|
sandman420
Saint PP



Registered: 06/17/04
Posts: 5,384
|
A Word On HEPA Filters and Static Pressure 13
#27288836 - 05/01/21 08:20 AM (2 years, 8 months ago) |
|
|
Static pressure is the common term used to describe the state of pressure of a filtered air system. It is commonly measured in inches water gauge (" w.g.) and it literally means how many inches the pressure will push water up a 1" diameter tube.
There are other units used such as Pascals but for the USA you commonly see inches w.g. and that is what we focus on here. The units can be easily converted.
Each filter has a pressure drop from one side of airflow to the other. The pressure drop is the resistance to airflow passing through the filter.
Technically the proper term is Pressure Drop for the resistance of a single filter and static pressure is the resistance of the entire ducted system.
Static pressure can be replaced just with the word pressure.
Typically the terms are used interchangeably in the OMC (online Mushroom Community) when discussing DIY flow hoods but there is a distinction.
Pressure Drop is not static....It is a function of the velocity of air going through the filter. More air makes higher pressure and less air makes less pressure. The pressure drop of a filter obviously starts at 0 (inches water column) at 0 airflow, right? Now keep that idea in your head to understand what I am saying. It starts at nothing and moves up as the air flows and meets resistance, and gets higher and higher the harder you blow through it.
Here is the data for AirflowTek HEPA filters for instance.

So when you see a filter is rated for so and so inches water gauge you have to look at what velocity the filter was tested at. This is a particularly huge issue with 12" thick filters, because they actually have half the pressure drop of a 6 inch filter and are tested and designed for much higher velocities.
Why do 12" thick filters have twice the pressure drop as a 6"?
Here is the anatomy of a HEPA

You can see that the filter media is actually about 1/4" thick piece of fiberglass paper basically. The 1/4 inch thick sheet is continuous and simply folded around the metal separators to fill the thickness of the filter frame. So a 12" thick filter actually has twice the surface area of filter media resulting in roughly half the pressure drop at the same velocity vs a 6 inch thick pack. The more surface area has to flow the less the airspeed will be going through the filter.
If you look at 12" thick filter spec sheets you will see that they are tested at much higher speeds than 90-100FPM, usually 250-500FPM. The pressure drop given is for that test speed. So when you are blowing the desired 90-100 fpm you are making way less static pressure than at the rated speed.
here is an image for the BioMax HEPA to illustrate my point

You can notice that the 24x24x11.5 and the 24x24x5.88 filters both say 1.0" initial pressure but the rated flow for the 11.5 is nearly double the 5.88 filter, and it has more than double the media area square feet.
------------------------------------------------------------------------------------------------------------
I will show you that these 12" filters only have about 0.27" of pressure drop @ 90 FPM, NOT 1.0". And most typical 6" filters have between 0.45-0.7" pressure drop at 90 fpm.
Calculate the fpm the filter was rated/tested at (cfm/sq ft.) Take the stated pressure and FPM and put it into a ratio where the FPM comes to 90.
To easily calculate the actual filter face size in square feet: Multiply the circumference length of the frame (in inches) by 1.25 and divide by 144. Subtract this number from the sq ft of the total filter size to get the actual sq ft.
Ex: 24x24 (4 sq ft) actual size = 24+24+24+24= 96, 96x1.25=120, 120/144=.833, 4-0.833=3.167 sq ft of actual filter material when you remove the adhesive area and frame.
For example on that 24x24x11.5 filter above. It was tested at 1050 cfm, so 1050/3.167 square feet surface area = 331 FPM test velocity.
When you plug it into a ratio calculation it would look like 1.0/331::x/90 and then x=0.27.
So to make 90 fpm out of this filter you want a fan that blows 3.167x90 = 285 CFM.
285 CFM @ 0.27" pressure drop + 0.2" prefilter pressure drop = 285 CFM @ 0.47" w.g. STATIC PRESSURE.
If you would have put a 400 cfm @ 1.2" rated fan on there you would be blowing waaaay too fast to get good proper laminar flow out of the filter.
Now let's look at the 24x24x5.88" thick HEPA from the BioMAX data. Tested at 525 CFM / 3.167 = 166 FPM test speed. Here's our ratio again 1:166 :: x:90 x=0.54
So the 6" thick has 0.54" pressure drop at 90 fpm vs 0.27" on the 12" thick filter. This may be handy for a weaker fan but less resistance is not desirable for laminar flow.
------------------------------------------------------------------------------------------------------------
Last I want to talk about pre-filters. Furnace filters of most types are designed to operate at 300 fpm. So you need to calculate your pre-filter size with a simple formula: CFM / 300 = sq ft of filter to achieve 300 FPM. Then you can pick out a size that best matches the square ft required to achieve 300FPM through the pre-filter and get your approx.. 0.2" of static pressure from it as you calculated on your original design.
So in short, pay attention to the pressure at 90-100 FPM not at whatever the filter is tested at. And 6" filters are more desirable because they have a higher pressure drop at 90-100FPM which makes a more even flow across the face of the filter aka LAMINAR FLOW. 0.27" for a 12" filter vs 0.6" for 6".
Edited by sandman420 (03/28/22 09:50 AM)
|
tiptrippy
The Mechanic



