Hmm. Think there's a bit of synchronicity about!
I took a quick look into the excellent (so many members, such a wide range of input) Sawmill Creek forum in the US after making the above post.
What should be at the top of the list but a 2005 thread on dust particle size distribution and the relative effectiveness of the some of the differing types of cyclone type dust system commercially available in the US that somebody had just found and added a post to:
http://www.sawmillcreek.org/showthread.php?t=22907 There's also some feedback from guys who built Pentz systems.
Bill Pentz in a 2005 post in this thread answers at least in outline the question of particulate size distribution in wood dust collected from machines:
'Still, for those considering these units (some other models of cyclone discussed in the thread) I again clarify that these are not efficient fine dust separators, so are best used without filters and should be exhausted directly outside. Pumping lots of fine dust into filters quickly ruins filters from over cleaning and the high silica (glass) content in wood rapidly cutting up our filters quickly turning even good fine filters into sieves that pass most of the finest dust. Likewise, the cost and time to buy then modify one of these often exceeds the cost to build or buy a far more efficient separating cyclone based on my better separating design.
OSHA gathered sawdust from hundreds of large woodworking shops, and measured that dust. They found on average woodworking dust is about 85% heavier sawdust and chips and about 15% fine airborne dust. Airborne dust consists of small dust particles that do not immediately fall out of the air. Most airborne dust is sized 30-microns and smaller, with 30-microns being about one third the diameter of a human hair. We all know from working with MDF and doing some operations like power sanding that we sometimes generate close to 100% fine dust and other times like when planning we generate near zero fine dust. So the amount of airborne dust we make will vary considerably.
Going after that 85% heavier dust is known as “chip collection” and means collecting the same stuff we would otherwise sweep up with a broom. Going after the remaining 15% finer airborne dust is known as fine dust collection, something most hobbyist tool, dust collector, and cyclone vendors do not provide in spite of advertising claims. Sadly, vendors like this cyclone maker test with heavy chips to make incredible separation claims, yet decades of testing shows cyclones based on this basic design provide about the same 85% separation by weight that we get from a far less costly trashcan separator.
This remaining 15% of the finest airborne dust poses serious short term and long term health problems. We can quickly become sensitized to some of the more toxic woods if we have too much exposure. That sensitization can lead to serious immediate reactions such as severe asthma attacks and worse. We all also develop long term damage from too much fine dust exposure. Our bodies do a good job clearing dust down to about 10-microns, but have trouble with finer stuff getting lodged in our tissues. Particles sized 1-micron and smaller pass directly into our blood and can get lodged anywhere. Over time the chemicals and toxins in that dust, plus the silica (glass) will cause almost all to get ill. At typical hobbyist exposures this may take a decade or two, but we have over forty years of insurance data and medical tests that show all are affected with about one in eight developing serious problems. Sadly this often occurs so late in life that many do not tie their respiratory problems to this exposure.
Dust sized 10-microns and smaller is invisible without magnification, creating a big problem for hobbyist woodworkers. Unlike most large woodworking facilities that blow the finest dust away outside, we mostly trap this fine dust inside where it often builds to dangerously unhealthy levels even in shops that do minimal woodworking. It takes six months or more for this dust to dissipate. During this time it gets broken down into finer particles plus gathers extra toxins as it is broken down my molds, mildews, bacteria, yeasts, etc. I know from personal experience this buildup can be dangerous, especially when working with the more toxic woods like walnut, red oak, cocobolo, rosewood, etc. I thought I was well protected because I used the “best” advertised cyclone and “best” oversized 1-micron filter. After landing in the hospital, I did not believe my new respiratory doctor, so had my shop tested. Although praised for such a clean looking shop, my shop after over three months of no woodworking still failed its fine airborne particle counts from just turning on my cyclone with doing no woodworking. My near new fine filter stored up and freely passed most of the fine dust.
This cyclone you modified is near identical to almost all other current hobbyist cyclones and the one I bought. They are all built on plans from the New York Dept. of Labor, Division of Hygiene Engineering published in August 1962. This cyclone design has its roots in a basic agricultural cotton cyclone built with very high internal turbulence to break sand and dirt from cotton fiber. When used with woodworking these agricultural cyclones do an excellent job of “chip separation” breaking the fine dust from heavier and dropping close to 100% of the heavier sawdust and chips into a collection bin while and separating near 0% of the airborne dust particles that it blows right through the cyclone. In short, these cyclones provide almost exactly the same 85% separation efficiency that we get from a trash can separator lid. Instead of just blowing this fine dust directly into the outside air as done by large commercial woodworking cyclones, Delta built their cyclones to blow the fine dust into very open filters that freely pass the airborne dust. These big filters catch the chips and sawdust if the dust bin gets full because cyclones with a full bin blow everything right through. The big commercial units don’t have this problem because they have automatic bin emptying, or sensors that shut down the cyclone when the bin becomes full.
Even with all the changes from my Cyclone Modifications pages this basic cyclone design will still pump nearly 60% of the 30-micon and smaller particles right through. With this fine dust full of silica (glass) that trees use for strength, that dust loading will soon cut and tear its way through fine filters leaving them wide open and of little use to protect your health from the finest unhealthiest dust. Worse, unless you use a particle counter or pressure gauge to know when it is time to change filters, you are not going to see any problems until well after the finest dust levels have become dangerously unhealthy. That is why I strongly recommend venting these cyclones directly outside with open filters and no air returned into your shop.
My own testing showed my cyclone design was over 90% efficient at separating off these fine particles before the filters. This greatly increased filter life, reduces cleaning needs, and provides much better long term health protection. Recently I received the early testing results from a major medical school. They provided some nice praise for my design saying it was more than twice as efficient as any other hobbyist cyclone they tested showing better than 90% separation on the 30-micon and smaller particles.
bill'
One thing this material maybe suggests re my post prior to this one is that the essential factor that determines how well the differing cyclone solutions work is probably their performance on very fine invisible dust. It highlights as well why there's sense to exhausting outside when you can, and not relying on filters...
