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PAINT BOOTH EXHAUST DESIGN CONSIDERATIONS

By Klaus Raddatz

This is an updated version of the fan selection guidelines posted on the Hobby Heaven Message Board last year. Primarily, the SP values have dropped a bit due to a change in criteria for the calculations. I made a few other changes too that will, hopefully, make it easier to follow.

Spray Booth Design and Fan Selection

This is not intended to be the final word on spray booth design, fan selection, or a criticism of anyone's spray booth, but to share what I know about paint application with respect to booth design, and provide a guide to take some of the experimentation out of building your own spray booth.

When it comes to fan selection, there are usually concerns about drawing too much air into the booth and contaminating your paint with dust/dirt. In determining the airflow through your booth keep in mind where your spray booth will be used, the type of spray equipment that you’ll be using, and the ambient condition. The idea is to exhaust the paint fumes without changing the barometric pressure in your workshop.

Two things to consider before selecting a fan for your spray booth are spray booth size and the duct diameter and length.

Spray Booth

Booth size and proportion is really dependent upon need. Make sure it will accommodate your largest part and you can comfortably paint within those confines. If you’re not sure about proportions, build a mock-up out of cardboard first and try it. Then transfer those dimensions to whatever material you’re going to build your booth out of. Something to keep in mind when you’re deciding on size: the larger the booth the more fan you’ll need to exhaust it, and fan prices go up with CFM and SP ratings. Also, consider adding a plenum chamber between the fans and the filters. Production type spray booths use plenums to even out the airflow across the filters and reduce the windiness inside the booth.

Another consideration is the direction of draft. Most spray booths are either a cross-draft type or down-draft type. The difference is that a cross-draft booth pulls air across the booth into filters located directly opposite the booth opening, and a down-draft booth pulls air downward into filters located in the bottom of the booth. Cross-draft booths are easier to build, but draw all of the air across the part. Down-draft booths tend to pull less dirt into the paint because the air is pulled downward as soon as it enters the booth (the design of the booth opening also has an effect on how well this works). The downside is they are a bit more difficult to build. Otherwise there’s not much difference.

As for venting to the sides or top of the booth, it's typically not done that way because the draft pulls the atomized paint away from the part. To correct for this effect you may have to spray with higher pressures to force the paint against the part, and that can lead to a number of paint finish problems. This is compounded by the limited number of adjustments on siphon feed airbrushes versus pressure feed spray guns, and limited solvents and reducers available for hobby paints.

Fan placement on 1:1 booths is usually not a consideration with respect to dust control. In many cases they have an air make-up system that replaces the exhaust air with filtered/temperature controlled air, especially where decorative finishes are concerned.

My spray booth is an 18” tall, 24” wide, 18” deep cross-draft booth. The top of the booth is set back about 6” to provide clearance for overhead spraying, and I’ve added a 2” plenum chamber to even out the airflow. An added plus of the plenum is that if I find I’ve installed too much fan, I can vent the plenum chamber to allow the fan to draw in outside air, thus reducing the airflow through the booth. I've also added a door, so I can close the booth to prevent dust from settling in the paint.

Air Flow Requirements

Once the spray booth size is determined, calculations for fan requirements can begin starting with air movement through the booth, known as face velocity. Current industry standards indicate that the face velocity should be between 100 and 150 feet per minute (FPM) for most applications. Some applications require more or less velocity depending on the type and quantity of material being sprayed, or the applicator being used. Airbrushes and rattle cans deliver much less volume than full size spray guns, so face velocity can probably be reduced if that’s all you’re going to use. My booth has a face velocity of about 75 FPM and seems to work well, although a bit more fan would be better.

To calculate the cubic feet per minute (CFM) of air required to produce the face velocity, multiply the inside height of the booth by the inside width by the face velocity desired. Depth is not a consideration.

For example, the inside height and width of my booth is 18” x 24”. To calculate for industry standards, multiply the height and width by 100 FPM and 150 FPM:

1.5’ x 2’ x 100 FPM = 300 CFM, and

1.5’ x 2’ x 150 FPM = 450 CFM.

To meet industry standards, I need a fan capable of moving between 300 and 450 CFM of air.

For my booth: 1.5’ x 2’ x 75 FPM = 225 CFM.

But don’t buy a fan yet, read on…

Ductwork

Figure out your exhaust duct routing to determine the length of the straight sections and how many 90 and 45-degree elbows you’ll have. Measure the straight sections, and refer to the Elbow to Straight Duct Conversion below to convert the elbows to straight duct, and then add it all up.

My system has 4’ of 4” straight duct, one 4” 90-degree elbow, and one 4” 45-degree elbow. The elbow to straight duct conversion for a 4” 90-degree elbow is 6’, and a 4” 45-degree elbow is 3’. Add up the duct lengths, 4’ + 6’ + 3’ = 13’, and I find my system has the equivalent of 13’ of straight duct.

Elbow to Straight Duct Conversion:
3” x 90 elbow = 5’
3” x 45 elbow = 2.5’
4” x 90 elbow = 6’
4” x 45 elbow = 3’

Static Pressure

Next, determine the static pressure in the ductwork. Static pressure (SP) is the resistance to air movement in the ducts, and is important in choosing a fan. The fan you choose must be able to deliver the required CFM at the static pressure level inherent to your system.

