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Old 10-18-2013, 09:44 PM
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Squirrel cage fan for NEGATIVE pressure???

ok, Im just going to ask it...even thought I know there are many posts about squirrel cage fans. I HAVE BEEN RESEARCHING FOR HOURS!!! Are squirrel cage fans good for negative pressure? The reason I'm thinking squirrel cage is because it is belt driven. Im trying to pull exhaust out of a small booth, like 6x13 ish.
Must I use these type of fans for positive pressure only? Of course positive pressure is safer because it is not in the stream of the fumes. But if I do make it negative, I would build the motor outside the fume path anyway. I just can't bring myself to spend $1000 on an explosion proof fan. I'm also assuming that I need to push about 2000 cfm. Sound right? Can anyone point me in the right direction? Thanks!

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Old 10-18-2013, 10:28 PM
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Oh yea. They will pull neg. It will work. Trust me. I make my living playing with fans.

I am guessing 800 cfm would work, furnace fan with a 10 inch round intake. Motor would be on the outside then.
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Old 10-18-2013, 10:43 PM
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Thanks for the reply and expertise! Do furnace fans usually come with the motors or am I just after the squirrel cage, then buying a aftermarket motor? I would assume I can salvage both off a furnace?
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Old 10-18-2013, 10:54 PM
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AN older one will have the motor on the outside of the fan, belt driven. New ones are built onto the fan inside the blower ( not explosion proof ). You can
change the motor pulley, bigger = faster / more air. A half hp motor on an old
furnace will go up to about 1600 cfm. In your booth that would be about 2 changes per minute? The only draw back is a squirrel cage gets dirty easy and needs cleaning. A squirrel is better than an axial in the sense axial do like to have the discharge choked down. An axial is almost always found on a wall because they hate duct work, can be done though with duct work.

You have a bit of messing with an old furnace blower to make it safe, but they can be found for free almost at scrap yards or behind plumbers shops.
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Old 10-18-2013, 10:57 PM
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If I had a dime for every paint booth I stated up I would have like 100 bucks cash!

Installed a lot years ago
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Old 10-18-2013, 11:07 PM
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Thanks. Upon further inspection, I see that the motor is built inside the squirrel hole. I'm thinking I'd have to fabricate a rod to go into wheel (like the motor does), relocate motor to outside, then attach a pulley to both rod going Into wheel and Rod from motor. Sound hard? Or should I look for old one? I do know a hvac guy. Ill be asking him anyway. Any ideas on size of the pulleys? Oh yeah, I will be filtering air before it goes through the fan too for particulate.
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Old 10-18-2013, 11:20 PM
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Do not try it. It will shake and rattle into pieces in 3 minutes.

Go find an old one. 12 bucks tops or free for an old one. Get the motor on the
outside. No way you can extend or mod that direct drive. Vibration kills.

Just go 4.25 inch pulley on a 1725rpm motor ( stock motor is 1725 ) do not mess with blower pulley.
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Old 10-18-2013, 11:41 PM
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Ok. I'll be on the hunt for a older unit. Are they hard to find? I was reading they are getting scarce...
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Old 10-18-2013, 11:47 PM
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I'm looking at pics of older ones... Looks like even though motor is on the outside seems like there might be some seep through because the pulley side isn't sealed. Will the air suction keep it out of motors way, or should I think of sealing it? If seal it, how?
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Old 10-19-2013, 07:32 AM
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Be advised that even though the belt driven units will work they are not explosion proof. The key is the level of explosive vapors in the air. It would have to get pretty heavy with suspended particles to actually reach an explosive level.

Vince
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Old 10-19-2013, 09:56 AM
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True, but if he puts the fan *outside* the booth, and ducts the intake in, chances are good he is ok. There is always risk. Even static is a risk if you have build up. A true explosion proof fan is going be thousands of dollars and not much different than what he is looking at doing. I wouldn't pay for it.

Besides, I have done what he is looking at before. The primer scares me more than the paint.
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Old 10-19-2013, 10:05 AM
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Hi i am building a home paint booth, one u can take apart and put together, i was thinking about a 12 v radiator fan from a car to pump air in and an exhaust fan blower to pull fumes out is that stupid ?
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Old 10-19-2013, 11:06 AM
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A radiator fan has the motor right behind it. It will be in the path of vapors. Not recommended. Motor/belt driven fan has the motor away from path of vapors.
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Old 10-19-2013, 02:24 PM
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Just a question..Why negative pressure..why not positive pressure on the booth..??

Sam
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I have tried most all of it and now do what is known to work..
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Old 10-19-2013, 02:45 PM
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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
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