Pre-filter to final filter staging strategy

The economic case. Final-filter media is expensive, denser, higher capture, more material per cycle. Pre-filter media is cheap, lower density, lower capture, less material. By placing the cheap pre-filter upstream, the expensive final filter sees only the fine particulate that the pre-filter let through. Result: final filter cycle extends, total operating cost drops.

Where staging applies. - AMU side: AMU pre-filter (cheap MERV 8-11) protects intake-ceiling diffusion (more expensive higher-grade media). - Intake side: intake-ceiling diffusion is itself a "pre-filter" relative to the spray-zone air the panel sees. - Exhaust side: primary exhaust pad captures the bulk of overspray; secondary exhaust filter (where present) handles bypass particulate. - Aerospace Subpart GG 3-stage: stage 1 + stage 2 + HEPA-class stage 3, each catching what the previous let through.

Cycle math interaction. Pre-filters typically cycle 2-4x more frequently than the final filters they protect. The AMU pre-filter at 60-90 days vs intake-ceiling at 30-60 days is an inversion of this, explains why those two specific positions have the cycle relationship they do.

Substitution pitfalls. Substituting a finer-grade pre-filter (higher MERV) than spec doesn't necessarily extend final filter life, it can restrict airflow, shifting booth pressure balance and accelerating wear elsewhere. Match the spec.

When staging fails. Pre-filter break-through means downstream final filter takes the contamination load. Final filter cycle compresses dramatically, finish defects appear, total operating cost spikes. Staging discipline (replace pre-filters on cycle, don't let them go to break-through) is what makes the strategy work.

Multi-stage filtration is required by Subpart GG (3-stage with HEPA-class final). Other regulatory frameworks specify capture-efficiency outcomes rather than staging directly, but multi-stage is often the practical way to meet the outcome cost-effectively.