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Invisiblerickyswamps
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Help understanding Blower needed for Flow Hood
    #25944641 - 04/19/19 09:44 PM (5 years, 8 months ago)

So Im a bit confused.  Here is a graph of the filter I bought.



My math says that I need 400 CFM from the blower.  Add 20% for friction loss, HEPA clogging over time, that comes to 480 CFM.  According to the above graph, at 480 CFM the filter has a resistance of .45 inWG.  Add .2 for a prefilter, that comes to .65.

So I need a blower that delivers at least 480 CFM @ .65 inWG ?

When I read some teks they are basing the resistance off of the tested number on the filter.  In my case that is 1.0 inWG @ 1000 CFM.  And then adding .2 for a prefilter.  Bod's tek says to find out the resistance at the flow you are going to use (if I am understanding it correctly).

Maybe someone can clear up if I need a blower that delivers:

480 CFM @ .65 inWG resistance
or
480 CFM @ 1.2 inWG resistance

The difference between the 2 is pretty big.

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Invisiblerickyswamps
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Re: Help understanding Blower needed for Flow Hood [Re: rickyswamps]
    #25945310 - 04/20/19 08:53 AM (5 years, 8 months ago)

I cancelled buying that filter after reading about 12 inch deep filters not having enough resistance to provide good laminar flow...

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OfflineSFS96
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Re: Help understanding Blower needed for Flow Hood [Re: rickyswamps]
    #25945402 - 04/20/19 10:00 AM (5 years, 8 months ago)

480 @ 1.2 I do believe :thumbup:


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InvisiblebodhisattaMDiscordReddit
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Re: Help understanding Blower needed for Flow Hood [Re: SFS96]
    #25945494 - 04/20/19 10:57 AM (5 years, 8 months ago)

Mine is 11.5" deep. People told me it was the wrong one because people read old quotes and take them as gold. People went so far as to try arguing with me only using other people's misinformed quotes as evidence. The manufacturer specifically said in a reply my filter produces laminar flow at 80-120fpm face velocity.

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Invisiblerickyswamps
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Re: Help understanding Blower needed for Flow Hood [Re: bodhisatta]
    #25945529 - 04/20/19 11:17 AM (5 years, 8 months ago)

That was what I was thinking too.  That it was just something people used to say.  I read through your tek on the build and you came to a conclusion that it had around 0.8-1.2 in WG resistance at your airflow.

Here is a graph for the filter I was looking at.



Seems to have a much lower resistance at 400-500 CFM than yours…But I still don't really get why it wouldn't work if you got the 100 flow per square foot out the front.  :shrug:

The number written on the filter is 1.0 in WG @ 1000 CFM.  Which seems to be what most builds are based off.  Ignoring that you always use a blower with less flow than that.

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InvisibleAyePlus
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Re: Help understanding Blower needed for Flow Hood [Re: rickyswamps]
    #25945721 - 04/20/19 01:21 PM (5 years, 8 months ago)

I would shoot for 700 cfm @0.6 personally, basically as long as stuff isnt blowing off the table and the flow is laminar you’re good to go. I’ve built three hoods now with different sized filters and different fans and so far liking the slightly overpowered one the best, I think its about 180 fps at the face. I have another that blows about 105 and it works but seems underpowered even though it does work just fine.  Commercial fan filter units usually blow around 90 but theyre meant to be used in clean rooms by people wearing tyvek suits and presumably also wearing masks. Afaik anywhere from 90-300 is workable and its only gonna slow down as it ages so build it to last. That filter ahould work fine but I’ve built all mine with 6” so they dont have to be as bulky. I’m also impatient so I order from stamets because he keeps them in stock whereas many others only make them once they’re ordered. I buy the fans from electric motor warehouse or grainger though.


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Re: Help understanding Blower needed for Flow Hood [Re: AyePlus] * 1
    #25945778 - 04/20/19 01:57 PM (5 years, 8 months ago)

Above 120fpm face velocity your airflow breaks up into non laminar flow from what ive gathered on how laminar flow gets produced from filter faces

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InvisibleAyePlus
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Re: Help understanding Blower needed for Flow Hood [Re: bodhisatta]
    #25946348 - 04/20/19 07:37 PM (5 years, 8 months ago)

Source of that tidbit? I’m no physicist but I dont see why it would make a difference, laminar is laminar no? Regardless The math on my 18x24 hood works out to about 178 or thereabouts and I’ve never had any issues, I have the same fan on a 24x24 and it works just as well but feels slow so Imm extra careful using it, and I know Gr0wer uses one closer to 300 with no issues either. And mushpunx has one that to me sounds like it would be waay underpowered.


