# What size and capacity extraction fan for a spray booth?



## stefang (Apr 9, 2009)

I will be building a spray booth for my new Earlex 5500. I plan to only spray waterborne finishes onto small to medium sized projects. I would like opinions on what would be an appropriate size/capacity fan. I know many go by the bigger the better philosophy and I can understand that, but I would really like to know what would work starting at the lower end and up to max needed. I will appreciate any help I can get.


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## patron (Apr 2, 2009)

one off the problems with extractor fans
aside from explosive finishes
is the finish clings to the blades

i did read a post here
about having a fan before the booth
and let the over pressure force the air out
without clinging to the blades
or being in the stream if you do use volatile finishes
anything that moves the air past the in filter
(you do want clean filtered air )
and out past the exit filter should work just fine
(i have a special fan that is spark free in mine
but it cost $400 when i got t)

i also read that water mist before spraying in the booth
will act as a magnet for any airborne particles of dust
plan to do that in my booth
when i get it up and running
and do have good lighting
so you can see the finish right
from all angles


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## bigblockyeti (Sep 9, 2013)

I have a few old cast aluminum spray booth fans made by Dayton and they were rated for a maximum of 3500rpm, despite their small diameter, they should provide substantial air flow. With these, you'd have to have an explosion proof motor to keep from potentially igniting the aerosol overspray, provided you were spraying a combustible finish. I've seen three phase motors used with a remote VFD to offer adjustability to the airflow based on the requirement of the job.


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## stefang (Apr 9, 2009)

Thanks David and BBY. I won't be spraying oil based finishes so I won't need an explosion proof fan. The RPM rating doesn't tell me much. It's easier to relate to the CFM rating which give some idea about it's actual capacity. I am thinking max would be 6000cfm, but those are pretty expensive here (about $330), so I'm hoping a lower rating (and thereby cheaper) will still work, aided by a fan providing filtered input air. I know you can get pretty cheap box fans at the big box stores in the States, but not here. I am trying to find one in the U.K. at a more reasonable price, but without much luck so far. I did find an 18" 3 speed fan for about $150 which is rated at 2200cfm, but I don't know if that will do the job.


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## TopamaxSurvivor (May 2, 2008)

These are really common in the US. Not sure how you would go about getting one to Norway and it will be 120 volt. Putting 2 in series would be cheaper than a transformer to drop the volts and double the air flow.


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## stefang (Apr 9, 2009)

Ridiculously low priced compared to here. You do have a good idea there though Bob. Maybe I should think about using 2 smaller fans. I hadn't thought of of that.


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## canadianchips (Mar 12, 2010)

We used these in our welding school at the oxy.ace. cutting table. 
Downside in welding, the filter plugged to fast. Paint particles might be similar.
It was portable.
It had squirrel cage fans inside. 
More cfm at lower rpm.


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## GregD (Oct 24, 2009)

I have only a small amount of relevant experience but I think a simple box fan or two like Topamax suggested would be enough to keep up with the overspray mist for typical woodworking projects. I don't get so much airborne mist using my Earlex 6900 unless I'm spraying a latex type paint. By contrast I get a lot of overspray from heavy bodied paint (latex, etc.), using an airless sprayer, and/or spraying large surfaces that wouldn't fit in a spray booth (interior walls/ceilings).

When I do occasionally set up a booth it is in a utilitarian garage where loosing a bit of overspray from the booth won't be noticeable, so I use simple and inexpensive setups. If I were regularly spraying finish and my spray booth was in a nice, clean workspace, I might put more effort into more complete capture of overspray.


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## stefang (Apr 9, 2009)

Thanks CC and Greg. I did find an article this afternoon by Jeff Jewitt in Fine woodworking com where he had shows how to make a simple homemade spray booth. He said a fan rated at 2000 CFM should be sufficient and he suggested using at least a 16" diameter fan. That sounds pretty good to me. I checked Ebay and others in the U.K. and they have inexpensive 20" box fans, but none show the CFM rating. I guess I will just have to keep looking.


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## TopamaxSurvivor (May 2, 2008)

Mike, I was suggesting 2 in series to use 220 volts you have, but 2 small fans should more more air. The inexpensive 20" box fans you found probably move enough air. The one I linked moves 2200 cfm. Most fans that size should be fairly close even with the 50 vs 60 hz difference.


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## stefang (Apr 9, 2009)

Thanks Bob, I will keep that in mind.


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## JAAune (Jan 22, 2012)

The needed CFM depends upon the size of the spray booth. A larger opening requires more filter surface and more CFM.

Binks Airflow Reference


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## stefang (Apr 9, 2009)

Thanks JAAune. I checked it out, but the terminology throws me. Not sure what he means by 'Filter banks' and he is talking about oil based finishes from the sounds of it. The back 'filter' wall on Jeff Jewitts's little booth was about 5' X 7' and it had some side walls set at an angle to direct air flow towards the back wall.


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## JAAune (Jan 22, 2012)

Sounds like 3500CFM would be ideal for that size of booth. It's not necessary because you're dealing with water-based products and aren't dealing with OSHA. However, using OSHA standards will ensure you've got enough airflow to pull out the overspray and keep the shop and furniture clean.

The filter bank is just the area at the back of the spray booth that is covered in filters.

Binks makes a nice filter material which I've linked below.

AF Booth Filter


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## stefang (Apr 9, 2009)

I'm thinking more like 2,000 cfm for the size booth I have in mind, although I can think that the bigger the better, but I don't want to pay more for one than I have to. I was thinking about building a big booth, but I have changed my mind because I can't do any really large painting upstairs in the loft anyway due to the difficulty of getting big stuff up there.


