air flow in paint room

[55chevylover]

I think it was Len that posted about how many times the air is turned in a paint room. Could you tell me how I would calculate this Thanks

[Jeffw5555]

I'm just building a new booth, and just researched this very thing. There are actually a lot of laws in various areas that specify the amount of air flow in paint booths, (for commercial businesses, obviously, that don't necessarily apply for hobbyists in their own homebrew booths) that give optimum air flows in Feet per Minute, rather than the number of air changes.

Most commercial booths have flows in the range of 50-100 FPM, with downdraft booths getting better results with the 50 FPM, and crossdraft booths needing the higher 100 FPM.

How to calculate? Well, for a downdraft booth with 50 FPM, multiply the width X Length X 50 = cubic ft/minute fan flow. Example: The downdraft booth I'm building is 13 1/2 feet wide, 20 feet long, so 13.5 X 20 X 50 = 13,500 CFM flow. For this amount of flow, I would need a 34 inch tubeaxial fan with 5 hp to drive it. (I have a 24 inch tubeaxial fan with a 3 hp motor; it will flow about 10,000 CFM, which should be OK)

For a crossdraft booth that is 14 ft wide by 9 ft tall, the ideal flow would be 14 X 9 X 100 = 12,600 cfm.

Jeff

[Pot]

I'm just building a new booth, and just researched this very thing. There are actually a lot of laws in various areas that specify the amount of air flow in paint booths, (for commercial businesses, obviously, that don't necessarily apply for hobbyists in their own homebrew booths) that give optimum air flows in Feet per Minute, rather than the number of air changes.

Most commercial booths have flows in the range of 50-100 FPM, with downdraft booths getting better results with the 50 FPM, and crossdraft booths needing the higher 100 FPM.

How to calculate? Well, for a downdraft booth with 50 FPM, multiply the width X Length X 50 = cubic ft/minute fan flow. Example: The downdraft booth I'm building is 13 1/2 feet wide, 20 feet long, so 13.5 X 20 X 50 = 13,500 CFM flow. For this amount of flow, I would need a 34 inch tubeaxial fan with 5 hp to drive it. (I have a 24 inch tubeaxial fan with a 3 hp motor; it will flow about 10,000 CFM, which should be OK)

For a crossdraft booth that is 14 ft wide by 9 ft tall, the ideal flow would be 14 X 9 X 100 = 12,600 cfm.

Jeff


Jeff, you've obviously done some research so you may know exactly what you want without my opinion. If it isn't too late, you may want to consider extending your booth to 24' length. I think you will find 20' to be quite short for many mid-size and larger vehicles. Ask me how I know. :)

[Stan (in NC)]

:) I totally agree with Pot: My shop, which is also my booth is 24' deep and I can't get a full size pickup in there and spray around the front and back with enough room at each end.

[Jeffw5555]

I agree with Pot & Stan; 24' would be a better length as 20' is pretty tight. Problem is that I have a beam in the way that somewhat forces me to the 20 foot length. Maybe I will have to build around it somehow.

Thanks for the input.

Jeff

[paintnbcool]

Mine is 26' long and gives plenty of room

[bravenrace]

For clarification FPM (Feet Per Minute) is air velocity, like in MPH. Units for air flow would be like CFM (Cubic Feet Per Minute). Velocity refers the the speed of the air and flow refers to the volume of air.
A more direct way of calculating air flow would be to multiply the velocity by the area perpendicular to the direction of flow. For simplicity, if you were pulling the air lengthwise through a rectanglular shaped booth that is 12 feet wide by 10 feet high by 20 feet long, at 50 FPM, the calculation would be


Area = 10 x 12 = 120 square feet
Flow = 120 sq feet x 50 ft/min = 1000 CFM (Edit - That's incorrect, it should be 6000 CFM)

But, this is only if the velocity in the booth averages 50 FPM. If that is the velocity through the fan, then the area used to calcuate the flow would be the area of the fan, not the booth. In other words, you have to use the air velocity at the point where you want to know the flow in order to get a correct flow number. The reason for this is that the velocity can change through the booth but the volume (flow) doesn't.

