Squirrel cage fan for NEGATIVE pressure??? - Page 2 - Hot Rod Forum : Hotrodders Bulletin Board
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  #16 (permalink)  
Old 10-19-2013, 03:09 PM
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When we design clean rooms we put those under positive pressure for the reason that if there are any leaks then the dust is blown out instead of being sucked in as it would be in a negative pressure situation. All you will need is a small furnace and just mount it up in the rafters out of the way and just run it on summerfan setting. Only time I have ever used a negative flow is in a dust hood similar to a kitchen exhaust hood for dust and fume extraction purposes such as over a welding bench in a factory..

You may wish to put some sort of damper on the fan as those things blow a lot of air and you may need to cut the flow of air somewhat..

Sam

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Old 10-19-2013, 07:54 PM
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I am going to delete this and shut up
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Old 10-19-2013, 08:41 PM
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Nah Craig...I enjoy your knowledge of homemade booths. I think push is a good idea because it doesn't require an explosion proof fan. And that it will push dust out. The problem is for me personally, I will be doing grinding and sanding inside this booth. It WILL have loose particles and I don't think a large stream of air will be good for that even if it is tapered off somehow. I am making a mobile paint booth in a 6x12 trailer. I will only be shooting small items. And the booth is more for overspray control and staying warm in the winter (heat lamp) than keeping junk out of clear. While I'm sure a pressure booth is great, my application leans to pull. Thanks for helping out Craig.
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Old 10-19-2013, 10:07 PM
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Thanks to you as well one more time
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Old 10-20-2013, 09:36 AM
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cfm is cfm, the thing that stirs up dust is velocity.
getting rid of velocity is as easy as putting a baffle or a plenum in front of a fan.
that's how the big boys do it
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Old 10-20-2013, 09:43 AM
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Just food for thought. Since you are mounting this on a trailer, you may want to look at these.
Air Vent Inc. 24" Whole House Fan 54301 Attic & Whole House Fans - Amazon.com Air Vent Inc. 24" Whole House Fan 54301 Attic & Whole House Fans - Amazon.com
They still have the belt drive with the motor off to the side. We actually used one similar when we built my dad's booth. I have a squirrel cage in mine, but if it ever gives out on me I will replace it with one of these. We mounted the fan on the outside of the booth, pulling air from inside the booth. Mounted the shutter on the inside so it basically seals itself when not in use, then pulls the shutter open when turned on. Most are 2 speed. My dad's booth measures 24'x26' and he usually runs the fan on low when painting, high speed almost pulls too much, so you may have to slow it down a little for your application.

Kelly
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Old 10-20-2013, 12:24 PM
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Quote:
Originally Posted by jsm1847 View Post
It's a small booth, 6 ft high, 6 ft wide, and 12 ft long. To have that much air blowing in, in my opinion, would cause dust to stir. From my thoughts, it has very little room to dissipate the draft. I have been in a negative pressure booth this same size. Seemed to work pretty good. Sam, I'd be glad to know your opinion on this matter if you have a different idea. Thanks
Negative pressure is best if you are spraying paint. Like you said much less turbulence. Put in a block of furnace filters (2or 3) at the opposite end of the room. I like to stack them from floor to ceiling to get good flow. I spray the filters with Aquanet hair spray to pick up the little stuff that might get drawn through them.
Mark
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Old 10-20-2013, 03:15 PM
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Oh boy, will a squirrel cage fan make negative pressure. Mine is an old furnace fan, sucks the walls clear in on my 'booth.' My fan is mounted to a pulley, which is driven by a belt driven from a pulley mounted to the shaft of the motor. And contrary to what's been stated, mine runs just fine, I run it for HOURS, the old body guy I got the set up from ran it for HOURS too. I built a wooden enclosure that goes around the motor to protect it from overspray. I'm not worried about the fumes because this set up changes the air out completely every few minutes. If the room starts to get kind of foggy, stopping for a few minutes clears it.

The back of my booth is a large sliding door. I built a wooden insert to go in the opening and the door closes up against it. The insert has a hole in it cut for the 8" ducting from the fan, which goes out the hole and blows outside, as you can see in the pic.
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Old 10-25-2013, 04:34 AM
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We have an old....literally found on side of the rd...sq cage.Have had several and this one is nice because it's a sm one.

It's the kind with the motor riding piggyback on top....belt drive.Made a sheet metal sq to round transition of sorts that allows us to run 24x24 filters.

We have it hung in our cabinet shop as an ambient air scrubber.It's designed "racetrack" style.Meaning,the air is in a circular flow pattern around perimeter of space.vs a center pull(or whatever its called).Anyway theres another non filter'd fan at the other end.....and round N round she goes.

Posted to say,this thing runs non-stop some days,all day long.Hung from ceiling framing with dog chain,instead of direct bolt up.It's pretty quiet,and moves a boatload of air.
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Old 10-25-2013, 10:44 AM
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Quote:
Originally Posted by OneMoreTime View Post
Just a question..Why negative pressure..why not positive pressure on the booth..??

Sam
Because positive pressure means you will draw dirty air in, even if the intake is filtered, and a fine enough filter will restrict air intake. It's a lot easier to create negative draw and not pull in outside contaminants.

On the original squirrel cage fan question. How well it works depends on the fan size, and the housing the fan comes with. I've seen large squirrel cage fans that flowed a lot of air with a properly shaped housing, and others that had too loose a tolerance and didn't flow much.
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