Ask the Expert Question-and-Answer Archive

by Ron Joseph

August, 2003

Airflow (Velocity) in Spray Booth and HVLP Spray Guns

Q. Is it possible to change standard exhaust chamber (dry filters) to smaller (with less air output) if we are changing from simple spray guns to HVLP spray guns?

A. The current guideline in the US is for the airflow rate in a spray booth to be such that the concentration of solvents in the booth never exceeds 25% of the lower flammable limit, (LFL). Since most people don't know how to calculate 25% LFL the old guidelines that call for the air velocity to be greater than, for example, 100 ft/min are still being followed by most painting operations. The 100 ft/min guideline did not apply to all painting operations, since lower velocities were allowed when electrostatic spray guns were used, or when painting in very large spray booths or uneder other special circumstances. Therefore, in this answer I use 100 ft/min only as an example.

Since the implementation of HVLP guns into the US market during the 1980s and 9s, I have not seen any new guidelines that differentiate between conventional and HVLP guns. Remember, the criterion, according to the National Fire Protection Association, (NFPA) is 25% LFL and not air velocity.

Even though many companies use HVLP spray guns, in my travels across the US I have observed extensive abuse of these spray guns. Painters often set their fluid and air pressures so high that they do not significantly benefit from HVLP. Under these circumstances there would be no justification to lower the air velocity in a spray booth.

If I were in your situation I would either calculate the air velocity that will lower the concentration of solvent vapors to 25% LFL, or if I didn't have the knowledge, time or inclination to do this, I would keep the air velocity the same as before.

Finally, we tend to forget the airflow through a spray booth is not only there to lower the concentration of the solvents, but also to remove the paint particles (overspray) from the working areas of the painters. That is the only reason for having filters in the first place. Filters don't do a single thing to lower solvent concentrations, they serve ONLY to capture the particulates. If you lower the air velocity more paint overspray will fall onto the floor and/or contaminate your freshly painted surfaces.

So my final recommendation is that I would not lower the air velocity on account of the HVLP spray guns.