Preventing the infiltration of outdoor airflow into the clean area.
I. Pressure Differential Standards (General Cleanrooms)
Clean operating areas must maintain positive pressure relative to the outdoors.
Standard clean areas: ≥10 Pa; high-risk aseptic / PCR workshops: recommended 12–15 Pa.
Pressure gradients across adjacent buffer rooms / changing rooms: Clean operating area > Buffer room > Corridor > Outdoors.
Strictly prohibited: Clean area pressure lower than adjacent rooms; negative pressure allows unfiltered outdoor air to rush in directly.
Example of pressure gradient logic:
Clean operating area (12 Pa) → Buffer room (6–8 Pa) → General corridor (2–4 Pa) → Outdoors (0 Pa)
II. Pressure Differential Adjustment Methods (Practical Adjustment Sequence)
1. Fresh Air Supply (Primary Method)
Positive pressure in clean areas is maintained by the continuous supply of fresh air.
Increase AHU fresh air damper opening → Room air intake increases → Positive pressure rises.
Decrease fresh air damper opening → Positive pressure drops.
⚠️ Relying solely on return air is insufficient; stable positive pressure cannot be maintained without continuous fresh air makeup.
2. Exhaust Air Adjustment (Key Pressure Relief Path)
Room pressure differential = Air intake − Exhaust air / Air leakage volume.
Clean areas feature process exhaust and local exhaust (e.g., universal extraction arms, fume hoods): closing exhaust dampers increases room positive pressure; opening them decreases it.
If uncontrolled leakage is excessive, adjusting fresh air alone makes it difficult to stabilize pressure; prioritize controlling exhaust airflow.
3. Residual Pressure Relief Valve (Passive Pressure Relief for Stability)
Installed on the partition wall between the clean area and a lower-grade area.
Set opening pressure: Typically 8–10 Pa.
Function: Automatically exhausts air when fresh air intake is excessive (preventing over-pressurization) and closes when fresh air is insufficient (preventing backflow of outside air).
Key Point: These valves only exhaust air outward and do not allow intake; the selected model must include a one-way check mechanism. 4. Sealing of doors and enclosure structures (a common root cause of pressure differential failure)
The infiltration of outdoor air is often caused not by airflow volume, but by leakage through gaps:
Aging door seals or excessive gaps in clean area doors; poor sealing of pass-through boxes.
Air leakage at color steel panel joints, wall penetrations for utilities, FFU frame gaps, and observation window sealant joints.
In cases of severe leakage, the pressure differential cannot be maintained even with maximum fresh air intake, as outdoor air continuously backflows through the gaps.
Remediation: Apply sealant, replace door seals, and seal wall penetrations.
III. Typical Faults: Diagnosis and Handling of Outdoor Air Infiltration
Phenomenon 1: Fluctuating pressure differential; negative pressure occurs upon opening the door, causing an influx of outside air.
Cause: Opening the door disrupts the pressure differential gradient; insufficient pressure differential in the airlock (buffer room).
Countermeasures:
Strictly enforce the interlocking mechanism for the two doors of the airlock; simultaneous opening is prohibited.
Provide independent air supply to the airlock to maintain positive pressure; do not use the airlock merely as a passageway.
Slow down movements when entering or exiting to minimize transient airflow disturbances.
Phenomenon 2: Fans operating normally, yet clean area pressure differential remains consistently low or even negative.
Troubleshooting sequence:
1) Significant air leakage in the enclosure structure;
2) Excessive process exhaust airflow;
3) Clogged fresh air filter resulting in insufficient fresh air supply;
4) AHU supply fan belt slippage causing reduced airflow;
5) Damaged or stuck-open residual pressure valve causing continuous air leakage.
Phenomenon 3: Pressure differential meets standards, yet odors persist or outdoor air infiltrates.
Likely cause: Disordered pressure differential gradient.
Example: Clean area at 10 Pa, but airlock at only 2 Pa and corridor at 10 Pa → Corridor air infiltrates into the airlock and clean area.
Solution: Rebalance supply and exhaust airflow for each room to establish a proper step-wise pressure gradient. IV. Standardized Adjustment Procedures
Close all external doors and pass-through hatches to ensure the room reaches a sealed, steady state;
Read the differential pressure gauge and record the current pressure differential between the clean zone and the outdoors;
If the target pressure differential is insufficient: prioritize slightly opening the AHU fresh air damper; if there is significant exhaust airflow, moderately reduce the process exhaust;
If the pressure differential is too high: slowly increase the process exhaust or fine-tune the residual pressure valve setting;
Allow the system to operate stably for 15–30 minutes after adjustment, then re-verify the pressure differential;
Test door-opening scenarios individually to observe for any transient negative pressure; if significant negative pressure occurs upon opening the door, appropriately increase the static pressure differential setting.
V. Additional Important Considerations
Impact of outdoor wind pressure: During high winds, fluctuations in outdoor static pressure can cause pressure differential drift in the clean zone; consider incorporating automatic VFD-based fresh air/exhaust regulation;
Do not rely on keeping doors closed to "trap" pressure and maintain the differential; excessive leakage over the long term leads to massive system energy consumption;
Ensure pressure differential sampling tubes do not face directly into airflow outlets to avoid reading inaccuracies; calibrate pressure differential instruments regularly;
Note that logic is reversed for biosafety or negative-pressure isolation rooms (where negative pressure prevents internal air from leaking out), whereas clean operating zones almost exclusively utilize positive-pressure modes.