Spray Booth Explosion or Fire

[sberry]

why is there no such thing as a positive pressure booth?

on a positive pressure booth you force air through a filter system in at one end and it naturally exits the other end bringing with it paint fumes. it works exactly as a draw through only in the opposite. its much better as you can exit the room if needed and you don't **** in dust. also if your using a visqueen booth it helps hold it up and won't let dust in any little cracks there might be.
Jay D.

I was trying to get a simple link to explain this but it is a FUNDAMENTAL principle for painters and paint booths. Forced air is for a clean room, the pressure inside is basically the same everywhere, as I said balloon. Yes some air is forced out but the pressure inside the envelope is the same, trying to force air out everywhere and it does not clear.
All booths are negative, they do use some forced make up air especially when the booth is in another building, doors open, doors close, maybe changing conditions.
Look at ready made booths, most do not come with forced air but all come with an exhaust fan.
The proper term would be pressurized air filtration. A pressure room would shove air out any opening, not just the exhaust, this would contaminate the air in the building. It must be all drawn out the exhaust.

 

[sberry]

finishing academy has fugged up a good page, that is about useless, I have it in PDF but dont know how to post it, There are good drawings on some old pages, they must have changed them.

 

[Spokerider]

Well, I sprayed some epoxy primer and some Imron in the booth yesterday with no problems. It seemed to work well.

I am using polyester quilting material for the exhaust filter........and it does catch a lot of the over spray. After spraying 600cc of Imron, the filter was totally painted on one side with still some white fibers showing on the back side of the filter.

 

[LS6 Tommy]

Flammable/combustible vapor is not the only problem. Overspray (and most any other fine particulate dust) itself is highly flammable. If the exhaust fan has a start capacitor (very likely), every time you turn it fan on, there is a spark when the centrifugal switch opens. I realize 99% of the time the fan will be turned on before you start spraying, but the risk exists.


Tommy

 

[markharleypainter]

Hell.back in the 70s-80s i sprayed cars all over with polyurethane with a kerosene turbo forced air fan.i had the garage so fogged up till i couldnt see.then turn on an old box fan. It got so bad in there the air filter on the heater would clog up and make it shoot out flames. No problem at all !

 

[ptschram]

The last time anything about the topic of paint booths came up, I got blasted pretty good in spite of my having professional experence with the EH&S engineering component of the process having been the EH&S manager for the largest painting operation that Lear Corp had in the US.

My two cents worth.

Drop the incoming fans and replace with filters. Use those fans as your exhaust. Wear a full-face respirator (FWIW we sprayed a LOT of isocyanates and I was in those booths daily wearing a full face respirator and have no issues with sensitivity yet-although I now can't be around most other paint solvents).

While you probably do not have reliabale airflow rate data on the fans you're using, you can find simple enough calculations to determine the amount of airflow necessary to maintain the internal concentration of flammables well below the LFL/LEL so that you'll be safe from over-exposure and the risk of fire.

It is pretty easy to exceed the minimum airflow rates and if you have the airborne concentration below the LFL you'll never need worry about ignition sources.

Remember, if you are in an environment where the airborne concentration is above the LEL, YOU are the ignition source.

Now, W/R/T dirt, etc. I knew BASF's Dirt Doctor-yes, he really did have a PhD in surface finishing and the reduction of dirt in paint. put simply, you can't get it clean enough at home.

 

[rustyjames]

Hell.back in the 70s-80s i sprayed cars all over with polyurethane with a kerosene turbo forced air fan.i had the garage so fogged up till i couldnt see.then turn on an old box fan. It got so bad in there the air filter on the heater would clog up and make it shoot out flames. No problem at all !

Same here, turbo heater to get things warmed up, then shut it off to keep the dust down. The wood burner helped to knock the chill off .

 

[flat tire]

I painted cars/trucks in a 24' x 40' garage. a large coal furnace in one end and two
24" box fans blowing out under the 16' door with cardboard filling in the rest of the opening
I would open the top door on the furnace and it would draw in the over spray

 

[John in OH]

With a modern spray gun shouldn't need to worry. If it clears fast its good. You don't always need fans on the intake, I don't use it on mine. I actually had too big exhaust fan for a while but mine is tuned quite well and dont filter any air coming in but pull it mostly over top of the end wall, makes for a warm downdraft.
It taks a bit of experience for tuning a room to become second nature. A good experiment is a simple twin box fan in a common bedroom, use the door as a regulator and notice too tight and no air movement, too loose and no "pull" or air clearing.
Used to smoke in dorm rooms.

How do you "tune" the airflow in your painting area? Is the big axial fan on the end of your painting space variable speed?

 

[sberry]

I did tune it by changing the design to slow it down a bit, I was losing a lot of heat and moving more than I needed. But once the speed is generalized it is tuned by changing the air intake to the building a bit and then to the "room" which is the booth. We want to restrict it so there is draw on the room, not so tight there is no air flow.
A super good way to get the feel for this is get one of those little twin box fans for a window, put it in a bedroom window and play with the door opening to see how this changes air flow. You can have several incense going in the room and walk by the outside door and smell nothing when its working correctly.

As the man advised in post 46, drop the incoming fans.

You are not trying to blow fumes out but to draw them out thru a stream or movement of air.