# Air Changes vs. Air Velocity – Gauging Paint Booth Ventilation Performance

Recently we received a technical question from a visitor to the GFS Finishing Academy site regarding the number of air changes necessary for a paint booth. This is a common concern of many shops, but you may be surprised to learn that the performance of a paint booth is more linked to the actual velocity of the air within the cabin than the total number of times the air is exchanged. Here’s the question we received and the answer that was provided by the GFS engineering team:

Q: How many air changes are required for a 16 by 14 by 40 paint booth?

A:  There is no code or standard that requires a specific air change for paint spray booths.  It is more common for the ventilation performance of a spray booth to be specified by the average velocity through the booth or in the vicinity of the painter and product.  From the design velocity you can calculate ventilation flow rates and air changes.

For example, if your booth is 16 feet wide by 14 feet tall and is a cross draft design, typical velocity in the booth would be 100 feet per minute (fpm). The total flow through the booth would be 22,400 cubic feet per minute (cfm) (16 x 14 x 100). The volume of the booth is 8960 cubic feet (16 x 14 x 40). The air changes can be calculated by dividing the flow rate by the volume. In this example, the air changes would be 2.5 air changes per minute (22400 / 8960).

However if this is a downdraft booth, typical velocity in the booth would be in the 50 to 100 fpm range. For this example let’s use 50 fpm. In this case the total flow through the booth would be 32,000 cfm (16 x 40 x 50). Dividing the flow by the volume of the booth would give 3.5 air changes per minute (32000 / 8960). The velocity is half that of the previous example, but the air change is higher.

From a fire safety standpoint, NFPA 33 – Standard for Spray Application Using Flammable or Combustible Materials, 2011 Edition and the 2012 Edition of the International Fire Code both require that the ventilation rate shall be able to maintain the concentration of flammable vapors in the exhaust below 25%. So the minimum ventilation flow rate is a function of how much paint is being sprayed and the volume of flammable materials in the paint. After establishing this minimum, the ventilation rate may be increased in order to maintain a minimum average velocity through all openings to prevent the escape of overspray from the spray booth and to achieve the desired collection of overspray toward the exhaust filters.