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Paint Booths & Safe Ventilation?

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You've miss read what the sq ft relates to. It is not the size of the working area, it is the size of the opening. It may seem a trivial difference but has to do with the velocity of the air not simply movement, both are moving 100 cfm but the larger opening allows for slower moving air and areas of little movement.

If a closed 2x3 ft working area only has a 14" x 20" opening (fairly standard size filter) then you are looking at 1.9 sq ft, not 6. 1.9 x 50 (downdraft) would put the 100 cfm fan at the appropriate size.

I would be concerned that the 3" ducting is inadequate though and could be restricting the blower from seeing its full potential. I picked up a section of 6" metal flex duct at Home Depot, it is cheap, easy to use and provides for very little friction loss.

Here is an easy to use online airflow calculator. If you use it you will see that the 3" duct has 4x the restriction of a 4" duct and 30x the restriction of a 6" duct.

http://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html

Thanks for the info. It makes a lot of sense. I wasn't sure if I had the right size fan, but visually, there never was any sort of fogging of the interior while spraying. That led me to believe that I was in the ballpark.

My painting technique is a work in progress but I've recently moved to lacquer paint and really like it. I apply 3 or 4 mist coats and 2 or 3 wet coats. I wait about 5 minutes between mist coats and about 10 between wet coats. . There's never a lot of continuous spraying and the booth stays clear.

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You've miss read what the sq ft relates to. It is not the size of the working area, it is the size of the opening. It may seem a trivial difference but has to do with the velocity of the air not simply movement, both are moving 100 cfm but the larger opening allows for slower moving air and areas of little movement.

If a closed 2x3 ft working area only has a 14" x 20" opening (fairly standard size filter) then you are looking at 1.9 sq ft, not 6. 1.9 x 50 (downdraft) would put the 100 cfm fan at the appropriate size.

I would be concerned that the 3" ducting is inadequate though and could be restricting the blower from seeing its full potential. I picked up a section of 6" metal flex duct at Home Depot, it is cheap, easy to use and provides for very little friction loss.

Here is an easy to use online airflow calculator. If you use it you will see that the 3" duct has 4x the restriction of a 4" duct and 30x the restriction of a 6" duct.

http://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html

I will respectfully disagree with that logic. Greatly exaggerated to make my point, following the method you stated, one could in theory properly exhaust a full-size 8' x 10' x 8' room through a 1' x 6" filter with a 25 CFM fan as that combination would have a proper rate of flow at the filter. There is no respect to the area being cleared and outside of the immediate area of the fan the pull would be virtually non-existent.

I have read and re-read Klaus' article many times over the years and have exchanged communications with him several times. In his guide, he always refers to the square area of the box dimensions when calculating the first portion of the flow needs.

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I used one of those for awhile. Not enough CFMs to move paint out of the booth (I had the 8" version). They are used in HVAC systems to move air to dead or low performing areas.

Do yourself a favor, go to grainger or similar and spend $100-$125 for a squirrel type fan. They mount easily to the box (mine is mounted on the bottom and ducted with 6" tube) and move a lot more air.

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I will respectfully disagree with that logic. Greatly exaggerated to make my point, following the method you stated, one could in theory properly exhaust a full-size 8' x 10' x 8' room through a 1' x 6" filter with a 25 CFM fan as that combination would have a proper rate of flow at the filter. There is no respect to the area being cleared and outside of the immediate area of the fan the pull would be virtually non-existent.

I have read and re-read Klaus' article many times over the years and have exchanged communications with him several times. In his guide, he always refers to the square area of the box dimensions when calculating the first portion of the flow needs.

I actually used the same article when building my booth and it worked out great.

Something to ponder, he never takes cubic feet into account. Using his formula a booth 2 ft x 3 ft x 3 ft (18 cu ft) and a booth 2 ft x 3 ft x 10 ft (60 cu ft) will both recommend a blower of 300 cfm (down draft) or 600 cfm (cross draft).

Try an experiment with your own booth, get a large piece of cardboard and cut a hole 1/2 the size of your current booths opening. You will feel a dramatic increase in the velocity of the air entering the booth, it may even suck the paint right into the filter making it hard to actually paint the model.

Your exagerated example would be ineffective no doubt, but the actual opening does play an important part. I'm not an aerodynamic engineer so I can't tell you where that point is.

Edited by Aaronw

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Search for. Jenn air oven blower Or jenn air oven fan. The motor is seperate from the fan and fumes can not get into the windings

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As stated above the inline fans are not a good idea, unless you are only spraying acrylics.

The computer fans are not really safe either. ANY fan in the air flow would needs to be explosion proof, not practical for a home spray booth. Although the computer type fans are brushless they will still shoots sparks out when they decide to go bad. I have had a few burn up in my computer, it's not pretty.

Go with the squirrel cage type and play it safe.

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The movement of airflow is measured by the size of the openings the air flows through, not the size of the box. With the closed system the only incoming air is through the airbrush. He doesn't have an opening of 2x3 ft.

I too have read Klaus' website and built 2 of my own paint booths. All of his examples show the incopming air coming through the large open front.

As long as the air is being sucked out as fast as it is entering, he should be good.

I would highly recommend either a larger smooth hose or a shorter run to increase efficiency.

