Paint and ventilation/air flow.

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PostPosted: Mon Sep 04, 2017 7:59 pm
Is there a formula as to how many cfms of air intake and air exhaust a person needs nased on the size of the booth? I am thinking of making a temporary booth of 10x10 to paint small parts, fender, hood, etc. I know you generally want more air in than out, but is there a reference as to the ideal? Any advice is greatly appreciated.

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PostPosted: Tue Sep 05, 2017 8:09 am
Do a search for Homemade Paint Booth or similar and there is a lot of information.

Here are some figures I copied from a spec page for a commercial booth supplier.
Notice the velocity for a side draft is figured at 100 ft/min. and the downdraft at 50 ft/min. Either way you look at it the air is moving right along howbeit en masse.

Paint Booth Airflow Calculations

Scenario #1 - Air moving from ceiling at back of booth to the exhaust filters at far end of booth.
Based on these assumptions:
Surface area (ft2) = 4.4m (wide) x 2.2 m (high) x 10.76 (m2/ft2)
= 102.4 ft2
Air velocity = 102.4 ft2 x 100 ft/min

= 10,240 ft3/min
Scenario #2: Air entering at ceiling and moving downward toward the filters immediately above the floor along the sides of the booth.
Based on these assumptions:
Surface area (ft2) = 6.6m (long) x 4.4m (wide) x 10.76 (m2/ft2)
= 312.5 ft2
Air velocity = 312.5 ft2 x 50 ft/min

= 15,625 ft3/min
If you want uniform air distribution throughout the booth you should size the air intake filters accordingly. If you make the filter section too small, you can expect turbulence in the booth and this will cause paint overspray to settle on the walls, ceiling, lamps, etc. In addition, turbulence lowers transfer efficiency.
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PostPosted: Tue Sep 05, 2017 1:46 pm
How do I figure this when the only thing given on the fan is cfms? Like a box fan or hf ventilation fan?

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PostPosted: Wed Sep 06, 2017 11:31 am
Booth size.
Multiply the width x height to get Square footage.
Multiply this number by 100 to get the CFM requirement for a Cross draft Booth.

Example: 15 ft. wide x 10 ft. high = 150 square feet.

150 sq. ft. x 100 feet per minute = 15000 CFM

You will need a fan or fans that can produce 15000 Cubic Feet per Minute.
Your filter banks need to be of sufficient size to hand this flow as well.

Remember these are ideal flow rates, hence the high cost of paint booths.
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PostPosted: Thu Sep 07, 2017 9:17 am
'68 Coronet R/T wrote:...

Remember these are ideal flow rates, hence the high cost of paint booths.


The typical box fan only produces about 2000 cfm, but at very low pressure. In other words, they will bog down and produce less air flow if they have to push against very much back pressure at all (this back pressure comes from pressure drop across filters and any restricted ventilation cross sectional area).
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PostPosted: Thu Sep 07, 2017 10:24 am
You don't compress the air so what you displace you replace.

If all your doing is painting small parts, (hoods, fenders) you really don't need air movement, just a clean dust free area.

That being said, a small amount of air flow is not going to hurt anything just make sure your not introducing more dirt into the paint area and be careful of fumes and electrical motors!

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PostPosted: Thu Sep 07, 2017 12:59 pm
Marq wrote:You don't compress the air so what you displace you replace.
...


Fact 1. Air is a compressible fluid. Consequently, there will be a small degree of pressure development when using a feed fan arrangement... It (pressure) is very small, but it is there UNLESS you have BOTH a feed fan AND an exhaust fan. The pressure may exist only between the fan and filter (if the fan is in front of the filter), but it will be there. The only way there would be no pressure in the booth is if there is a stronger exhaust fan or the open area for exhaust air is so large that the pressure drop through the open area is virtually non-existent.

Fact 2. In the two fan scenario described above (both feed and exhaust fans), you will still have a slight amount of pressure unless the fans are perfectly matched. This scenario's pressure development can be either positive or negative, depending upon which fan is the strongest and how much pressure loss there is across the openings and the filters.

Verification of pressure development... What is your booth cover doing (assuming it is plastic, tarp, or something similar)? Is it billowing out, hanging perfectly flat, billowing around the bottom edges, or getting pulled into the booth space? Billowing outwards proves there is a positive pressure in the booth, and getting pulled into the booth space proves a negative pressure, or vacuum, inside the booth.

Obviously, if the booth is made from all hard surface materials, the proof lies in whether or not air is leaking into or out of the booth joints and seams while the fan(s) is(are) running.

It's just plain and simple physics!
Pete
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PostPosted: Fri Sep 08, 2017 9:37 am
I also have a large round floor fan that puts out about 13,000 cfm I think. It came from a factory but plugs into a regular outlet and is belt driven. I could use it to push air into a nank of furnace filters. Not sure how many it would require.
My origional intention was to keep the booth soze on small size and paint hood, fenders and doors off of the car. However the color my grandfather intended to paint the car was Jaguer metallic racing green, and I understand you cant paint a car this way???
This will be my first time painting a car....



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PostPosted: Fri Sep 08, 2017 9:48 am
Is it better to have both intake and exhaust fans? I have another rather powerful fan but it is not explosion proof. It is a factor fan used to ventilate tractor trailers when they are being loaded and unloaded from a shipping dock.



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PostPosted: Fri Sep 08, 2017 11:03 am
You guys are putting too much thought into this.

When I had my shop and was painting small parts I wouldn't even turn the fan on, wet the floor down and paint. Your in there for 1-2 minutes for a hood or fender.

You need some air movement when painting a car, mainly just to be able to see, but that's a lot more time!

Clean, dirt free area is more important that air flow.

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