What you should know
- Filter efficiency must be read together with its test standard, particle size and certified test result.
- 1200 × 600 mm and 600 × 600 mm are common nominal face sizes for terminal filters and FFUs.
- Airflow estimated from face area is a starting point, not a substitute for the manufacturer's rated duty.
- Final selection must include pressure drop, ceiling fit, seal type, integrity testing and available fan pressure.
HEPA and ULPA are performance classifications
HEPA and ULPA describe high-efficiency particle filters, but a label alone is not enough for engineering selection. Different standards classify and test filters using defined procedures, including performance near the most penetrating particle size (MPPS). Always record the governing standard, filter class, rated airflow, initial resistance and individual test certificate.
The familiar statement that a HEPA filter is 99.97% efficient at 0.3 µm is widely used in some markets. It should not be treated as the universal definition for every filter supplied worldwide. ISO 29463 uses MPPS-based classification and testing, so compare products on the same standard and duty point.
| Item | HEPA | ULPA |
|---|---|---|
| Typical role | Most pharmaceutical, healthcare, laboratory and industrial cleanroom terminal duties | Very high particle control where the process and selected standard require it |
| Selection basis | Certified class and efficiency under the specified test standard | Certified class and efficiency under the specified test standard |
| Pressure impact | Usually lower than an equivalent ULPA duty, depending on media and construction | Often higher for the same face area and airflow, so fan energy and reserve pressure need review |
| Best choice | When it satisfies the process risk and regulatory requirement | When the added performance is required and the complete air system can support it |
Common 1200 × 600 mm and 600 × 600 mm face sizes
The table below gives geometric face-area airflow at three example face velocities. These values are useful for early layouts and quantity estimates. A filter's effective media and open face, casing dimensions, gel or gasket seal, plenum, grille and manufacturer's certified rating can change the final selection.
For conversion, 1 m³/h is approximately 0.5886 CFM. Use the manufacturer's rated duty for final schedules.
| Nominal size | Face area | 0.35 m/s | 0.40 m/s | 0.45 m/s |
|---|---|---|---|---|
| 1200 × 600 mm | 0.72 m² | 907 m³/h | 534 CFM | 1,037 m³/h | 610 CFM | 1,166 m³/h | 687 CFM |
| 600 × 600 mm | 0.36 m² | 454 m³/h | 267 CFM | 518 m³/h | 305 CFM | 583 m³/h | 343 CFM |
Terminal filter or fan filter unit?
| Decision point | Ducted terminal filter | Fan filter unit (FFU) |
|---|---|---|
| Air source | Central AHU and duct system | Ceiling module with integral fan, normally supplied from a plenum or conditioned source |
| Balancing | Damper, terminal box or duct branch balancing | Local speed command plus system-level balancing and controls |
| Fan heat | Mainly handled at the AHU | Motor heat is near or within the clean space air path |
| Maintenance | Filter access and upstream system access must be planned | Each fan, controller and filter needs safe access |
| Useful when | A central plant and conventional duct distribution suit the project | Modularity, high ceiling coverage or individual control are project priorities |
A reliable filter selection workflow
Define the requirement
Confirm the room state, target class, critical particle sizes, process risk and applicable standard or regulation.
Calculate room airflow
Determine airflow from contamination control, heat and moisture loads, exhaust replacement, pressure balance and recovery requirements.
Choose the arrangement
Compare terminal filters, FFUs and other supply arrangements against zoning, ceiling space, controls and maintenance access.
Select a certified duty
Check filter class, rated airflow, initial resistance, final recommended resistance, casing depth, seal and test certificate.
Check the pressure budget
Include dirty-filter allowance, terminal box, grille, damper, duct fittings and the critical path back to the fan.
Commission and record
Verify installation, scan or integrity test where required, balance airflow, record pressure drop and retain traceable filter data.
Pressure drop, seals and integrity testing
Initial resistance is measured at a stated airflow and normally rises as the filter loads. The fan and control method need enough usable reserve to maintain required airflow without exceeding the terminal, duct or room noise limits. A clean filter selected at the edge of the fan curve leaves little operating margin.
Leak-free performance also depends on the housing and seal. Gasket compression, gel seal engagement, frame flatness, clamping, penetrations and access method should be inspected as a system. A perfect media certificate cannot compensate for bypass around the frame.
- Record manufacturer, model, serial number, filter class and test standard.
- Record rated airflow and initial pressure drop at that airflow.
- Confirm actual casing and ceiling-grid dimensions, not only the nominal module size.
- Define the installed integrity-test method and acceptance criteria before procurement.
- Provide safe access for testing, replacement and cleaning without damaging the seal.
Frequently asked questions
How much CFM can a 1200 × 600 mm HEPA filter deliver?
Using the full 0.72 m² geometric face at 0.35 to 0.45 m/s gives about 534 to 687 CFM. Final selection must use the manufacturer's certified rated airflow and pressure drop because the active face area and construction differ.
Is ULPA always better than HEPA?
No. ULPA may provide higher particle-removal performance, but it can add resistance, energy and cost. Use it when the process risk, classification scheme or client specification requires it and when the air system can support the duty.
Are 1200 × 600 and 600 × 600 mm actual filter dimensions?
They are common nominal module sizes. The actual casing, neck, face and ceiling opening dimensions must be confirmed on the selected manufacturer's drawing.
When should a HEPA filter be replaced?
Replacement should follow the facility's approved limit or risk-based plan, considering pressure drop, damage, failed integrity testing, contamination events and manufacturer guidance. A calendar date alone is usually not enough.
Technical sources
Confirm the current controlled edition of every standard and the requirements that apply to your facility.
- ISO 29463-1:2024: Classification, performance testing and markingInternational Organization for Standardization
- ISO 14644-3:2019: Test methodsInternational Organization for Standardization
Prepared by 360S Projects as a practical learning reference. Final design, validation and compliance decisions should be made by competent professionals using current project requirements, standards and manufacturer data.