Garage filter question

Dave C 5

Member
I’m finally getting around to making some kind of booth/filter design for cleaner paint job - does the intake side need to be roughly equal to the exhaust side ? I’ve read all about calculating proper air flow. Just trying to improve on what I do now which is box fans under my garage door. If my exhaust area is going to be 30 square feet - do I need the same for intake - my exhaust is low on back wall and intake will be on opposite wall up high near ceiling - I’m thinking it should but keep in mind it’s a garage and I’m limited due to garage design- my exhaust fans ( recent acquisitions ) are 2 big furnace fans welded together- don’t know CFM’s but they’re BIG AND HEAVY ! Clears out cloud in a few minutes under garage door
 

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That is how osha regulates shops, positive pressure in the building. Equal inlet as what is leaving and you have a good shop. If its warm enough and you can open the door using a wall of filters, it will work, but keeping it closed you need at least equal if not positive pressure.
 

Dave C 5

Member
Ok - now if the actual hole in the wall is small (exhaust) but the surface area of filter is much larger- building a “box” around the exhaust fans - are we looking at filter sq ft or hole in the wall sq ft ? What is equal ? Holes or filter banks ?
 
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Chris_Hamilton

Trying to be the best me, I can be
The intake area to exhaust area ratio is more like 1:1.5. If in doubt always have more area on the exhaust side than the intake. Area being the filter area. Smaller hole is offset by the power of the fan. In a commercial booth fans are usually 30-36 inches diameter. Sometimes 2-24 inch fans. Try to make everything as big as is practical. Too small a hole (wall) will ultimately limit how much air you can move. Determine how much air (cfm)you need to move, and build toward that or as close as is practical. Good idea to also include a way to throttle it down in case you have a lot of air moving. That can be as bad as too little air movement.
This is all assuming that you are putting the fan/motor behind the exhaust. (Negative pressure) Placing a fan/motor in front of the intakes (positive pressure) is far less efficient and will require a much bigger fan/motor (probably approaching 2x as big) and a very well designed plenum to move the same amount of air. Every commercial booth I've used or have read literature on uses negative pressure.
 
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Dave C 5

Member
Thank you ! As you can see from the picture I am using furnace fans - I'm guessing 1000 cfm a piece ? They’re big ! Was gonna construct a fairly large “plenum” box around it - I have another fan that I can add to that area- regular exhaust fan- about 2000 cfm. There will be only static air being drawn in - no fan - in this case we are concerned about sq foot of filter space ? Correct ? Cause the actual hole of the intake will be much larger than the actual hole in the wall for the exhaust
 

Chris_Hamilton

Trying to be the best me, I can be
One reason that you want a larger exhaust filter area than intake area is because your effective filter area is getting smaller as you use the booth due to overspray. The actual ideal amount of area for exhaust and intake? I'm not really sure. Judging by how commercial booths are made more is better with both. So I would go into it looking to make the intake area and exhaust area both as large as possible keeping a bias towards the exhaust side.

As for amount of air, a typical semi down draft booth (cross draft booth with intakes located at the front top) uses a 30" tubeaxial style. Typical booth would be about 3300 cubic feet. (26 x 14 x 9) So for 2 air changes a minute you would need 6600 CFM. So that amount of air is exiting through a 30" opening. Google "air flow formulas" to help determine what your openings will flow. Keep in mind too that even one air change per minute will work and be better than nothing.

So are you thinking positive pressure or negative pressure? If it was me doing it I would only want to do negative pressure. Open area lined with intake filters (you could frame them in your door) and then some form of an exhaust plenum and ducting where the fans would mount. If you have enough room you could build a exhaust plenum with filters against your back wall and open up a hole higher up for the exhaust to escape. Shoot it out the wall, roof the opening to protect from rain. Something to block it off when not in use. This is assuming you have enough room and willingness to put another hole in your block(?) building. The hole isn't a big deal actually. Open it up and line it with steel flats (plate) and weld them up. That would be my preferred way to go about it. If because of circumstances you can't, then you have to work with what you have. I just don't like the idea positive pressure, inefficient and dirtier. Have to have a good intake plenum and good gasketed filtering, because of the positive pressure trash could get around a filter that isn't sealed perfectly. Harder to clean your intake air that way than by drawing it through. Intake air will have much more velocity entering and less velocity leaving, which IMO is the exact opposite of what you want and why expelling the exhaust is much less efficient with a positive pressure system.
Anyways, just some things to chew on. :)

Just to add, using negative pressure you want to have the exhaust exit as high as possible, like a chimney, because like a chimney the pressure differential will help move the exhaust air. Makes your fan more efficient.
 
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The problem with negative pressure is you will be pulling in dust from any opening in the building. Areas around doors, windows, sill plates, alot of places you are not filtering the dust away. If the rest of your building is sealed well, then the air will come in your designed opening at a greater velocity so you have to control the dust. The easiest way to see if you have positive or negative pressure is unlatching a door and seeing if its harder to push open or wants to blow open on its own. Probably hard to tell with a 1000 cfm, but running a few 20000 fans at the same time its really obvious.

