Design-Based Differential Pressure in Cleanroom Spaces Ⅱ

4. AHU Control

AHU control directly impacts room pressurization:

In primary/secondary systems, control schemes can apply to secondary AHUs.

Avoid series flow control to prevent system conflicts from measurement errors.

AHU fans and primary air should be controlled by supply duct static pressure. If room pressure control restricts return airflow to the AHU, primary air supplementation is needed to avoid cascading failures.

5. Differential Pressure Measurement and Pressure Cascade Specifications

Regulatory Objectives:

Maintain 10–15 Pa pressure difference across airlocks when doors are closed; sustain airflow direction when doors are open.

ISPE guidelines recommend at least 5 Pa between same-classified rooms.

Well-designed HVAC systems should limit pressure fluctuations to ±2.5 Pa, triggering alarms beyond this range and activating alerts for deviations ≥5 Pa.

Pressure cascades ensure positive pressure from Class A to D and from Class D to non-classified areas, preventing cross-contamination.

Airflow to Adjacent Rooms:

To maintain pressurization, make-up air must replace "leakage" airflow to lower-pressure rooms (e.g., through door gaps or penetrations).

Maintaining drop seals and door closers is essential. Door seals are often omitted for maintenance simplicity, ensuring controlled leakage to meet FDA requirements for outward airflow during door openings. Grilles, dampers, or pressure stabilizer dampers can assist.

6. Differential Pressure Monitoring and Signal Processing

Monitoring Methods

i.Room-to-Room Measurement: Easy to implement without automated calculations; suitable for direct single-room pressure measurement, but may miss inter-suite pressure issues in large facilities.

ii.Measurement to a Public Reference Location: Used for complex facilities, connecting all rooms to a neutral reference zone to calculate inter-room pressure differences and trigger alarms; requires a stable reference point.

Recommendation: Connect low-pressure ports to a public manifold linked to a 3–4-inch diameter pipe or expansion vessel, placing the reference point indoors away from unstable influences (e.g., external airflow or vibrations).

Specific Strategies and Measures:

Dynamic Monitoring: Deploy differential pressure monitoring systems for real-time data collection, ensuring stability during dynamic conditions (e.g., personnel entry/exit).

Regular Calibration: Calibrate pressure transmitters every 6 months, archiving records for accuracy.

Data Recording: Automatically log pressure data to a central system, with shift-by-shift checks and reports for anomalies.

Trend Analysis: Use monitoring software to analyze pressure trends and identify potential issues (e.g., equipment failures or airflow anomalies).

Instrument Selection:

Avoid ranges far exceeding requirements; choose ±0.25 Pa accuracy with partial negative scales (e.g., -10–100 Pa for 50 Pa differentials), ensuring durability against operational and calibration fluctuations.

Instrument Signal Processing:

·While aiming for rapid response, stabilize control using rolling or time-weighted averages to reduce false alarms from unstable pressurization signals (2.5–50 Pa), with at least short delay settings.

·Signal Smoothing: Apply 5–10-second rolling averages to pressure signals to filter out transient fluctuations (e.g., from door openings).

·Delay Settings: Implement 3–5-second delays to filter brief disturbances and stabilize control systems.

Suzhou Pharma Machinery Co.,Ltd.

2025/06/27

Gino