CFM for paint booth

IceMasterIceMaster Posts: 1,333Member ✭✭✭✭
Looking at getting a better exhaust fan for my paint booth but not sure how big I'll need. Guessing, my paint booth is about 5' long x 3 1/2' wide x 6' high.

Right now I have 2 fans that I'm trying to decide between. Both are 2 speed fans, one runs 360/300 CFM and the other is 690/580 CFM. Now I was thinking to just go with the bigger cfm of the two but didn't want to end up putting something in that was too many cfm and then be back at square one. Getting more air INPUT isn't an issue, I just need to sort out the exhaust part of it.
Which fan would you recommend?



  • MidOhioHydrographicsMidOhioHydrographics Posts: 9,404Member, Moderator, Business Ninja El Moderator
    edited February 2016
    For OSHA requirements You want about 120 linear feet per minute across the user (for wearing a 1/2 face respirator, under that you need a supplied air respirator) . The depth of your booth doesn't matter for air flow. Just take your opening sq ft. which I think is 3.5 x 6 = 21sq ft, and then divide your CFM by that. So 360/21 = 17 or 690/21=35, so your larger fan is still pretty undersized. Install both and you're still undersized but better. 1050cfm/21 = 50ft/min
    About 2500cfm is what you ideally want.

    As a comparison, my booth opening is 10' x 7' = 70 sq ft and my fan draws 8750cfm, 8750/70=125 ft/min.
  • IceMasterIceMaster Posts: 1,333Member ✭✭✭✭
    What gets confusing is that every time you read something different regarding this, you get different answers and different equations. For instance, reading what I found here:

    To calculate the amount of exhaust air needed for the booth, a simple calculation is used.

    CFM = Booth Face Area in square feet multiplied by the required flow (typically 100 FPM)

    For example, and 8 X 10 filter bank (80 square feet) would require an exhaust of 8000 CFM (100 X 80) to achieve the required 100 FPM velocity.

    To calculate the existing velocity knowing the exhaust volume the following formula is used:

    Velocity = Exhaust in CFM divided by the booth filter area.

    For example, a fan that exhausts 9000 CFM with a 10 X 10-filter bank would have a velocity of 90 FPM.

    By using their calculations, the larger fan would be enough because my filter bank is 3' x 2' and the requirements here in Ontario are a minimum exhaust air velocity of 30 m/min. (98 feet per minute) So, 6 x 98 = 588 cfm

    I'll have to dig around more in my area, maybe talk to some auto paint places and see if they know exactly what's required for here
  • MidOhioHydrographicsMidOhioHydrographics Posts: 9,404Member, Moderator, Business Ninja El Moderator
    My best guess is that their calculations are for a professional built booth where the filter area is propertioned adequately to the booth opening. Remember what the booth is designed for: to protect the user from fumes. You want to calculate the amount of air pulling across the user to remove those fumes. If you had a single 30cm x 30cm filter in the back of the booth, and a 2m x 2m opening, their calculation would be grossly insufficient.
  • IceMasterIceMaster Posts: 1,333Member ✭✭✭✭
    ya, got to thinking about that more after I posted that and thought, that didn't make any sense at all. How could the fan CFM possibly be based solely on the size of the filter bank??
  • MidOhioHydrographicsMidOhioHydrographics Posts: 9,404Member, Moderator, Business Ninja El Moderator
    Any other openings, like conveyor openings, etc. are also calculated into the opening size, and the fan cfm must me increased accordingly. I actually just purchased a used booth w/ conveyor openings so I just became aware of this. We are closing the openings off, so the fan is actually over-sized for the application. Need to change the pulleys to make the flow correct. That I'll leave to the professionals. lol
  • IceMasterIceMaster Posts: 1,333Member ✭✭✭✭
    Well, going tomorrow to pick up a new exhaust fan for the paint booth. Found one that is a 3 speed and goes up to 3,000 cfm, so no matter what, I should be good with this one for the size of my booth. Thanks for your time and help @MidOhioHydrographics, really appreciate it!!
  • MidOhioHydrographicsMidOhioHydrographics Posts: 9,404Member, Moderator, Business Ninja El Moderator
    No problem! Gad to help where I can. I learned most of this from Trevor @WileECoyote. He's a paint equipment guru.
  • WileECoyoteWileECoyote Posts: 6,400Member, Moderator, Business Ninja El Moderator
    @MidOhioHydrographics thanks for the shout out! You are almost perfectly correct with your information (brings a tear to my eye) but the OSHA requirement is 110 cfm not 120 for a half mask respirator, and 60 for supplied air. Honestly, those flows are PLENTY for most of us (we have 80 or so in our booth and think it's just right). Keep in mind that using Aqualac (ding!) makes it a bit friendlier to your body because it's water based, so you can get away with cheating that a bit. The only downsides of TOO MUCH flow are amount of heat you pull out of the shop, transfer efficiency, and more air over the part means more dust... Eliminate those possibilities, and the sky is your limit. @IceMaster both of you guys good catch on the flaw in the logic from that binks info. 

