Spray booth and activated carbon filter

Yep, it's a downdraft booth. When I started the project I planned for a cross draft booth but from the tips Gip gave I swithched to downdraft as the fan units would have a hard time with a cross draft booth.

What I did to get started and to get a feeling for the size I neede I built a booth out of cardboard. This way I could try diffrent sizes and location. I found out the a width of around 25in would be fine I went to the hardware store to have a look at the options foor perforated flooring to the booth. I found that the majority of options had a width of 60cm(23,6in). The biggest plane that fit in my booth is a 1/32 scale beaufighter, any bigger than that I will paint the old way with a respirator. Test your needs with a cardboard box, if you don't have the planes yet make som carboard models of them to see how they fit.

The bottom of the booth is built so that the hoses from the fans coms in 2in to each side of the centerline(in width). Then there is a 2,2in plenum. On top there is a metal screen, then a filter to catch the particles and then another metal screen. These all three are on top of each other kept in place by gravity. In my case the filter is not filtering any fumes, itjust catches particles. The booth is vented outside.

I measured mine with a professional meter that a friend borrowed from his workplace. The size of the front opening has a great influence on the air flow so I have now put a 2in wide cardboard on one of the sides of the opening to reduce the opening size a little. This way a have over 140fpm across the whole opening area. 

And remeber to do the calculation for the price, if you need to get everything new the price can be high, I think the total was around 250-300dollar. But it was fun to build and I got a both that fits my needs perfectly. And there are no commercial units available for a reasonable cost where I live so it was the only options.

I can post some detail photos if you need.

And thaks to Gip for the help.

Michael

So this is basically a "Downdraft" designed spraybooth?  I need to build one myself so I can get back to building my aircraft models but am at a loss as to how to vent the fumes and exhaust from the booth.  I was thinking furnace filters mounted under the screen floor of the booth with a couple fans extracting overspray and fumes, but I'm unsure of the size of the fans as I don't want to suck all the paint out of the booth as soon as it leaves the airbrush. The booth needs to be big enough to handle 1/72nd aircraft like C-130's, KC-135's, and so forth as well. I want to get to work on this project in the next few days so any help is appreciated.

Now the spray booth is done, thanks everybody for the comments. I had a friend over to measure the airflow and it looks good. The value at the front opening was between 52 and 110 fpm depending on speed setting on the fans and if I was in the opening as well(partially blocking off the area). The size is 24" wide, 16,5" deep and 17,5" high. The front clear plastic can be opened. The height of the front opening with the clear plastic lowered is 11".

The model in the booth is a 1/48 scale meteor.

The hose from the bottom of the booth to the fan is 3inch in diameter. The fans are connected to the same box and from there it is a 4,4inch hose to the outside. The fan units are connected in series and powered by an adjustable power supply drawing around 1,7A @ 25V 

 Havent sprayed in it yet but will have a go tomorrow, just thinking on putting some papper on the inside walls to protect the wood and it would be easy to replace when to much overspray has coated them.

Michael

smoffo wrote:

Making the plenum 2inches high would be enaugh? Michael

Hi, Michael. 

Glad to be of help, my friend.

As far as the 2-inch plenum, I need to do a little research tomorrow at work (my ventilation workbook with all the tables and drawings are there), and I'll try to get back to you on that one.  Thanks for your patience.

Gip

Gip, thanks for the calculations.

I just made a cardboard mockup of the booth so I could verify that my measurements where ok for working in it. The size is 2ft*1,3ft(width*depth) and the front opening is just a percent larger. I'm going for a down draft version.

When calulating whit these values I got a 130cfm for the fan, ~160cfm when added 25%. Using two fans of ~85cfm each it would be aroun 170cfm. That should be enaugh, I will also reserv a place for a third fan.

Making the plenum 2inches high would be enaugh?

Have to get som plywood on monday to start building the box.

