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Hepa filter specifications/ blower match

  1. Feb 23, 2017 #1
    Greetings, Im new here

    I hope I don't come of rude by just posting like this, but Im in need of assistance.
    Im making an air purifier myself. It will consist of a fan(blower Hepa filter and active carbon filter.
    I have no problem with carbon filter and fan specifications matching, but Im having a hard time putting Hepa in the equation, because Im so confused about calculating appropriate Hepa specifications in relationship with the static pressure drop. And at the moment I need this purifier in a very short time and hence I don not have the time to research the science behind it.

    Can somebody just tell me what size and thickens of Hepa filter I should buy, to mach with my fan. I was planing on buying a 400m3/h blower that has 300Pa static pressure.

    Should I buy half of foot square Hepa H12 filter of 2,5" or is it ok to get 1 square foot Hepa H13 filter of 3inch thickness??

    Please help, any recommendations on what the dimensions and thickness of Hepa(h12 or H13) is much appreciated.

    Thank you in advance
  2. jcsd
  3. Feb 23, 2017 #2


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    Staff: Mentor

    Welcome to PF!

    Your filtration requirements drive your choice of filter. If you don't have filtration requirements, then your choice of filter (and airflow) is arbitrary. Either way, once you have selected the airflow and filter model, you will size the filter based on the manufacturer's recommended face velocity.
  4. Feb 23, 2017 #3
    Hi, Thanks for the reply. My choice of filter is based upon the selection of my blower. Im just trying to calculate what size and thickness(and number) of Hepa filter that I need to buy in order to work correctly and dont overload my fan. The fan is rated at 400m3/h and has static pressure of 300Pa. I just dont know how(and dont have time to learn now) to calculate the filter specifications. So if any kind soul could tell me what Hepa specifications(size, thickness and number) are most appropriate, I would be thankful very much.

    I Have Hepa H13 302*302mm, thickness 78mm and the smaller H12 200x120mm, thickness 48mm

    Which one of these is better suitable and/or should I get some other one?

    Thank you for help in advance
  5. Feb 24, 2017 #4


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    Staff: Mentor

    There are no calculations to do. Just pick a filter out of a catalogue that meets your needs. At 300pa, you will have any easy tiime finding one, as most HEPA filters have much lower starting pressure drop.
  6. Feb 24, 2017 #5
    Ok, sorry.
    Its preety obvious that I do not have a catalogue for these Hepa filter available or else I would just do it.
    There must be some general specifications chart for different Hepa thickness sizes and what fan/blower is suitable for them.

    But like I said I havent found it and am not with enough free time to research and Ive been searching for it long enough so I decided to ask here. If anyone can PLEASE rely i the can help me with this .

    Here are MERV ratings for some merv ones but they dont mention the size o the filter

    I also found this but it doest mention the dimensions or the thickness or type of Hepa filter
    https://www.shroomery.org/8491/I-want-to-construct-a-HEPA-hood-How-do-I-match-a-blower-to-a-HEPA-filter [Broken]
    Last edited by a moderator: May 8, 2017
  7. Feb 25, 2017 #6


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    Staff: Mentor

    Usually the specs are available online, but for those, I'm not seeing them. I would email the vendor if that's the filter you are set on. For typical HVAC applications, though, 12" is a pretty common depth:

    As you can see, that document lists the maximum airflow at a specified pressure drop. Others sometimes provide a graph of airflow vs pressure drop. Remember: a HEPA filter needs to be protected by a pre-filter too, so you need to include one as well.

    What is this information for? It would probably help us to help focus your questions if we knew what your purpose was.
  8. Feb 28, 2017 #7
    Hi Russ, Thank you for your replies.

    I am making a "portable" diy air purifier. When I say portable I mean just to distinguish that its not typical entire home HVAC system/
    It will just be a active carbon filter and HEPA filter in front of a fan, I will assemble it all together, i have no problems with doing that manual work, its just that I have concerns of how will it be performing so hence I try to learn more. I guess I will find out soon if it works :D
    I already bought a fan, its a 1000m3/h, 110W, 350Pa fan. But I am always skeptical about manufacturers claims when it comes to values.
    My filtration needs are not that much based on airflow with this device, but Im more interested in having the highest filtration possible, thats why I had Hepa in mind, but Im not sure if it will be possible with this fan, because I will also be putting a carbon filter in front and a prefilter for HEPA.
    I dont have pressure drop spec of a carbon filer part, it just says it has 300m3/h airflow and its a granulated (not pellet active carbon).
    The Hepa filter I found has 250Pa pressure drop. Ive read that carbon filters can have up to 0,5 to 1" (250Pa) pressure drop.
    My filer looks like this.

