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A little while back I built a spray booth. I was in a hurry to get done with it and did not think it through good. As always when something is rushed, it turned out to not work so well. The problem is in the way it exhausts the spray mist outside. The hood that I built that tapers to the window, gets to small, so a lot of air is being bounced back into the room. I wanted something that was air tight, especially when I wasn’t spraying, so here is what I came up with.      

This is in the top of my shop where my spraying is done.     Here is the front of the booth.




Now for my question. I am planning on taking off the tapering hood that I made and connecting a flex hose to the rim around my fan. I will need to remove my window and put something there to allow the air to flow through, but can also be closed off. What about a fan shutter? How air tight are they? The reason I went with this ridiculous plan in first place is because I did not think a fan shutter would be very air tight, and I wanted to avoid taking out the window. Also, since I am spraying into a box I thought that the spray mist would be pulled out better than installing the fan in place of my window. Anyway, I will appreciate any info you can give on this matter

Sounds like you need a bigger fan, since I can't see it and you don't mention it, we can only guess.

Hoffman uses a 18" Explosion Proof Fan, which will probably suck the hat off your head.

I've been using a 20" box fan and praying the Lord doesn't decide to take me home each time I pull the trigger.

I heard that you only need a small movement of air through the booth. If the flow is too high it takes most of the spray out the window. If it's taking your hat off, it's probably taking a lot of money in lost lacquer out the window as well. I'm sure there's a balance...perhaps someone knows how to determine the optimum air flow.

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JJ
Napa, CA
http://www.DonohueGuitars.com

I bought an explosion proof fan from Northern tool co. it is made by Shaefer ventalation equipment.

it is made to be installed in grain silo`s and is hung in its own frame, so all you have to do is take the windo out and and pop it in and hook it up.

I also have a dimmer control switch that you can buy with the unit which allows me to slow the fan down to my liking, otherwise it will suck your hat off your head. I tend to spray with the fan as slow as I can have it go and still evacuate the fumes.

Bruce you really should`nt be spraying with that box fan!

Good looking unit Matt.

Nice fan Matt. Can you remember what it cost?

The problem with mine is not that the fan is to small, it could be that it is to big though. Here is a pic off the fan before I built the tapering dust hood.

I can’t remember the size of the fan but it would barely fit in the space of the window if it had to.
The problem is that so much air is being blown into the dust hood, (that tapers down to 6") that some of the air is coming back out toward you. You can stand in front of the booth before you even spray with the fan turned on, and you can feel some air from the air from the fan bouncing back. Imagine that fan blowing into a tapering hood like that without an opening in the end of the hood. All the air from the fan would blow back on you. That is somewhat the effect you get with my setup, it has to taper so small to be able to go through the opened window.

Thanks for you input guys. Anybody else have any helpful info?

Colby,

Don't have any suggestions for you, but wanted to say that is a nice looking spray booth you built.

Also, would it not take a very heavy concentration of airborne lacquer fumes to ignite? Has anyone ever experienced this, or know of someone who has? I don't doubt that it is possible, but it would seem that it is probably something that would be more likely in a production environment. Not in a shop where someone is spraying a guitar or two. I don't take chances if at all possible, but an explosion proof fan is no small expense. Of course, neither is a trip to the emergency room!

Jeff

I see in your picture that your tapered it to a 6" circle. Then you had to block off a big chunk of your window. Why not make the taper go to the width of your window. You might leave the height the same. If the output is reachable from the inside of the booth, you could make a plug to put in there when the booth is not in use.

Mike, good tip, I probably could fix up something like that. I just wonder if widening would fix the problem. I’ll keep that thought in mind though. Thanks.

Does anybody use a fan mounted in the wall or window that closes of the air flow with fan shutters? If so please tell me how air/temperature tight they are. I am still considering this option.

You effectively cut your fan down to about six inches in size. Whatever the sq. inches are of the fan, you should have that at least as an exhaust. Not an engineer, but just common sense..... ?

Alright ,...I better sound off on this one . I have 18 yrs of HVAC Mech. Engineering .
The fan is over kill ,..for the booth size. That things probably 600cfm.+
If you continue With this current fan size ,.... (Mike hit the nail on the head) You need to increase the discharge opening to at least the Fan size . So if its 18 " fan ,...than you should atleast provide the same square inch opening,...be it rectangular, Square or round.

