Paint booth venting
 

All,

I live in northern Alabama and the fluctuations in temperature and humidity makes year around guitar finishing a problem. So.....

I'm building a paint booth in the corner of two walls in one of the rooms (8’X12’) in my shop. The walls are covered with plywood (instead of sheetrock). I was going to encase a 4'X4' area around the spraying area with walls made out of heavy plastic. I'm going to install a filtered fresh air vent and a dehumidifier in the room. I was going to control the winters with an electric heater. The summers, I'm not sure of yet. This room is usually 15 degrees cooler than the outside air temp so I would like to see if just running the dehumidifier will do the trick. I'm installing the heater & dehumidifier outside of the spraying area encased in plastic.

So the questions I have are concerning venting…

One idea based on reading the forums was using 20" box fans with filters mounted to the intake. I would install a sheet of plywood across the corner (forming a triangle with the other two walls) to mount the fans and act as a manifold for the fumes. I was going to use two vertical fans and possibly one overhead fan that will blow the fumes directly into the manifold. I would then vent the fumes out of the shop using dryer ducting. I am aware of the explosion hazard using the fans but I have read where they are safe using household filters on the intake. I’m not convinced a common manifold is safe with all of the fumes blowing around the back of the fans even if I put a filter on both sides of the fan. What are your opinions? Should I run the vent line (dryer vent line) out the top or bottom of the manifold, or does it matter? The fumes seem to naturally rise but with all that air pressure in the manifold seeking an exit point, I wasn't sure it really mattered. Should I encase the manifold with plastic to prevent the fumes from permeating into the plywood?

2. The other idea I read was to pressurize the room with filtered intake air (blown in with a fan) and then a simple exhaust duct venting to the outside. This seems safer but not as effective in exhausting the fumes. Thoughts on this idea?


Thanks

A positive pressure booth is the best. I own a body shop. I have a spray bake booth and a crossdraft. The spray bake is a positive pressure down draft. It has a slightly higher volume of air being forced in than the volume being sucked out. It allows you to open the entry door without sucking dirt in and if you have gaps like around doors it doesnt allow dirt to suck in. Youll have a hard time pressurizing a room that isnt extremely well sealed up. Just install some intake filters in one wall and install a fan with some exhaust filters on the opposite wall. You dont need a huge draw to move air. Just remember the better the room is sealed up the better movement youll get.

Yeah, positive pressure down draft, if you can manage it. It is pretty standard these days in the auto refinishing industry, and we are shooting for a higher level of finishing then you can get away with in the car business. If you can build a raised floor, then you can use the space below for your exit vents, just 2x4's on edge would be enough. Also, a distribution manifold for the inlet is a really good thing to keep actual drafts to a minimum while moving enough air to pull overspray away from your work. Intel's production clear room has a vertical air movement of about 5mph across the whole facility, and that is what you should be shooting for.

Is it a good idea to use fans with encased motors, so you don't' ignite any fumes, or is that not really a risk?

As long as you can see your hand in front of your face not much chance of igniting anything. The fumes would have to be really concentrated.

Concerning the positive pressure downdraft system....Are exit fans necessary? Again, thinking about the potential for an explosion. My question comes from my job that feeds my addiction to guitar building; airline pilot. I think of the way an airplane pressurization system is set up. You have air conditioners "packs" that take compressed air from the engines and push it into the cabin. The pressure differential/ cabin altitude is controlled by a "simple" outflow valve that is opened or closed by small amounts. The air naturally seeks that valve because the air pressure on the other side of the valve is lower. So if we are pumping air into the spray room & it sealed very well, this air should seek out the exit vent. Shouldn't this work or is the pressure differential just not high enough?

If you use fast volatile solvents like lacquer thinner an explosion proof fan is mandatory.

By the time you have the equation for explosion you would be passed out on the floor from lack of oxygen.

But then you explode.

:lol: :lol:

I am not sure I understand your point.
When dealing with potentially dangerous materials I found it useful to always err on the side of caution.
Explosion proof fans are required by code for all professional spray booths, for good reason.
I would add that your fire chief will be more than happy to inspect your spray booth.