That is why you may end up wanting to put the same intake fan in to match the exhaust fan. If you load the filters quickly, then you can water down the area outside the opening to try to keep the dust from getting in.

They have panel filters, they are like the 20x20 we all use, but are attached, like 5 across to make a box type filter to slow velocity. It will still work if you end up putting a fan in the opening. .
 

Chris_Hamilton

Trying to be the best me, I can be
The problem with negative pressure is you will be pulling in dust from any opening in the building. Areas around doors, windows, sill plates, alot of places you are not filtering the dust away. If the rest of your building is sealed well, then the air will come in your designed opening at a greater velocity so you have to control the dust. The easiest way to see if you have positive or negative pressure is unlatching a door and seeing if its harder to push open or wants to blow open on its own. Probably hard to tell with a 1000 cfm, but running a few 20000 fans at the same time its really obvious.

That is why you may end up wanting to put the same intake fan in to match the exhaust fan. If you load the filters quickly, then you can water down the area outside the opening to try to keep the dust from getting in.

They have panel filters, they are like the 20x20 we all use, but are attached, like 5 across to make a box type filter to slow velocity. It will still work if you end up putting a fan in the opening. .
I get what you are saying but I don't think it's a real issue. I'm assuming he's trying to build a booth of some sort for his shop. In a booth environment it's most definitely not an issue. If he is making a booth sealing it up isn't hard. Negative pressure is the only way commercial booths are constructed. If he is just wanting to exhaust fumes in his garage it still isn't a real issue because the pressure differential will not be very much. Take a very high powered fan to create the kind of draw you are referring too. An exhaust of some sort exiting in the back wall would be far more efficient than trying to push it through the shop. If Dave is just trying to get the fumes exhausted more efficiently in his garage/shop then simply mounting an exhaust fan with a simple filter plenum would be a good choice. I've done something similar with mine and there is nowhere near enough of a pressure differential to draw a lot of dust, dirt etc from crevices sills etc. It just slowly sucks away the fumes. If you had a leaky booth drawing a lot of air with a high powered fan and no way to throttle it down then yes it would be a problem. But caulking up the booth would solve the leaks and you can always throttle down a fan with a speed control (proper name escapes me atm) box wired into the fan.
 

Dave C 5

Member
Just trying to exhaust a garage- quickly. I just ran an experiment- filled the garage with smoke- waaaay more than I would ever create in overspray- the furnace fans in my picture took 4 1/2 minutes- I also have a 2 speed exhaust fan in a window ( large cfm)- took 3 1/2 minutes- both together took 2 1/2 minutes- the smoke was so thick I was tripping over stuff I couldn’t see. I understand the negative pressure argument but I really can’t rig up an intake fan- just gonna draw air from the attic area - the garage is hardly sealed- my decision now is if 2 fans will draw too much ?( dirty paint job) or less is more ( slower evacuation but cleaner) - thoughts ? I tend to over do things
 

texasking

Promoted Users
The 2 furnace fans will probably be fine until you get to clear, then you may need the second fan if doing a large area, IMO.
 

Dave C 5

Member
Good point- most of the time I’m either doing an all-over without taking them apart or several large parts placed throughout the garage - definitely a large clear fog compared to base
 

dhutton01

Promoted Users
Before I had my commercial style booth my home made booth was positive pressure. I used a large attic fan to push air through filters and into the booth. I did this for a couple of reasons. My fan was not explosion proof and I was worried about passing paint fumes through a fan that might ignite the fumes. The other was I felt it would keep dirt from getting drawn in through any cracks. The booth worked ok but I decided to move to a commercial style booth when we moved.

Don
 
Just trying to exhaust a garage- quickly. I just ran an experiment- filled the garage with smoke- waaaay more than I would ever create in overspray- the furnace fans in my picture took 4 1/2 minutes- I also have a 2 speed exhaust fan in a window ( large cfm)- took 3 1/2 minutes- both together took 2 1/2 minutes- the smoke was so thick I was tripping over stuff I couldn’t see. I understand the negative pressure argument but I really can’t rig up an intake fan- just gonna draw air from the attic area - the garage is hardly sealed- my decision now is if 2 fans will draw too much ?( dirty paint job) or less is more ( slower evacuation but cleaner) - thoughts ? I tend to over do things
That is the experiment. I dont know if you want to do it again, but its turning one of those fans the other direction and seeing how long it takes might help you to settle and not overthink. Sounds like you just put the furnace fan outside blowing in. The engineers that want to sell you a spray booth will tell you that positive building pressure is the only way to make an exhaust fan work efficiently. Our booth has a filter box on top, 12 20x20 prefilters, then adjacent, 12 more finer filters. When those filters are new, you can open the man door easily, when they are clogged with dirt, its harder to open the man door. so when you do open the man door, it sucks in whatever dust is outside of it like a shop vac.
 

Dave C 5

Member
I might try that - thanks ! I guess it’s possible to put the fan in the ceiling- might suck insulation though and clog the fan- maybe I can figure something out
 

Dave C 5

Member
Exhaust is done - will work on intake tomorrow
 

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