Is there a “magic number” on the manometer at which paint booth filters should be changed?
Regardless of the filter used, it is a matter of how much total resistance is designed into the fan in question and the relative condition of the fan. All fan units are designed to operate within a range of resistance, that includes the ductwork, plenum and filters. Without the exact fan curve data that should have been supplied in the operator’s manual for a particular booth, the best one can do is base the change out resistance on industry “norms”.
The exhaust filters of most spray booths are designed to be changed at approximately 0.5 to 0.75-inch water column, but with variable frequency drive systems this may be extended to 1 inch or beyond for a single-stage exhaust filter.
On intake filters, the change-out final resistance depends in part on the type of booth: non-pressurized or pressurized; and with pressurized booths, whether it is a recirculation or has independent supply and exhaust fans.
For non-pressurized booths, such as a traditional crossdraft, a preventative maintenance schedule of three to six times annually is most common since the manometer or magnehelic is used to measure the exhaust fan only.
For pressurized booths, there may be a separate manometer or magnehelic for the supply air or recirculation air plenums, and a photohelic that would measure the differential pressure of the intake and exhaust air balance. Recirculation filters would typically be changed at 0.5 to 1.0-inch water column dependent on fan capability. Air make-up filters in a separate fan unit would be changed similarly. For diffusion filters in a pressurized booth, a preventative maintenance schedule is probably the most preferred method of maintenance: once per year for booths that have MERV8 or higher supply air filters, and twice per year for filters with lower efficiency ratings.
Again, these are generalized and not specific to the equipment in use at this application. More critical than any speculative or default method of determining filter change parameters would be to consult the operator’s manual or the manufacturer for the specific equipment.