Michael

smoffo wrote:

Thanks for the comments, going to scrap the idea of using the respirator cartridges. Just tested on of the fans(have two identical) and it seems to be moving air nicely. Specs says 2400litres/min. They are deigned for extracting gasoline vapor, so they should not ignite any vapor from airbrushing. For the next three to four months I can ventilate outdoors. I have the possibilty to adjust the speed of the fans as they are connected to a variable voltage supply. The problem that is the biggest is how to measure the air speed at the front of the booth? There are those small windspeed meters but I'm not sure if they are any good at this low speeds. For the filtering I have to do some thinking. Michael

Michael,

Here in the U.S., 2400 liters/min is the same as ~85cubic feet per minute (cfm), not nearly enough to power your booth. 

Let's say you want to make a booth with a face size of 2 feet X 2 feet.  If you calculate the face area (2ft X 2ft = 4 square feet) and multiply that by a 100 fpm face velocity (recommended minimum), you come up with 400 cfm, the minimum needed to power a spray booth that size. Once you figure in filtration, duct diameter and length, any baffles, etc., you're faced with a bit of velocity loss.  Depending on the variables, bumping up the cfm rating by as much as 25% will put you in the ballpark for correct fan performance.

Here are the metric conversions:

4 square feet = 0.37 square meters or 3716 square centimeters.

100 fpm = 30.48 meters per minute or 3048 centimeters per minute.

So, minimum face velocity = ~11,327 liters/minute  (5 times your fan's capacity).

Now, if you were to construct a downdraft booth, you could almost halve the velocity recommendations (50 fpm instead of 100 fpm face velocity = 200 cfm for the same 2 X 2 booth), and you could possibly consider the use of an activated charcoal filter for that setup since velocity is slower.  There would be some design differences, such as a plenum to even out the airflow underneath, and a perforated surface to spray on, but you could use a smaller (less expensive) fan.

By the way, those small portable vaneometers (airflow meters) work pretty reliably for checking airflow.

Gip Winecoff

Thanks for the comments, going to scrap the idea of using the respirator cartridges. Just tested on of the fans(have two identical) and it seems to be moving air nicely. Specs says 2400litres/min. They are deigned for extracting gasoline vapor, so they should not ignite any vapor from airbrushing. For the next three to four months I can ventilate outdoors.

I have the possibilty to adjust the speed of the fans as they are connected to a variable voltage supply. The problem that is the biggest is how to measure the air speed at the front of the booth? There are those small windspeed meters but I'm not sure if they are any good at this low speeds.

For the filtering I have to do some thinking.

Michael

One thing to consider is the velocity of the air in relation to the filter.  If air velocity is too great, the organic vapor fraction will simply blow past the carbon filter without allowing ample time for adsorption to take place.  Respirator cartridges take that into account in their overall design, and therefore should not be used in spraybooth applications, IMO.

The Artograph downdraft booths are moving air at somewhere around 50fpm, and are equipped with an activated charcoal filter that performs quite well at that velocity; however, that same filter may not perform adequately in a back drafted-style spray booth where minimum face velocities need to be about 100fpm.

Also keep in mind that as air enters a smaller area, velocity increases.  If you are pulling air at 100fpm at the face of a hood, airflow inside the duct is going to be significantly greater than that---not a good place to put a charcoal filter.

Hope this helps some.

Gip Winecoff

The problem with buying one is that as I live in Finland we have a 230V system, I have talked to many vendors in the US but they will not sell me one because the spray booths are not CE marked.

The only one I found here in europe is around 900USD so it's a little bit expensive.

If someone knows of a retailer online in europe that sells spraybooths then I could consider that as an option.

If I build my own I'm thinkin on using activated carbon filter from respirators, the manual says to change every three months.

Have to test a fan tonight to see howmuch air it moves.

Michael

Will the activated carbon filter clean the air sufficiently to vent it back into the room?

If you get one of the commercial units designed for this, and change the filter when you should, the answer is a qualified "yes". The qualifications are that there are some things it won't take out, and it can be difficult to know when to change the filters.

I'm also considering to use a kitchen hood as it has an activated carbon filter before the motor, this should prevent the vapors from reaching the motor?

No. These filters are primarily intended for cooking vapors, and do not have the capacity to deal with larger quantities of highly volatile organics.

This is NOT the sort of thing you should home-build unless you are an industrial hygenist and know how to do ALL the math, not just the flow rates! Both Bear-air and Dixie Art have such units for very reasonable prices, last I looked.