Danger

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Yes, Klaus is using the angled-face 18" x 24" opening dimensions in his calculation which is correct. But, the point you both have missed is that the actual cross-sectional area of his booth is around 17" x 24". So when the air leaves the opening of the face and enters the body of the booth the airflow is actually slightly accelerated by being forced through the smaller area. By calculating for the slightly larger opening you can ensure that the column of air through the booth maintains a sufficient velocity. Air is not empty space. It is full of molecules that have to be treated like a fluid where motion is concerned. So in the case of a booth you have to move all (or a great deal) of the molecules that are in the space between the fan and the opening. In the case of this booth, you have exactly the opposite scenario of Klaus' example. You have an extremely small opening and a large cross-sectional area. Once the air passes through the inlet filter into the box, it will decelerate rapidly as the molecules coming in have to fill the much larger column of the booth. That 100 cfm fan has to move all (or a great deal of) of the molecules in that 2' x 3' column before it can exact a draw on the smaller inlet. To actually obtain 50 fpm at the fan in this case of the enclosed booth, the air may have to hit the inlet filter at nearly 100 fpm to overcome the inherent deceleration of hitting such a large working column.

If you want an example, take a drink with a straw from a glass and see how easily it pulls. Next take the large end of a funnel and put it over your face and try to take the same sip with the same effort. It's not going to happen.

The straw has the same inlet and working area so it is relatively simple to draw your column of fluid through it. The funnel has the same roughly the inlet size as the straw but a much larger working column and as such requires a great deal more pull to achieve the same effect.

Quite simply put, you have to maintain the rate of flow through the largest cross-sectional area of the column of air. It does not matter whether that is the opening (as it is with the angled opening of Klaus' example) or the large working area of the booth in this thread. On the exhaust side or second part of the equation, the ducting is so much smaller than the area being vented that the molecules of air can get stacked upon each other and create a back pressure. So in picking a fan you have to have enough pull to maintain velocity on the inlet side and overcome back pressure on the exhaust side.

Edited by LOBBS

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As stated above the inline fans are not a good idea, unless you are only spraying acrylics.

The computer fans are not really safe either. ANY fan in the air flow would needs to be explosion proof, not practical for a home spray booth. Although the computer type fans are brushless they will still shoots sparks out when they decide to go bad. I have had a few burn up in my computer, it's not pretty.

Go with the squirrel cage type and play it safe.

If I ever rebuild by booth I will be using a squirrel cage fan. Hopefully because I want to and not because it blew up...

Since the issue is sparks from an electric motor and both types of fans have them, what makes the squirrel cage safer?

Dale

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If I ever rebuild by booth I will be using a squirrel cage fan. Hopefully because I want to and not because it blew up...

Since the issue is sparks from an electric motor and both types of fans have them, what makes the squirrel cage safer?

Dale

Dale, On the squirrel cage fan the motor and all electrical connections are outside the flow of the flammable exhaust.

A loose wire connection, even on 12volt, can cause plenty of spark to ignite paint fumes. On real explosion proof motors the electrical box is also explosion proof as well as the conduit feeding the motor. The conduit is sealed with a special fitting that is packed with a fiber wadding and poured with a cement made for that purpose so vapors can't enter the electrical system.

Open light bulbs near the spray is also a very dangerous situation.

I have been criticized on here about going overboard on safety, but having been a state licensed electrical contractor for 30 years, I've seen way too many disasters from improper wiring.

You can NEVER be too safe!

Edited by AzTom

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Not the type you show there. They are nothing more than a large bath fan type motor. Jenn-Air and I'm sure others make high end kitchen fans that do use a large squirrel cage blower, good for a large booth. I think they are 6in duct.

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You might consider a bilge blower. That's what I use. Do a search here for "glove box style spray booth".

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Has anyone built a spray booth using boat bilge fans?

They are made to vent gasoline fumes so wouldn't they

be explosion proof? The ones I am eyeing are 130 cfm

so for a decent sized booth you would need 2, but they are

priced reasonably on E-bay. Any thoughts?

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Here's a link to the pics of my booth. I'm using a bilge motor that pulls about 130 cfm and it works great.

http://smg.photobucket.com/user/baddogracing/library/Glove%20Box%20Paint%20Booth?sort=9&page=1

PS I got mine on eBay for about $20, including shipping.

Edited by Miatatom

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That was the only thing I was not sure, but I thought that you would need a 12 volt power supply for the bilge pump. Looks like in the photos by the author above, that he converted a computer power supply for that purpose.

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Looks like in the photos by the author above, that he converted a computer power supply for that purpose.

Luckily, my brother in law works on computers and had that one laying around and was nice enough to give it to me. You may be able to find one at a computer repair shop.

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Is there another way to vent the spray booth instead of using window if so can you explain how its done

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Google indoor clothes dryer vent. You will get some ideas, but it will not remove fumes from the paint.

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I have a charcoal filtered, paper filtered suction box, purchased from an air filtration company. The paper filter can be replaced by myself, but the charcoal portion has to be serviced by the company. It was designed for the purpose of cleansing the air of paint fumes and is a stand alone unit. I incorporated it into my paint booth. The purchase price at the time I bought it was $50, and the servicing of the charcoal filter is $15 I have it serviced every 6 months. Paper filters are changed every 2 paint jobs.

Edited by vypurr59

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Hello everyone,

I started airbrushing Gunze acrylics recently. I'm using a respirator in a not very well ventilated area and I sometimes feel dizzy and have a headache after spraying. My question is if this is because I'm not using any kind of glasses and gloves (I've heard that paint can get into your body through skin) or simply because there is only one window in the room. Another thing I was wondering is why even after a month my models still have a slight smell of paint.

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