Re: Setting up a paint booth in The Knowledge

I decided that this ought to be filed under the relevant description.

As an alternative to a supplied air respirator system and painting in the carport, I'm evaluating the feasibility of safely using my shop and it's exhaust fan as the basis of an open front spray booth. The goals are to reduce VOC emissions to both the outdoor atmosphere and my breathing zone without blowing anything up or starting a fire. Improving the quality of my paint jobs is not a goal, but I wouldn't mind it.

A couple of initial reality checks:
1) Two of the three legs of the fire triangle exist in a spray booth: Oxygen and fuel. Only ignition is controllable.
2) There is no such thing as an explosion proof blower; only blowers with spark resistant construction (SRC) exist.
3) This is not going to be a production booth; potentially combustible mixtures will exist for a total of about 2 or 3 hours per year, in batches of a few minutes, followed by fresh air flush (between coats, fan running continuously, obviously). That constitutes a Class 1, Division 2 environment (C1 meaning the existence of combustible gas mixtures, D2 meaning that the C1 condition does not normally exist, it's extremely unusual).

The requirement, then, is to prevent electrical arcing and ignition temperatures in the potentially combustible zones without making my shop C1D2 compliant.

The few outlets on the fan end of the shop ( Flickr ) will have nothing in them; everything will be unplugged; and I can physically seal them off if I feel the need. The cord for the fan will be replaced with a longer unit so it can be plugged in at the air supply end of the shop ( Flickr ). There are no switches anywhere near the fan half of the shop. The fluorescent lights on the ceiling: While technically within the 3 or 5' exclusion zone near the face of the booth (I believe that's the general standard) they'll be well flushed by incoming air. They don't generate ignition temperatures (nowhere near them) and arcing at the pins seems a pretty remote possibility. Nothing else about them would arc short of a catastrophy (broken tube, ballast exploding, wire shorting) and I can tape the lens to the receptacles so as to pretty effectively seal them anyway. And I'm not going to smack the lights and torch off the overspray.

The motor is TEFC and so suitable for C1D2 environments. I'll tape, to hermetically seal, the connections between the motor leads and the power cord, or hermetically tape off the whole make-up-box. I'll also make certain that air can't leak into the motor can via the motor lead access; I can pot it if necessary....shoot, I could pot the entire make-up-box for that matter. The power cable is exposed and so not C1D2 compliant but given that they're in good shape, inspected and not going to be damaged and cause an arc, as would eventually happen in an industrial environment, I'm comfortable with that.

Spark Resistant Construction (SRC): I've read everything I can find about SRC for metallic fans and blowers and it all seems to boil down to protecting against sparks due to rotating parts making contact with stationary parts, and implicitly in the context of mechanical failure or grossly excessive wear as you might find in full time operation and/or poorly maintained equipment. Static charge accumulation and subsequent discharge doesn't seem to be an issue (as it can be for plastic fans and pipes) assuming that everything is properly grounded (as it is in my case). My fan has generous clearances between the blades and framework nozzle, is in excellent condition with no detectable play in anything and is seriously overbuilt relative to the ¾ hp motor that I installed (the fan is rated for much more power). Catastrophic shaft and bearing failure seems highly unlikely so in-spite of not being made with SRC the likelihood of blade/frame contact and arc seems remote. I do need to double check for thrust displacement of the shaft now that I think of it.

If I go this route then I'd install filters in front of the fan and on the windows where the make-up air is introduced (there are four). Sheets would be hung from the ceiling and secured to the floor by weights to form the sides and fan end of the booth.

My tentative judgement is that the likelihood of an ignition event, under these specific conditions, is very low, almost non-existent. I also tend to think that, given the rather large flowrate of the fan, the flammable gas concentration won't reach the LEL. I might be able to borrow a combustible gas detector from a former colleague to confirm.

That's my train of thought. Being prudent and professionally timid about such stuff (handling rated areas on the cheap) I'm far from certain that I'll employ it, and I do not recommend that others do so, but if nothing else it was an interesting exercise. The safest way to spray is to do so where there simply aren't ANY potential ignition sources (including synthetic clothing on a dry day).....for me that's the carport with the lights turned off (the ambient light is adequate). If the list has any other process/process control/HAZOP engineers with experience in dealing with rated areas I'd be interested in hearing your reactions.