I’ve scoured the internet in search of an answer to no avail, so it’s time to ask the experts!

My background is more Chemistry and Biology, not Physics, or specifically fluid dynamics, so bear with me! Also, this is a little long, so I will do my best to make it easy to follow.

The Problem: I am trying to derive a formula to calculate air flow rates (in CFM) based on the static pressure (Inches W.C.) of the HEPA filter I am blowing through for a laminar flow hood that I am building. ( en.wikipedia.org/wiki/Laminar_flow_cabinet ) I am trying to figure this out because many HVAC application fans have ratings of their output at certain static pressures, many, many more do not, and it would be really helpful to beable to derive this information on my own.

Information I have available to use:

--Standard fan information, from the manufacturer (Brake Horsepower, Fan wheel dimensions, RPM &/or CFM).

--Volume of the Plenum Chamber ( en.wikipedia.org/wiki/Plenum_chamber ), in this case >= 4sq.ft. (12in. x 24in. x 24in. as it is generally accepted that the size of an adequate plenum must be >= the volume of the filter).

--The Static Pressure of the HEPA filter, in my case 1in. w.c., and although I don’t know how it relates to anything or what it means, I also have the resistance range of, 0.69.

--The desired volume/rate of air on the intake of the filter, calculated out to 400CFM @ 1.00in. w.c. SP. ( Based upon this formula (Desired FPM Output) x (Volume of HEPA Filter) = Required CFM Input. ( http://www.fungifun.org/English/Flowhood#match ). --The desired output of the entire unit, 100-200FPM, ideally.

Questions developed from research & attempts to solve:

--Some fans are rated at 0.00” SP all the way up through 1..5+” SP. What are the standard conditions for these specs? I.E. in chemistry you have SC = 1.0ATM @ 0degC. If a fan is rated at 500CFM without any information given about Static Pressure, what would be the static pressure? Is it 0.00” w.c.? above, or below?

--I have stared at the Affinity, or Fan Laws ( http://www.greenheck.com/library/articles/10 )for countless hours now. Specifically Law #2. SP2 = SP1( CFM2 / CFM1)sqrd. I was hoping that this would solve my problems, but I have been trying to solve for CFM1, since I know I want SP2 to equal 1.0” and I know I want CFM2 to equal 400 @ SP2. I have been operating under the assumption that a fan, at it’s highest RPM/CFM is operating at 0.0” SP. But then you can’t calculate for the obvious reasons, so this has to be wrong. So back to standard conditions for fans.

Thanks! This has been degrading my life for a week now, any help or insite is very appreciated! -Science!

P.S. Sorry about the broken wiki links, stackexchange was grumpy with my too many links, so I "broke" them.

  • $\begingroup$ You might be better off posting to the engineering.stackexchange.com site. $\endgroup$
    – Dai
    Commented Apr 9, 2015 at 19:04
  • $\begingroup$ I would look up orifice flow and work from there. $\endgroup$ Commented Apr 9, 2015 at 19:54
  • $\begingroup$ Thank you, sorry for the wrong area. As mentioned above, not my area of expertise! $\endgroup$
    – ItsScience
    Commented Apr 9, 2015 at 23:18
  • $\begingroup$ Consider to spell out acronyms. $\endgroup$
    – Qmechanic
    Commented Aug 22, 2023 at 14:17

1 Answer 1


The HEPA filter manufacturer should have the data for pressure drop for each airflow already tested.

The fan manufacturer will also have a fan curve.

  • 2
    $\begingroup$ While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review $\endgroup$
    – Miyase
    Commented Aug 22, 2023 at 15:56
  • $\begingroup$ The links are just pictures of the equipment data I'm referencing $\endgroup$
    – StefanW
    Commented Aug 22, 2023 at 21:59
  • 1
    $\begingroup$ That's not the point. The message was a standard Stack Exchange warning because what you posted was not a self-contained answer. Since this a Q&A site, answers are expected to be useful without the need to follow an external link. $\endgroup$
    – Miyase
    Commented Aug 23, 2023 at 7:51
  • $\begingroup$ The answer involves using these charts since there is no formula that can be used for all fans or filters. They each have unique properties based on their geometry that is then tested in an ASHRAE 33 test rig and thus a chart is produced. $\endgroup$
    – StefanW
    Commented Aug 25, 2023 at 21:17
  • $\begingroup$ @StefanW why not include you previous as part of a more detailed answer? $\endgroup$ Commented Aug 28, 2023 at 22:52

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