Official Luthiers Forum!

Hello everyone,

I am in the process of building a knock down spray booth in my shop. It will be 48" wide, 40" tall, and 36" deep. The back wall will be filters with a fan pulling from behind it.

My question concerns the exhaust fan. I would like to buy something that I could use for other purposes as well, like an ambient room cleaner. My thinking for the mini booth is that I would exhaust the fan into a very large refrigerator box sealed from the inside and with filters along one face to empty back into the room (passing through 2 sets of filters first). I cannot send the exhaust outside unfortunately and I am nervous about sending it through my dust collector canister. I will be spraying waterbornes so I am not looking to get an explosion proof fan. I am looking at 3 fans in 2 styles that pull what seems like a decent cfm draw and are in my current price range but they are different styles and I am not sure if it makes a difference as to how they will pull air.

The first is a classic exhaust style:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... K:MEWAX:IT

The second looks very similar but is bigger, has a higher cfm rating but is less expensive? Is there a difference I am not seeing?
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... K:MEWAX:IT

The last is a different style altogether, smaller and contained differently:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... K:MEWAX:IT

I apologize for the linking to ebay auctions but I was not sure how else to do it.

This may be a simple question to answer and if anyone even has a link to somewhere that could explain the different styles of fans that would be much appreciated.

Thanks everyone.

_________________
Burton
http://www.legeytinstruments.com
Brookline, MA.

It sounds like, by your description all you need is a ....fan. My "air filter" is one of these with a furnace filter taped to it.
http://www.lowes.com/pd_95021-1621-3733_0_?productId=1114391&Ntt=box%20fan&Ntk=i_products&pl=1&currentURL=/pl__0__s?newSearch=true$Ntt=box%20fan

Since you'll have filtration before the fan, and you won't be using flammable materials it should work fine. I'm sure a heavier duty fan could last longer, but it's only $15 at my local Lowes. 3 speeds, with 2500 CFM on high.

_________________
I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

Yeah, I guess that makes sense. Is the only difference that those fans are more heavy duty? I figured there was some special twist I was not figuring out.

Thanks Woody.

_________________
Burton
http://www.legeytinstruments.com
Brookline, MA.

The main factors for selecting a fan/blower are cfm at a given pressure rise, noise, and input power. The cfm stated in advertisements is the flow at no pressure rise (zero resistance / no filters), which is generally meaningless for filter applications. It's often difficult to estimate pressure drop through filters, but it can be anywhere from 0.5 iwc (inches of water column, a common unit of pressure) to 8 iwc or more. In general, axial flow fans are best for high cfm and low pressure rise applications (<0.25 iwc) -- circulating air. Most of them won't pull much, if any, air through a dirty filter. Centrifugal (squirrel cage) blowers are much better for pulling air through a dirty filter -- depending on design, they can pull between 1 and 15 iwc at high cfms.

Flow and pressure are inversely proportional, though the shape of the "fan curve" can vary a lot:
http://www.cartage.org.lb/en/themes/sci ... nCurve.htm
http://www.greenheck.com/library/articles/10

For the fans you listed:
The Dayton can pull 535 cfm at 1/8 iwc
The FlowPro doesn't give any iwc spec
The 1236 CFM fan is that cfm at 0 iwc, and says its max pressure rise is 410 Pa (1.6 iwc, fairly respectable for an axial). Very roughly, it would probably flow 600 cfm at 0.8 iwc (halfway point)... might work at first, but as filters load it will probably choke.

I'd look for a centrifugal blower for your app. I can probably find you one on ebay if you like... went through this myself a number of times. Let me think a bit on specs.... I'm guessing around 800 cfm @ 4 iwc would be conservative. I'd suggest looking up the osha specs for cfm as a starting point.

_________________
David Malicky

Filter material choice will be critical. I would think you will need something cheap and easily replacable and with some form of vapour trap before it.