Also you must have a fresh air intake.(30%-40%)
of 18" discharge ,...7" rd will work) This provides negitive pressure release in the shop ,...so your fan can properly pull air mass.(locate in shop anywhere or crack a window)Cool or humid air stinks but you must have an intake to have proper discharge.
   The only way to gain intake air that is tempered is to install an air to air heat exchanger. Which cost from $650-$1200.
Explotion proof fan is a good Idea ,...but if funds are limited ,..you can safely mount a 200 cfm bath fan to the proper duct set up and trick the air flow out the top of the booth and right outside ,...without crossing the motor brushes. So the fumes never cross the motor.
If anyones interested ,...it is quite simple ,...and much safer than holding your breath for the big kaboom!

I am interested. How would you set something like that up?

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

JD,

Please explain your bath fan method, I think many of us are interested. Thanks for the imput.

Jeff

Okay JD show us how to trick a fan.... to be explosion proof?

JD,
I am also interested in doing something like that. I'm tired of waiting around for a day warm enough or dry enough to spray.

Alright ,...I am going to build another one in two weeks for a friend .
I will document pics and show you guys ,...I am not crazy.

    The motor and small twinblower wheels are on the up side of the draft . No over spray or gas reach the blower compartment.
Also the blower compartment is feed by an insulated fresh air intake.

I will make entire booth from sheet Metal.

   So stay tuned ,....

      I can not mention factory name of this blower set up on the post . But will be glad to give info. in an email.

I know you said you'd post what you're doing for a friend, but maybe you can tell me if this is what you're envinsioning...

Are you perhaps talking about setting up some sort of Venturi effect? (I think I'm remembering the right principles ). Venturi may not be what I'm thinking of but, basically, here's what I'd envision you getting at:

You have two air "supplies", one being the booth itself that you're trying to evacuate and one being fresh air. You'd have a blower inline with the fresh air, restrict it on the positive pressure side (output side), and feed that side into your main exhaust duct at some point in the line behind the booth, and have it forced in the main, outward, exhaust direction (out the window or whatever).

In theory this would cause a negative pressure situation in the booth itself, and "encourage" a flow from the booth, out the window. Depedning on the force of the fresh air blower, that "encouragement" could be as strong as any fan, and, as you say, the flow would always be away from the blower.

Here's a super-crude drawing of what I'm thinking you mean... forgive the 1-minute crudeness of it:



Is that kinda what you're talking about?

Hello ?

Hey we need to know your name, so we can thank you for explaining the concept. Welcome to the forum also.

JD, pictures would be great. Welcome to the forum, incase we forgot to tell you.

Thank you.

Jeff

   Yes ,...this is exactly the principles that apply to this set up .

The picture you show is correct ,..but with one missing element.

You must have a splitter baffle starting at the entry of air supply and extending just past your supply opening .(At the Y )

Baffle will allow maximum draw ,..with no chance of any positive feedback towards booth .

Give me a couple weeks and I will document with pics .
               

   JD Myers38416.3974074074

Thanks for the info guy's! I'm going to check the size of my fan and see about rebuilding the part that fits in the window. There is no point in my fan being explotion proof, all I spray is waterborne stuff.

See if you can calculate the CFM's removed from the spray booth, thanks.

Charlie Hoffman indeed does use a 18 inch fan with filters in front of the fan catching overspray, then on the door of the spray booth somewhat filtering Fresh Air coming from his shop area. He says it's passed the Minneapolis fire inspections many times.

I'm not finding this fan at Northern...?

This may be looking at the wrong side, the first is fiberglass, the second is PVC. And one is polyethylene. None say explosion proof. What kind is yours Matt? Model number, thanks..

Okay here it is at another vendor...



America's Pride Online.com

1-800-348-4244


$301.oo

Doesn't mention explosion proof? You spraying nitro Matt?Dickey38416.5281134259

This is a great thread...need to keep it alive and on the first page. Lance...Is there anyway to do that?

_________________
Dave Bland

remember...

"If it doesn't play in tune...it's just pretty wood"

How can you tell the cfm of a fan/blower if there's no label. I've got a couple of old fans that might work, which will save $$$ and I would just like to get some idea how much air they move. Is there a low-tech way to check them?

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DES - Shepherd, MI