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X-Ray Cat
Sees through wooden doors

Registered: 05/12/20
Posts: 134
Last seen: 6 months, 23 days
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Does my HEPA filter have enough resistance?
#26835533 - 07/21/20 07:11 AM (3 years, 6 months ago) |
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24 x 18 x 5 7/8ths filter.
At 100FPM the true CFM would be 243, this has a resistance of 0.46"
But, the standard 100FPM x 3sq ft = 300CFM, which has a resistance of 0.57"
The filter was sold by fungi.com so I figured it was fine for laminar flow because why would they sell one that isn't? If there isn't enough resistance has anyone had success with puting a prefilter directly behind the HEPA filter to add resistance?
Any help would be greatly appreciated, as I've spent $330 on this damn filter!
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"It says number one son on the license plate. That's me. I'm the number one son." "What about me, Mom?" "You're number one son too"
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dfwerydfhg
Stranger
Registered: 05/04/20
Posts: 194
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Re: Does my HEPA filter have enough resistance? [Re: X-Ray Cat]
#26835636 - 07/21/20 08:10 AM (3 years, 6 months ago) |
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Resistance is not important, flow, velocity, and efficiency are.
I don't know what you're doing with your equations either: the filter is 1.5' x 2', so 3 sqft area. If you want 100 FPM, you need 100 * 3 = 300 CFM. What do you mean by "true CFM" and where did the 243 figure come from?
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X-Ray Cat
Sees through wooden doors

Registered: 05/12/20
Posts: 134
Last seen: 6 months, 23 days
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Re: Does my HEPA filter have enough resistance? [Re: dfwerydfhg]
#26835680 - 07/21/20 08:46 AM (3 years, 6 months ago) |
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It came from the lady at HEPA who I've been emailing. It's the actual square footage of the filter itself (not including it's casing). I did the math and it's 22x16, so an inch all around the filter.
Thanks for responding btw. Some people, a lot of people on here, have said that you need .5" or .8" at least, to get laminar flow. That's why I was asking. Before I order the wrong blower.
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"It says number one son on the license plate. That's me. I'm the number one son." "What about me, Mom?" "You're number one son too"
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Gan
Wielder of Narya



Registered: 08/26/19
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Re: Does my HEPA filter have enough resistance? [Re: X-Ray Cat]
#26835692 - 07/21/20 08:50 AM (3 years, 6 months ago) |
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Resistance does matter... at least for a laminar flow hood/cabinet
And yeah OP, those HEPAs from Fungi Perfecti (fungi.com) do have a little lower resistance than others it seems. However, I have the 24x36 hepa from there and it works fine. I know others that have the 18x24 or 12x24 filters from there and they work fine.
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dfwerydfhg
Stranger
Registered: 05/04/20
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Re: Does my HEPA filter have enough resistance? [Re: Gan]
#26835705 - 07/21/20 09:00 AM (3 years, 6 months ago) |
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Well yeah, I simplified the matter in this instance 
Obviously you need enough dP to get good distribution across the face, assuming a reasonable plenum/baffle system. But at the design stage, neither of those numbers posted should present a problem.
So to X-ray: given the accuracy/precision of most fan curves, the difference in those pressure drops is not super important. Use whichever flow you like, pick the appropriate resistance, add a bit for a pre-filter (if you intend to use one), and see which fan curve gets you closest. I guarantee you none are going to go through the exact point you're looking for anyway!
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Stipe-n Cap


Registered: 08/04/12
Posts: 7,623
Loc: Canada
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Re: Does my HEPA filter have enough resistance? [Re: dfwerydfhg]
#26835768 - 07/21/20 09:46 AM (3 years, 6 months ago) |
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Quote:
dfwerydfhg said: Resistance is not important, flow, velocity, and efficiency are.
I don't know what you're doing with your equations either: the filter is 1.5' x 2', so 3 sqft area. If you want 100 FPM, you need 100 * 3 = 300 CFM. What do you mean by "true CFM" and where did the 243 figure come from?
Resistance is essential.
OP just go ahead and click the link in my sig, please.
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bodhisatta 
Smurf real estate agent


Registered: 04/30/13
Posts: 61,889
Loc: Milky way
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Re: Does my HEPA filter have enough resistance? [Re: Stipe-n Cap]
#26835904 - 07/21/20 11:18 AM (3 years, 6 months ago) |
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It's 0% important in regards to the questions being asked.
For figuring out what blower you need its 100% important.
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Stipe-n Cap


Registered: 08/04/12
Posts: 7,623
Loc: Canada
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Re: Does my HEPA filter have enough resistance? [Re: bodhisatta]
#26835945 - 07/21/20 11:47 AM (3 years, 6 months ago) |
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He asked "does my filter have enough resistance ". He would like to know if .46 or .57 inWG is sufficient for laminar flow. So yes, it is absolutely important. Dfwery responded by saying that the resistance is not important, this answer is incorrect. The reason why the resistance (static pressure) of a filter at a given velocity is relevant is because it will dictate whether or not laminar flow is even possible. His concern is whether or not he is purchasing a filter capable of laminar flow.
But you already know that, bod.
If the OP reads my math template it describes in adequate detail the steps required for selecting a filter and then the necessary steps to match that filter to an appropriate blower. Every question he has raised in this thread would be answered by the time he's finished reading my thread.
Edited by Stipe-n Cap (07/21/20 12:05 PM)
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X-Ray Cat
Sees through wooden doors

