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Thread: Which Air Filtration is Better??

  1. #61
    Thanks Alan,

    I have recently upgraded my filters to MERV 13, and don't regret it. I also need to than Bill Pentz for all the work he originally put into cyclone design and testing...

    Ray

  2. #62
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    I am curious why everyone is happy with MERV13 or 14. Why would you not want to achieve HEPA levels? Efficiency is at greater than 99% at the smallest particles, compared to ~75%

  3. #63
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    Quote Originally Posted by Mike Nathal View Post
    I am curious why everyone is happy with MERV13 or 14. Why would you not want to achieve HEPA levels? Efficiency is at greater than 99% at the smallest particles, compared to ~75%
    First of all, HEPA is not a dust level. Dust levels are measured in particles or mass per unit volume, e.g., PM2.5 is the mass of dust in microgram per cubic meter with particle size less than 2.5 micron.. HEPA is a filter efficiency.

    An ambient air cleaner removes dust from the air at a rate determined by the product of particle concentration, airflow rate and filter efficiency. For example, if we start with a concentration of 1000 microgram per cubic meter and pass that through a fan moving 1000 CFM and then a filter with 75% efficiency we're removing 1000*1000*.75 = 750,000 microgram per minute from the air. If we change to a 100% efficient filter then we might expect to remove 1,000,000 microgram per minute. The problem is that a higher efficiency filter will usually have more resistance to airflow than a lower efficiency one. So if the fan can't push more that 750 CFM through the filter, the HEPA filter actually reduces the removal rate, making it take longer to get to a low dust level.

    Alan's excellent data clearly shows this effect. Looking at the Jet 1000B we see that the total flow is reduced from 1341 CFM to 919 CFM when the small particle filter efficiency is increased from 67% to 79%. That means it takes 25% longer to reach the same level of cleanliness with the higher efficiency filter.



    This is an example of dilution ventilation. There's not a 'clean side' and a 'dirty side' in a shop. It's all mixed together.

    "HEPA" does not mean "better", it's just a measure of filter efficiency. There's more to keeping the air clean than just that.
    Attached Images Attached Images
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  4. #64
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    Alan,
    check Out Filti in Lenexa KS. They will sell you the filter material used in n95 masks.

  5. #65
    I would suggest that this is not good enough. I like Winn Environmental for affordable filters.

    NOTE: Just a happy customer...

    Ray

  6. #66
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    Quote Originally Posted by Roger Feeley View Post
    Alan,
    check Out Filti in Lenexa KS. They will sell you the filter material used in n95 masks.
    They are only 1” thick filters. As per the discussion above, it might perform worse. Or maybe better. Only way to know is to test, and I’m a little tired of all the tests. Plus, not cheap. But an interesting thought.

    Does anyone make HEPA / MERV 16 20x20x2” filters?
    - After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
    - It's above my pay grade. Mongo only pawn in game of life.

  7. #67
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    I have a friend in Reno whose family has been suffering through unbelievably awful air quality for the last few weeks due to the Dixie fire and now in addition the River fire. They've had AQIs in the 300s. In Tahoma, the AQI is 667 right now. The PM2.5 is 753.

    I gave him the directions to quickly build a few of those duct tape air filters and buy an AQI meter for the house. He's promised to send me some before and after AQI readings from the house, but I really worry about them. A few small kids, and having to breathe this stuff 24/7. He's upgraded all the house HVAC filters to MERV13, but it's a lot to ask of an air filtration system to filter out that massive amount of material.
    Last edited by Alan Lightstone; 08-18-2021 at 9:06 AM.
    - After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
    - It's above my pay grade. Mongo only pawn in game of life.

  8. #68
    The MERV13 filter in my furnace is indeed overwhelmed during pollen season in my area. It will be interesting to see how well they work for him. I suspect they will have good results. I also suspect they thought you "not of right mind" when you gave them the instructions.
    Thanks,
    Fred

    Seasoned professional possessing unremarkable proficiency at innumerable skills.

  9. #69
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    Quote Originally Posted by fred everett View Post
    The MERV13 filter in my furnace is indeed overwhelmed during pollen season in my area. It will be interesting to see how well they work for him. I suspect they will have good results. I also suspect they thought you "not of right mind" when you gave them the instructions.
    He would have thought I was crazy, but I gave him the link to the Ask This Old House feature about it as well as he knows my MIT / MacGyver ways since we were partners a few years ago, so he actually rushed out to follow the directions.

