Cleanroom pressurization cascades

A cleanroom suite is designed so that air flows from cleaner areas to less clean areas, never the other way. The HVAC system creates this by supplying slightly more air than it removes from the cleanest rooms, so they are at a higher pressure, and stepping pressure down room by room toward the corridor. This arrangement is called a pressure cascade.

Where the process involves hazardous material, the goal changes. The cascade may need to keep contamination in as well as out. That leads to more complex arrangements using airlocks as pressure bubbles or sinks. This guide explains how cascades are designed, built and controlled.

Why pressure differentials work

Air moves through any opening from high pressure to low pressure. In a well-sealed cleanroom, the openings are door gaps, pass-throughs, cable penetrations and the doors themselves when opened. Keeping the cleaner room at higher pressure means that air leaks outward through these paths, carrying particles away rather than drawing them in.

A pressure differential only protects the room when doors are closed. When a door opens, the differential collapses almost immediately, and the protection comes from the volume of air flowing through the doorway and from airlocks.

Typical pressure differentials

Larger differentials are not automatically better. Above about 20 to 25 Pa, doors become hard to open or close, whistling noise appears at gaps, and the airflow needed to maintain the differential rises. Very small differentials, below about 5 Pa, are difficult to measure reliably and are easily upset by door operation and wind effects on the building.

Airlock types

Airlocks separate rooms of different classes so both doors are never open at the same time. Their pressure relative to the rooms on either side defines their function.

Cascade airlock

Pressure steps down from the cleaner room, through the airlock, to the less clean area. Air always flows outward from the clean side. This is the standard arrangement when the product needs protection and the material is not hazardous.

Bubble airlock

The airlock is at a higher pressure than both rooms it connects. Air flows out of the airlock in both directions. This protects the cleanroom while also preventing contaminated air from the cleanroom reaching the corridor. It is used where the cleanroom handles potent or biohazardous material but the product still needs protection, such as aseptic processing of potent compounds.

Sink airlock

The airlock is at a lower pressure than both rooms. Air flows into the airlock from both sides and is exhausted. This keeps hazardous material from escaping the process room while preventing corridor air from reaching it. Sinks are common for containment labs and some hazardous drug handling.

The right choice depends on a risk assessment of both product protection and operator and environmental protection. Containment facilities are discussed in containment labs: BSL-2 and BSL-3 HVAC.

Calculating the airflow offset

Pressure is created by an airflow offset: the difference between supply and the sum of return and exhaust in each room. The offset equals the air leaving through the room's leakage paths at the design pressure.

Leakage through an opening can be estimated with the orifice equation:

Q = Cd x A x (2 x dP / rho)^0.5

where Q is airflow (m3/s), Cd is a discharge coefficient (often taken around 0.6 for sharp-edged openings), A is the leakage area (m2), dP is the pressure differential (Pa) and rho is air density (about 1.2 kg/m3 at sea level).

Example

A cleanroom has a single door with a 3 mm gap around its perimeter and a total leakage area, including other cracks, of about 0.02 m2. To hold 12 Pa:

1. (2 x 12 / 1.2)^0.5 = 4.47 m/s.
2. Q = 0.6 x 0.02 x 4.47 = 0.054 m3/s, or about 54 L/s (115 cfm).

That is the offset the room needs. The estimate is only as good as the leakage area, which depends on construction quality. Designers commonly add margin, and the final offset is set during balancing. Well-built modular panel rooms often leak much less than drywall rooms with many penetrations.

Controlling the cascade

There are two main control strategies, compared in detail in room pressurization control: offset vs direct pressure.

• Airflow tracking (offset) control measures or sets supply and exhaust airflow and maintains a fixed difference between them. It is stable and responds quickly to door openings without chasing pressure. Pressure-independent devices such as venturi air valves are often used.
• Direct pressure control measures room pressure and modulates supply or exhaust to maintain it. It works well in tight rooms with closed doors but can overreact when doors open.

Many cleanrooms use airflow tracking for the control loop and direct pressure measurement for monitoring and alarms, or a cascaded combination that slowly trims the offset based on measured pressure.

Monitoring and alarms

Room pressure monitoring is a core requirement in GMP, USP and containment facilities.

• Install a pressure sensor for each critical room, referenced to the adjacent space or to a common reference.
• Provide a local display at the room entrance showing whether the room is in or out of range.
• Use time delays, typically tens of seconds, so normal door openings do not trigger alarms, but long enough excursions do.
• Record data for regulatory review where required.
• Calibrate sensors at regular intervals.

In regulated environments, the monitoring system may need to be independent of the control system and validated.

Commissioning a cascade

1. Verify room construction and sealing before balancing.
2. Balance supply, return and exhaust airflows in each room.
3. Adjust offsets to achieve design differentials with all doors closed.
4. Check pressures with each door opened in turn, and confirm the cascade recovers promptly.
5. Test fan failure scenarios and confirm that containment rooms never become positive.
6. Verify alarms, delays and local displays.

Key takeaways

• Pressure cascades keep air flowing from clean to less clean areas, with typical differentials of 10 to 15 Pa between zones.
• Cascade airlocks protect product; bubble and sink airlocks add containment for hazardous materials.
• The airflow offset equals leakage at the design pressure and can be estimated with the orifice equation.
• Airflow tracking control resists door-opening upsets, while direct pressure measurement supports monitoring and alarms.
• Commissioning must prove the cascade with doors open and during fan failures, not just at steady state.