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Thread: DC chip vs fine dust collection

  1. #1
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    DC chip vs fine dust collection

    I keep seeing it repeated (and repeated and repeated) that you need more cfm to collect the fine dust than the larger chips. Can anyone point me to where this is documented, not just who originally repeated it? Or at least a calculation of why. My rational brain would suggest the opposite. Table saw as a case in point. The tip speed of a 10" blade at 4000 rpm is 119 miles per hour. The blade produces chips and dust of varying size, presumably ejected at 119 mph. Grab a handfull of chips and a handfull of sanding dust and see which you can throw the farthest. Wouldn't the heavier chips travel further, just as a baseball will travel further than a shuttlecock when hit by a bat? Wouldn't it be more difficult to collect the fast moving chips? With my DC running, I am more likely to see chips at my feet than fine dust in the air when I am not able to use my table top collector. Nuts, I'm confused...help me out here.

  2. #2
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    Everything you said is correct.

    But people worry about the finest dust because it can be inhaled deep into the lungs, where it stays.

    So whatever very fine (respirable) dust is missed poses a health risk. The chips just sit on the floor.

    I know many don't agree with me, but I do not think you need large cyclones and 6" piping to get the finest dust. 1-1/2 horse single-stage DC's (with good filters) combined with air cleaners do an excellent job of keeping the air clean. Cleaner than the air outside, or in my office.

  3. #3
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    Your instincts are right, but slightly misapplied. It all makes sense if you look at it in the right way.

    The ratio of mass to surface determines how strongly air drag affects the flight of objects. The large chips have greater mass/surface, so air has a smaller effect on them. They fly farther and straighter. It takes a strong blast of air to divert them from their path. They also fall out quickly due to gravity if the air isn't moving fast enough to support them. Fine dust has a small mass/surface area, so air drag easily affects it. Fine dust is quickly brought to a relative stop by the air and then floats in whatever direction the air is moving. It will continue to float for a long time - in fact, sufficiently fine dust will float nearly forever.

    For chip and dust collection this means several things. First, for larger chips you need a hood that will intercept their flight path and guide them into the collector. Because most of them fly in a pretty straight path from the cutter, it isn't very hard to make a hood that catches most of them. However, because it takes a strong stream of air to direct them, once they escape the hood there is almost no chance of pulling them back in, and they end on the floor. If they bounce off parts in the machine, it becomes very hard to collect all of them. Once they are in the DC ducts, you need enough air velocity to keep them suspended, else they will drop out and cause clogs. This is the source of the air velocity specs for DC systems.

    Fine dust, as noted, scatters rapidly because it is easily brought to rest by air drag and floats in the air. To capture it, you need to draw in the air that suspends it. Unfortunately, the rapidly moving cutters in a machine act like a fan, causing a flow of air that spreads the fine dust quickly. Some machines address this by having a relatively tight hood around the cutters, so that there is a net air flow into the DC from all directions despite the fan effect of the blade. Festools are famous for this. But most conventional designs have very large, loose hoods (or no hoods), and the only way to capture the fine dust is to suck in all the air that the dust scattered into. That is the basis of the large CFM requirements usually cited for DC systems.

  4. #4
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    For fine dust collection you're going to need lots of airflow.

    For chip collection you're going to need lots of airflow.

    Most machines aren't set up to capture either, unless you buy a Euro machine and they come with certification for their dust collection performance, based upon a standard.

    If you consider a table saw, the old rules said about 400 CFM was good, yet we all know what our shops look like after we saw a piece of MDF with only 400 CFM of airflow to a cabinet port only.

    You need better design of the tools, as Steve indicated, so you need above and below the blade collection which can require twice as much airflow. Now we're at the 700 to 800 CFM to do a good job of capturing the chips and dust.

    As Steve indicated the fines don't travel in a ballistic trajectory, their mass/area ratio causes them to disperse after being affected by air currents. Obviously we want to capture them before they leave the machine hoods, so lots of air, and better hood design.

    So no, there's nothing wrong with a good single stage collector with a good filter, the issue is that you really need 800 CFM or more to capture the dust and chips.

    A good single stage collector with a good filter and 6 inch ducting will collect it just fine. The two issues with single stage collectors however are;

    1) Many of them have grossly inflated performane specifications

    2) The rapid plugging of the filters due to no pre-separation of the dust affects their performance, and makes them more labour intensive.

    Regards, Rod.
    Last edited by Rod Sheridan; 01-26-2012 at 10:44 AM.

