Fan Power (question aimed at Mr. Stauffer, in particular)

Several of my first attempts used pretty wimpy fans, and they didn't work very well.  I have since found that I need at least 100 cfm.  I use the muffin fans, AC type. I understand they are brushless.  I also find that a flange for a dry vent hose will fit over the muffin fan outputs nicely if you drill the same hole pattern as the fan mounting holes.  Then that flange will be held on by the fan mounting screws.

I used to use auto air cleaner filters- I now use furnace filters.  They seem to have less pressure drop than auto filters, so the combination of more efficient filters and larger fans seems to make my more recent booths work much better than my old ones.

I don't know what it is- I can never seem to be satisified.  My current booth works fine but is too heavy when I take it down to replace the filter, the filter is too hard to replace (umpteen screws) and it is a heck of a job to clean.  I am designing a new booth to fix these three objections.

Thanks Don

I get a feeling that the 100 cfm * sq ft @ the opening of the booth is possibly a little on the high side, being it is a commercial standard for industrial shops. However, I'm reluctant to start cutting and sealing this contraption if I'm going to have to replace the fan

I'm also going to use furnace filters - the fiberglass type, in fact. A standard size will fit nicely in the back of the hood. I'm also going to place some filter material inside the guard that came with the fan, and attach that to the lid to flow clean air through for drying

My biggest concern is whether the fan is powerful enough. It's hard for me to imagine that it won't move the whispers of fumes coming from an airbrush, but I just don't know. I won't until I wet test it. I hope it works. If it doesn't, I'll have to get a fan from Grainger, and build an enclosure for it. But if that's how it goes, so be it

Okay, I made a mock up with a cardboard box - 25" x 18", same dimensions as my hood within 0.5". For the first wet test, I sprayed air freshener with no filter and only the elbow attached to the fan's outlet. I could spray, laterally, several inches (~ 3 or 4) in front of the opening of the box without any blow-back. For the second, and final, test, I installed a furnace filter, and attached aluminum flex hose to the elbow and ran it out of an open window. I sprayed a large amount of Tamiya White Surface Primer from a rattle can. The amount sprayed was at least three times as much, and as aggressively, as I would ever do on any kind of model. There was no blow-back at all. Immediately after spraying I could only detect a faint smell with my face up to the opening of the hood. Standing upright, immediately in front of the hood, I could not smell any vapor whatsoever. I also left the room for a few minutes, to remove any sensory adaptation (very common with smells), and upon return, could not detect that any painting had been done at all. Within five minutes, even the faint smell inside the booth was completely gone. I call this test a success

From this, and the information that Mr. Stauffer has shared, I'm thinking that the 100 cfm rule, in regards to small hobby booths, may apply more to the plenum opening more so than to the opening of the hood. I may be wrong about that though. Regardless, testing, is always the best producer of reliable results

@jetmaker

What kind of fan did you use?  What cfm?  I bought a small spray booth 3 years ago, but I plan to build a little bit bigger one.

Hey Chris, it's a Broan Model QT140E. It's 140 cfm, and I believe 1.5 sones (it's VERY quiet). I got it at Lowe's. It was around $130, give or take ten bucks. It's a bathroom fan, and it has it's own metal enclosure - steel, I believe - that measures about 10" x 11" x 8". It has a 6" round exhaust port, but comes with a 6" to 4" reducer. I used clear silicone to seal the enclosure - there's a few little machined holes here and there, but the silicone goo sealed them right up. Wiring it was extremely easy as well. I just attached a plain old three-prong cord, but if you wanted to wire a switch that would be easy too - in fact, I still might do that

I'm kind of both surprised and not surprised by the results. I had a strong feeling, after wiring the fan and dry testing it, that no spray inside the hood would be able to escape the fan's gentle, yet firm pull, But, what blew me away was not being able to smell even a hint of the Tamiya primer - INSIDE THE BOOTH - within five minutes (leaving the fan running, of course)

Thanks for the info.  I thought about buying a 130cfm 230mm PC case exhaust fan (about $25) and adding an AC-to-DV adapter and building an enclosure and an exhaust port.  It looks like buying a high quality bathroom fan would make my life much easier.  

