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Thread: DC chip vs fine dust collection

  1. #46
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    No prob Dave, I had a look at the Dylos and most certainly am not knocking the use of particle counters. The point was just that the fine end of the dust produced by most machines comprises only a small part of the output, and also of the permitted airborne dust in the case of the 1mg/m3 limit. They seem to work very well, and are basically the only show in town so far as making objective measurements is concerned. I just wish i could afford one right now...

    What I was trying to point up was more the personal sense that very much in keeping with Chris' comments above that dust collection (while very well known field in the professional engineering sense) is for us DIY guys not an exact science. (this page of technical topics by Camfil Farr gives a feel for how well developed the topic is in industry: http://www.farrapc.com/articles/This one here on recirculation of filtered air make some interesting points: http://www.farrapc.com/articles/recirculating/)

    There's a learning curve involved for most of us, and the task is made significantly more difficult by time/budget limits, varying dust collection capabilities and needs on different machines, and the poor availability of good performance data and design information for most of the available dust system hardware.

    The consequent reality that it's not always easy to (without experience) precisely predict outcomes using the fairly basic data and design rules used at this level - especially if using minimally specified equipment to save money. ( the latter part of no 6 here shows for example how the output of a marginally specified blower can be very sensitive to small changes in system resistance/pressure drop http://tiny.cc/ljbh3)

    So while if we could each afford to go to a professional supplier and contract for delivery of a system guaranteed to meet regulatory requirements it'd all be very straight forward, the reality for most of us is that there's inevitably a degree of ongoing investigation/development/trial and error ('development' is perhaps upmarket trail and error) involved in getting to a good a dust system.

    We're probably not doing badly if in selecting the core components and configuring our systems (blower size, filter capablity, duct size, dust separation capability - plus layout) we find our way to a system that works well (maintains clean enough air in the workshop to be so far was we know safe) on our major applications, and is capable of being tweaked into working well on the remainder.

    i.e. It's probably not unreasonable to say that while a few bugs are pretty inevitable, the key requirement in planning a new system is to try avoid painting ourselves into a corner that's not easily recovered from. (installing a system that can't do the basic job, or hasn't got the legs to enable ongoing improvement)...

    ian
    Last edited by ian maybury; 02-07-2012 at 5:30 PM.

  2. #47
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    Very well said Ian,

    I design dust collection systems and dust collection equipment. I don't want to mess with the details of it when I get home in my workshop. A DC system is very personal, I've seen this all over industry and I imagine it is true for most hobbyists. Everyone comes up with there own solutions to common problems. A lot of them work very well, some can certainly be improved. There are some inherant truths. Here are a few things I try to do on a basic level:

    Get the hood as close to the dust source as possible.
    Use the dust source to your advantage. Position the hood so dust is thrown into the hood, not away from it.
    Keep hose runs as short as possible to reduce pressure losses.
    Avoid sudden contractions. These add pressure drop, reduce flow, and can cause plugging.
    Avoid straight tees, use laterals. The pressure drop from a tee affects the branch. The pressure loss between a tee and lateral is significant.
    Reduce duct sizes appropriately to maintain constant area (velocity). Don't bring two 4" ducts into another 4" duct.
    Use long radius elbows where possible. Two 45s are not necessarily better than one 90. If the 45s are the same radius as the 90s, you get slightly more loss through the 45s. Not because of the elbows, but because of additional straight duct required between them. Maybe hard to measure, but its there.


    I have a Jet 1100 DC with a 6" inlet. I use a 6x4x4 wye on the inlet and have two legs of 4" duct to varius locations. I only run one leg or drop at a time. I haven't measured the flow or static pressure (like I said, I don't like to mess with this stuff at home), but I estimate this gives me the best performance versus having two ducts open at any one time. Yes the fan would pull more CFM with two ducts open, but some amount of it will come through the second duct. If you have a dedicated open duct, you have to constantly balance it depending on which other drop you are running to maximize performance, unless all of the other drops have the exact same resistance to flow, which is doubtful. It would probably be best for me to have a network of 6" duct instead. Then alter any hoods to accept a 6" tap instead of necking down to 4". The collector works fine, the primary issue is hood design (getting the dirt in the pipe.)

