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remediator
IllustriousPotentato


Registered: 02/23/06
Posts: 137
Loc: NOLA ATM
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Positive Pressure box issues...(PICS)
#5395462 - 03/13/06 02:49 PM (17 years, 11 months ago) |
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Damnit. I went through all this trouble to make this box nice and sealed, even did some soldering to get the power supply worked out, then when the construction is done (and i mean done, the housing for the fans is sealed with RTV) One fan is pushing more than the other and causing suction, or the fans are too close together... I can't figure it out. When i block one fan, the other blows. and vice-versa. But when both are running, there is suction at the friggin arm holes and the air inside becomes a cyclone, circulating air that has been sucked into the box. The fans are both blowing, the labels face into the box. before i put the housing with the tyvek on it, the flaps were flying off the front of the box. Any suggestions on the physics I'm apparently missing here?
Will mounting the fans on the outside affect the way the air flows? Is the cut of the plastic on the inside of the box causing redirected air to swirl?
I can't figure it out. Man, it was gonna be so great. any help?
Edited by remediator (03/13/06 03:22 PM)
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musher_420
Stranger

Registered: 08/01/05
Posts: 2,691
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Re: Positive Pressure box issues... [Re: remediator]
#5395495 - 03/13/06 02:58 PM (17 years, 11 months ago) |
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I'm not sure why you'd get suction if the fans where blowing into the box.
But I can tell you tyvek doesn't have the same filtering capability of a HEPA and nor do I think those fans have the kind of power they need to pull much air though the tyvek. They will likely just circulate air around inside the box.
I'd suggest you just switch it over to a still air. just cover those fan holes with the tyvek. This will help with gas exchange and allow prints to dry better inside the box. Assuming your using for printing.
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remediator
IllustriousPotentato


Registered: 02/23/06
Posts: 137
Loc: NOLA ATM
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Re: Positive Pressure box issues... [Re: musher_420]
#5395524 - 03/13/06 03:04 PM (17 years, 11 months ago) |
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thanks for the suggestion. I have had great luck for years with still air, but these boxes over at the nook: http://www.thenook.org/archives/2630.html inspired me to do this. Several of those boxes achieved positive pressure through a hepa with these chintzy fans. I knew my pressure would be weak, and that there really isn't a substitute for a flow hood, but these things i had ion hand.... i actually have some old jar filters, and in due time i am going to build a better flow box with an actual HEPA, but today there is oyster spawn coming to my house, and i wanted to do it up fancy...
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cappa
Nerd
Registered: 02/12/06
Posts: 854
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Re: Positive Pressure box issues... [Re: remediator]
#5395666 - 03/13/06 03:44 PM (17 years, 11 months ago) |
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You are blowing more than you are sucking! haha
1. Double check your polarity.
2. Install a poteniometer or resistor inline with the 'blowers' to reduce 'blowing'.
-------------------- Their are 10 types of people. Those that understand binary, and those who don't. ~Cappa.
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remediator
IllustriousPotentato


Registered: 02/23/06
Posts: 137
Loc: NOLA ATM
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Re: Positive Pressure box issues... [Re: cappa]
#5398151 - 03/14/06 08:31 AM (17 years, 11 months ago) |
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cappa, the fans are def both blowing, the polarity is right. One of the fans is .20 a and the other is .30 a Maybe thats part of it, But i can't help thinking that there is a simple solution. I guess I'm just going to remove the housing and mount the fans on the inside of the box, with the tyvek on the outside. I sure wish i could spare the money for a HEPA, But I'm doing all of this work in addition to my REGULAR life, as a volunteer in a project... so i can't just go all out. Non-profits... Nobody?
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RogerRabbit
Bans for Pleasure


Registered: 03/26/03
Posts: 42,214
Loc: Seattle
Last seen: 11 months, 22 days
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Re: Positive Pressure box issues... [Re: remediator]
#5398585 - 03/14/06 10:49 AM (17 years, 11 months ago) |
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You can't build static pressure with a computer fan. The blades are simply cavitating the air causing turbulence. Still air gloveboxes are best in my opinion. You can't replace a flow hood with an envelope from the post office.
Also, if those are AC fans, you can't install a resistor or rheostat in the line to control speed. Speed of an AC motor is determined by the frequency(hertz) of the supply voltage. RR
-------------------- Download Let's Grow Mushrooms semper in excretia sumus solim profundum variat "I've never had a failed experiment. I've only discovered 10,000 methods which do not work." Thomas Edison
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Roadkill
Retired Shroomery Mod