    The air first goes through carbon tube (that also has a prefilter on it ) then it goes through prefilter for Hepa and then HEPA before passing through the fan.

    I'm mostly interested if anyone can tell me what can I expect if I overload the fan?? in other words if the pressure of filters is too big for fan to operate.? Will the fan overheat and malfunction? or will it just work at lower airflow through the filters? I would be fine if it will be pulling 100m3/h airflow through these filters but my instinct tells me its probably not good in long term if any at all.
    My fan is backward inclined centrifugal type.

    I will test it all in the end and see, in the worst case I will have to lower my filter desires or just get a much better fan. but I would love to have MERV16 filter and carbon at least.

    Thanks again for your thoughts in advance

    Attached Files:

  9. Feb 28, 2017 #8


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    Staff: Mentor

    A worthy exercise: I once bought a $300 one from a major/reputable manufacturer for a girlfriend. It was just a fan and filter in a poorly made plastic and foam box (there is a scathing review of it somewhere on this forum...). I'm sure I could do better and considered doing it myself...
    As you should be. What, precisely, does the spec sheet say? 1000m3/h, 350P and 1000m3/h @ 350P are very, very different things. The best thing to do is find a fan curve, if the manufacturer can provide it (more discussion below...). Anyway, the airflow is in the ballpark of what a room-sized HEPA should be.
    Sounds reasonable.
    Fans have performance curves and if you choke them off by having a higher resistance than the chosen operating point specifies, you end up further back on the curve at a lower airflow and higher pressure. They don't overheat unless you stop the airflow completely: fan runs at lower power input when they are choked-off.
    A high quality/efficiency fan type.

    You're doing fine with your project.
  10. Mar 1, 2017 #9
    Yes! Thats exactly why I decided to do it myself, when I recalculated the cost of parts and doing it myself instead o buying a branded one, plus I love making stuff and learning.
    The sheet says exactly that what I wrote , its a typical curve, The highest static pressure(350Pa) is when the an airflow is lowest, and 1000m3/h is when there is no pressure. if Im reading the curve correctly.
    But I got completely confused how all these work together when assembled, because the filter has its own curve and Im trying to calculate how it will all work together. As I learn more I found out there is initial pressure drop in filter and when it gets clogged up the pressure will only get bigger so I guess the fan is definitely weak I thing, but if I read the filter curve correctly lets say that the filter has 1000m3/h airflow at 250Pa pressure drop. Does that mean that at lower airflow there will be less pressure drop in filter? Or will it always be the same 250Pa and only rise above as it gets clogged and with more airflow required?
    I just cant seem to wrap my head about how the fan and filter combined together will work. For example there was a smaller an with 270Pa and "only" 300m3/h airflow, maybe I should have taken that one instead of this one.
    Either way I should be buying the filers in the next few days so I will see what will happen. I was thinking of also making my own inclined manometer to measure the pressure and learn more as I go. But my biggest concern is how the fan will behave and survive if it gets little airflow and big pressure for example running at 100m3/h at 300Pa all the time
  11. Mar 2, 2017 #10
    Hey, What are you trying to grow over there ;)
  12. Mar 2, 2017 #11
    Very funny :D I'm trying to clean my indoor air. Oh, and if I was trying to grow something there would be no need for a HEPA filter , just carbon :)) btw, Im still interested in anyone opinion on overloading my fan with pressure and thoughts on how it will work. Anyway I buy my filters tomorrow and should start building it
  13. Mar 2, 2017 #12
    The filter media that you called out will do you reasonably well. The wild card here is the existing air quality. As you are filtering the air at a high level the media will over time plug. This will show as an increase in pressure drop.
    My recommendation would be to rig some type of differential meter across the filter. Change your media when your starting to drop excessively. If the filter change interval is too frequent you need more media for the air your working with change up the filter on the next purchase.
    With no indication of the suspended solids level in your air there is no way to predict the adequacy of a particular filter.
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