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flowhood build


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#21 sandman

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Posted 11 January 2021 - 08:32 PM

The static pressure of a filter changes based on how many cfm is blowing through it (more air = more static pressure)

The static pressure is basically how "hard" it is to blow through the filter if you will.

 

The 12" Thick filters (11.5" actual) are designed for high velocity such as air ducts, the 6" filters are designed for lower velocity use. As such the 12" filter have lower static pressure vs the 6".

 

The static pressure of the filter is the main thing that helps the filter have a laminar air flow so a higher static pressure is preferred.

 

Also 12" thick filters make the hood bulkier. 

 

Some people are successfully using 12" filters though, but it is not ideal.


Edited by sandman, 11 January 2021 - 08:32 PM.


#22 jrh

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Posted 12 January 2021 - 06:51 AM

@ideaman I'm with you. It's all so complicated. I guess I'll see if I can find any youtube videos to get some of the basics down.



#23 sandman

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Posted 12 January 2021 - 07:47 AM

HEPA filter media is just an approximately 1/4" thick piece of fiberglass paper. 

 

6" Filters have a 6" deep accordion fold to fill the filters frame in with media.

 

12" deep filters have a 12" deep accordian fold to fill the filters frame.

 

They both have basically the same actual media, there is just twice the surface area of media in the 12" thick filter so that more air can flow through it easier.

 

media thickness.jpg

 

You want the filter to be harder to blow through than easier given the choice, this helps keep the flow even across the whole filter.



#24 sandman

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Posted 12 January 2021 - 07:55 AM

I don't ever see anyone considering the static pressure @ THE RATED SPEED for their filters is my point.....The 12" thick filters say 1.0" of static pressure but at four times the velocity that you want to use...When you are blowing a lower velocity of air than that, you are getting less pressure and there could be a more uneven, non-laminar flow of air that may lead to a lot of sucking ass. Try to get a 6" filter.

 

Or really just look at the rated static pressure and cfm and calculate the feet per minute, you want around 100 fpm of flow give or take 10. So if a certain 24x24 filter is rated 1.0" SP @ 800 cfm for easy maths sake.... 4sq feet of filter at 100 fpm ideal is 400 cfm. So you are going to be blowing 40% of the tested airflow through the filter you can expect approximately 40% lower static pressure at that speed, or 0.4" static pressure instead of the rated 1.0"  for example.


Edited by sandman, 12 January 2021 - 08:00 AM.

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#25 sandman

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Posted 12 January 2021 - 08:02 AM

And yes I know there are some people that have 12" filters that will go on and on about how good they work, good for them no one said it wouldn't work it's just not the right way.


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#26 Jrotten

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Posted 12 January 2021 - 01:56 PM

My 12” Flanders filter is extremely even, bends a nice 45*@12”, and it was calculated to use a simple duct fan rather than a much larger squirrel cage fan. When it was new the prefilter brought it down very well. Aerosolized oil made the filter a little less permeable, but that was resolved very well with a washable 1micron prefilter meant for a duct fan that size in a MJ grow room which doesn’t have much effect on cfm.

I don’t think 12 or 6 matters so long as you have room for it and the fan is matched to the filter. Mine is over 5 years old and the only modification has been to change the prefilter. When I was building mine everyone advised 12” to make sure the air spent enough time in the filter to make it laminar. I have 2” hepa filters for an air scrubbing machine I’ve never considered using because the air flow through those will pressurize an entire room, or drop the pressure in a room depending how it’s used with very compact and inexpensive fans. The whole unit body, prefilter, fan, and hepa weighs about 20lbs and is intended to be carried around. I’ve considered it for creating a positive pressure clean room, but never a flowhood.

#27 ideaman

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Posted 12 January 2021 - 08:35 PM

Thank you so much for schooling me./taking the time to help me. I hope that info will help many in the future.

 Im waiting on a response from 2 places. What am i looking for Flange gasket down stream or upstream or no flange gasket at all.  

 

 

 

https://www.airfilte...ers/ulpa-99-999



#28 Jrotten

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Posted 12 January 2021 - 10:20 PM

The gasket depends on your design. Mine is upstream to be out of the way. It’s metal framed and I just ran a heavy bead of silicone serpentine around the frame of the filter then close up the box.

#29 newmoon

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Posted 12 January 2021 - 10:34 PM

Out of curiosity, can anyone clarify why a certain static pressure would be necessary for laminar flow in this application? This is not obvious to me. I'm more ignorant than I should be about fluid mechanics, but in general my understanding is that laminar flow is easier to achieve with lower pressures and velocities (think of water exiting a faucet, for example)?



#30 Jrotten

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Posted 12 January 2021 - 11:32 PM

Ultimately you want about 100ft/s velocity at the filter face and the pressure drop of the fan will affect the required cfm and static pressure of the fan. The fan has to be sized to the filter. Ideally you want the pressure drop curve of the fan (as air pressure increases so with the pressure drop of the filter) and the static pressure or performance curve of the fan. Where they intersect is the working point and you want that to give you 100ft/s

https://learn.freshc...inar-flow-hood/

I mostly used this as a general reference though my design differed. Prefilter pressure drop also has to be considered and can also be used to adjust wind velocity so long as you don’t over restrict your fan since the fan is cooled by the air. Also some people, me included in summer, like a little higher wind speed. It will blow things around a bit more so you want to be careful, but my hood is in an uninsulated space and it can easily be I the high 90’s, the higher wind speed keeps me from sweating all over my work.

Edited by Jrotten, 12 January 2021 - 11:40 PM.


#31 newmoon

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Posted 12 January 2021 - 11:57 PM

Yes, definitely. I was unclear, sorry: I'm not sure why one would need 1" water of static pressure across the filter to get laminar flow?



#32 sandman

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Posted 13 January 2021 - 06:11 AM

The higher the static pressure is, the more your filter will resist having an uneven flow from your amateur design in the corners and center.

 

12" filters have roughly half the static pressure as a 6" filter at 100 fpm.

That's really all there is too it.

 

A 12" filter is just more likely to have uneven flow from a poorly designed plenum or fan layout.

 

Anywhere the air is uneven pressured in the plenum will experience twice the change in flow as a 6" filter would in the same design.


Edited by sandman, 13 January 2021 - 06:24 AM.

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#33 ideaman

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Posted 23 January 2021 - 11:02 AM

Im giving up looking for a Hepa filter 30x30x5-7/8. I can only find them with the 11 inch depth. I would be willing to get a 30 x 24 but  can only find them at 99.997 efficiency. Should i just abandon getting what I really want and just get the next best thing? 24x24?

https://www.spycor.c...ggb2424a6ax.htm






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