NAFA's Maintenance of Acceptable Booth Airflow
Per OSHA requirements, a means must be established for maintaining booth airflow so it remains at or above minimum acceptable levels, typically, 100 FPM average across the face of the booth (60FPM for electrostatic operations). However, it is the operator’s responsibility to ensure that airborne concentrations of chemicals do not exceed the maximum levels allowed by OSHA 1910.107 and State and Local regulations, and that these concentrations never exceed 25% of their lower explosion limits (LEL). One method for measuring booth airflow velocity is with direct-read velometer or thermal anemometer. However, many velocity-sensing instruments are delicate and not well-suited to daily use in a spray environment since they are susceptible to fouling by airborne overspray particulate.
An alternative method for maintenance of booth airflows at or above the minimum acceptable levels is to utilize the relationship that exists between booth airflow and pressure drop across the exhaust filters.
How to Relate Filter Pressure Drop to Booth Airflow
Pressure-sensing instrumentation such as manometers and Magnehelic® gauges are commonly employed for this purpose. These instruments are discussed in more detail in the NAFA Installation, Operation, and Maintenance of Air Filtration Systems Manual, Chapter 2, page 7. Gauges are to be installed using the manufacturer’s installation instructions.
A full set of clean overspray filters should be installed and the fan should be started. The differential pressure of the clean filters should be recorded and posted near the pressure gauge. Filter loading is then simulated by placing sheets of paper in a uniform pattern over the air entering side of the exhaust filters. The average velocity is determined through the use of a velometer/anemometer and the restriction of the airflow using paper sheets until the average airflow velocity drops close to the OSHA minimum (see http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9734). Average velocity is typically determined by averaging one reading approximately every eight square feet of filter surface area. Another option for extremely large booths is to safely take airflow readings over time as the filter loads to ensure the minimum average velocity is above the OSHA requirement. The differential pressure is recorded and posted near the gauge. This pressure becomes the filter change out point.
Multiple stages of filtration require the monitoring of pressure between each stage. Overspray-laden filters should be replaced when the OSHA minimum velocity can no longer be maintained. End users may choose to install clean filters before the minimum airflow is reached based upon other priorities such as shift changes, etc. Pressure drop instrumentation and all other penetrations through the booth wall, such as taps and connective tubing and hoses, must be kept in good working condition as part of a booth maintenance program. Tubing, hoses and small tap orifices, in particular, are subject to clogging and obstruction due to their small diameter and exposure to paint overspray.