  3. #48
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    Everyone keeps on about the "best desigh" as if it is the holy grail. The basic design paramaters of the SYSTEM are known and if this is followed you have gotten that part right and can move onto the next bit. The next bit is the hardest, getting the dust and debris into the duct and it is here that needs the attention, not the ducting or the DE itself but the hoods and machines as they are presenting the biggest problem to hobbyists given the poor design from manufacturers and the difficulty of dust collection on stuff that we make like router tables etc, a high speed machine that we make with no real knowledge of how to deal with the high speed debris that come off it. If and when we get stuff like this right then the system is complete and works well but until the stuff we attach to it is designed well the problems will remain.

    The comment above by Michael that the more marginal the performance of the DE the better the system has to be is something I have touched on earlier in this thread. As I said earlier their are a lot of myths and assumptions plus downright lies that have built up around this stuff for whatever reason and every time someone finds something different they are dealt with opposition as happened to me recently with power figures on motor draw. More on that shortly in a different thread....
    Chris

    Everything I like is either illegal, immoral or fattening

  4. #49
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    I need to retract what I said about more losses in the two 45s. It is probably actually a little less loss because the hypotenuse would be shorter than the two legs added together. So you probably have a little more duct with one 90. However, one 90 is typical because you have fewer fittings, and fewer joints, and results in a lower cost system to install. It would probably be difficult to measure the difference in loss, without a high quality instrument. The greater the offset (larger the right triangle formed) the more pronounced the difference.

    "Best Design" can have several definitions. If you have a clean sheet of paper and deep pockets, "best design" could mean one thing. You could get all the dust, return clean air throuhg HEPA filters, and have a very clean shop. While, if you have an existing shop, and you pick up a used dust collector because it was too good of deal to pass up, then you start connecting it to your dust sources (like I did); the definition of "best design" is probably different. This is why I think dust collection systems are somewhat personal, especially with hobbiests or those that do not want to spend mucho $$$ for collection equipment. I'm not advocating one way or the other, probably somewhere in between the two makes the most sense. The intent is to help someone improve their current situation or help provide ideas and concepts (based on inherant truths and/or proven methods) that help solve a particular problem.

    When it comes to dust collection, more CFM is generally better. If you don't return the air to the shop (discharge outside) you have to take into account the cost of bringing in and conditioning (maybe) supply air.

    I have heard it referred to as "Ideal State" and "Target State". "Ideal State" is where you would be if money and time were plenty. The "Target State" is a compromise based on your current state and what you can (or are willing) to realisticly do. Its usually best to define the ideal state first, then evaluate where you go from there.
    Last edited by Michael W. Clark; 02-11-2012 at 12:53 PM.

  5. #50
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    Mike why didn't you go with 6" instead of two 4" ducts? Cost? Availability of 6" PVC?

    I think too much attention has been given to reducing duct size to "maintain constant velocity." Velocity only matters where there is a danger of dust precipitating out of the moving air mass and clogging the duct. It certainly depends on the system, but that is typically only a problem in shops with extensive ducting and mainly when more than one blast gate is open.

    Interesting discussion, however.

    Unless I missed it, I think a key issue WRT the Dylos, air cleaners, and various dust standards was overlooked by Witter and not addressed here either. Did anyone read the comments below the FWW Witter letter at the earlier link? One poster made an astute comment - most OSHA and other dust standards are based on either 8 hrs. or 40 hour work week exposure. While that might apply to some here, I think the comment suggested that what is needed for most of us is an STEL (short term exposure limit) and the potential effects of exceeding that. Is exposure to 8X the 8 hour limit for 1 hour then exiting the shop the same as or worse than exceeding the 8 hr. max over 8 hrs.?

    The problem with most of these threads is that, most (not all) of the participants generally understand dust collection priciples, and if money were no object, would have the best system they could afford, likely better than they have now. But as mentioned, there are so many variables, the woodworker who is just starting out and/or uniformed about DC basics, doesn't know what or who to believe.

    Oh, the comment about Bill P. re heresy and being burned at the stake (or at least being banned from the Pond! ), would apply to most posters to this thread not that many years ago.

  6. #51
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    Quote Originally Posted by Alan Schaffter View Post
    Mike why didn't you go with 6" instead of two 4" ducts? Cost? Availability of 6" PVC?