There are also AC muffin fans in the same form factor as the PC fans.  They install easier than bathroom exhaust fans, and make adapting to exhaust hose quite easy if you wish.

Yeah, Chris, I'm glad Don chimed in. I was gonna suggest asking him about fans. I was actually looking at muffin fans, and found some very powerful ones - for very reasonable prices too. The only reason I went with the bathroom fan is because the 90 deg draw angle of a blower type fan works better for me architecturally - though a decent muffin fan through the side of a plenum, which would be very easy to construct, would probably work quite well. In fact, if I ever build another booth, I might try that. That's another good thing about the fart fan I got is that it comes in a square metal housing that acts as a plenum, but, yeah, an axial fan would definitely be easier to install - not that the fart fan is that difficult, just that a muffin fan would be a breeze (pun sort of intended)

One of the things I've gathered from this, so far, is that a simple hood can be completely adequate. I'm so blown away by how well the cardboard box mock-up functioned that the only favorable reason for using the plastic storage container is the lid that I can use for drying. Other than that, a cardboard box ($2.00 from Lowe's) would work very well, especially if you lined the floor with a piece of sheet acrylic

I have no clue when it comes to air ventilation.  Based on the discussion above, it looks like a plenum is needed.  Could you explain why? 

Not sure if it's needed, but I think it's probably helpful. It evens out the negative pressure (suction), so the pull from the fan is balanced throughout the painting area (hood), which minimizes dead spots, and - I believe - leads to better air dispersal. They're used in commercial booths. We use them in making 1:1 airplanes to keep fuel from sloshing inside the tanks. It's pretty ingenious. It's an easy way of helping an exhaust fan function better

With auto air cleaners I was able to avoid a plenum, but with furnace filters they are just too big for any fan intake to cover the filter.  I used the auto type filter that brings air in through edge to a center opening, cut a plywood piece to cover the filter, with the fan in the center of that piece. In effect that center area makes a plenum.

BTW. all the ones I make with the auto air cleaners were made from cardboard boxes, cut opening so the filter would fit in.  The cardboard was a bit bigger than filter, so the filter/plywood/fan would not drop through hole.  Very simple to build, with a boxcutter and a jig saw to saw the 1/4 inch plywood.

I've found this Web site on building a spray booth.  A lot of good information is there.

http://www.modelersite.com/Abr2003/english/Spray-booth-design_Eng.htm

Okay, I just decided that I'm going to ditch the plastic container hood and go with the cardboard box. The geometry is just too much better than the plastic tote - no compound curves to deal with, fan installation is practically nothing, and sealing/reinforcing it is a breeze. I weighed out the pros and cons, and tried mock-ups of both, and the cardboard box won hands down in every category. Not only is it extremely simple and cost-effective to build, but it's also more comfortable to use. The only advantage to the plastic tote, again, is the lid, but that's hardly a show-stopper. Heck, simply draping a towel over the box is a workaround. Plus, my room is not very dusty to begin with. With the simple geometry of the box, a decent, filtered lid can be fabricated for a couple bucks worth of materials within minutes

Don, that idea of using an automotive air filter is brilliant. If I ever make another booth, I am probably going to try that. That is an excellent idea

Chris, that's a great source of information, especially regarding how the different draft models work and the effect of static pressure. I actually used that as part of my research into designing my booth. My only contention with Mr. Raddatz' article, and it's actually pretty slight, is that I believe the formula for 100 cfm/sq ft @ the hood opening may be unnecessarily aggressive. I believe the critical point should probably be the plenum opening. After reliable testimony from Mr. Stauffer and others - including the painters at the aircraft company where I work - and the results of my own tests, I've come to believe that 100 cfm/sq ft at the plenum (note: this still includes taking static pressure into account) is adequate to exhaust both particulates and vapors from airbrush spray, and even aerosol cans. I use the terms "probably" and "may" and "I believe" because I am far from an expert on the subject. That is an excellent article, though, and I would advise anyone thinking about designing their own spray booth to check it out

My hobby room is not dusty at all.  But a few minutes before starting a paint job I turn on the booth fan and blast the air from my airbrush into the spray booth to blow dust that might be there.  By the time I'm ready for airbrushing, my booth is dust-free.  I got this tip from an Airbrush forum.