Ask the Expert Question-and-Answer Archive

by Ron Joseph

July, 2003

Paint Spray Booth Air Velocity

Q. At what height should the velocity be checked from the base filter in a down draft system and what should be the ideal value be for that velocity?

And, one more question:

Can we increase the air velocity by increasing the CMH (or cfm) inside the paint booth? As I have noted, for a given space different manufactures give different CMH blowers. The paint booth with a higher CMH value give a higher velocity inside the booth and a better overall performance.

A. The air velocity should be measured at the same height as the painter's face when he is painting. One of the goals of a spray booth is to remove the solvents and paint particulates from the face of the painter. When I measure air veocity I do so approximately 4-6 ft (1.2 - 1.8 m) from the ground, since this is where the painter holds the spray gun most of the time. If the painter holds the spray gun higher up, then you can measure there too.

The rule of thumb is for the air velocity to be grater than 100 ft/min (30 m/min). In very large spray booths where the vapors do not build up significantly the velocity can safely be dropped significantly below 100 ft/min (30 m/min).

Yes, you can definitely increase the air vlocity by increasing the CMH (cfm).

Velocity (m/min)= volumetric flow rate (m^3/ surface area of filters (m^2).

Since the surface area of the filters remains constant, the velocity must increwase as you increase the volumetric flow rate.