    I think too much attention has been given to reducing duct size to "maintain constant velocity." Velocity only matters where there is a danger of dust precipitating out of the moving air mass and clogging the duct. It certainly depends on the system, but that is typically only a problem in shops with extensive ducting and mainly when more than one blast gate is open.
    Alan,
    Yes, cost and the fact that the system kind of evolved. I think the 6" system would be best if I had to redo it. I used sewer pipe in a lot of places, I think 6" is readily available at home centers (the green pipe). Don't discount the duct velocity issue. If you plug the duct with planer or jointer chips, you lose airflow and therefore dust capture efficiency at the source. I have to steal time away to work in the shop as probably most do here, so I don't want to spend it disassembling ductwork and clearing plugs. Not my idea of fun. Like you said, probably not as big of an issue in a home shop system because of the small number of drops. Sorry, I just have it engrained in me about duct velocity, espicially with all of the current dust codes in industrial applications.

  7. #52
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    It's tough to know what the practical reality is on duct velocity. On the one hand the standard figure you see about is 4000 fpm in verticals, and 3,500 in the horizontal. Against that some run set ups that sound unlikely to make that sort of velocity - 3,500 fpm is very close to 350CFM in a 4in duct - and some of the smaller systems with ducting hung off them may not be getting to that. Some run larger than normal ducts on bigger fans and seem to do fine too.

    Maybe I've just missed it, but you don't seem to hear a lot about duct blockage or problems caused by dust build up. (potentially a fire risk, impeller damage, filter issues and so on) Maybe it's because the numbers have a lot of slack built in, but maybe too it's that people don't like to report problems with self built systems - especially if they are thinking of selling on an under performing unit....

    ian
    Last edited by ian maybury; 02-12-2012 at 1:32 PM.

  8. #53
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    Plugging is more common when doing something like planing. My planer hood plugs almost everytime I use it (Delta put a a very thin slot at the end and the stringy material has a tendancy to plug it, Delta 22-580). I've used 4000 FPM for the woodworking systems I've designed. Codes require each section of the system to be at or above the minimum conveying velocity. We ere on the side of conservatism, as bad installations and lawsuits are not good for business. A 4" (nominal size) duct pulls about 305 CFM at 3500 FPM and 350 CFM at 4000 FPM. I only bring up conveying velocity because it may be a problem for someone. Duct plugging would reduce capture at the hood, putting more dust in the air, relating back to the original poster's question. Duct velocity also affects fan performance requirements for a given system. The lower the duct velocity, the lower the losses, the more CFM the fan can pull and bring dust into a given hood. If you reduce the velocity too much, you can plug.

    The reason I say that I would be better off with the 6" is that I don't believe that 350 CFM is enough for a lot of the tools we use such as TS, Planer, Jointer, maybe even the router table. Sure, I am probably pulling more than 350 CFM when only using one drop with the 1100 DC, but I doubt that I am pulling twice 350 CFM. (The duct losses go up with the square of the velocity change). If I used 6" duct and run at 4000 FPM, I would have about 780 CFM and have similar static pressure requirements, maybe less. If the hoods stay the same, 780 CFM will do a better job than 350 CFM, probably capturing more of the large chips and fine dusts as well. The higher volume on the same hood, results in more capture velocity, more persuation to get the dust into the duct (both small chips and fine dust).

    There are situations where more CFM is not better, Festool uses an interesting concept with their sanders. They use less CFM, capture the dust, and reduce swirl marks. Their system seems to work very well. Its a system where everything is designed to work together.

  9. #54
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    Other than at planer hoods, I can't recall seeing a report of a duct clog. An interesting thought- a partial duct clog will reduce effective diameter which essentially will drive up velocity- a self-optimizing situation?

    I have always wondered why most ROS manufacturers are still relying on holes in the pads for dust collection. The size and number of holes theoretically have little effect- they are mostly blocked by having the pad in close contact with the stock- and if the pad isn't making contact, it isn't sanding!!! If the pad is in contact and sanding the work there will be little to no flow or pickup because the air must migrate in from the perimeter of the pad, That is hard to do without a pad having marble-sized grit! The best opportunity for collection is beyond the edge of the pad, like provided by these after-market shrouds especially the one with a brush skirt:






  10. #55
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    Quote Originally Posted by Michael W. Clark View Post
    Plugging is more common when doing something like planing. My planer hood plugs almost everytime I use it (Delta put a a very thin slot at the end and the stringy material has a tendency to plug it, Delta 22-580).
    My 22-580 used to plug using a 1 hp Penn State one bagger unit, but since I went to a 2 hp Oneida cyclone, no problems. It uses a 5" vertical from the 4" short hose then goes to a 7" main, so it gets a lot of suckability.

