Fan in a portable spray booth; Safety question

[Scott Brandstetter]

Do I remember reading correctly in the past that you can't just use any box fan to exhaust the fumes when using oil based stain and poly. In a previous thread I mentioned the need for a portable booth and now that I have some design idea's, want to make sure I'm not creating a safety hazard.

I was planning to get a cheap box fan and put a furnace filter in front of it to exhaust. Any comments would be appreciated.

[David Hawxhurst]

its recommended to use a explosion proof motor if it will be exposed to flammable fumes. in this case the air flow is directly over the fan motor so there maybe some risk of a spark in the motor that could set of the fumes. most professional set ups have the motor offset from the fan blades so the motor is not in the air flow path.

why would you spray oil based poly, it takes so long to dry. i would consider using waterborne poly instead. there are several good ones out there. with most the waterborne ones you can typically get all three coats done in a day vs three or so days for oil based.

[Howard Acheson]

Position the fan so it is pushing fresh air into the spray booth. Position the exhaust opening on the side opposite the fan. That way the fan motor will be in fresh air that is not loaded with finish or thinners.

[John TenEyck]

What both the prior posters said. Especially, why would you want to spray OB poly? Whatever the over spray lands on will be an incredibly sticky mess. WB products are just as good in all respects and spray easily, and the over spray is dry before it hits the floor.

John

[Jim German]

Plus with the water based you can clean your gun with soap and water.

I'm partial to the Target Coatings waterbased polys.

[Keith Weber]

Probably the best way of doing it is with a tubeaxial fan. The air flows through a ducted fan, and the motor sits outside the housing on a mount. The fan is turned by a belt. This keeps the motor away from the air being exhausted. You'll probably get sticker shock on the price of a new one, but there is a fairly good used market for them, which brings the price way down if you don't mind it not being shiny.

[John TenEyck]

The odds of getting a fire/explosion with oil based poly or stains is probably very, very low. Regardless, I would not spray them into a fan that has a steel impeller and housing, where a spark could be generated if the fan gets out of alignment or a piece of steel happened to get sucked in. You need an aluminum fan. Better yet, you need an explosion proof fan/motor. That's why they make them.

All said and done, there's no good reason not to use water borne products.

John

[Dave Cullen]

Safe fans are the reason that portable spray booths are so expensive...

http://www.tcpglobal.com/PAS-HSSB-22-16.html?sc=113&category=3420186#.ViOv29KrQsY

[John Sincerbeaux]

I have sprayed countless gallons of NC lacquer through a box fan. I have a knock down spray booth and use a standard 20x20 furnace filter in front and back of the fan.

[Steve Jenkins]

Of course I can't reccomend using a box fan. When I had my shop in a rented warehouse I had a commercial spray booth but after moving to a building behind my house I now use a 4' box fan that I got from Home Depot. When I first got it I set outside and for a test sprayed lacquer thinner directly into it and at the motor. I'm still using it 9 years later.

[Scott Brandstetter]

Thanks for the replies everyone. In regards to oil based stain and poly, I don't have a good answer, just never used a lot of the water based. I have started a test to get familiar with the water based and I'm sure I will convert just based on the responses in the thread.

[John TenEyck]

I have sprayed countless gallons of NC lacquer through a box fan. I have a knock down spray booth and use a standard 20x20 furnace filter in front and back of the fan.


Many people remove the guard and splitter on their TS, too, and never have a problem, until they do. Explosion proof fans were invented for good reason.
John

[Keith Weber]

Many people remove the guard and splitter on their TS, too, and never have a problem, until they do. Explosion proof fans were invented for good reason.
John

^^^ What John said!

[John Sincerbeaux]

[QUOTE=John TenEyck;2480361]Many people remove the guard and splitter on their TS, too, and never have a problem, until they do. Explosion proof fans were invented for good reason.
John[/QUOTE

I am also in the no TS guard or splitter camp. Could one day I cut off my fingers if I do something careless? Of course!
Since I learned how to properly use a tablesaw in high school shop 40 years ago, I have never used them. As far as spraying into a box fan, trust me, I did some research and asked some very informed people on the subject. I think if you did the same, you would find the chances of having a flash explosion caused by a box fan is near impossible. I have used the same box fan for over 12 years now. Go on you tube and look up garage spray booths for cars. Look how many videos there are of all the spray booths that guys are spraying auto paint and clear coat finishes. And look what all these guys use for exhaust fans..... Box fans.
Through knowledge, and experience, I don't fear box fans.