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InvisiblebodhisattaMDiscordReddit
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Re: Help understanding Blower needed for Flow Hood [Re: AyePlus]
    #25946358 - 04/20/19 07:52 PM (5 years, 8 months ago)

Something to do with Reynolds number

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Re: Help understanding Blower needed for Flow Hood [Re: bodhisatta] * 1
    #25946417 - 04/20/19 08:33 PM (5 years, 8 months ago)

“Faster moving air is not necessarily better,” says Brande. “It may seem counterintuitive, but lowering airspeed can provide better protection for a critical area. Increasing airspeed may even be counterproductive by increasing turbulence and a reverse air flow around obstructions.”

In December 1963, 90 fpm was codified when Sandia Laboratory produced Federal Standard 209 “Clean Room and Work Station Requirements, Controlled Environment,” which stated in a nonmandatory appendix, that clean rooms should maintain an airflow velocity of 90 fpm (within ±20 fpm).

Dan Milholland of Dan Milholland and Associates recalls that the 1994 LUMS (Lilly, Upjohn, and Merck) project that investigated the use of an isolator for aseptic filling found that a unidirectional airflow at significantly less than 90 fpm was efficient in dealing with particle contamination.

The electronics industry has found that greater cleanliness can be achieved at lower air velocities due to reduced turbulence around objects in the flow path.

The electronics industry has found that greater cleanliness can be achieved at lower air velocities due to reduced turbulence around objects in the flow path,” explains Milholland. “The typical microelectronics room is designed to operate at 55 to 70 fpm.”

He notes that modern semiconductors are built in isolators and never exposed to air. These mini environments operate with 25% to 33% filter coverage with a filter face velocity of 65 fpm, or the equivalent of approximately 20 fpm with raised floors, rather than sidewall returns.

Semiconductor isolators operate two orders of magnitude cleaner than the FDA requirements for pharmaceutical aseptic operations. Milholland recently measured particle concentration in the duct upstream of the ULPA filters at 175 particles 0.1 μm and larger per cubic foot of air. This meets ISO Class 3 at 0.1 μm before the ULPA filters. “That’s pretty clean!” he says.

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Re: Help understanding Blower needed for Flow Hood [Re: bodhisatta]
    #25946425 - 04/20/19 08:39 PM (5 years, 8 months ago)

I have a 11.5 and it works just fine 3 years and counting ..


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InvisibleAyePlus
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Re: Help understanding Blower needed for Flow Hood [Re: bodhisatta]
    #25946434 - 04/20/19 08:44 PM (5 years, 8 months ago)

Quote:

Increasing airspeed may even be counterproductive by increasing turbulence and a reverse air flow around obstructions.”




This makes sense, but It doesn’t imply that laminar flow above 120fps turns into turbulent flow, merely that at higher speed turbulence is more likely to occur around obstructions, and proper FH technique would be working in “virgin” air. Once again it all falls back to proper technique. But I do see now that 100 fps is probably better than 170. Luckily I have a speed controller built into that hood and I can dial it back.

That last  bit also explains why mushpunx has been successful using a slower fan.

:grampofapproval:


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InvisiblebodhisattaMDiscordReddit
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Re: Help understanding Blower needed for Flow Hood [Re: AyePlus]
    #25946921 - 04/21/19 06:07 AM (5 years, 8 months ago)

The faster air goes the quicker it breaks up into turbulent flow. So the faster your speed the shorter distance from the hood that laminar flow happens. if you get fast enough no laminar flow gets made at all. Of course laminar flow can happen at any speed but at those high speeds it could only happen inside of a pipe with fins to guide the air. and upon exiting it would be turbulent.

A Reynolds number above 3000 = turbulent flow.


https://en.wikipedia.org/wiki/Hydraulic_diameter

given the faces of our filters are so big I would think much above 100fpm would be putting us into non-laminar territory or making the distance of laminar flow from the filter very short.

https://en.wikipedia.org/wiki/Reynolds_number
like the candle picture in the article faster flow will make that transition to turbulence faster shortening your working distance

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Invisiblerickyswamps
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Re: Help understanding Blower needed for Flow Hood [Re: bodhisatta]
    #25947036 - 04/21/19 07:51 AM (5 years, 8 months ago)

Great info.  If it drop to 55 fpm is still clean laminar air.  Wonder how far from the filter face that is true?

