sizing exhaust fan for auto body shop

Paul S_3

hey everyone, I recently got called to a auto body shop by one of my customers. a previous company installed this exhaust fan made by "Jen co fan" the manufacture told me it can move up to 12,000 cfm's. the problem is when they are painting the cars the exhaust fan does not move enough air and the paint spray stays in the shop. the rotation is going the right way, I checked amperage to motor and its within range. ive never sized a exhaust fan before can this be undersized? spray room is 25' x 25' x15'. any advice would be appreciated thank you Paul S......p.s ive read something on the internet that said if you take the total cubic feet of the room , the fan cfm should be 3 times that? I don't know how accurate that is

bmwpowere36m3

Can't comment on sizing… unless its a dedicated paint booth with downdraft setup. My father paints cars in an addition he built to his 2 bay shop (roughly 25x40x15). However it only has two small (maybe 16") fans that he uses to "ventilate" the area.

That said, after spraying… there's "mist" or paint in the air for a while till it clears. Not an issue with finish… more so the random speck of dust that needs to be wet-sanded and buffed out.

Otherwise, he "borrows" time at a decided booth down the street that has a downdraft setup, filtered and heated.

rick in Alaska

If that is the return air above the fan, then there is no way that I can see it can work. The air will just cycle right from the return air into the fan, and not pull in any fumes. Return air needs to be across the room to work right.
Rick

Paul S_3

No that is the blower motor housing above the fan....the return is below it where u see the blower wheel.....the supply is ducted through roof to a "gooseneck"

Pumpguy

Back in the early 90's we built a 9000 sq ft pump repair shop. As part of that, we included a make-up air heater to replace air exhausted when using the paint booth.

IIRC, the make-up air heater had 14,000 CFM capacity. Unfortunately, I don't have any of the sizing figures. I would expect it would be related to the booth's exhaust fan capacity.

One nice thing about this was if it was cold outside and we had the overhead doors open for any length of time, we would turn on the make-up air heater to keep from loosing ambient warm shop air.

Tinman

You have 9375 cubic feet of air in the room and your fan is capable of moving up to 12,000 cubic feet of air per minute. Is it too small? Maybe. It depends on what static pressure it's up against. If it's really moving that much air (check with a velometer), I'd say it's not too small.

Tinman

More than an air change per minute is a boat load of ventilation.

Tinman

What kind of fan is it? Tube axial?

ratio

You'll have to talk to an engineer about the exhaust air requirements, or maybe an Old Guy who's been doing it for years, but as for working, assuming it's sized right, there are a few things to check.

Verify that is wired for the right voltage, belt is tight. Others mentioned the round pointing towards the suck side of the fan, what is that for? Maybe you can cap it off or pipe it to the other side of the room, down low for cross ventilation. There's no way it's flowing 12000 cfm, but it can't be helping you suck air outta the space.

Check the rating of the fan, it might only be 12000 cfm at a static pressure of 0 in wc, that is, with no duct on either side. The mfr should have a table or a graph showing cfm vs static pressure, take a reading on the outlet duct & see where you line up in the table/graph. You could traverse the outlet duct & cypher up the actual volume of air moving, but that's tedious at best.

After that, you need to make sure you have 12000 (or whatever) cfm of make up air coming into the space. Being as it's a painting booth, you probably want to run it slightly negative to keep the fumes from entering the rest of the building.

RJ_4

I am concerned that the motor on this fan is in a possible explosive atmosphere, what is the code in your area. Most installs I have seen use a utility style exhaust fan set on the roof.
Working with what you have I would get a fan curve from Jen co and start making some readings with a inclined manometer, you have to take in consideration that screen on the fan intake. I remember the old ww Grainger catalogs had usefull sizing info in the back. Is there a makeup air fan to go with this install. Last but not least get Jenco involved on sight asap

KC_Jones

Has anyone even thought about EPA regulations on this install? A very close friend of mine has been spray painting for 20+ years and every shop he ever worked in used an EPA certified spray booth. I think you can only be exempt from SOME regulations is if you spray a small amount per day. But as far as I know ANY business spraying paint must apply for EPA permits/certs. I don't even see any filter banks in those pictures? Are they just dumping the paint fumes into the air? Also as RJ mentioned is that an explosion proof motor?

Tinman

I just looked at the specs of that fan. Roughly 8000 cfm @ 1"esp

Tinman

And, it's definitely not explosion proof or approved for hazardous location.

JUGHNE

Not only the motor but the fan/belt itself. Also lights are required to be at least explosive resistive. Most body shops buy a pre-fab paint booth that contains the correct fan and lights. Have doors with filters installed to maintain clean air flow. The heated air of the rest of the shop flows thru the door filters and out thru the exhaust. For one I had to add an electric solenoid valve on the air line several feet from the door, fan had an interlock that the fan had to be running before air was supplied to the paint gun. NEC and probably OSHA gets fairly involved for paint booth and the factory approved prefab unit was the simplest way to go if it required inspection.

BobC

I had to retrofit a paint booth years ago to get OSHA off our backs. If you are going to do this make sure you read the rules that govern paint booth safety.

Once you touch it you may own it.

Bob

Paul S_3

I checked the rotation etc all checks out....i spoke to manufacture they told me the most i can get is about 7100 cfms ....they also told me this fan isnt designed for a automotive spray area...im in NYC ive never dealt with a autobody exhaust fan so i dont know the code around here....just by looking at it it doesnt seem right...yes they are just exhausting fumes into outside air...there is no actual booth its just a room with sheetrock walls and wooden sliding doors seperating the paint area from the rest of shop..i dont think its a tube axial fan....is a tube axial fan more like a propeller than a squirrel cage?...the fan is more like a squirrel cage blower....thanks Paul s.

Brewbeer

@KC_Jones is probably correct, the booth is likely not compliant with environmental regulations.