[Julie Moriarty]

I set up a small spray tent in my basement. I made a frame out of PVC pipe and exhausted the air through a 20" box fan that had several filters on the intake and exhaust side of the fan. I was spraying a WB finish with an HVLP spray gun and I was more concerned with suspended particulates ruining the finish on the cabinet parts I was spraying than with a safe working environment. I was fully suited and wore goggles and a solvent approved respirator. I sprayed about 4 gallons of Endurovar with that setup.

The spray tent kept overspray from ruining the finished basement. After that, I can only say it served a purpose.

Later I started spraying NC lacquer on guitars. I modified the exhaust by installing a 24x24 return air panel in the suspended ceiling. It reduced to 6" round. I then ran 25' of flex foil duct to the nearest window. I made up a 6" elbow that housed a booster fan and vented to the outside. Over the 24x24 return panel I placed a spray booth filter. I sprayed about 4 quarts of NC lacquer with that setup.

When I broke it down, I could see a slight buildup at the fan. It was a cheap booster fan and didn't move a lot of air but it maintained negative pressure in the tent and kept the house from smelling. I tried to do a positive pressure setup but I couldn't sufficiently seal the tent to keep odors from permeating the house.

One time I took a picture of one of the guitars just after spraying.
http://i867.photobucket.com/albums/ab233/jules42651/Radocaster/lacquer_cloud_zps85cb9791.jpg

The flash on the camera picked up the airborne particulates. I could not see them. But this shows you just how much can be suspended in the air if you don't have sufficient ventilation.

[keith micinski]

Position the fan so it is pushing fresh air into the spray booth. Position the exhaust opening on the side opposite the fan. That way the fan motor will be in fresh air that is not loaded with finish or thinners.


Only do do this if you want to introduce a lot of contaminates from outside the spray booth the filter didn't catch. It's much easier to keep a small spray area relatively free of contaminates then it is the whole outside.

[Malcolm McLeod]

A box fan or ODP motor are a risk if used in a spray booth. Even explosion-proof motors are a risk if not installed to appropriate NFPA Class/Div recommendations. There is supporting evidence for anyone willing to look.

I'm not a statistician, so won't even guess at the odds you'll have a problem. And I'm sure lots of people have used a box fan in the manner described with NO problems. But keep in mind that when a 'problem' does occur, it is called an 'explosion'. Or if you are lucky, the insurance adjuster will merely call it a flash fire. Kind of like the guard/splitter, it won't be a problem....until it is.

There are dozens of flammability factors at play: from %RH, (dust) concentrations, to total volume. Is it dry dust? Or flammable aerosols?

If you're going to the effort to build a home-shop* spray booth using a box fan, a few extra steps could save you a great deal of after-the-fact-head-banging. Buy the fan, a couple of pillow-block or flange bearings, a piece of CR shafting, 2 sheaves, and a v-belt. Mount the fan impeller, the bearings, and a sheave on the shaft. Rig a support bracket for this assembly in the fan housing. Rig the motor and 2nd sheave to a support outside the housing and install the v-belt. Ideally the belt sits on the suction side of the impeller. Now the motor sits outside the main stream of volatiles (the previously mentioned tube-axial fan). Maybe $30-$40 (+ fan), and a significant reduction in risk. Reduced risk, not eliminated.

For what its worth, I suspect even a water-borne finish poses some risk. As I understand it, it all depends on if the (dry) finish will burn. Overspray can and will dry while suspended. At that point, if it will burn, it is no different that grain or coal dust, subject to the same flammability factors as above.

I am not a big nanny-state advocate, so please do as you see fit. Just please make sure you do so with all the facts and understand the risks.

(* For Pros, please ignore this post. OSHA will not be amused by your DIY efforts.)