The following is some emails exchanged between Phil Thien and me. I thought I'd share them here (with Phil's permission), because some of the ideas might be useful to others as well.
I thought your shop vac/separator articles were very well done and nicely balanced.
Many people have looked at my site and concluded that I feel my design is a replacement for a conventional cyclone. And certainly, many people have had some degree of success using it as such.
Going back a few years, my original purpose was to save on shop vac bags. I was using bags in my vac to prolong the life of the filter. But when I got an Inca jointer/thicknesser (my first heavy chip producer), I started filling those bags pretty quickly. And it got expensive.
After trying some commercial and shop-made units, I settled on my own design with my baffle. But my intention was always (and still is) to continue using a bag in my shop vac. To this day, I use both a drywall bag (which fills extremely slowly with the finest talc-like powder imaginable), and a filter.
It was always my assumption that a conventional cyclone would separate much better than one of my designs. I have actually seen some tests and a bit of anecdotal evidence that true cyclones don't separate nearly as well as I had assumed, but I still have no doubt they outperform my design. Of course, there are tradeoffs (real cyclones are substantially larger, for one, and a bit more complex to build).
I think it would be interesting to extend the outlet tube further into your separator. This may reduce resistance and bypass a bit.
Also, I wonder how your blower handles the resistance of the separator compared to my large Ridgid shop vac. I know the separator puts a giant hit on CFM. I have my unit attached to a large Ridgid shop vac, and plumbed to my shop with 2.5" clear pipe and seven blast gates. I use it on my Dewalt 12" planer, and it keeps up with wide boards pretty well. A bit escapes. When i connect the separator directly to the planer it gets everything. So I just wonder if the difference is in the blower.
I did want to write, though, and thank you for spending so much time and saying what I feel are nice things about the design. Like I said, I know it isn't perfect, it is a compromise, but it works well enough for what it is, and many people seem happy with them. I think you covered it very fairly.
So thank you.
-Phil Thien Milwaukee, WI
Back-pressure is a huge problem with the one I built. And I think it's an
unacknowledged weakness of cyclones, especially for Thien baffle style
I think I lose somewhere around 40 to 50% airflow from the separator. But then again, my filter has very low air resistance, so the separator my biggest source of air resistance. If your vacuum has one place where the air comes out, it's easy enough to measure airflow with a pitot tube like I did.
I imagine with a shopvac, the back pressure is less of an issue. My shopvac will do about 1000 mm water of static pressure, whereas the blower I'm using, and my large dust extractor, both only do about 220 mm.
If you inject the air with an elbow, like with your garbage can lids, the back of the elbow slows down rotation a bit, which helps airflow, but also decreases separation. But then again, if you suck the air through a vacuum cleaner downstream, it's ok as much as 5% of the stuff makes it thru. I was concerned about getting enough rotation in mine, but I think if anything, I have too much.
I'm a bit concerned about the whole microparticle thing. It's definitely an occupational hazard for people working with wood full time. I don't know of hobbyists running into health problems. I wish I could measure it somehow. Had a bid for $500 in on a Fluke 938 particle counter (for measuring fine particulate in the air) on eBay, but it ended up selling for over 2k! Researching this thing, I'm pretty sure cyclones are not effective at getting the microparticles out, and I don't know how much faith I should have in my furnace filter.
You mentioned a bag and a filter - does your shopvac have the capability to take both? If I build another one, I think I'd try an air cleaner HEPA filter.
Phil Thien writes:
Jumping around here a bit, sorry...
(1) Have you seen the Dylos particle counters? I think they're a bit of a compromise, but provide some good relative data. And they're pretty inexpensive.
(2) My shop vac (as many others, as well) can use a bag, along with the filter. I think many of the Ridgid and Shop-Vac(tm) models that have the inlet on the front of the bucket can handle the bags in combination with the filters.
(3) At one point, I had ordered a plastic cyclone made for dentists. I found a cheap one on eBay (I think $20). Dentists use these to recover fillings and bits of teeth, etc., and flush any water/saliva down the drain. I think they're typically connected to their little j-shaped suction gadgets. I actually called the manufacturer of this cyclone before placing my bid to ask whether they thought it would work for sawdust, and they were very encouraging and said they used these same cyclones in their production facility, on their radial arm saws.
In experimenting with this plastic cyclone (which looks almost identical to a well known one on the market), I found that the separation efficiency was about the same as what I was already getting with my design. Without a bag in the vac, I was still getting very fine wood flour on the filter. This was several years ago, and I had thought (at the time) that the design of that unit was too specific for the intended purpose (water/saliva and fillings).
It was only later that I saw the nearly identical one on the market, and some anecdotal test results, that I realized that no cyclone gets 100%. I guess I had known that, I had just expected any true cyclone to vastly outperform my gadget.
I know any separator puts a giant hit on CFM. My design perhaps more than others. But I have not performed exhaustive testing and quite frankly, there are many variables. Some people build my design with a top inlet, others a side inlet. The distance from the top to the baffle always fluctuates. Some people use them with shop vacs, some people with a DC blower. Sometimes the separators are separate, sometimes the baffle gets added to the ring of a DC.
Making any generalizations is hard, to say the least. I'm happy saying a conventional cyclone probably outperforms my design (in terms of separation, and resistance). But quantifying any differences isn't so easy. I might even be selling myself short, I don't know. I could buy an Oneida and perform some testing as related to shop vacs, but would anyone trust my results if my design came even close?
(4) I am somewhat concerned about the fine dust. Many unknowns there, and a lot of misinformation. After I discovered those Dylos meters and got a group buy done, we found (from reports coming in from buyers) that using a decent DC and a decent ambient air cleaner resulted in a shop with air cleaner than the air in the rest of the home, or outside.
Until that point, there were quite a few people saying that there was this invisible fine dust that never settles, leading many to believe shops were some sort of toxic wastelands.
But still, many more people today are using exotics that are apparently more likely to trigger immune system responses. So a tiny bit of some types of dust could be doing substantial damage to our sinuses or lungs. Again, I just don't know.
In regards to fine particulate, I have been sent a link to Bill Penz's pages on cyclones by a few readers
I've been hesitant to link to these pages - reading over these pages, it seems that anything but the most extreme measures of dust collection are inadequate, and quite possibly more harm than good.
What Bill Penz doesn't mention directly on his website, but did mention in an email to me is that he has had developed a severe allergic reaction to wood dust. I would guess that makes him much more sensitive to dust than most of us. But he's not the only one. I cam across this neat respirator hack by Jake VonSlatt:
I'm sure exotic woods make it worse, and I'm not keen on making whole pieces out of the stuff. Even walnut is something I'm hesitant about, but then again, I also prefer the look of lighter coloured woods. I hear the fungus in "spalted" wood can be risky as well, but I don't much like the look of it. I'd prefer my wood to not be partially rotten :)
I have been experimenting with looking past the front of a 300 watt halogen light, in a way that I'm looking at something dark but can see the dust illuminated by the 300 watt bulb. I found that my bedroom is actually a much dustier place than my workshop, at least while I'm not working on stuff. And outside air has also a fair bit of stuff floating in it. But my guess is that I'm only able to see larger particles - maybe 4 micron, just a guess. Rubbing my sleeves, even on a freshly washed shirt releases quite a lot of particles visible this way, a does clapping my hands, even if I just washed them a few minutes ago.