  10. #70
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    This is what his filter looked like after being used for one day.
    65126269418__CA82BA41-E6F6-4FBF-BE86-73C7F2FC6FDE copy.jpg

    The 20"x20"x2" filters have still not arrived for him, so this is just with 1" filters.

    OMG!!!! They are breathing this stuff 24/7, and worse outside.
    Last edited by Alan Lightstone; 08-22-2021 at 2:41 PM.
    - After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
    - It's above my pay grade. Mongo only pawn in game of life.

  11. #71
    Quote Originally Posted by Alan Lightstone View Post
    This is what his filter looked like after being used for one day.
    65126269418__CA82BA41-E6F6-4FBF-BE86-73C7F2FC6FDE copy.jpg

    The 20"x20"x2" filters have still not arrived for him, so this is just with 1" filters.

    OMG!!!! They are breathing this stuff 24/7, and worse outside.
    Yikes, that's how the filters look in me shop after a couple months.
    Thanks,
    Fred

    Seasoned professional possessing unremarkable proficiency at innumerable skills.

  12. #72
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    I am bumping this thread intentionally. There is a lot of good hard data here, and there is at least two folks posting in here who either are engineers or who chose to go into physics instead.

    Recently I built a box out of 1 20x20 sheet of plywood, 4 20x20 furnace filters, with a 20x20 box fan to make up the sixth side of the cube. In my shop, it kicks butt. I do not own an AQ meter.

    Tonight I took the item, a Corsi-Rosenthal filter system, to the upstairs of my home where I am running a woodstove, a Blaze King Ashford 30 woodstove in 1200 sqft of common American ranch house.

    I want to reconfigure the Corsi -Rosenthal box so the fan blows out one side, with three furnace filters on the other sides, keep a plywood platform for the floor side - but locate another sheet of plywood some inches above and parallel with the furnace filter that makes up the top side of the cube. It is inevitible that sooner or later me or the wife will trip over the cat in the middle of the night headed for the bathroom and punch a hole in the upper filter.

    The question is this: to How big a gap do I need between the top filter and the shielding sheet of plywood above it to keep good efficiency?

    Fan (20" Lasko) will be running on low for most of my heating season, pushing cold air along the floor of the hallway towards the stove room. When I have a good convective loop setup as described I can keep the bedrooms down the hallway within 1-2 degrees F of the temp in the stove room. I set my top outlet C-R filter near the wood stove this evening and (subjectively) have a better view of the TV from my chair than I can recall ever after about three hours of runtime; but my convective loop is broken, it is about 10 degrees warmer in the stove room than it is in the back bedroom.

    In spring pollen season and the more intense summer forest fire seasons I will be tempted to run the Lasko fan on high, would prefer to have modest to minimal efficiency losses for those occasions with whatever top gap as built.

    Even as a relatively skilled operator of my woodstove I recognize there are some indoor AQ issues with woodstoves in general, but an appreciable increase in perceived picture quality on my television with a very efficient burner was suprising.

    Planning to build in something pretty like hickory. Does anyone have any experience sticking clear packing tape to any of the common North American woods?

    Also, in recent air quality threads I have seen a couple questions about HEPA filtration. For household use I think MERV 13 is plenty good filtration. MERV 13 is going to capture mold spores, pollen, forest fire smoke, and a pretty fair chunk of plague virus particles. As a Registered Nurse I spent 16 of my 25 years at the bedside harvesting boogers from folks that may or may not have tuberculosis, the words "rule out TB" in a chart always made my short hairs curl. If/when I am going into a patient room to harvest some lung butter I want HEPA filtration on the room to protect my colleagues and a FIT tested N95 on my face (along with some other PPE) to protect me, it is an "aerosol generating procedure" and I got to be closer than arm's length to the patient's airway.

    For regular household use I think it is perfectly fine to start with MERV 13 if you are having issues or want to prevent them, and then look at HEPA filtration if you or a family member is still struggling. FWIW it seems like MERV 17 is entry level HEPA filtration. MERV is a new 4LA (four letter acronym) to me in the last couple weeks. Compared to MERV 13, HEPA/MERV17 is going to be a dramatically more expensive filter with much higher resistance to airflow and minimal benefit for ordinary household situations.

    My guess is a 4 inch gap for a plywood shield above the filter cube should be adequate with a Lasko fan on low, but that is a straight up guess. Appreciate your input, esp for pollen and forest fiore seasons.