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    Effective dust and chip collection is very machine and shop specific. Small machines and small shops can be well served by smaller collectors and overhead cleaners. Large collectors not only are necessary for large or older machines but are much more effective at whole shop cleaning than many overhead cleaners. Obviously there are many sizes available but keep in mind that a system that cleans the air every hour means you are breathing dust much of the time. It's easy to turn an over engineered system down. The only way to speed up an under engineered system is to breathe faster. Not a good plan B. Dave

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    As the guys have said. Chips broadly speaking are available for collection in the locality of the hood if it's doing any sort of decent job if intercepting them. Because picking them up requires maintaining only an albeit fairly high velocity airflow through this small volume the CFM required can be fairly low. (but not necessarily - chips have lots of momentum, and if they escape the hooding there's no pulling them back with any reasonable level of airflow)

    Whatever gets missed makes a mess, but the key point is that by ending up on the floor it isn't a significant health risk.

    Fine dust remains largely airborne and ready to breathe in, and quickly drifts away from the point of production to fill the workshop. Testing has shown that in order to prevent the escape of medically significant quantities of dust into the air we breathe that it's necessary to maintain an inwards airflow (about 50ft/min) in a decent sized sphere around this point. (the principle is similar to the inwards flow/negative pressure approach used in fume hoods in chemistry labs) This sphere gets larger as David says with larger equipment. (e.g. a wide planer/thicknesser) More significantly less dust escapes if the airflow is increased enough to significantly increase the size of the sphere, and testing has shown that current machine hoods require this sphere to be quite large.

    The practical problem this leads into is that since the volume of a sphere increases very rapidly as the diameter is increased, the CFM required to maintain this 50ft/min inwards flow increases very rapidly too. Which is what gave rise to what you could call 'high CFM' dust systems.

    There's a secondary air cleaning effect as he says that kicks in on high CFM systems too - air is pulled in from the shop, filtered to HEPA standard or whatever and returned to it in volumes high enough to have a very noticeable cleaning effect.

    The numbers most around here reference are these tables by Bill Pentz on his pages:http://billpentz.com/woodworking/cyc...uirementsTable Which in turn it seems come from testing by the ACGIH (American Conference of Governmental Industrial Hygienists) on presumably some sort of 'average' industry machine for each type. The EU numbers he quotes are very similar. So far as I know most of these originated in regulatory standards for equipment, workplace air quality and the like established in Germany by groups made up of e.g. regulatory, medical, employer liability insurance and presumably equipment manufacturing groups. They have now been applied EC wide.

    As Phil says you can collect pretty effectively with less air on smaller equipment, but your hooding (and indeed ducting) needs to be well optimised to do so. Against that there's quite a few of us who have found out that heading for higher CFM takes the pernickedtyness out of both chip and dust collection...

    ian
    Last edited by ian maybury; 01-26-2012 at 4:05 PM.

  7. #7
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    IMHO you can get away with much less than 300 to 400 on the blade guard. Getting 300 to 400 there would likely require 4" hose, and a lot of above the table guards can't handle 4" hose anyhow.

    Keep in mind that a 360-CFM DC can evacuate a 2.2' diameter sphere every single second. If you generate fine dust in the center of that sphere, it either has to escape the boundaries of that sphere in one second, or it is going into the DC system.

    And even if it escapes, if it is fine dust then the makeup air (the air replacing the air you already evacuated) might carry it right back (provided you aren't blowing a fan through the middle of your work area--which does happen sometimes with air cleaners).

    So how large a sphere can an 800-CFM DC evacuate in one second? About 2.95'. Basically, you have extended the sphere by 4" in every direction.

    If anyone is following along, a $150 Harbor Freight DC can do a 2.2' sphere every single second. A $1500 cyclone setup gets you to 2.95'.

    That is the way volume works.

    IMHO, to get truly substantial improvements in performance, you need exponential increases in CFM.

    So let's say we wanted to increase our original 2.2' evacuation sphere (accomplished with our $150 DC) to 4.4'. Doing a bit of math, we find we need 2676-CFM to get that done.

    In summary, you can accomplish a great deal with the intelligent use of a single-stage DC (IMHO). Add an air cleaner, and you can work in a very clean shop. Dust collection, like just many other things in life, is governed by the law of diminishing returns.

    When I was taking a lot of economics courses we studied post-purchase rationalization quite a bit in an effort to understand the behavior of consumers that purchased expensive items like luxury automobiles. If you want some interesting reading, google "post-purchase rationalization" and "principle of commitment and consistency."

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  9. #9
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    Quote Originally Posted by Dan Hintz View Post
    Don't worry, Phil, we all love you
    LOL, where did everyone go?

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    We think you're great in fact!

    ian

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    Crickets.......



    LOL.


    IMHO, while collection at the source is vital (and sadly, many of our tools are designed poorly in this regard), I've been fascinated by watching my Dylos Air Quality Meter over the past few months.

