Air Filtration
There is a pitch past which more pleats make the filter worse. More media, more cost, more pressure drop.
— Pa
Media plus channel losses at the design airflow
The channel loss coefficient is a lumped fit standing in for a real viscous-flow solution in a tapering wedge, and the optimum pitch is only as good as that fit — recover it from pressure drop measured across at least two pleat pitches on the same media before trusting the recommendation. Both terms are clean-filter values: dust loads unevenly along a pleat, blinding the tips first, so the loaded optimum sits at a wider pitch than the clean one. Pleat stability under flow is a separate constraint and often binds before the pressure optimum does, since tall thin pleats deflect and close their own channels.
HEPA Filter Pleat Geometry to Airflow Optimisation — free, with the formula and a worked example, at Textile School.