Resistance Is Not Futile — Measuring Paint Booth Filter Performance

We recently received a great question from a customer:

“Is there a “magic number” on the manometer at which paint booth filters should be changed?”

While it sure would be nice if there was a universal ‘change-your-filters-now’ number that would make everyone’s maintenance a lot easier to manage, there’s just too many variables in paint booth design that come in to play. However, there are some general guidelines for most finishing operations that you can use to better plan out your paint booth filter changes and get more life out of your filters.

Here’s the answer that our experts came back with!
While the question doesn’t specify which paint booth filters are involved: intake, exhaust or recirculation, the key to answering the question boils down to 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”-0.75” water column, but with variable frequency drive systems this may be extended to 1.0” 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 PM schedule of 3-6 times annually is most common since the manometer or magnahelic gauge is used to measure the exhaust fan only. For pressurized booths, there may be a separate manometer or magnahelic gauge for the supply air or recirculation air plenums, and a photohelic gauge that would measure the differential pressure of the intake and exhaust air balance.

Recirculation filters would typically be changed at 0.5”-1.0” 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 Preventive 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 manual or the manufacturer for the specific equipment involved.