to the branch- to the main to the collector, particularly at major tools like the Table Saw, Jointer, Planer, Drum Sander, etc. Some tools have two connections (like your table saw might have a cabinet duct and an overhead blade guard duct) and they should be connected by a wye fitting, and go from there back to the collector with the proper duct size.
Keep in mind that the current filter bag is new, with little dust cake on it. But, the singed felt is supposed to hold onto the proper amount of dust cake and let the rest fall off when you tap the outside of the bag. Regular felt will get loaded up ( I saw this when I cleaned the old ones to store away—an amazing fine dust pile was dumped out of what looked like a relatively clean inside surface).
In the future, with the dust cake in place, I don’t expect a huge drop in CFM since the filter bag was oversized on purpose to account for this. I’ll take future readings and keep you posted on what happens.
I got the BIA filtration efficiency paperwork with these filters showing 96-98% efficiency at 1-micron, for clean bags. This one was for 12 oz. singed felt, so my 16 oz. singed felt will perform even better. As the dust cake develops, it’ll only get better with time.
CFM vs. Static Pressure
Your dust collector should have an associated “Fan Curve”, which tells you what CFM it’ll give you at certain static pressures. For example, if I calculate that I have 4.75” static pressure after adding up all of my ductwork and filters, etc., the curve might tell me I should get 655 CFM. But I calculated that 4.75” static pressure at a CFM I guessed at. So, I take the 655 CFM from the fan curve and re-run my static pressure calcs using 655 CFM.
This will determine what the new static pressure would be at 655 CFM. I keep “reiterating” with the new CFM’s until it narrows down to where both numbers are true (meaning that the CFM works for the static pressure calculation, and the fan curve).
If I move my finger along the curve to a lower static pressure, I can find the higher CFM I want and upgrade my ducts, fittings, and/or filter to get that static pressure. So, if you’re starting your design today, do some layouts and calculate the resulting static pressure at the longest duct run, and check your higher CFM-eating tool duct runs as well, so you know what collector to buy.
Your calculation needs to give you the recommended CFM at each tool (assuming you run one at a time) at the static pressure associated with your shop’s duct run to that tool. In another article, I’ll detail how you can do that, and when I finish my spreadsheet, I’ll share that with you too.
If you’re about to buy a dust collector, and a manufacturer refused to give you the fan curve, just don’t buy it. That means they’re embarrassed to show it. I have a Penn State Industries catalog and they publish theirs. You might be able to search online for tests others have done on the model you’re planning to buy.
While I’m messing with the dust collector, I noticed that I have a long-radius 6” diameter elbow connected directly to the fan impeller housing. I know from my engineering time that a “system effect” occurs when you have fittings too close to your fan inlet. The ACGIH manual and SMACNA help you calculate the pressure drop that results from this, and it does make a good bit of difference.
For now, know that your goal should be to have straight duct into your fan at approximately 6 duct diameters in length. This means if you have a 6” main riser, you need 6” x 6 = 36” straight duct at your inlet before you install an elbow. This may not fit your shop, so you might compromise this a bit, but do the best you can. Here, some is quite a bit better than none.
For my shop, I can easily fit some additional straight duct here, so that’s what I’m doing. I angled the dust collector in its corner so I can fit exactly 3 ft. or 36” of straight duct from the inlet to the duct riser. As I say in my workshop layout articles, why does everything have to be parallel to the walls? Position stuff at an angle if it works better that way.
Using the spreadsheet I’m making, I entered in 700 CFM to a 6” duct to see what the static pressure would be with:
- 6” (1.5 R/D) elbow right at the fan inlet
- 36” of straight 6” duct at fan inlet then the 6” (1.5 R/D) elbow
With the elbow right at the inlet, this condition causes 1.21” static pressure! If I add the 3 ft. of straight duct (6 duct diameters), then it’s only 0.285”. This is quite a difference. That’ll buy you several elbows, or partially make up for other poor design choices just by doing this simple thing. No wonder I’m getting so much more CFM, even more than I predicted.
What Goals I’ve Achieved with the New Bag Filter
The goals I was aiming for (listed above) were to filter smaller particles, lower static pressure, increase the CFM, streamline the dust emptying process, and reduce the hassle of filter cleaning maintenance.
So how did I do with each?
On the filtration front, I no longer see fine dust build-up on the dust collector frame, even after running some MDF through the table saw. Before I’d see this all the time, and when I noticed it after cutting padauk, I wiped it clean and after a few cuts it was back. Well no more. I know I’m breathing better air.
I purposely chose the 16 oz. singed felt (I could’ve gotten 10 oz. or 12 oz.) to get the fine particles; to me if I’m gonna pay to upgrade these it might as well be a close to the best available as possible.
These 16 oz. singed-felt bags are amongst the best I could choose so they filter really fine particles even with no dust cake. Clean filter performance is critical to check. Know that what you’re buying has been tested and rated as such because it’ll be awhile before the dust cake fully develops.
