Pressure cascades in containment: pascals, not cubic metres

01The fallacy: critical room, more exhaust air

In laboratories, radionuclide areas and biosafety zones, the same reflex appears early on: a room is “critical”, so it gets more exhaust air. More air feels like more safety. Technically, however, it offers no protection. A room can be operated with a high air change rate and still lose containment – if supply and exhaust air are equal or the offset has the wrong sign.

02The principle: a staircase of room pressures

A pressure cascade is a sequence of room pressures along a route, for example corridor – airlock – laboratory. Each step amounts to only a few pascals, but clearly determines the direction in which air flows through door gaps and penetrations. What matters is not the absolute value, but that the direction is correct along the entire route. Swiss radiological protection legislation puts it exactly this way: the pressure in rooms with a greater risk of contamination must be lower than in rooms with a lesser risk.

The direction follows from the protection objective. If the hazard is to remain in the room – radioactive, biological or chemical substances – the room is kept at negative pressure. If contamination is to remain outside, for example in aseptic manufacturing, it is kept at positive pressure. Where both logics meet, an airlock with its own pressure regime belongs in between.

03The lever is airtightness, not airflow

The pressure step results from the offset between supply and exhaust air. It must be as large as the air that escapes through the leaks in the envelope at the desired pressure differential. This leakage increases only roughly with the square root of the pressure differential, but directly with the leakage area: a fourfold pressure step costs roughly twice the offset air, half the leakage area saves half.

≈ 110 m³/hoffset air for 15 Pa negative pressure with an assumed effective leakage area of 100 cm²

The large airflows of a laboratory stem from heat and substance loads, not from containment. Anyone who cannot hold the pressure step therefore usually has an airtightness or control problem – not an airflow problem.

04Where cascades fail in operation

• Door opening causes the pressure step to collapse abruptly – airlocks with mutually interlocked doors absorb this.
• Variable air volume fume cupboards change the exhaust air within seconds; the supply air must follow quickly enough.
• Filter loading shifts the fan curve; without pressure control, the cascade drifts.
• Retrofitted penetrations for cables and pipes increase the leakage area.
• Part load and night-time operation: the direction must be maintained even at reduced airflow.

05What this means for your project

Require a pressure step plan at an early stage, with pressure differential, direction and airlock concept for each room – it is the actual control document, not the airflow table. Specify the airtightness of the envelope as a testable value, have the offset documented for each room, and have the pressure differentials at the relevant doors monitored and alarmed. Accept the cascade in two stages: first with the doors closed, then under defined door-open scenarios.

And not every sensitive room is a pressurised room: for electron microscopes, for example, vibration, stray fields and temperature stability are what count – a sharp cascade would only bring draughts there.

Takeaway

A pressure cascade contains hazardous substances with pascals, not with cubic metres – and the most economical way to get there is an airtight envelope with a cleanly controlled offset.

SEDC AG · Article from our internal training series “Technik Praxis”, abridged for clients and users.