So a fan that is say 36" in diameter and moving the air at 50 FPM would have a flow of:

Area = Pi x radius squared = 3.14 x 1.5 feet squared = 3.14 x 2.25 = 7 SQ FT
Flow = 7 SQ FT x 50 FPM = 353 CFM.

So as you can see, the velocity is the same, but the airflow is not, and this is because the area is vastly different.

[Len]

You'll find that you don't need a lot of air flow through the room as long as you don't cause a fire hazard and you have decent breathing apparatus. In a shop that is not designed for spraying paint there are usually a lot of places for dust to settle and it's picked up by air turbulence and deposited on wet paint. With less air flow you have less chance of dust nibs in the final finish.

If you have a strong air flow in a garage you should try to have the air being pushed in rather than pulled out. The positive pressure will help stop dust from coming in every nook and cranny and the paint will remain much cleaner.

The same goes for breathing apparatus... the positive pressure systems work much better than the negative pressure masks and are much safer when using paints with isos in a closed room.


http://autobodystore.net/Merchant2/graphics/00000001/hobbyair2.jpg
Hobbyair Breathing System Link (http://autobodystore.net/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=HP)

[Jeffw5555]

snip.. For simplicity, if you were pulling the air lengthwise through a rectanglular shaped booth that is 12 feet wide by 10 feet high by 20 feet long, at 50 FPM, the calculation would be

Area = 10 x 12 = 120 square feet
Flow = 120 sq feet x 50 ft/min = 1000 CFM

end snip.

bravenrace, your math is a little off, should be 6,000 CFM. ;)

In order to meet "industry standards" for the above booth example, it should be 100 FPM, which is 12,000 CFM fan size.

I agree that you should be careful about the distinction between FPM and CFM. In my research, seems that most commercial booths have fans in the 8,000 to 15,000 CFM range. I figure that if I am going to the trouble of building the booth, I might as well do it right from a ventilation standpoint.
My last homebrew booth had two 24" window fans and was totally inadequate for ventilation. I'd spray a section, and have to wait until the fog cleared to do the next. It was a crossdraft design. My new homebrew will be downdraft.

Also, I am talking about using a tubeaxial fan, which is a totally different animal that a standard window or exhaust fan.

I'm trying to decide at the moment whether to raise the floor, or jackhammer out the floor for the outlet vent raceway.....

Jeff

[bravenrace]

Right you are, Jeff, but I'm going to blame it on a calcuator malfunction ! :)

[John Salzer]

After reading the above, I am somewhat at sea about airflow in a temporary booth. I am going to set up a 10' wide x 20' long x about 9' high booth in the driveway to paint my roadster, and my calculations have been based on one air change per minute. The math would indicate about 2000 cfm.

I've scrounged four ex-AC air handler blowers rated at about 1000 cfm each, so I can play with it... Jeff's math would indicate about 90x10x50=4500 cfm for 50 FPM (high filtered input in one end of the booth, low filtered output at the other end plus leakage-it is a Sam's Club fabric temporary garage).

Does anyone have any real world experience with this kind of setup? I'd be interested in your opinion.

Thanks.

[bravenrace]

After reading the above, I am somewhat at sea about airflow in a temporary booth. I am going to set up a 10' wide x 20' long x about 9' high booth in the driveway to paint my roadster, and my calculations have been based on one air change per minute. The math would indicate about 2000 cfm.

I've scrounged four ex-AC air handler blowers rated at about 1000 cfm each, so I can play with it... Jeff's math would indicate about 90x10x50=4500 cfm for 50 FPM (high filtered input in one end of the booth, low filtered output at the other end plus leakage-it is a Sam's Club fabric temporary garage).

Does anyone have any real world experience with this kind of setup? I'd be interested in your opinion.

Thanks.

Your booth will be 1800 CU FT (10 feet x 9 feet x 20 feet = 1800 CU FT). If you want to change the air every minute you need a fan that will produce 1800 CFM. Keep in mind that the CFM ratings for fans are in free air, so airflow will be reduced when restrictions such as filters are present.