Spray Booth Ventilation, Back Pressure?

A fan that works well mounted in a window has trouble moving air through a 50-foot duct with three bends. December 9, 2010

I just installed a 50' run of 18"D clear PVC tubing to ventilate my spray booth. I have a great industrial fan that has done amazing for my last two booths when window mounted. In this case, the fan seems to be unable to accomplish full speed and the suction on the booth end of the line is insufficient. Is this a resistance to flow/back pressure situation? Is there anything I can do to get the fan up to full speed?

Forum Responses
(Finishing Forum)
From contributor C:
What diameter is your fan? Sounds like your pipe is too small. Also, I do not think PVC would meet code. Also creates a lot of static.

From the original questioner:
My fan is 26"D and it's super powerful. No info on the motor housing as to hp. We only spray waterborne finishes, so I think the PVC will work. The difference in fan performance is astoundingly better when we disconnect the duct. It just can't get going when pulling through such a long distance.

From contributor R:
How airtight is your shop? Is there enough make-up air available? Does the exhaust fan flow better if a door is open on the other side of the shop?

From the original questioner:
There isn't even a door on the spray booth yet. I've opened every door and window possible and no improvement. I even put a box fan on the booth side trying to push air through the duct work to help out the main fan, but no dice. Maybe the run is too long and the three turns in the run are creating too much resistance?

From contributor J:
For one, you have a 26" fan trying to push through an 18" pipe. Your pipe is too small. Two, every foot of pipe adds resistance; 50' and 3 elbows adds a substantial amount of resistance. In summary, your pipe diameter is too small for the length of the run.

The pipe should be made of metal. Plastic can build up static electricity, which can lead to a spark, which can lead to a boom. Especially dangerous with all the solvent vapor.

From contributor E:
I think you hit the nail on the head. 50' is a long run; add in 3 turns and that's a lot of resistance to overcome. I'm not sure if that would cause the fan speed issue. You may have two different problems here.

Can you rethink the exhaust location for a more favorable (closer) one? Generally you want your spray booth either on an outside wall for horizontal venting, or inside only if you're going through the ceiling. 50' just seems like a tough situation to make work well.

From contributor S:
Make a plywood tunnel.

From contributor J:
Take a garden hose and try to breathe through it. That is what you have done to your fan.