    As much as my 5HP Oneida cyclone can remove at the source (and this is vitally important), when using the table saw or especially the drum sander, the particles in the room quickly reach unacceptable levels. What quickly makes these go down is the overhead air cleaner.

    I often wondered whether buying the air cleaner was money well spent. Since being able to measure the environment with the Dylos meter, I don't work without the overhead cleaner on. And at high speed when sanding, for sure.

    The air cleaner gets the particle count down to approximately 100 when running. The ambient air in the shop is normally 500. And with the sander running (hooked up to the cyclone and an additional 4" port sucking in surrounding air), the air can easily get up to 15,000 particles (measured 30 feet away !!!!!)

    So again, IMHO, get the best dust collection you can swallow purchasing, modify (enlarge) the ports to 6" if possible (usually is) but save enough in your budget for an air cleaner.

  12. #12
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    It's an interesting question Alan as to whether it's realistically possible on most operations even at high CFM and with enough effort on careful hood set up to capture all the dust produced. The answer technically has to be no, in that the chances are that something escapes - but the big question is how much actually needs to.

    It's got to be very situation and machine specific - both in terms of what works by way of hooding/capture arrangements, the dust system set up and the machine type/size. And that's before we start talking about cyclone separation and filter retention.

    Wonder if there's any chance of some government money to do some testing and figure it out???

    ian

  13. #13
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    Hi Alan, I also have a Dylos, an Oneida cyclone with HEPA filter, and no ambient air cleaner.

    When I come into the shop the air is around 500 counts, run the cyclone for 10 to 15 minutes, down to 100 or less.

    I will admit that the only power sanding I do is a ROS with a festool vacuum, I mostly scrape or hand plane for a finish process.

    My only dust producing machine in any amount is the sliding table saw and it has a blade shroud and overhead guard, which makes an enormous difference.

    The important thing is to measure and make sure you're doing well, buying the Dylos was a great decision...........Regards, Rod.

  14. #14
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    Quote Originally Posted by Phil Thien View Post
    IMHO you can get away with much less than 300 to 400 on the blade guard. Getting 300 to 400 there would likely require 4" hose, and a lot of above the table guards can't handle 4" hose anyhow.

    Keep in mind that a 360-CFM DC can evacuate a 2.2' diameter sphere every single second. If you generate fine dust in the center of that sphere, it either has to escape the boundaries of that sphere in one second, or it is going into the DC system.

    And even if it escapes, if it is fine dust then the makeup air (the air replacing the air you already evacuated) might carry it right back (provided you aren't blowing a fan through the middle of your work area--which does happen sometimes with air cleaners).

    So how large a sphere can an 800-CFM DC evacuate in one second? About 2.95'. Basically, you have extended the sphere by 4" in every direction.

    If anyone is following along, a $150 Harbor Freight DC can do a 2.2' sphere every single second. A $1500 cyclone setup gets you to 2.95'.

    That is the way volume works.

    IMHO, to get truly substantial improvements in performance, you need exponential increases in CFM.

    So let's say we wanted to increase our original 2.2' evacuation sphere (accomplished with our $150 DC) to 4.4'. Doing a bit of math, we find we need 2676-CFM to get that done.

    In summary, you can accomplish a great deal with the intelligent use of a single-stage DC (IMHO). Add an air cleaner, and you can work in a very clean shop. Dust collection, like just many other things in life, is governed by the law of diminishing returns.

    When I was taking a lot of economics courses we studied post-purchase rationalization quite a bit in an effort to understand the behavior of consumers that purchased expensive items like luxury automobiles. If you want some interesting reading, google "post-purchase rationalization" and "principle of commitment and consistency."
    Phil, great info. One comment I have: when going through this process myself, I found that DC's start to lose performance immediately and continue to worsen as they collect more dust. At least that is what I read. A cyclone does not. So, if true, your calculations may only be valid when you first start your DC with clean bags.

  15. #15
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    I doubt there's too many with higher CFM systems around (with or without particle counters) that would agree that the difference between say a 350 and a correctly functioning 800CFM+ system is negligible in practice....

    Correctly functioning, in that CFM is far from being the only differentiator of performance. As before it's clear that poor hooding, bad system layout, leaky or dirty filters (which latter may raise the issue of cyclone separation efficiency on fine dust) or any number of additional factors can quickly overcome any advantage.

    The need to use a separate dust filter surely raises serious questions as to the capability of any system - large or small. Should the need for one arise then it's surely a pretty clear indication that the primary system (whatever its size) is not doing a very good job. A large system is probably going to scrub the room air at about the same rate as one anyway...

    ian
    Last edited by ian maybury; 01-27-2012 at 4:32 PM.

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