Filter Pressure Drop and Fan-Energy Penalty
Clean and change-out pressure drop by MERV / HEPA class (velocity-scaled, user-overridable cut-sheet defaults).
Example
You enter
- Filter class merv13
- Face area (ft²) 4
- Face velocity (fpm) 300
- Fan total efficiency (0-1) 0.6
- Runtime (hr/yr) 4000
You get
- Airflow 1200
- Pressure drop (clean -> change-out) 0.35
- Final dp in wc 0.7
- Fan power (clean -> change-out) 0.0821
Details, formula, and sources
The fan power at each via brake-HP, and the annual fan energy and loading penalty over a clean filter. Per ASHRAE 52.2-2017 and first-principles fan power.
Airflow (CFM) = face area × face velocity. Pressure drop scales ~linearly with face velocity (Darcy regime): dp(V) = dp_ref × V / 300 fpm. Fan power (kW) = (CFM × dp_in_wc / 6356) / fan total efficiency × 0.7457. Average drop over a linear loading cycle = (clean + change-out) / 2; annual fan energy = average-drop kW × runtime; loading penalty = (average − clean) kW × runtime.
ASHRAE 52.2-2017 (Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size) by name; manufacturer published cut sheets for the pressure-drop values.
ASHRAE 52.2 TOC free at ashrae.org; full standard licensed; manufacturer cut sheets free at each vendor site.
Estimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
Field names used by the API: filter_type, face_area_ft2, face_velocity_fpm, fan_total_efficiency, runtime_hr_per_year, airflow_cfm, clean_dp_in_wc, final_dp_in_wc, clean_fan_kw
- Air horsepower constant AHP = CFM × dp(in. WC) / 6356fan engineering identity
- Velocity scaling dp ∝ face velocity (first-order Darcy regime)fibrous-media engineering practice
- MERV 13 reference drops 0.35 in WC clean / 0.70 in WC change-out at 300 fpmrepresentative pleated-filter cut-sheet value
- Default fan total efficiency 0.60 (fan × motor × drive)engineering practice