Registered: 12/11/01
Posts: 22,674
Loc: Montana
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Re: Positive Pressure box issues...(PICS) [Re: remediator]
#5398771 - 03/14/06 11:40 AM (17 years, 11 months ago) |
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-------------------- Laterz, Road Who the hell you callin crazy? You wouldn't know what crazy was if Charles Manson was eating froot loops on your front porch! Brainiac said: PM the names with on there names, that means they have mushrooms for sale.
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remediator
IllustriousPotentato


Registered: 02/23/06
Posts: 137
Loc: NOLA ATM
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Re: Positive Pressure box issues...(PICS) [Re: Roadkill]
#5399142 - 03/14/06 01:14 PM (17 years, 11 months ago) |
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I am going back to still air until i have a few hundred spare bones for a big hepa,
but
These designs seem to have worked for their creators and i couldn't help posing the question Really a matter of fluid dynamics and the power of the fans and the resistance of the tyvek. No arguments, just inquisitiveness...
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cappa
Nerd
Registered: 02/12/06
Posts: 854
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Re: Positive Pressure box issues...(PICS) [Re: remediator] 1
#5399213 - 03/14/06 01:36 PM (17 years, 11 months ago) |
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Okay I have looked at it a little closer. Some quesions:
When you turn them on and stick you hand right in front of them, inside of the box, do you feel they are blowing? Is this suction you notice only occuring at the arm holes? Does it occur with the lid on, off, or both? Does you air have a place to escape near the top of the box?
You may have a venturi effect going with those arm hole tubes. The cycloning air is passing by your tubes at right angles. This creates a vacuum called the venturi effect. Are you planning on installing some gloves on those arm hole tubes? Block the arm holes and see what happens to the air around the lid. You can use a lighter or something to see which way the air is travelling.
-------------------- Their are 10 types of people. Those that understand binary, and those who don't. ~Cappa.
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mogur
regnartS

Registered: 11/15/05
Posts: 322
Loc: Puget Sound
Last seen: 11 years, 2 months
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Re: Positive Pressure box issues...(PICS) [Re: Roadkill]
#5399273 - 03/14/06 01:50 PM (17 years, 11 months ago) |
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You're gonna have to do what RR and Road suggest until you get a workable filter. Your main problem is the Tyvek. It is designed to block air flow. Grab a scrap of Tyvek and try to blow through it. You can, but you won't be much of a threat to any birthday candles. It is permeable (>58 perms) to both air and vapor, allowing it to breath, but at the same time it is designed to be a barrier to air infiltration. Meaning that large volumes of air are hard to push through it. Those little fans will probably pull enough air through a hepa to positively pressurize your box, but the way you have it now pulls more through small leaks than it can get through the tyvek.
Doesn't take much money to buy a small hepa filter, since vacuum cleaners and small room air cleaners are competitive markets now. Here's one at Home Depot for about $20-

Some of the vacuum cleaner hepas are under $15, or half that on ebay.
But, if you need to use it right away, it will work as it is for a still air box. The tyvek will block contams and actually help equalize any pressure differential, reducing air turbulence inside.
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remediator
IllustriousPotentato


Registered: 02/23/06
Posts: 137
Loc: NOLA ATM
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Re: Positive Pressure box issues...(PICS) [Re: mogur]
#5399310 - 03/14/06 02:00 PM (17 years, 11 months ago) |
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Sweet. I'm back to a still box now after removing all my apparatus. Tried installing the fans inside the box, with the tyvek on the outside without the "chamber" to no avail... still some strange cyclonic stuff going on. I used smoke to determine air flow, and there is definitely something not right.
CAPPA - the second set of photos is from the archives at NAns' nook, the link for which you will find earlier in the thread. I merely posted those photos as justification for my logic in thinking that my method might succeed.
In new orleans right now it is a far trek to major retail giants, you sort of have to leave the actual city and go to the outskirts for target, wla-mart, home depot and the like. I don't have access to a car, so even if i had money to spare, i couldn't get out shopping. I know it seems inconceivable, but i am quite poor, and N>O> is quite effed up. I actually built my first still air glovebox in 2001, and have probably built 8 or 9 of them using basically the same materials every time, all with great success. These boxes are somewhat randomly spread about the country at this point...
anyhoo. thanks for your time folks, i consider this idea dead in the water, despite the tantalizing photos at the nook... peace
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cappa
Nerd
Registered: 02/12/06
Posts: 854
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Re: Positive Pressure box issues...(PICS) [Re: mogur]
#5399412 - 03/14/06 02:25 PM (17 years, 11 months ago) |
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I prefer still air myself but I figure if you wanted a still air GB, you would have built one. The vacuum filter idea is definately the best.
Just cause I love ya, I'm building something like what you have to test it out.