    Thanks

  13. #73
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    Scott:

    I've no clue as to whether a 4" gap is enough, but I'm suspicious that it is not. 4" gap x 20" length x 4 sides = 320 sq in of intake area. The filter is 20" x 20" which is 400 sq in of intake area (slightly simplified), so you are cutting down the intake area a little, plus adding, I would assume, a decent amount of turbulence, so I would think that isn't enough to not cut down air flow), but that being said:

    1.) If you had an air quality meter you could experiment with a couple of different gaps and see what the sweet spot is. But since you don't...

    2.) Why not install a metal grate any height you want over the Lasko fan? Build that into the cabinet, and the cat's whereabouts at 2am become less of an issue (except for potential hip fx, etc...)

    3.) Why keep the Lasko fan on low at night? I would think that since it's not particularly noisy, you would get far better clean/warm air distribution on high speed.

    4.) Are you planning on building 2 C-R filter boxes to circulate the air in your home?
    (BTW, not sure those guys should get total credit for the filter box. There is an episode of Ask This Old House that shows the exact same build months before them. That being said, I'm thrilled that they popularized the concept, and especially that massive build at UCSD was awesome!) https://www.youtube.com/watch?v=aw7fUMhNov8
    Ask This Old House DIY Air Filter.jpg

    5.) I agree that MERV 13 filters are great for this scenario. 3M also makes MERV 14 filters, but not in 2" thickness, and only available by mail order around here (not a big deal, as that's how I get all my air filters.) I haven't measured the air flow through them, but when I have to change the filters on my homemade box, I'm thinking of trying them (but will have to double up as they are 1" filters, so it won't be cheap. Ah, for the sake of science....)

    HTH,

    Alan

    Also, I feel your pain re: aerosolizing procedures. A multiple daily event in my past career as a cardiothoracic anesthesiologist. The only time I wore true PAPR gear before SARS Cov-2 was with a patient I had to intubate who had TB resistant to all known antibiotics. I wore a full space suit on that one. Boy it's tough to hear wearing that...
    We almost never wore N95 masks, even though we were fit tested and knew how to wear them. We weren't cavalier, that's just how medical practice was performed. I could write a book about how OR practice has changed since the pandemic. All these procedures and equipment that were commonly used were seen as bad things when examined in the context of a pandemic. Ah, but I digress...
    Last edited by Alan Lightstone; 10-11-2021 at 8:48 AM.
    - After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
    - It's above my pay grade. Mongo only pawn in game of life.

  14. #74
    Quote Originally Posted by Scott Winners View Post
    ... 4 20x20 furnace filters, ...

    The question is this: to How big a gap do I need between the top filter and the shielding sheet of plywood above it to keep good efficiency?

    ...
    The filter pleating throws a wrench in the works, but if you assume the basic filter design is suitable for a 20" x 20" duct (or 400in-sq area), then you need the same 'gap area' as a suitable duct would entail. So, 4 sides X 20" X 5" gap = 400in-sq. This is just basic simple intuition.

    And you can also do CFD analysis, and pressure drop, and velocity profiles, and average filter pressure drop, and filter efficiency, ..... and 4" will probably work just as well as 5"! (The air flow into this upper (exposed) plenum will just move a little bit faster through the 4" gap.)

    (And instead of top grating, bore some 1" holes in the center section of the top deck to even out flow to the filter. This will keep the outside edges from doing 90% of the filtering. Or do nothing, since as the filter slowly blinds off at the edges, flow will shift to the middle.)

    I don't know the specs on your fan, but doubt it can move enough air to actually need the total area provided by your proposed internal plenum. The extra filter area will just mean very long intervals between filter changes..?

    See how easy it is to over-think this.

    Edit - Reading back RE: smoke/particulate issues, I feel it pertinent to add "this" is the referenced gap above a filter. It has nothing to do with requirements for safe indoor air quality.
    Last edited by Malcolm McLeod; 10-11-2021 at 10:32 AM.

  15. #75
    Scott:

    My default approach, which I'll blame on my background, is testing. IMO the next logical step is the purchase of an air tester.....this as even though Alan and I went down "similar" filtration paths, variables in our shops required a "differing" approach. Without a baseline PM count everything you do in your space is a question mark. Having said that, a filter strapped to a box fan will certainly improve your air.....which If I understand correctly, obscures your view of the tv? If that's case I understand why you are looking to improve your air.

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