Registered: 09/09/20
Posts: 1,131
Loc: United States
Last seen: 1 year, 6 months
|
Re: A Word On HEPA Filters and Static Pressure [Re: sandman420] 1
#27289238 - 05/01/21 01:56 PM (2 years, 8 months ago) |
|
|
This is invaluable information. Thank you for your contribution.
Bookmarked.
|
sandman420
Saint PP



Registered: 06/17/04
Posts: 5,384
|
Re: A Word On HEPA Filters and Static Pressure [Re: tiptrippy]
#27290886 - 05/02/21 06:47 PM (2 years, 8 months ago) |
|
|
Thanks homey!
I added some info there at the end about prefilter selection.
|
Mycospore



Registered: 09/24/11
Posts: 74
Last seen: 2 hours, 6 minutes
|
Re: A Word On HEPA Filters and Static Pressure [Re: sandman420] 1
#27330812 - 05/31/21 09:43 PM (2 years, 7 months ago) |
|
|
Great info! Thanks!
|
HotMess
i'm your buddy
Registered: 12/05/09
Posts: 154
Last seen: 1 year, 10 months
|
Re: A Word On HEPA Filters and Static Pressure [Re: sandman420] 1
#27648207 - 02/06/22 11:47 AM (1 year, 11 months ago) |
|
|
I pmed sandman about this exact thing (found a 24x18x12 inch 99.99% filter for 100$ on craigslist.) This thread answered my question, thanks sandman.
|
sandman420
Saint PP



Registered: 06/17/04
Posts: 5,384
|
Re: A Word On HEPA Filters and Static Pressure [Re: HotMess]
#27648242 - 02/06/22 12:12 PM (1 year, 11 months ago) |
|
|
Glad to help bud!
|
Bobbins
Stranger
Registered: 02/02/22
Posts: 445
Last seen: 1 year, 3 months
|
Re: A Word On HEPA Filters and Static Pressure [Re: sandman420] 1
#27649606 - 02/07/22 02:28 PM (1 year, 11 months ago) |
|
|
This is exceptionally helpful, thank you.
-------------------- DeALeRsHrOoMs
|
sandman420
Saint PP



Registered: 06/17/04
Posts: 5,384
|
Re: A Word On HEPA Filters and Static Pressure [Re: Bobbins]
#27649629 - 02/07/22 02:47 PM (1 year, 11 months ago) |
|
|
No prob.
I made some edits to the prefilter calculation...You ever read something you wrote in the past and couldn't understand it? LOL my bad...
Hopefully this is easier because I have no idea what the fuck I was trying to say before... I intend to redo this thread a little to make it easier to understand and include the common calculations including frame size and proper 90fpm calculations. 100fpm is fine but ISO standard is 90fpm+/- 10fpm.
|
Gorguss
Chaotic sums


Registered: 02/03/10
Posts: 633
Last seen: 3 days, 11 hours
|
Re: A Word On HEPA Filters and Static Pressure [Re: sandman420] 1
#27675887 - 02/27/22 05:52 PM (1 year, 10 months ago) |
|
|
Posting so I can reference, Idk if there's a favorite function
-------------------- ------------ ------------
|
sandman420
Saint PP