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## JAAune (Jan 22, 2012)

Actually, bigger is not better. If the airflow gets too high, you'll end up affecting the spray pattern and/or causing the solvent to flash off before it hits the wood (dry spray). You're shooting for just enough airflow to pull the over-spray into the filters.


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## crank49 (Apr 7, 2010)

Measure the open area of the booth inlet.
The inlet means where you will be standing.
If your booth is 6 ft high and 8 ft wide with a top and a floor, your opening is 6×8 = 48 sq.ft.
You want at least 50 FPM (ft per minute) of air flow into the opening.
48 sq.ft. x 50 FPM = 2400 CFM is the size fan you need.

By the way, if you were sand blasting or grinding the inlet velocity should be increased to 400 FPM.

If you put furnace type fiberglass impingement filters across the back wall in front of the fan, the fan needs to develope at least .25" static pressure while moving the required CFM.


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## stefang (Apr 9, 2009)

Thanks JAAune and crank49. I believe that around 2,400 will be about right for what I am planning. I can see my bigger is better comment wasn't correct as it can be difficult to spray in a hurricane.

That's the positive part of starting something new. It always leads to new skills and a steep learning curve. On the negative side, it usually costs a lot too, but at least with all the good advice I've gotten here I should be able to avoid the worst pitfalls.


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## joeyinsouthaustin (Sep 22, 2012)

Late in on this thread. Many great comments and aides. The one thing I would echo is that with both water bourne and solvent based getting the right airflow is more important than just getting air to flow. I can really affect the final product way more than you might anticipate. A diffuse, gentle, even flow at the right way is important. I would encourage building some kind of sliding gate at the intake fans, if you go that route, so you can adjust the air flow. Also think of how to diffuse the air, so you have an even flow and not a draft or wind.


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## stefang (Apr 9, 2009)

Thanks Joey, that makes sense. I will keep your suggestion in mind when I build my paint booth.


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## Dark_Lightning (Nov 20, 2009)

I personally have the luxury of the benign, dry weather in southern California to spray my projects anywhere I want, and almost any time of year. I've even painted my cars in the driveway, early in the morning before the wind comes up. There is no dew during certain times of the year, and I take advantage of that. I usually use lacquer or water-based poly (which sprays quite nicely, BTW, despite the instructions to the contrary) on my wood projects. I use breathing protection with these, in any event. In my experience, I've used fans on the supply side with a filter downstream of the fans. This makes for positive pressure with a clean air stream. This ensures clean air flowing in the proper direction. But I generally do small projects that way. I brush or wipe large projects.


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## stefang (Apr 9, 2009)

Thanks for your input DK. I plan to use a mask even for waterborne. I grew up in So. Calif. and even got married to my Norwegian wife there. We had a house in Simi. There were a lot of orange groves there then.


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## Dark_Lightning (Nov 20, 2009)

Heh, it's almost all houses, now! We've only lived here 12 years, so the orange groves were pretty much long gone by the time we got here. More houses, probably condos or town homes, are being built right up against "Happy Face Hill", it you lived here to see that. If you didn't, it's no big deal, just a little small town joke.


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## OSU55 (Dec 14, 2012)

You can search the web for info - doing it by US code results in a pretty big fan. From Binks' website:

A properly designed and operating spray booth in a finishing facility is a major ingredient in the achievement of the finish required by the manufacturer. Either of following two methods can analyze the proper airflow:

Air Velocity (Feet per Minute - FPM)

Air Volume (Cubic feet per Minute - CFM)

Prior to 1995, the National Fire Protection Assn. (NFPA) book 33 listed the proper flow for spray booths using non-electrostatic spray equipment at 100 FPM. That was determined to be adequate flow for the protection of the operator. Current regulations stipulate the proper flow is:

The air volume required to dilute solvent concentration below 25% of lower flammable limit (lfm).

My booth has a 100 sqft of area (width x height), with an 800 cfm fan resulting in a velocity through the booth of 8 ft/min. This has worked well too keep overspray off other parts in the booth but not too fast to cause dry spray issues (waterborne and shellac). I have two 20" x 20" filters.


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## stefang (Apr 9, 2009)

Thanks for that OSU55. I can think my booth will be about that size or a little smaller, say 80 sq. ft. It seems most are using fans for booths that size with a capacity of about 2000 CFM or a little more. I wonder if the filters change that CFM calculation since I would think that they restrict airflow somewhat? That said, I would be happy if an 800 CFM fan can do the job.


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## OSU55 (Dec 14, 2012)

The 800 cfm is the fan rating at 0.8" WC I think. It is an HVAC blower. Not sure of the actual flow with the 2 filters. I think it's close to the 800 cfm with clean filters. I use the non electric electrostatic filters and then put the disposable paper paint filters over them. Others say 2000 cfm or more, but I don't come across many comments where folks have an actual setup and experience. A 20"x20" filter panel is needed for each ~500cfm, so bigger blowers need more filters as well. The NFPA used to require 100 fpm velocity through the booth, but that is for commercial solvent based booths. I haven't spent the time to calculate current NFPA requirements for waterborne based on flammability. I suspect it would be fairly low.

I typically hang or set all parts in the booth, and move each part up by the blower for spraying, that way I don't spray adjacent parts. I have sprayed large parts like kitchen table tops, large sideboard/buffet, bookcases, etc. I leave the blower on between coats to help with drying. I use waterborne and shellac and do not vent the exhaust outside in the winter. After each coat I open doors that create cross flow through the whole shop to let fresh air in.


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## stefang (Apr 9, 2009)

Thanks OSU55. Lots of usedful info.


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## OSU55 (Dec 14, 2012)

Stefang it's been a while but I measured the flow out of my blower. The filters as described were in place and a bit dirty, so it's a good number. I used a prop type device an HVAC business owner friend loaned to me. The flow was ~ 1020 cfm.


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