Registered: 05/12/20
Posts: 134
Last seen: 6 months, 23 days
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Re: Does my HEPA filter have enough resistance? [Re: Stipe-n Cap]
#26835973 - 07/21/20 12:04 PM (3 years, 6 months ago) |
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Thanks for the help everyone, I truly appreciate it.
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"It says number one son on the license plate. That's me. I'm the number one son." "What about me, Mom?" "You're number one son too"
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bodhisatta 
Smurf real estate agent


Registered: 04/30/13
Posts: 61,889
Loc: Milky way
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Re: Does my HEPA filter have enough resistance? [Re: X-Ray Cat]
#26836421 - 07/21/20 03:52 PM (3 years, 6 months ago) |
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Filters that produce laminar flow can have all sorts of different resistance values. It's meaningless. You can get whatever laminar filter you want and worry about the resistance later when matching the blower
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dfwerydfhg
Stranger
Registered: 05/04/20
Posts: 194
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Re: Does my HEPA filter have enough resistance? [Re: bodhisatta]
#26836542 - 07/21/20 04:40 PM (3 years, 6 months ago) |
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Resistance is needed because to ensure even distribution over the filter, the dP getting to the filter (i.e. through the plenum, through any baffling, etc) must be << than the dP across the filter. Otherwise distribution gets raggedy.
Think of a 100 ft hose with small holes drilled every foot. Turn it on. Is the flow out of each hole perfectly even? Answer is it depends. If the dP over the hose, on its way to the hole, is a lot less than over the holes themselves, then the holes at the end aren't going to have as much pressure behind them, and the flow there will be lower than at the start.
Now if you swap out the hose for a 12" pipe, but keep the holes and the water pressure the same, all the holes will flow nice and even. The hose/pipe is your plenum, and the holes are your filter. It doesn't matter what the filter dP is, as long as it's sufficiently far above losses elsewhere in the system.
There were two numbers mentioned: 0.46" H2O and 0.57" H2O. If anyone thinks either that a) these are significantly different in the context of this guy's DIY flow hood, or b) that the absolute values are too low to provide laminar flow with that filter, you should say so now or stop confusing the matter.
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bodhisatta 
Smurf real estate agent


Registered: 04/30/13
Posts: 61,889
Loc: Milky way
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Re: Does my HEPA filter have enough resistance? [Re: Stipe-n Cap]
#26836555 - 07/21/20 04:47 PM (3 years, 6 months ago) |
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If it's being sold as a laminar flow hepa it obviously has enough resistance 
So the resistance doesn't matter. Until you match a blower
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Stipe-n Cap


Registered: 08/04/12
Posts: 7,623
Loc: Canada
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Re: Does my HEPA filter have enough resistance? [Re: bodhisatta]
#26836580 - 07/21/20 04:57 PM (3 years, 6 months ago) |
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Quote:
bodhisatta said: The resistance is probably too low for laminar flow[i/] The thickness has little to do with resistance. But the 12 inch thick filters are usually for clean rooms, high flow and low resistance You need like 0.8" w.g. static pressure across the filter for laminar flow. [i/] most are 0.8-1.2 That 12" thick one you found is probably testing at 0.4
bodhisatta said: There's an initial resistance, a certain pressure is required to even begin to move air through the filter.[i/]
You don't agree that there must be a minimum resistance for laminar flow?
Quote:
bodhisatta said: If it's being sold as a laminar flow hepa it obviously has enough resistance 
Considering he started a thread asking if his generated enough resistance I'd wager that it wasn't obvious to the OP.
Edited by Stipe-n Cap (07/21/20 05:16 PM)
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bodhisatta 
Smurf real estate agent


Registered: 04/30/13
Posts: 61,889
Loc: Milky way
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Re: Does my HEPA filter have enough resistance? [Re: Stipe-n Cap]
#26836624 - 07/21/20 05:20 PM (3 years, 6 months ago) |
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Hence the enlightening answer. If it's sold for laminar flow use it's got enough resistance 
And you can always email the people that are selling the filter
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X-Ray Cat
Sees through wooden doors

Registered: 05/12/20
Posts: 134
Last seen: 6 months, 23 days
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Re: Does my HEPA filter have enough resistance? [Re: bodhisatta]
#26836662 - 07/21/20 05:39 PM (3 years, 6 months ago) |
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I will say that I did email the people at fungi.com and they didn't know shit, lmao. They directed me to contact Hepa Corporation, which I did through their website. The lady that helped me from Hepa was absolutely amazing, even sending enquiries to their engineering department for the exact answers that I needed.
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"It says number one son on the license plate. That's me. I'm the number one son." "What about me, Mom?" "You're number one son too"
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bodhisatta 
Smurf real estate agent


Registered: 04/30/13
Posts: 61,889
Loc: Milky way
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Re: Does my HEPA filter have enough resistance? [Re: X-Ray Cat]
#26836666 - 07/21/20 05:40 PM (3 years, 6 months ago) |
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Well fungi.com is overpriced paul scammets stuff tho
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X-Ray Cat
Sees through wooden doors

Registered: 05/12/20
Posts: 134
Last seen: 6 months, 23 days
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Re: Does my HEPA filter have enough resistance? [Re: bodhisatta]
#26836849 - 07/21/20 07:05 PM (3 years, 6 months ago) |
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I just learned this recently, after I made my purchase. Very disappointing.
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"It says number one son on the license plate. That's me. I'm the number one son." "What about me, Mom?" "You're number one son too"
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