Hold the phone, you want to recycle this air BACK into your workspace??? No, you really don't want to do that. Even if waterborne products are not prone to explosion, doesn't mean the compounds they contain are safe to inhale or non-toxic. Getting them down to safe levels in the recycled air will be a huge challenge, involving activated charcoal and other technologies. Fan selection is the least of your worries. I think you need to rethink this.

Presumably you're reluctant to exhaust to open air due to bylaws or other regulations. In that case, I would either break the bylaws (for your own safety), or find somewhere else to spray where you can set up a well-ventilated environment, diluting the vapours by exhausting to open air.

David, thank you for those links. It does help me to understand a little more what the differences are. I can do some more searching now that I have an idea of what to look for.

Tim, I know this is a problem. The way I see it though it would still need to cure in a controlled environment which cannot have air being continually exhausted and the fumes would still be off-gassing there. For the short bits I would actually be spraying (1-2 guitars every couple of months) I am not looking at it as such a huge problem. Not ideal, but not the end of the world so to speak.

_________________
Burton
http://www.legeytinstruments.com
Brookline, MA.

On the fumes issue (excellent points, Tim) -- you probably know to wear a charcoal respirator during and after spraying, then air out the house. For curing, it would help to put it in a small room with a small fan exhausting fumes out a window.

On centrifugal blowers, there are a number of types, each with ads and disads...
http://www.cincinnatifan.com/blower-wheels.htm
And 2 others:
- Backward curved: like a backward inclined, but quieter
- Airfoil: like a backward curved, but quieter (hard to find these)

As a rule of thumb, the fewer and bigger the blades, the more pressure a blower can pull. But avoid anything with "radial" in the name -- these are very loud (e.g., dust collectors).

Forward curve v. backward inclined (or curved) wheels (p. 2-3):
http://www.tcf.com/aerovent/pdfs/ED2400.pdf

If you have a lot of filter area (pleated), don't spray much, and replace the filters often enough, you could probably get away with a forward curved blower. These usually top out around 2-3 iwc, but are quiet and plentiful on ebay (or an old hvac blower at a garage sale).

_________________
David Malicky

Filters will help remove the solids but not the chemicals and here is the major health concern. Vent to the outside not back into the work space. All finishes can be a concern

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Thanks for the concern.

To satisfy my own curiosity, what is the air contamination level of spraying a few blasts of a waterbase finish (lets say Target EmTech 6000 for this case) vs. an open can of naptha and a rag of it being rubbed over a guitar body, or say brushing on that same waterbase finish? Are these situations comparable in terms of the dangerous vapors or does the spraying somehow make it much worse? I am not trying to make a gotcha point here, I honestly don't know.

I am not trying to be irresponsible here either, although in technical terms I guess I am. I would wear my respirator anytime I was in the room and use circulation inside of the room while I was not in it (which I would not be between spraying sessions). My shop is not heavily sealed and there is a slow transfer of air in and out of it. The only other thing I spray in there is shellac, which is totally non toxic (and would not be sprayed near a fan with an exposed motor). In my head the small amounts of finish I would be spraying seemed like a manageable risk considering I would not be in the room for a good amount of time after I sprayed and the space around my shop is fairly large and usually very empty. Also, the other option is brushing the exact same finish in the same shop and that just creates more sanding, which I am actually more nervous about since I am in the middle of it while it is happening (I do have a makeshift downdraft table but it still gets all over you when doing it by hand).

I do realize that exhausting the air outside is the optimal solution.

_________________
Burton
http://www.legeytinstruments.com
Brookline, MA.

If you build a small enclosed both you can make it a positive pressure booth. You do this by having the fan force air into the booth with an opening near where you are shooting lacquer to exit. Old furnace blowers work great for this type of system
You filter the incoming air before the blower and if you wish you can filter the out going air.

_________________
"Try to be the person your dog thinks you are."

http://sites.google.com/site/bertsguitarshop/