Bubble vs. Cascade vs. Sink Airlocks: Cleanroom Pressurization Guide
A bubble airlock holds higher pressure than both rooms it connects, a cascade airlock steps pressure down from clean to dirty, and a sink airlock holds lower pressure than both adjacent spaces. Each scheme controls airflow direction at the door threshold. Each protects a different asset – the product, the operator, or both. Choosing the wrong one contaminates your process or exposes your staff. This guide compares the three so you specify the correct pressurization strategy the first time.
What Is a Cleanroom Airlock Pressurization Differential?
A pressurization differential is the measured pressure gap between two adjacent controlled spaces. That gap forces air to move from high pressure to low pressure whenever a door opens. Air carries particles. Control the air direction and you control where particles go.
ISO 14644-4 sets the design framework for cleanroom construction and pressurization. It does not fix a single mandatory number. Instead, it establishes that adjacent zones must maintain a stable, directional pressure cascade sized to the contamination risk.
The industry works to the 10–15 Pa rule. This is the standard differential between two classified spaces of different grades:
- Below 10 Pa: Door movement and foot traffic disrupt the differential. Directional control fails.
- 10–15 Pa: Airflow stays directional and stable under normal operation.
- Above 15 Pa: Doors become hard to open. Whistling, turbulence, and door-seal stress follow.
Hold the differential inside the 10–15 Pa band and your airlock does its job. Drift outside it and containment breaks down.
Comparative Breakdown: Bubble vs. Cascade vs. Sink Airlocks
The three schemes differ by one variable: how the airlock pressure relates to the rooms on either side. That single relationship decides what each design protects.
| Attribute | Cascade Airlock | Bubble Airlock | Sink Airlock |
|---|---|---|---|
| Function | Step pressured down from clean to dirty in one direction | Pushes air outward into both adjacent rooms | Pulls air inward from both adjacent rooms |
| Pressure Relationship | Pclean > Pairlock > Pdirty | Pairlock > Pclean and Pairlock > Pdirty | Pairlock < Pclean and Pairlock < Pdirty |
| Ideal Application | Standard aseptic and sterile product manufacturing | Isolating a high-grade room from a lower-grade corridor | Potent compounds, cytotoxics, and biohazard containment |
| Risk Mitigated | Particle ingress into the cleaner zone | Cross-contamination between two protected zones | Escape of hazardous material to surrounding areas |
Deep Dive into Each Airlock Scheme
Cascade Airlock
The cascade airlock is the default for most sterile manufacturing. Pressure decreases in steps from the cleanest space outward. Air always moves toward the dirtier zone, so particles never travel back toward the product.
| CLEAN ROOM | 🡺 | AIRLOCK | 🡺 | DIRTY CORRIDOR |
|---|---|---|---|---|
| (+30 Pa) HIGH PRESSURE |
(+15 Pa) MEDIUM PRESSURE |
(0 Pa) LOW PRESSURE |
||
| Airflow Direction: Continuous One-Way Flow (Clean Room ──▶ Dirty Corridor) | ||||
Use a cascade airlock when the product is the sole priority and the surrounding area carries no hazard. It is simple to balance and predictable to monitor.
Bubble Airlock
The bubble airlock sits at the highest pressure in its zone. Air pushes outward through both doors. Nothing from either adjacent room enters the airlock, and the airlock isolates two clean spaces from each other.
| ROOM A | 🡸 | AIRLOCK | 🡺 | ROOM B |
|---|---|---|---|---|
| (+15 Pa) LOWER PRESSURE |
(+30 Pa) HIGHEST PRESSURE |
(+15 Pa) LOWER PRESSURE |
||
| Airflow Direction: Outward Both Ways (Protects Airlock from Adjacent Rooms) | ||||
Use a bubble airlock to separate two high-grade rooms that must not share air. It is common between a filling suite and its supporting corridor when both need protection.
Sink Airlock
The sink airlock holds the lowest pressure. Air flows inward from both rooms and gets captured. Hazardous particles cannot escape the airlock into surrounding spaces.
| ROOM A | 🡺 | AIRLOCK | 🡸 | ROOM B |
|---|---|---|---|---|
| (+15 Pa) HIGHER PRESSURE |
(0 Pa) LOWEST PRESSURE |
(+15 Pa) HIGHER PRESSURE |
||
| Airflow Direction: Inward Both Ways (Traps Contaminants Inside Airlock) | ||||
Use a sink airlock for containment. Potent active pharmaceutical ingredients, cytotoxic compounds, and biosafety work all demand inward airflow to protect the operator and the wider facility.
How to Calculate and Monitor Airlock Pressure Differentials
Airlock pressure is a function of airflow offset. You create a differential by supplying more air than you extract, or extracting more than you supply. The volumetric imbalance determines the pressure gap.
Estimate the required offset with the orifice flow relationship:
Estimate the required offset with the orifice flow relationship:
- Q = airflow through leakage paths (m3/s)
- A = total leakage area of the room (m2)
- ΔP = target pressure differential (Pa)
Follow this sequence to set and verify each airlock:
Follow this sequence to set and verify each airlock:
- Define the target differential. Set each door threshold to a 10–15 Pa gap based on the grade change.
- Measure the leakage area. Account for door gaps, pass-throughs, and wall penetrations.
- Calculate the offset airflow. Solve for Q to size the supply-versus-extract imbalance.
- Balance the system. Adjust supply and return dampers until each differential holds. Precise HVAC air balancing is what makes the calculated numbers hold in the field.
- Verify at every door. Confirm readings with all doors closed, then during single-door operation.
Monitoring keeps the differential inside spec after commissioning. Install Magnehelic differential pressure gauges at each airlock for continuous visual readout. For data logging, alarms, and audit trails, connect the airlocks to facility monitoring systems that record every excursion.
Material transfer deserves its own strategy. High-frequency door openings collapse a differential fast. Use pass-through boxes to move materials without breaching the pressure cascade, and reserve the personnel airlock for people.
Frequently Asked Questions
The accepted minimum is 10 Pa between adjacent classified spaces, with a working range of 10–15 Pa. Below 10 Pa, door movement and foot traffic disrupt directional airflow and containment fails. ISO 14644-4 does not fix a single number, but requires a stable differential sized to the contamination risk. The 10–15 Pa band satisfies that requirement in most applications.
Yes. Complex facilities often use hybrid layouts where an airlock behaves as a bubble against one room and a cascade against another. This happens when a single airlock separates zones of differing grade and risk. Each door threshold must be calculated and balanced independently, because the pressure relationship differs on each side. Continuous monitoring at both doors is essential to confirm the intended airflow direction holds.
Specify the Right Airlock the First Time
A miscalculated airlock differential contaminates product, exposes staff, or fails your next audit. ACH Engineering designs cleanroom HVAC systems and balances airlocks to hold precise, directional pressure across every classified zone.
Talk to our cleanroom design engineers. We translate your process requirements into compliant pressurization documentation and verified field performance.
Consult ACH Engineering for cleanroom HVAC design and airlock balancing →
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