Made from checkbook box, silicone, tyvek, electrical tape. Will test it in 1 hour when the silicone is cured enough.
-------------------- Their are 10 types of people. Those that understand binary, and those who don't. ~Cappa.
Edited by cappa (03/14/06 02:41 PM)
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remediator
IllustriousPotentato


Registered: 02/23/06
Posts: 137
Loc: NOLA ATM
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Re: Positive Pressure box issues...(PICS) [Re: cappa]
#5399441 - 03/14/06 02:37 PM (17 years, 11 months ago) |
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Well I'll be a monkey's hongo...
Just as i give up, too. Well, let me know. I take it those fans are going outside of the box, eh? What for the filter? I have a couple of filter disks from years ago laying around, think they might be somewhere between hepa and tyvek? I really am beginning to think that although the amount of air through the tyvek was the major restricting factor, the proximity of the fans to one another created some uber-tornado effect, thereby reducing pressure enough in the box in some areas to create considerable suction ... I'm anxious to see what happens to yours...
Edited by remediator (03/14/06 02:40 PM)
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cappa
Nerd
Registered: 02/12/06
Posts: 854
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Re: Positive Pressure box issues...(PICS) [Re: remediator]
#5399467 - 03/14/06 02:45 PM (17 years, 11 months ago) |
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I'm going to guess that running the fans at full RPM is the main culprit. The tyvek will only allow so much air through it in a given amount of time with the force you have available.
Hypothesis: Lowering fan RPM to match the volume of air that can pass through the tyvek will make it more effecient.
Will post pics and results within a couple of hours.
-------------------- Their are 10 types of people. Those that understand binary, and those who don't. ~Cappa.
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cappa
Nerd
Registered: 02/12/06
Posts: 854
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Re: Positive Pressure box issues...(PICS) [Re: cappa]
#5399894 - 03/14/06 04:41 PM (17 years, 11 months ago) |
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Okay the results are in.
Hypothesos: disproved.
Result: From 500rpm to 2500 rpm the fans do not acheieve desired results.

Interestingly enough, they do pull a very small amount of air through the tyvek. But as Mogur said, you wouldn't be much of a threat to a bday candle.
 Not a very good pic, but the flame was definately being pulled into the filter.
What happens is the inner part of the fan blades are moving slower than the outer part of the blades. Becasue of this the inner area doesn't generate enough of its own pressure to overcome the pressure generated by the outer part of the blades. The result is that air moves back into the fan near its center, and is blown away from the fan near the outer areas. Also noteworthy is that their is a 'dead spot' of equally pressurized air in between.

But overall you get this:

It becomes easier to suck air into the contraption through the inner part of the fan blades than it is to suck air through that amount of tyvek at certain rpm.
Myth busted. You cannot simply use computer fans in this manner to bring in filtered air through tyvek. I also have my doubts that a vacuum hepa filter will yield any better results.
My friend came up with an interesting and cheap solution that has so far worked for him. He just cut 2 holes in his box at each end. Put tyvek over the holes. Then he installed a piece of plastic pipe onto the box, over one of the holes. This pipe had the same O.D. as his vacuum cleaner hose I.D. When he wants to use his GB. He opens it and puts in the stuff he wants to mess with in there. Then he sprays it down with lysol, closes the GB,waits half an hour, and sucks the air out with his vaccum cleaner.
Anyways, don't feel bad. I've seen a generous amount of pictures showing GB's made with computer fans. Supposedly they work, but now I'm a little sceptical.
-------------------- Their are 10 types of people. Those that understand binary, and those who don't. ~Cappa.
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remediator
IllustriousPotentato


Registered: 02/23/06
Posts: 137
Loc: NOLA ATM
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Re: Positive Pressure box issues...(PICS) [Re: cappa]
#5400255 - 03/14/06 05:55 PM (17 years, 11 months ago) |
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Jesus. What an effort. Thanks a lot for going thru that man... Now i don't feel so alone.
you da man.
Bout to get down with some G2G, old school.
peace bro... cappa cappin myths...
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mogur
regnartS

Registered: 11/15/05
Posts: 322
Loc: Puget Sound
Last seen: 11 years, 2 months
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Re: Positive Pressure box issues...(PICS) [Re: remediator]
#5400906 - 03/14/06 08:21 PM (17 years, 11 months ago) |
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Damn, Cappa, a tip of the hat to a fine piece of work, there.
Haven't built one myself, so this is pure speculation, but, that photo you posted above, remediator, is two axial fans and a hepa. Seems like it should work, since this isn't a flow hood. You don't need 100 lin/ft/sec flow velocity, just some nice steady, positive pressure. Those fans are much stronger than the 80mm axials on your box, but here's a link to some 130 cfm axials for only $5 each.
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