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How to Know CFM for a Paint Booth & Exhaust Fan Size

Why Paint Booth CFM is Important

Paint booths are designed to meet a minimum air flow requirement to be compliant with national codes that govern spray booths. Important codes that paint booths comply with include National Fire Code 1, NFPA 33, OSHA, and a few others. For a good overview of all paint booth code compliance check out this article. The important thing is that to meet code compliance a paint booth is required to exhaust an appropriate amount of air. The amount of air a paint booth exhausts will depend on the style of paint booth. The three main styles of paint booths are cross draft, down draft, and side down draft paint booths. Their air flow requirements are summarized below.

  • In the past NFPA dictated paint booth air flow needed to be 100 FPM for non electric static coating application, downdrafts were usually between 50 – 100 FPM. Revisions to the code now dictate that the exhaust must be sufficient that solvent concentration is below 25% of the lower flammable limit.  To properly measure the solvent concentration requires expensive tools and so most of the time air flow rates will help to know if your booth is running approximately how it should be, to be safe consult with the booth manufacturer to know you have a proper sized paint booth fan replacement in mind.
  • If you do not have proper air flow it is a safety hazard, it can effect the finish quality of your product, and can cause your painters to have trouble working in the paint booth due to visibility problems

How to Know about the CFM Your Paint Booth Should Be exhausting

Cross Draft Paint Booths

For a Crossdraft paint booth the booth should remove approximately 100 FPM across the face opening of the paint booth plus any additional openings should be included. SO you take the width and height of the paint booth x 100 FPM and will provide the exhaust speed in Cubit Feet a minute. Lets take two quick examples to best understand the math

  • Crossdraft Paint Booth is 6 feet wide x 8 feet high your air flow will be 6 feet x 8 feet x 100 FPM = 4,800 CFM per minute
  • You have a Crossdraft Paint Booth with a 6 feet by 8 feet opening with a 4 feet by 4 feet crane opening you take the face opening 6 * 8 + (4 *4 ) then take this x 100 = 6,400 CFM

 

Down Draft Paint Booths

A Downdraft Paint Booth will remove approximately 50 FPM across the floor print of the paint booth. Let's take the following for an example

  • You have a downdraft paint booth that is 30 feet long x 14 feet wide this means the booth will remove approximately 21,000 CFM (30 x 14 x 50)

Side Down Draft Paint Booth

Side downdraft paint booths work on the same speeds necessary as downdraft paint booths. However, a side down draft paint booth will often use a few smaller paint booth fans. This makes side downdraft paint booths often require each fan size to know what is a good replacement fan.

Special Situations (Exhaust Chambers )

The majority of regular paint booths will have a Totally Enclosed Paint Booth fan. If you are using an exhaust chamber rather than the full spray booth fan it is important to remember that it is most likely not a spray booth, you will want to make sure to coordinate with your local authority to ensure all proper safety processes are in place. Additionally, it can be good to consider an explosion proof fan motor as this will reduce the chance of your fan causing a spark. Unlike a regular paint booth their is a little more chance that the fan could be a sparking source.

Paint Booth Fan Types & There CFM

  • Totally Enclosed – A totally enclosed fan is the typical tube axial fan that most paint booth utilizes it prevents air from outside of the fan to flow across and motor repair which reduces the chance that solvent laden air would be ignited by the fan
  • Explosion Proof – An explosion proof fan will have designed features that reduce the risk of a spark forming
  • Variable Pitch Sheave Design – A Variable Pitch Fan allows for adjusting of the fans speed which can allow for adjustment of the fans speed.  This is a great option for a fan replacement as it will allow for balancing of an older paint booth by adjusting the air flow
  • Each fan will have a varying CFM rating and you will want the fan to be rated for an appropriate static resistance based on the length of duct you have, elbows in the duct, and similar factors. If you need help with knowing the right size fan its best to speak to the provider of the booth to ensure it is appropriate. 
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