Ask the Expert Question-and-Answer Archive

by Ron Joseph

April, 2008

Measuring Booth Velocity

Q: Where do you take the measurements in a large spray painting booth (25 feet wide x 15 ft high x 60 feet deep) to compare to OSHA 100 linear feet per minute (29 CFR 1910.107)? At the face of the booth, midway of booths or at the open end. Or do you take measurement every 8-10 feet? Also, is the 100 fpm the "standard" to go by?

A: When I measure booth velocity, I stand a few feet from the filters, and I measure in various locations so that I can get an average. You will find that as you get close to the side walls the velocity will usually be close to zero; therefore I don't stand within 3 ft from the walls. If you have poor seals in the double doors at the entrance of the booth, you will find that there is a spike in air velocity in the center of the booth; therefore, I try to avoid measuring air flow in the center.

My method produces the real air flow at the points where the painter stands, which is, after all, the intention of the regulation. On the other hand, you can measure the velocity in front of each filter and then average your readings. Multiply this by the area of all the filters to get the volumetric flow rate (CFM) passing through the booth. Since your filter plenum is probably narrower than the width of the booth, you can now divide the volumetric flow rate by the cross sectional area of the booth (in your case 25 ft W x 15 ft H) to arrive at the average velocity across the booth. This velocity will usually be considerably higher than my measurement, because often the air travels along the ceiling of the booth and not where the painter stands.

As you know, 29 CFR 1910.107 provides Table G10 that lists the air velocities for different situations; however, for a typical USCG side draft spray booth, 100 ft/min is a good number to work with.

A2: My friend and colleague Rich Thelen of Global Finishing Systems provided the following answer:

Spray booth velocities are not cited in NFPA-33. They are vestigial requirements from US 29 CFR 1910-107 (OSHA) that list velocities in certain circumstances, like electrostatic painting or other painting. Some of these standards have worked their way into other codes also in various fashions. The US Military guides (UFC-3-410) list different velocities for several different spray techniques, including HVLP, electrostatic and airless.

OSHA has deferred to NFPA-33 in the design of spray and has their interpretation online at http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=21875

OSHA considers the use of other velocities than those listed in 1910-107 as “de minimis” violations. De minimis violations are violations of standards which have no direct or immediate relationship to safety and health and shall not be included in citations. But these velocities must conform to NFPA-33.

The focus for velocities, then, is on NFPA-33 which simply states that the airflow shall be sufficient to keep the concentrations of solvents below 25% of the Lower Flammable Limit. NFPA is concerned with fire protection. OSHA is concerned with human health and safety issues. Jointly they cover health, safety and fire issues and both agree that NFPA-33 is an acceptable rule. So much for legal issues!

A spray booth must also remove overspray in a manner that gives a good paint finish and at the same time provides for high transfer efficiency. These are production issues and are issues that the owner needs to address. Regulations will not guarantee a good finish! As spray booth manufacturers, we recommend air flows of 75-125 fpm in crossdraft paint booths and 40-50 fpm in downdraft booths. These velocities consider cross-sections that are normal to the booth airflow. We have found that these airflow velocities are well above the 25% LFL requirement and they yield good paint jobs.

Velocities below these numbers may also yield good paint jobs, but generally, the lower the velocity in the booth, the more likely the booth is to stratify and cause painting (and heating) problems. This happens particularly in large rooms (such as the example). Stratification in downdraft booths happens at lower velocities than in crossdraft booths. Downdraft booths are more efficient at removing overspray at low velocities than are crossdraft booths, since the motion of the overspray particle is downward and gravity helps in the removal of the particles. Stratification is also a problem that is very costly to remedy, as changing the exhaust and supply fans is a major project.

Good luck,

Ron Joseph