As for static pressure, I don’t have a manometer, but judging from the CFM increases, it looks like there was a significant impact there. I was able to find some fan curves for the Jet DC-1100 and from that I see that I may have decreased the overall static by about 1.5” to 2”.
The lower bag with the quick-release band clamp not only seals better, it’s MUCH easier to remove, and instead of snapping on the inside of the barrel, it wraps around the outside for easy access. Before a flexible steel ring snapped into the inner barrel, which sometimes would pop back at me and hit my face with dust.
For maintenance, you don’t want to wash, blast with compressed air, or otherwise overly clean your bag filters. You want the dust cake to help filter the finer particles. The AFF singed felt is designed to hold the proper dust cake, and let the rest fall off with a mild tap with a stick or something. So that’s my maintenance program. Beat it with a stick.
What Improvements Are Next
Now that this is done, I’ve been thinking about how to further optimize my system. I know that the capabilities of my current 1.5 hp motor/ impeller are limited, as good as the Jet DC-1100 is. After measuring the actual airflow to each tool with the anemometer, I’m mostly satisfied with the CFM I’m getting vs. the ACGIH book recommendations, but not quite with the “fine dust calculation” also detailed in the ACGIH book.
So, to increase your CFM, you want to do several things.
- Increase your motor Hp, say to 3 Hp (requires 220V for practical reasons, and if you want a cyclone definitely get a 3 or even 5 Hp version because of the huge 3”-4” pressure drop of the separator)
- Decrease your ductwork’s static pressure via better fittings, reduced run, or increased diameter ductwork
- Decrease your filter’s static pressure
For now, I plan on keeping my current dust collector, but I’m putting in a 220V outlet for a future upgrade. I just ran out of slots in my breaker panel, so I’d have to replace it or have a subpanel installed.
Next up is to add some sort of separator plate or dome similar to Jet’s vortex cone to protect the filter from violent chips, and to keep the felt from getting excessively loaded. As long as this device doesn’t decrease my airflow much (by increasing static pressure in the system), I’m ok with it.
Now that the “discharge” side is taken care of, I want to turn my attention to the “suction” side of things.
This means the duct main, branches to tools, and the tool connections themselves. I’ve developed a static pressure calculator in Excel to really figure out what my system’s doing. That way I can play around with duct sizing, elbow types, various routes, etc. to see what gives me the CFM I want at the proper duct velocities recommended by ACGIH.
I know, there are several duct static pressure calculators out there. Most are pretty good, but I wanted one that uses real fluid dynamics equations that more closely approximate the Moody diagram, and I wanted to incorporate all of the fittings I could find in the SMACNA and ACGIH publications. Some of the other calculators I’ve used have too many approximations in them, and by the time I factor in my own assumptions that we all have to make (especially with filters), there’s too much error in my opinion.
Since I’ve used static pressure calculation spreadsheets on large commercial air handling systems, a few dust collection systems, and even dryer exhaust ducts, I started from scratch and built one that is easy to use. By answering a series of detailed questions with yellow boxes to indicate where you enter data, just go down the items and the spreadsheet does the rest. I’m still in the process of putting in all the different fitting and configuration choices, but it should be done soon.
So, back to ductwork.
I still have a few short-radius elbows (1.0 R/D) that I will replace with long-radius elbows (1.5 R/D or 2.0 R/D).
Some branches are 4” which have a really high velocity that creates huge static pressure which ends up lowering the CFM dramatically. If I can increase the diameter while still staying in the “safe” range of 4,000 fpm velocity, I’ll get more air which will in turn capture more dust further away.
Some of my tool connections can be improved as well. Not only can I increase the critical ones from 4” to 5” or 6”, but I want the airflow intake to be as close to the source as possible, and the crossflow of air intake from outside a tool cabinet to be directed strategically. Installing chip/ dust deflection hoods would help to prevent particles from escaping the “capture bubble”. The ACGIH book has some diagrams suggesting hood designs for various tools. I’ll just tackle one at a time when I feel like it.
Other things I’ll be doing include re-sealing joints and seams( since I noticed some look in need of repair), buy smoother flex duct, and get those bridging hose clamps that are designed to fit nicely over the ridges of flex duct. Standard hose clamps don’t seat well all the way around because of the spiral geometry of the flex duct. My woodshop design is always evolving, and after living with certain things you find out what you’d like to improve.
There’s plenty to keep me busy just with dust collection. So because I have other stuff to make (like actual woodworking…), I’ll pick tasks and take care of them every few days. But it’s also fun since it’s a building project and I get to use it every day.
For more guidance in assembling your Workshop Design, click on the Starting? Go Here! category and read those first.
And, if you’re on Twitter, be sure to follow #woodchat every Wednesday night, at 6:00 pm, PST.
Gotta get more clamps,