    The discussion of 4" being too small and 6" being too large for a unit like the 22-580 is exactly why I went with metal duct so I can go in 1" increments.

    Another item that helps is that I went to the 4" clear expandable Dust Right hose from Rockler. Using it with the 22-580, it sucks down to about 18" long to minimise losses.

  11. #56
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    Quote Originally Posted by Alan Schaffter View Post
    An interesting thought- a partial duct clog will reduce effective diameter which essentially will drive up velocity- a self-optimizing situation?
    Alan, yes it increases the velocity at the plug. However, the plug does not usually optimize the situation. The best way to explain a partial plug is to think of it as a blast gate that is partially closed. It adds system resistance. The fan is required to operate at a higher static pressure to overcome the increased resistance which means it develops less CFM (per the performance characteristics of most centrifugal fans). The system will "self balance" but the result is usually a lower CFM than you had before in the partially plugged branch. The static pressure will be higher between the plug and the fan, but it will drop off between the plug and the hood. This is not necessarily intuitive but is helpful to know if you notice a lower airflow and need to troubleshoot.

  12. #57
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    Quote Originally Posted by Ole Anderson View Post
    The discussion of 4" being too small and 6" being too large for a unit like the 22-580 is exactly why I went with metal duct so I can go in 1" increments.
    I've thought about going to metal duct as well for the same reason. I don't think a 6" would hurt the 22-580 from an airflow standpoint, but it would be awfully bulky with the hood arrangement. I have to screw or tape my 4" connection to the hood as it is. Mine is on a flip-top cart for storage under a bench when not in use. If I left the planer flipped up, I would deffinitely do something different. I think your larger duct and bigger collector helps the argument for more CFM. I would bet you are pulling more than I am with a 1.5HP single stage, 10' of hose, and 6' or so of plastic pipe.

    Mike

  13. #58
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    Quote Originally Posted by Michael W. Clark View Post
    Alan, yes it increases the velocity at the plug. However, the plug does not usually optimize the situation. The best way to explain a partial plug is to think of it as a blast gate that is partially closed. It adds system resistance. The fan is required to operate at a higher static pressure to overcome the increased resistance which means it develops less CFM (per the performance characteristics of most centrifugal fans). The system will "self balance" but the result is usually a lower CFM than you had before in the partially plugged branch. The static pressure will be higher between the plug and the fan, but it will drop off between the plug and the hood. This is not necessarily intuitive but is helpful to know if you notice a lower airflow and need to troubleshoot.
    I was being facetious, but if dust is precipitating out and is not a "plug" caused by excessive amount of shavings or particle size or obstruction, it means the velocity is insufficient to support proper transport. That means the ducting doesn't match the blower or visa-versa. Without getting a larger blower, you'll never get better CFM than you have and will get even less, but may be able to prevent precipitation by increasing velocity. Before I would try something like that I would examine my ducting and try to eliminate all sources of SP.

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    It does seem like there are lots of posts about motors and blowers but not so many on marrying the pipe size and filter area to the fan size. Then 6" mains that seem to be the most used is really appropriate for a pretty narrow range of systems. Like a car, you can't choke the intake or the exhaust, or open them up too much. I have experience to fouling up them all. Dave

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    Quote Originally Posted by David Kumm View Post
    It does seem like there are lots of posts about motors and blowers but not so many on marrying the pipe size and filter area to the fan size. Then 6" mains that seem to be the most used is really appropriate for a pretty narrow range of systems. Like a car, you can't choke the intake or the exhaust, or open them up too much. I have experience to fouling up them all. Dave
    You need lots of airflow to utilise 6" with no problems and the usual cheap dust extractor will not provide it. For a 15" impeller 6" works and there is not much design required, keep the bends gradual, minimise the flex hose and open up the machine ports as well as design hoods that work. Actually the last is not so much design as copying what has been proven to work. If the workshop is a 100' x 100' then design is necessary but not for the average small workshop put the duct up with good bends/Wyes etc and it will work. yes, I know it is a bit unscientific but it works and we need to simplify this stuff not complicate it. People are confused enough now, keep it simple and encourage those interested to copy what has gone before them on known systems that work. I will now duck for cover!
    Chris

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