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Thread: tweak a design for low volume production help

  1. #1
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    tweak a design for low volume production help

    I am building cubes that take 20x20 furnace filters on some of the sides, and incorporate a 20 inch box fan on one of the six sides of the cube.

    Primary motivation is summertime wild fire smoke.

    482pm25.jpg

    I ran a filter array like I am taking about for about 9 months, inside my house, last winter. I burned about 7 cords of wood in my wood stove, we home a geriatric long haired cat, and the filter array made it through pollen season after having triumphed over heating season.

    I have filtered more particles out of the air in the last two months than I did in the previous 9 months.

    I have two potential customers asking "how many dollars?" to own a filter array of their own built by me. I will probably never sell 200 of these. So on the one hand I want to come up with a system, a plan, a template, a pattern; but I don't need to build an elaborate jig and bring in an air compressor and an air nailer.
    Last edited by Scott Winners; 07-09-2022 at 2:37 AM.

  2. #2
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    This permutation is for folks who don't heat with wood and have lots of sqft in their home.

    FWIW this is open source. All I have done is come up with a permutation on the Corsi-Rosenthal box, aka Comparetto cube. By building a wooden frame to hold all the components, I don't have to take the whole thing apart to change out a broken fan or replace the filters.

    It has casters under it to allow air flow for a filter panel on the bottom, and two of the adjacent sides have plywood blanks in the 20x20 inch openings. This can get shoved into a corner, blow upwards, and pull unrestricted through three filters.
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  3. #3
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    And for folks who do heat with wood, this one is a horizontal output to establish a convective loop between the back bedrooms and the room with the wood stove with the fan on low speed during heating season, but can easily accommodate full throttle fan at 1000 cfm for pollen or wildfire seasons.

    Casters again. I put a permanent plywood top surface on this one because the cat like to sleep on the darn thing. It is the one place she can be to spread her dander as far and fully as possible around the house- other than laying on the floor in front of the fan.
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  4. #4
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    General observations:

    1. The plywood panel with the round cutout to duct to the fan stiffens the assembly up a LOT. I am going to keep making those. If I just tape the filters to the fan with duct tape I have routinely excellent air quality in my house. I am not confident the round duct make a measurable difference in filter array efficiency, but I am confident the plywood sheet with the round hole will make significant contribution to structure longevity. I doubt leaving the round duct in or out will make a measurable difference on my electric bill.

    2. The Lasko fan is the one to get. I have tried 3-4 different 20 inch box fans now in various configurations. I am willing to believe they all pretty much deliver 1000 cfm on high. The Lasko is a bit harder to integrate because the sides have some bow to them, but it gets the job done without making as much noise as the other ones do.

    3. Glue. Use lots of glue. You can pare off the squeeze out after it is cured pretty easy with those cheap chisels you bought when you were getting started in woodworking. TB2 is working fine for me.

    4. Vibration. These are going to be vibrating a little bit in service. Not like a paint shaker, but use plenty of glue (qv) and put some diagonals in all the places. You can see in post 2, pic 3, I have diagonal furring strip glued down on both of the fixed plywood panels, and in post 2 pic 1 you can see a diagonal spanning one of the filter spaces as well. The most annoying thing about the top exhaust item in post two is the noise from the not Lasko fan, it is otherwise a docile unit. For the item in post three I do have a diagonal on the underside of the fixed top, and a diagonal on the inside of the plywood blank that will be filling one of the currently open sides.

    5. Finish. I am happy with spray shellac both inside and out. The insides stay pretty clean since they only ever see filtered air, but a whole lot easier to wipe down with some shellac between the wood and your rag. I have done one in baltic birch, the other is domestic birch, about one and a half cans of spray shellac each for me to feel good.

    6. Filtration level. I own I think 6 particle counters for PM10 and PM 2.5 and so on. My observation, for baseline healthy individuals, is that filtration at MERV 13 is appropriate. If your kid still has a runny nose or your wife is still coughing once you are at MERV 13, then look at HEPA filtration and big dollars. At team orange homestore, you want team orange rating "10" out of 10 to get to MERV 13. Among the 3M filters at team blue home store you want the Filtrete 1900 or Filtrete 2200 to start. I buy the Filtrete 2200s on Amazon by the dozen, for about $20 each.
    Last edited by Scott Winners; 07-09-2022 at 2:46 AM.

  5. #5
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    I see two hurdles to overcome on the way to production:

    1. I need better legs to attach the casters to. In general the floor of the box needs to be about 5" off the floor of the room to get unrestricted airflow to the filter in the floor of the box. I could buy 5 inch casters. I could use short lengths of 4x4 and find a way to glue the endgrain at one end of the leg to the bottom of the box, and then drive screws into the other endgrain to install casters under the legs. I am stumped on this one and could really use some input.

    2. I need to standardize the 4 sides and the top, 5 sides of the cube to one dimension, so I can just bang them out, glue them up and have standard parts. I can figure that out with some thinking time and a pencil. Two bottoms. For the side exhaust I need a platform to set the fan on, but for the top exhaust I can use a smaller floor. I could possibly inflate the dimensions so the top exhaust is a perfect cube with all six sides the same dimensions, but it would be bigger than it needs to be, the Japanese would make a smaller one that works just as good, and I would have to close up shop.

    Open to any and all ideas. Feel free to build one at your house for your family's air quality, I haven't done anything patentable here.

  6. #6
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    FWIW the item in post 2 is now in 24/7 use at my office in the building owned by my employer. Happy to provide the long version if anyone needs it. One of the guys came over with some test equipment, and I earned a sticker:
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  7. #7
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    One other tangential thing. In one or another thread here there are at least 3 folks who just use duct tape to slap one 20x20 filter onto a 20" box fan to filter their shop air. One registered user here (sorry on who) said, "I dunno, at least 20 years with the same fan." On the two I have been running I simply have not been able to find a meaningful temperature differential between the fan motors and local ambient. The idea that these two items might overheat and cause a house fire is just not a thing I worry about.

    I have been running two of them now 24/7 for >2 weeks with no problems found - but I am dealing with a metric short ton of particulates this summer. These two pictured filters are just about done, but for now, as pictured #3, they are still working.
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