Michael Dresdner » fire safety

Big fan, clean air

Michael — Sun, 04 Jul 2010 14:00:42 +0000

Q: Is a 7700 cfm fan adequate for a booth the size of the one in your video?
A: In terms of power, yes; a 7700 cfm fan is adequate for that size booth, but that assumes a small exit port paired with a well designed booth. Remember, a booth is a funnel that channels a large volume of air (from where you stand) through a small opening (the filter covered fan.) That means you will be standing in clean air because the airflow will go smoothly from behind you, past the piece you are spraying, through the funnel and out, as opposed to creating turbulence that just blows fouled air around the shop. It’s that directional flow that is critical, and that’s the crux of why you make a spray booth.
It’s also why you use a turntable. Once you make a booth, it is vital that you spray only in one direction; toward the fan. Only a turntable will allow you to spray all sides of an object and still be facing only one way.

Open face, closed room

Michael — Wed, 30 Jun 2010 14:00:12 +0000

Q: The only information I found on spray booths were for totally enclosed rooms, and indicated you had to use non-explosive fans and light fixtures if you are spraying solvent based finishes.
A: Actually, that has nothing to do with them being totally enclosed rooms. Open face booths share the same regulations. It has to do with the various agencies that regulate safety in the workplace and in commercial spaces. If you have employees, OSHA holds sway. If you rent a commercial space with a legal certificate of occupancy, you come under local rules governing fire safety, emissions and so on. In your own home you can do most anything you want, safe or not. However, you can forfeit your coverage after the fact. It’s a bit of a Catch 22; you have freedom, but that includes the freedom to lose everything on a bad decision. Negating your insurance without even knowing you did can turn a mishap into a nightmare.
However, let me add another word about fan safety, since we have raised the issue. In a recently written article (I believe it comes out in September of this year) I talk about the fact that spraying flammables in a booth, even a booth with a normal house fan and cardboard sides, is vastly safer than spraying with neither of those. That stops short of saying it is safe, but it’s certain that vastly safer is way better than not safe at all.

A big fan

Michael — Tue, 29 Jun 2010 14:00:02 +0000

Q: How big a fan will I need for a garage shop spray booth.
A: Though they have no legal sway in a garage shop, OSHA mandates 100 cfm (cubic feet per minute) per square foot of filtered booth wall, and in my opinion, those are good guidelines to follow. In other words, if you have a 2 foot square filter in front of a fan the same size, going out a window, you will want at least 400 cfm of air moving through it. A six foot square area wants 3600 cfm.
Choose your fan size or filtered wall size by the size of your booth and the amount of spraying you will do. Also, make sure you have enough clean intake air coming in to replace what is blowing out. I generally like three times as much filtered surface for my intake as is in my outgoing booth, to make sure you don’t overtax the fan motor.
By the way, make sure you don’t have anything on the other side of the outgoing fan. Even with filtering, it’s not unusual to get overspray on that car parked in your driveway, and I doubt you want that.

Spray week begins

Michael — Mon, 28 Jun 2010 14:00:04 +0000

Q: I want to put a spray booth in my garage shop to spray oil based paint. Is it necessary to use an explosion-proof motor for the fan?
A: You are asking a legal question, not a finishing question, and I am not a lawyer. My guess is that you want to know if you will negate your home owner’s insurance if you have a mishap spraying flammables in your garage. For that you should speak to your insurance company and your local fire marshal. They can tell you whether what you plan to do is legal and regarded as safe in your locality.

Spraying safely

Michael — Fri, 27 Nov 2009 17:00:39 +0000

Q: Is there a safe way to set up a separate room for spraying shellac? I was going to put a fan with a TEFC motor and exhaust through the window to the outside, and cut a hole in the room door for intake air.
A: There’s a bit more to it than that, but you certainly have the basic idea right. You want to evacuate the air in the room and replace it with clean air. You want to always be standing in the flow of clean air, and will want to efficiently move the solvent laden air out quickly and with good flow. That’s where spray booths come in. Simply put, a booth acts like a funnel. You stand in the large end, with clean air coming in behind you, and spray toward the small end which has a fan to gather the flow and move it quickly outside. Typically, there is a set of filters over the fan to clean the air of particulate before it goes outside.
All that is fairly simple, but here’s where it gets complicated. There are a host of laws and safety regulations regarding spraying of flammable liquids. They involve wiring, lighting, booth and room materials, types of fans and filters, and even the sheet rock on the walls and ceiling. A business subject to OSHA and EPA rules must be concerned with these. A hobby shop has the choice to ignore them, but they would be wise to speak to at least two people before they make that plunge; their homeowner’s insurance agent, and the local fire marshal. Get the blessing on your spray room design from those two and you are good to go.

An explosive answer

Michael — Wed, 25 Nov 2009 17:00:42 +0000

Q: I have a room off my shop that I want to set up as a spray room. I only use shellac and spray it. Is shellac highly explosive when sprayed?
A: Explosion potential is measured by LEL and UEL, or lower explosive limit and upper explosive limit, a measure of the amount of any solvent gas in air. To understand LEL, think of how a carburetor works; you don’t get combustion if there is too much air and not enough fuel, or if there is too much fuel and not enough oxygen. To answer your direct question, the LEL of ethyl alcohol (that’s what’s in denatured alcohol and most shellac) is 3.3 % by volume. Shellac resin itself is not explosive or flammable separate from the solvent, so its presence makes explosion less likely, not more. In short, if you evacuate the air by having a fan blowing out the entire time you are spraying, turning it on before you spray and off after the air is clear, and have clean air coming into the room, you are not in any danger of an explosion in the room, since you are highly unlikely to ever reach the LEL.

Hang it out to dry

Michael — Sun, 24 Feb 2008 06:10:00 +0000

I was saddened to hear that a member of my own guild lost his shop and tools last month to a fire started by a bunch of oily finish rags. It is a common cause of shop and worksite fires, but it is easy to prevent.

Oily rags are spontaneously combustible. That means that even in a very small pile, they will generate enough heat to ignite themselves in about two hours or less. Oil cures by oxygen polymerization, which is a fancy way of saying oxidation. Remember high school chemistry? Fire was described as “rapid oxidation.” This is the slower version of the same thing, and it too generates heat. In fact, it generates enough heat to catch the rags on fire. It can happen with any drying oil or finishing oil: linseed or tung oil, oil stains, and even oil varnish, polyurethane, oil based filler, or danish oil mixtures.

To prevent the heat buildup, lay out oily rags ONE LAYER THICK. Drape them over the edge of your workbench, trashcan, or even a clothesline. That will allow enough air flow to dry them quickly AND to dissipate the heat as it is generated. When the rags are stiff and crusty, usually in 24 hours, no liquid oil will be left uncured. At that point they are landfill safe and can go into the household trash.

Please, take extra care with oily rags. Lay them out to dry and don’t let them sit in piles. I don’t want to hear that any of my readers has had a spontaneous combustion fire in his or her shop.