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InvisibleJ. Jack Flash
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Re: Help understanding Blower needed for Flow Hood [Re: bodhisatta]
    #25947076 - 04/21/19 08:21 AM (5 years, 8 months ago)

i'm still in fluid mechanics. i want to act like i know what i'm talking about, but i do not. don't know how to figure the hydraulic diameter of the flow which isn't bounded once it leaves the filter. i am not smart. going to ask my professor about it.

however, a while ago i became convinced that slower air is better. reasoning goes: the air flow from the filter is assumed to be as sterile as can be. hands and arms and outsides of plates are not at all sterile. high speed air becomes turbulent more quickly/easily when flowing around non aerodynamic obstructions, like nonsterile hands and arms. the more turbulent the flow around such things, the more likely it is for vortices to draw contaminants into sterile things from nonsterile things. on the other hand, a more gentle breeze is all that's required to push microscopic, nearly weightless contaminants essentially horizontally away. not to mention i imagine it's more comfortable to face directly into while working.

thanks for the quotations above, bod. confirmation bias: activated. =]

ricky, i'd go with closer to 480@.65. 55fpm is just under 1fps. seems plenty fast to keep contaminants out.

nerd alert: i'm running numbers.
filter width (ft) 2
filter height (ft) 2

Area(ft^s) 4
Perimeter(ft) 8
Rh(ft) 0.5
Dh(ft) 2
density, rho 0.0765 lbf/ft^3
velocity, u 100 ft/m
        1.67 ft/s
kinematic viscosity, nu 1.63E-04 ft^2/s
Re 1.56E+03
=rho*u*Dh/nu

Re of 1560 indicates laminar flow

velocity, u 150 ft/m
2.5 ft/s
kinematic viscosity, nu 1.63E-04 ft^2/s
Re 2.34E+03 =rho*u*Dh/nu


Re around 2300 is right around the transition point between laminar and turbulent, so is roughly the upper limit of air speed through a 2x2ft filter. 150fpm.


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InvisiblebodhisattaMDiscordReddit
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Re: Help understanding Blower needed for Flow Hood [Re: J. Jack Flash]
    #25947077 - 04/21/19 08:23 AM (5 years, 8 months ago)

that's the thing since the filters are unbounded once the flow exits the face of the filter those numbers are like best case scenario is my guess. unbounded flow I would imagine breaks to turbulence even faster

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InvisibleAyePlus
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Re: Help understanding Blower needed for Flow Hood [Re: bodhisatta]
    #25947365 - 04/21/19 11:11 AM (5 years, 8 months ago)

Ok so this is totally talking out of my ass but wouldnt the bound cause the outer air to slow more making turbulence more likely? Or would the resistance generated by the air in the room be greater than the smooth aurface of the inside of a pipe/duct/whatever?


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Re: Help understanding Blower needed for Flow Hood [Re: AyePlus]
    #25947397 - 04/21/19 11:30 AM (5 years, 8 months ago)

Beats me. Physics can be counterintuitive

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InvisibleBoogieman47
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Re: Help understanding Blower needed for Flow Hood [Re: bodhisatta]
    #25947440 - 04/21/19 11:46 AM (5 years, 8 months ago)

I called the engineer of astrocel filters and he said the static pressure builds up in the plenum so if you have a deep enough or short enough plenum is hot to adjust your static pressure.. still didnt explain how that worked out Haha


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InvisibleJ. Jack Flash
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Re: Help understanding Blower needed for Flow Hood [Re: AyePlus]
    #25947475 - 04/21/19 12:06 PM (5 years, 8 months ago)

Quote:

AyePlus said:
Ok so this is totally talking out of my ass but wouldnt the bound cause the outer air to slow more making turbulence more likely? Or would the resistance generated by the air in the room be greater than the smooth aurface of the inside of a pipe/duct/whatever?




yes/no. it turns out that a no-slip condition is verified experimentally for fluids flowing through a pipe, channel or duct. that is, the velocity of the fluid relative to the pipe wall, at the pipe wall is zero. between the pipe wall and the pipe axis, there is a velocity distribution, meaning there's a range of velocities between zero and a maximum value. there's no such thing as a sustained uniform velocity distribution. the filter acts as a uniform velocity distribution generator. i think that's entirely the point, apart from actually being a filter. when the flow exits the filter, consider the outside air to be a pipe containing the flow from the filter, but with an inner wall which can move relative to the filter. the shear stress between the outside air and the laminar flow acts to slow the boundaries of the flow, which in reaction accelerates the inner wall of the pipe. reasoning through it, it would seem to sort of truncate the velocity distribution at some point greater than zero at the dimensions of the filter. i reckon this would scale with the square of the velocity, it's basically air resistance.

so, this would act to increase the hydraulic diameter of the hypothetical flow/duct, driving the reynolds number up and lowering the velocity threshold for turbulent flow. this could be mitigated by extending the frame of the filter beyond the filter itself as some hood designs do, so that the flow from the filter pushes air through a duct of the same dimensions.

:tldr:
a static bound slows the air at the bound to zero, which reduces the likelihood of turbulent flow at a given flow rate.

i could be wrong. if i am, i'd love to be corrected. finals are coming soon.

that part about using plenum design to adjust static pressure is fucking interesting. this makes me think about the voodoo that is speaker box design.


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