Registered: 06/17/04
Posts: 5,384
|
Re: A Word On HEPA Filters and Static Pressure [Re: Gorguss]
#27711289 - 03/28/22 08:09 AM (1 year, 9 months ago) |
|
|
I updated this with some more details and tried to word better. And also partially botches something lmao let me edit this again.
Edited by sandman420 (03/28/22 08:17 AM)
|
WeavieWonder
Time Travel Sucks



Registered: 08/18/16
Posts: 1,455
Loc: USA
Last seen: 1 month, 2 days
|
Re: A Word On HEPA Filters and Static Pressure [Re: sandman420] 1
#27711388 - 03/28/22 10:21 AM (1 year, 9 months ago) |
|
|
Bookmarked. Thanks Sandman! This will be a good reference for when I'm ready to pull the trigger on a DYI flowhood.
|
Tight Lunchbox
Drunk cat


Registered: 11/06/16
Posts: 2,116
Last seen: 4 months, 6 days
|
Re: A Word On HEPA Filters and Static Pressure [Re: sandman420] 1
#27711399 - 03/28/22 10:32 AM (1 year, 9 months ago) |
|
|
Very informative as always, sandman.
-------------------- "it's all a joke between mom contractions and coffin fittings" The most useful tool for noobs
|
hOxt
Life During Wartime

Registered: 02/19/22
Posts: 255
Loc: Gilded Splinters
Last seen: 9 hours, 19 minutes
|
Re: A Word On HEPA Filters and Static Pressure [Re: sandman420] 1
#27713067 - 03/29/22 03:33 PM (1 year, 9 months ago) |
|
|
This answers every question I had. Excellent.
|
baldur
Stranger

Registered: 06/30/21
Posts: 434
Last seen: 23 days, 21 hours
|
Re: A Word On HEPA Filters and Static Pressure [Re: hOxt] 1
#27714959 - 03/31/22 04:27 AM (1 year, 9 months ago) |
|
|
Amazing, thanks for the post!
|
hamloaf
Loaf of Fam.


Registered: 12/23/09
Posts: 20,192
Loc: Oklahoma.
Last seen: 1 year, 8 months
|
Re: A Word On HEPA Filters and Static Pressure [Re: sandman420] 1
#27715099 - 03/31/22 07:52 AM (1 year, 9 months ago) |
|
|
|
ElCann
Bailaor


Registered: 06/27/22
Posts: 3
Last seen: 1 month, 13 days
|
Re: A Word On HEPA Filters and Static Pressure [Re: sandman420]
#28016982 - 10/26/22 02:51 AM (1 year, 3 months ago) |
|
|
Thanks man, I'm still unsure if I understood this to the fullest, but that's because I'm dumb af sometimes. Let me see if I got this right, a 285CFM blower will suffice for a 24x24x6'' filter with the same specs you used here as an example?
|
Stuey1
We're all mad Here



Registered: 11/28/13
Posts: 740
Loc: Bat Country
|
Re: A Word On HEPA Filters and Static Pressure [Re: ElCann]
#28016985 - 10/26/22 03:00 AM (1 year, 3 months ago) |
|
|
How many posts can we make about this subject.
-------------------- Have you ever danced with the devil in the pale moon light?
 
|
MRB3
Just Browsing


Registered: 06/28/22
Posts: 37
Loc: TX
Last seen: 10 days, 3 hours
|
Re: A Word On HEPA Filters and Static Pressure [Re: sandman420]
#28022693 - 10/29/22 11:10 AM (1 year, 2 months ago) |
|
|
This is so good, I wish a techie person would turn this into a calculator so you could plug in your filter data and get motor data for those of us that skipped math class.
|
ElCann
Bailaor


Registered: 06/27/22
Posts: 3
Last seen: 1 month, 13 days
|
Re: A Word On HEPA Filters and Static Pressure [Re: MRB3]
#28023211 - 10/29/22 05:19 PM (1 year, 2 months ago) |
|
|
Fr, I feel you man, I'm on the ssame spot. I'm confused af
|
|