Refer to the Air Velocities section below and find your duct diameter and the CFM closest to the face velocity determined above. Note the velocity and go to the Static Pressure section. Find your duct diameter and velocity and note static pressure value. The static pressure value shown is for 100’ of straight aluminum duct. To calculate the static pressure for the duct length of your system, multiply 1/100 of your duct length by the static pressure value.

For example, to meet industry standards, we need to calculate for 300 and 450 CFM airflow. Multiply 1/100 of the duct length (0.13 in my system) by the SP value for the duct diameter and velocity:

300 CFM air flow:

4” x 300 CFM = 3500 FPM
4” x 3500 FPM = 4.37” SP
0.13 x 4.37 = 0.57” SP

At minimum, I need a fan capable of delivering 300 CFM @ 0.57” SP.

450 CFM airflow:

4” x 440 CFM = 5000 FPM
4” x 5000 FPM = 8.37” SP
0.13 x 8.37 = 1.09” SP

At maximum, I need a fan capable of delivering 450 CFM @ about 1.09” SP

In my system I use a fan capable of delivering 225 CFM @ about 0.30” SP
(0.13 x 2.31 = 0.30)

Also, if you’re using flexible metal duct multiply the SP values by 3. Flex duct is very restrictive.

I didn’t include losses through the filters because it varies with filter media. But furnace filters have minimal pressure loss when they’re clean.

Air Velocities:
3” x 100 CFM = 2000 FPM
3” x 120 CFM = 2500 FPM
3” x 150 CFM = 3000 FPM
3” x 170 CFM = 3500 FPM
3” x 195 CFM = 4000 FPM
3” x 220 CFM = 4500 FPM
4” x 150 CFM = 1700 FPM
4” x 175 CFM = 2000 FPM
4” x 220 CFM = 2500 FPM
4” x 260 CFM = 3000 FPM
4” x 300 CFM = 3500 FPM
4” x 350 CFM = 4000 FPM
4” x 390 CFM = 4500 FPM
4” x 440 CFM = 5000 FPM
4” x 475 CFM = 5500 FPM

Static Pressure in 100’ of Straight Duct:
3” x 2000 FPM = 2.18” SP
3” x 2500 FPM = 3.36” SP
3” x 3000 FPM = 4.58” SP
3” x 3500 FPM = 6.19” SP
3” x 4000 FPM = 7.91” SP
3” x 4500 FPM = 9.84” SP
4” x 1700 FPM = 1.14” SP
4” x 2000 FPM = 1.54” SP
4” x 2500 FPM = 2.31” SP
4” x 3000 FPM = 3.27” SP
4” x 3500 FPM = 4.37” SP
4” x 4000 FPM = 5.54” SP
4” x 4500 FPM = 6.94” SP
4” x 5000 FPM = 8.37” SP
4” x 5500 FPM = 10.12” SP

So, how critical is the SP to fan selection? Here’s an example from a catalog to give you some idea of how much the air volume is reduced as the static pressure increases. The free air (0” SP) rating of this particular fan is 320 CFM. At .5” SP that dropped to 50 CFM. In my system that would produce a face velocity of about 17 FPM. Quite a bit less than the 75 FPM I’m currently using, and a pretty safe bet that my workshop will smell like paint. Not all fans will lose this much volume, but without doing these calculations you’ll never know. Also, keep in mind that many fans are rated only in free air. If a SP value isn’t given, be careful. Contact the manufacturer to see if a SP rating is available. Some manufacturers have this information posted on their web sites.

Motors/Fans/Lights

Now, what type of fan? Bathroom, kitchen, induction motor, inside the booth, outside the booth…? You typically won’t find an electric motor in the air stream of a production type spray booth, unless it’s an explosion proof motor. An explosion proof motor is certified as such by one of several industry recognized certifying agencies. None of the previously mentioned fans are explosion proof. Kitchen and bathroom fans are probably the most critical because they typically have exposed stator windings. Solvents may deteriorate the varnish on these windings and cause the motor to short circuit. Exercise caution if you're using this type of fan.

Also, keep in mind that voltage and current levels inside a spray booth have to be kept below non-sparking levels unless the components are certified as explosion proof. Non-sparking voltage and current levels are, if I recall correctly, about 16 volts and 50 mA, way below the 120 volts and several amps that many small fans use. Ultimately, the best type of fan to use is one that keeps the motor out of the air stream.

There are several booths available that use computer type axial fans. These have induction motors located in the air stream, and while they’re not explosion proof, their design makes them a better choice than bathroom or kitchen fans. I’ve taken a few of these fans apart and found the stator windings embedded in epoxy. Epoxy typically has a high resistance to solvents, so I feel pretty comfortable that the solvents won’t migrate into the stator windings and deteriorate the insulation.

When it comes to lights, fluorescent are preferred. They run cooler, use less energy, and tend to be more color correct than incandescent lights. Lights should be mounted outside of the booth for the same reasons that apply to motors – voltage and current. Cut a hole in the booth, install a piece a plexi-glass, and mount the light over the top of that.

Last thing, if your booth is metal or plastic; make sure it has a proper electrical ground. I spent a number of years in robotic spray finishing, and I can tell you first hand that electricity, whether it’s AC, DC, or static, doesn’t mix well with paint, unless of course you’re painting with electrostatics. But that’s another story.

That’s it. I hope you find this useful. If you have any questions or need more specific information, feel free to e-mail me.

Klaus