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Air Changes vs. Air Velocity — Gauging Paint Booth Ventilation Performance

The ventilation performance of a paint booth is more closely linked to the actual velocity of the air within the cabin than the total number of times the air is exchanged.

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

Speedometer at 140 km/h

Airflow in a Crossdraft Paint Booth

If a crossdraft paint booth is 16 feet wide and 14 feet tall, typical air velocity in the booth would be 100 feet per minute (FPM). The total flow through the booth (16 x 14 x 100) would be 22,400 cubic feet per minute (CFM). The volume of the booth (16 x 14 x 40) would be 8,960 cubic feet. The air changes can be calculated by dividing the flow rate by the volume (22,400 / 8,960). In this example, the air changes would be 2.5 per minute.
Crossflow airflow diagram

Airflow in a Downdraft Paint Booth

If a downdraft paint booth is 16 feet wide and 14 feet tall, typical air velocity in the booth would be 50 to 100 FPM. For this example, let’s use 50 FPM. The total flow through the booth (16 x 40 x 50) would be 32,000 CFM. Dividing the flow rate by the volume (32,000 / 8,960) gives 3.5 air changes per minute. The velocity is half that of the previous example, but the air change is higher.

GFS Airflow model

NFPA 33 and 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 percent. 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. This maintains a minimum average velocity through all openings, preventing the escape of overspray from the paint booth and helping achieve the desired collection of overspray toward the exhaust filters.

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