DIY: air scrubber for a woodworking shop
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  1. #1

    Hmm DIY: air scrubber for a woodworking shop

    Good day everyone!

    I am a woodworker in need of information. Woodworkers or not I am sure you all know that in a woodworking shop you end up with a lot of dust. My shop is a small basement shop, about 1250 cubic feet. Despite trying to collect the wood chips and dust at the source an alarming amount of dust still remains floating in the air for a long time.

    As many other woodworker, I would like to build my own air scrubber. Generally from a recycled squirrel case blower, mine is about 300 to 500 cfm, a few layered furnace filters (coarse on the outside followed by a mid range one and sometime a higher quality one on the outfeed side). Generally I believe it is recommended to change the air in a wood shop 6 to 10 times an hour, So 500cfm should be quite sufficient.

    What I am planning is to use commonly available 16x25x1 filter. But people on the woodworking forum say that these can get clogged very fast. Probably depending of the blower driving them I would assume. But it made me think about doubling the filter area by using 2 opening of 16x25 instead of one, but that would also reduce the airflow on the face of the filter area by two. That is the CFM per square inch so to speak.

    So it occurred to me that there must be a lower limit as to the CFM that must pass through a filter for it to be efficient and probably an upper limit also. What would this limit be? That is more for general knowledge, I guess what I really want to know is Should I double my filter area or stay with one with my little 500CFM blower?

    Thank you very much in advance for your help and assistance.

  2. #2
    Join Date
    May 2011
    Location
    Southern Indiana
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    97
    I would rethink your design a little. Maybe it would work better if you used a bag type filtering system that collects the dust on the outlet side and a cheap filter on the inlet side to catch the larger particles along with a fresh air inlet.

  3. #3
    I am sorry if that was not clear the filtering steps would be as follow;


    Infeed side -> cheap filter merv4, then merv9 then the blower and following this on the outfeed side an merv12.

    The system would actually 2 modes, In the winter it would go through the Merv12 and be blowned into the next room. This would create a negative pressure in the basement and reduce the dust from going out the shop into the house. In the Summer, I would ventilate outside but because of the vent location I still would need to filter a bit.

    The negative pressure is not much of an issue because the house is electrically heated. The summer there would be a house wide negative pressure but the windows are always open as we do not have AC. In the winter the negative pressure is only at the shop level.

    Also, I did think about the pocket filters but they seem to be much harder to find, from what I can tell only in specialty shop and nowhere near me!

  4. #4
    Join Date
    Nov 2006
    Location
    Southeastern Pa
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    17,731
    First step: cyclonic separator.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  5. #5

    Frown

    Hi again

    I have a cyclonic separator on the chip/dust collection system. My question is more specifically for an air scrubber system to handle the dust that is not collected at the source by the chip collection system.

  6. #6
    Join Date
    Feb 2011
    Location
    Gainesville, Georgia
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    47
    The generally accepted maximum is 300 ft per minute face velocity for filter grills.

    That would mean 1 square foot per 300 cfm moved or 1-2/3 square feet for 500 cfm. That would be equal to 240 square inches. one 16x25 filter is 400 square inches so that sounds sufficient to me.

    Make sure to check the amp draw on the blower motor. You may need to restrict to outgoing, or what we call supply air in order to keep the motor from "over amping". Blowers are designed to operate a a certain static pressure and will require that to keep too much current from flowing to the windings ("over-amping").

  7. #7
    Join Date
    Aug 2006
    Posts
    2,579
    At work we have a real dusty lab. The commercial dust collector has three filters. First is a PSF-1 "Polyester Pad." It is on a square cage that is hinged on the end of the Air Handler. This gives a large surface area. We spray it with cooking spray to make it Tacky. Next is a four pocket 12" bag filter. Next is a two inch high efficiency pleated filter. The blower is way oversized to make up for the high negative static.

  8. #8
    Join Date
    Jul 2008
    Location
    Denver, CO
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    4,229
    Quote Originally Posted by timebuilder View Post
    First step: cyclonic separator.
    Without a cyclonic separator you will be changing your filters constantly. And loosing your airflow constantly.

  9. #9
    Join Date
    Nov 2006
    Location
    Southeastern Pa
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    17,731
    I thought he might not have a separator. Not everyone does.

    Depending on climate, I might settle for an exchange of air after the separator catches the big stuff. Maybe a percentage of fresh air, and for the air handler, a set of metallic screens that are washable to catch more dust, and then a good pleated filter that can be removed and shaken a couple of times before it gets replaced.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  10. #10
    Quote Originally Posted by Midlyph View Post
    The generally accepted maximum is 300 ft per minute face velocity for filter grills.

    That would mean 1 square foot per 300 cfm moved or 1-2/3 square feet for 500 cfm. That would be equal to 240 square inches. one 16x25 filter is 400 square inches so that sounds sufficient to me.

    Make sure to check the amp draw on the blower motor. You may need to restrict to outgoing, or what we call supply air in order to keep the motor from "over amping". Blowers are designed to operate a a certain static pressure and will require that to keep too much current from flowing to the windings ("over-amping").

    Thank you very much Midlyph that is exactly one of the number I was looking for. So this being the Maximum, would mean if I understand well, that it determines the minimum filter size required for efficient operation. I assume that passed this velocity the particulates would have too much energy to actually be caught in the filter.

    Now - the true number, I am looking for is the Minimum face velocity I believe as this will tell me exactly what I am looking for, that is the maximum efficient size of filter I can have.

    --------------------------------------------------------------------
    TimeBuilder - I live in Quebec, Canada. The winters are cold and long:-( Venting outside all year would be literally blowing money out the door! That is why I would vent in the summer but need to recycle the air in the winter.

    ----------------------------------------------------------------------
    madhat - the cooking spray sounds like a good idea, I'll try this. I can use that on my venmar air exchanger metalic filter.

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