TL;DR
Pressure differential loss is usually an FFU speed reduction, an exhaust air increase, or a wall panel seal failure. Check FFU face velocity and room seal integrity before adjusting any fan speed.
What you might see
- room pressure differential below ISO or site specification
- pressure alarm on the room monitoring system
- pressure dropping after a panel was accessed
- pressure differential normal at night but low during occupied hours
Likely causes
One or more FFUs running at reduced speed, decreasing supply air volume
HEPA filter loading increasing pressure drop and reducing air delivery
Wall panel or door seal damaged, increasing uncontrolled air leakage
Exhaust air volume increased from a process equipment change inside the room
Required tools
- Anemometer or hot-wire airflow meter
- Differential pressure manometer
- HEPA filter pressure tap gauge
- Wall panel sealant
Safety first
- Loss of positive pressure in an ISO Class 5 or better cleanroom is a product-quality event. Quarantine product processed during the pressure loss period until a risk assessment is completed.
- When adjusting FFU fan speed, increase speed gradually to avoid turbulence that could disturb the unidirectional flow pattern.
Procedure
- 1
Read the differential pressure from the room monitoring system and log the current value.
- 2
Measure the face velocity at each FFU with an anemometer. Flag any FFU below the design specification face velocity.[1]
- 3
Check all FFU speed controllers or the central BMS supply air set-point against the design document.
- 4
Inspect the room door seals and any recently disturbed wall panels for gaps. Re-seal any gaps with the approved sealant.
- 5
Check the HEPA filter differential pressure across each FFU (if the FFU has a filter pressure tap). Filters near or past the change interval will restrict air delivery.[1]
- 6
Schedule HEPA filter replacement for any filter exceeding the pressure drop limit.
- 7
Review any new process equipment installed inside the room for additional exhaust requirements. Balance the exhaust volume if a new exhaust source was added.
- 8
After corrective action, log the pressure differential at 15-minute intervals for 2 hours to confirm stability.
Sources
Terra Universal Terra Universal hardwall cleanroom Cleanroom Equipment / FFU general technical documentation, Terra Universal
Terra Universal cleanroom general pressure balance and FFU commissioning procedures per ISO 14644-3 (general)
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