    Joe, you can pick up a cheap velometer from Grainger or someplace online for testing flow in your booth (good preventative steps in case OSHA ever wants to poke around, they can check your years of recordings for employee safety). And then you can change the sheaves out yourself on that new booth. Motor rpm is typically 1760, fan size will not change, so the ratio is pully to pully size ratio. Let's say drive is 8" and driven is 10" for easy math, that makes it .80 for the ratio. So 1760 x .80 = 1480 rpm on the fan. If that gives you 110 through all the openings, then you close off the conveyors. (Do more math and figure out the percentage of area you are taking away, let's say 2 20sq ft openings and leaving an 640sq ft opening. 40/640= 0.0625)  Then you either subtract 6.25% from the drive. (8"x.0625=.5") So the drive goes to a 7.5" pully, or (10" x.0625=.625") the driven gets increased to a 10.625. 

    Or you get a velometer, and buy $5 sheaves till you get actual flow for actual conditions... 

  • OverCoatsOverCoats Posts: 350Member ✭✭✭
    @WileECoyote helped me with the sizing of the fan and filters for my booth and it works great!
  • IceMasterIceMaster Posts: 1,333Member ✭✭✭✭
    edited February 2016
    @WileECoyote - Not sure if you'll be able to help out with this but, the exhaust fan I just bought only says that it has 3,000 cfm but it's a 3 speed fan so I'm assuming the 3,000 cfm would be on high. Other then that, all it says is: SPEEDS - 1,100, 1,300 1,500 rpm.
    Based on the true measurements of my booth - 6.5' high x 4' wide x 6.3' long, and the back wall of my booth being a full filter bank, do you think this fan even on low speed (1,100 rpm) might be TOO much still? And, based on all this, what size would you recommend for a filter bank on the "incoming" air end of the booth?

    Just wondering if I should have went a little smaller fan
  • SingleAction52SingleAction52 Posts: 703Member, Business Ninja ✭✭✭
    @IceMaster, As I see it from what's been written above, you will need 2,860 cfm to achieve 110 feet per minute across the user. I believe @WileECoyote inadvertently typed 110cfm, (cubic feet per minute) instead of 110fpm, (feet per minute).

    If you go with 80fpm, you will need 2,080cfm.
  • IceMasterIceMaster Posts: 1,333Member ✭✭✭✭
    Well, this is one of the few times I'd say I'm lucky to be living in Ontario ;) Anyway, since we are only required to achieve 98 fpm, and high speed gives would give me 3000 cfm, I would think medium speed would be somewhere around 2600 cfm (rough guesstimate) which if I calculated right, should do the trick. 6.5' x 4' x 98 fpm = 2548

    Now, would I benefit from having the other end of the booth a complete filter bank for incoming air or cut that in half (top half only) to aid in exhausting?
  • SingleAction52SingleAction52 Posts: 703Member, Business Ninja ✭✭✭
    I'm not sure about the answer to your filter question but I am very interested as I'm also in the middle of completing a new spray booth. I was going to ask @WileECoyote, so I'll just wait and read his response to you.
  • IceMasterIceMaster Posts: 1,333Member ✭✭✭✭
    Guess my line of thinking is that if the entire back wall of my booth is a filter bank like the exhaust end, then it would cut down or eliminate any "dead" space within the booth.
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