closed-loop spray booth

[Aluminum]

Originally Posted by SJordan

I?m in the middle of painting my RV-4 in my homemade paint booth and boy was I suprised. I have A 7000cfm fan pumping air through 16sq/ft of inlet filter and I can still fully fog up the booth with the SPI high solid clear. I cant imagine trying to close loop this air. If you look in th second picture you can see how dirty the inlet filters already are after just shooting base coat on the fiberglass pieces.

Wait, what? You say your INLET filters get overspray!? As in, fresh outside air coming into the booth?

What kind of spray gun?

[1001001]

Make sure your activated carbon media/bed is adequately sized. There is always an equilibrium between adsorption and desorption. There is a narrow band in a carbon bed where the actual adsorption takes place. Upstream of that, the bed is spent and partially desorbing the stuff that is less strongly adsorbed. Downstream of the band, the fresh carbon is "polishing" what has been desorbed. Eventually the band will progress to near the end of the bed and you will begin to get breakthrough and the bed will be soon exhausted.

Depending on the vapor loading, bed mass and depth, and the superficial velocity, the heat of adsorption, if not carried away fast enough by convection via air or conduction through the bed, can build up rapidly and spark a bed fire. That's a bad day.

Make sure you contact your activated carbon manufacturer for proper sizing data.

P.s. charcoal and activated carbon are two very different things, but the terms are often nused interchangeably. Charcoal actually has very low adsorptive activity and is sometimes a precursor material in the AC manufacturing process. Medical applications (such as the carbon slurries used in emergency poisoning treatment) often improperly use the term "activated charcoal" to refer to activated carbon.

[Aluminum]

Originally Posted by 1001001

Depending on the vapor loading, bed mass and depth, and the superficial velocity, the heat of adsorption, if not carried away fast enough by convection via air or conduction through the bed, can build up rapidly and spark a bed fire. That's a bad day.

Very interesting! A quick googling for "carbon bed fire" comes up with great bedtime horror stories. Clearly this calls for further study.

First question that comes to mind is what's the best way to dispose of the saturated carbon media?

[salto]

Looks like your inlet air has too many places to go. Doesn't flow through your booth like a river. Just a suggestion.

[EXflyer]

Originally Posted by bret

You will need to move large amounts of air to keep the overspray down, I built a booth with three box fans, and two large inlet filters in the walk in door, it was not enough. I originally had exit filters but scraped that idea and just moved the cars out of the driveway highly recommend a fresh air hood or full face 3M mask with organic filters and peel away sheets. Full face has a protection factor of 50, a half mask is only 10. I did Base CC and lived....but barley......wife wanted to beat me every paint session, and I was pulling air from the house and exit the garage door........

And with all of the thinking going into this I see nothing about what kind of electric motors are being used on the fans. They should be explosion proof motors or all of the other bs could be just burned wood and garage if not used. I painted aircraft years back in the late 60s and even the lights and switches were done that way. You will need more area going in than out and also the exit has to be at the opposite side from the inlet or you wont remove much overspray.

[1001001]

Originally Posted by Aluminum

Very interesting! A quick googling for "carbon bed fire" comes up with great bedtime horror stories. Clearly this calls for further study.

First question that comes to mind is what's the best way to dispose of the saturated carbon media?

"In the industry" lots of activated carbon is reactivated. It is very similar to the final steps of the production process for "virgin" carbon. High temperature (1600 - 1900F) in a reducing atmosphere in the presence of water vapor. Not easy to do at home.

At home however the spent carbon can probably just be burnt as fuel. I wouldn't heat a house with it unless iI was monitoring the process very well (high temps are necessary to destroy the adsorbed materials). And I certainly wouldn't cook with it. But it could be put on a campfire probably. You might want to try a small amount first to see if it smelled unpleasant...

Pure solid carbon has a higher heating value of about 12000 Btu/lbm plus you would add the heating value of the adsorbates, so could be as good or better than coal, depending on the adsorbates.

All statements here are mine personally and not representative of my employer.

[Aluminum]

Originally Posted by Aluminum

This calls for more testing, to see if the vinyl would stick.

I applied a small patch of 3M automotive vinyl to pristine polysiloxane topcoat several days ago. Rubbed only, no heat gun. Today I tried lifting corners with 90 psi shop air. Good news: boundary layer won the battle. If I lift a corner using fingernail it peels off easily--adhesion is low but may be enough. It appears that vinyl decals over siloxane paint could work. Will revisit after flying with a decal for a while.