I think pressurization is pretty consistant regardless of volume. It does follow the path of least resistance. Problem with not having pull is that you cant adjust your your pressure, it would just be blowing out at what ever rate its blowing in. Basically a positive pressure booth is optimal at "0". Which is equal pressure but you do want a tad bit to the positive side so as not to suck in trash when you open doors or if you have cracks or voids. You want to control turbulance in your booth with being able to control in and out air.

You cant really achieve a positive pressure booth only blowing air in. You would basically have a crossdraft booth at that point. You have to be able to control the air exiting. The only way to do that is with an exhaust fan drawing the air out and both intake air fan and exhaust fan must have baffles you can open and close to adjust air flow to achieve positive pressure. I own a collision center and ive been doing this for 26 years. I have painted cars in garages and got great results. The main causes of trash in paint is to much air movement (ie:turbulence), dirty painter or dirty project. Make sure you are clean, whatever your painting is clean, your air is run through desicant filter and oil and water seperator. Ideally a compressed air dryer (refrigerated air) is the cleanest driest air you can paint with. Also leave your booth on till solvents are finished escaping. Lot of people turn booth off cause overspray is gone but you still have solvents floating around you cant see which will land on your project and give you solvent pop or dye back.

My booth fans actually have regular 3 phase motors but they are mounted on the outside of the fan housing. You should be sure to use alluminum blades on your fans though. No sparks.

Ive been painting cars for 26 years. I used to smoke cigarettes while i was painting a car. Back in my younger stupider days. Point is, its pretty damn difficult to get enough concentration of solvents to blow up. Ive actually never seen a fire in a paint booth. I would however not suggest having an open flame in your booth while your painting. I guess thats pretty obvious i just felt it nessacary to say.

Yes you can have a positive pressure booth by just blowing air in, you do so by adjusting the exhaust outlet size.

Unless you're willing to invest a LOT of money in a UL listed hazardous location fan unit, placing your air mover outside the booth, blowing in, is your only safe option. Putting a filter in front of a box fan will NOT make it safe to position it in the exhaust stream.

Accumulation of overspray buildup is another problem, you'll need a filter over your exhaust intake, and to clean the duct frequently. Accumulation of combustible, dried overspray is another very good reason you can't put a box fan in the exhaust stream.

In addition to the fan, you should have no electric fixture or light, radio, switch, or other source of ignition in the booth. Solvent vapors do separate from paint fog, so there is no way to determine if you're getting close to explosive limits by sight. Given the size of booth you propose, I'd say there's a good chance you could easily achieve hazardous levels of concentration from both a health and fire standpoint.

If you'll consider a carburetor's function, and compare that with what you're doing shooting lacquer in a spray booth, you can to begin to appreciate the hazard level. Except you don't stand inside an engine's cylinder.

Finally, from a quality standpoint, no matter how you heat/cool/dehumidify your booth, that conditioned, optimal air is going to be (literally) exhausted pretty quickly. Conditioned make-up air is a major consideration in a painting operation.

Not if you are properly protected. Positive pressure line feed respirators are the gold standard

Atmospheres in commercial booths generally don't reach flammable/explosive levels, that's what they're designed for, so accidents or idiots, don't start fires. Most of the paint operations I see today are pretty sophisticated, even small body shops, because partially of multi-component paints that demand an air-supplied respirator. Never seen anybody smoking in one of those things, but there's still time.
My experience is that most paint operation fires start in the "prep" area, (where you can usually see lots of overspray on the floor), when somebody has kicked a cigarette butt into a wad of paint-covered masking material, however, I feel it necessary to say I have seen booth fires in body shops, or I should say, the aftermath of them. They're called "total losses". One of the "perks" of being in the insurance bidness.

I quit smoking about 8 years ago so needless to say i havnt taken that chance again. I did see a dumpster on fire once. It was from used booth filters. They act kinda like compost as in if there is a bunch of them thrown away together they can start heating up and just combust. Interesting. There is a lot of stuff to watch out for.

What do you do for the insurance company? You a writer?