Positive Pressure

Positive pressure: make leakage flow away from the protected space. A room is positively pressurized when its air pressure is higher than that of a specified neighboring space. At an ordinary leakage opening, this difference tends to drive air outward, reducing entry of unfiltered neighboring air. It is one contamination-control measure, not a guarantee of cleanliness or a substitute for filtration, cleaning and controlled movement.

1. Pressure is always relative to somewhere. A cleanroom can be positive to a corridor yet negative to a cleaner adjoining room. A pressure cascade arranges these differences so leakage follows the intended direction between zones. Identify both sides of every measurement: “the room is positive” is incomplete without its reference space. The example values below explain the relationship; they are not design setpoints.

Three zones at decreasing pressures drive leakage from the protected room through the airlock toward the corridor Air follows the pressure difference Illustrative values, all relative to the corridor protectedairlockcorridor +20 Pa+10 Pa0 Pa Arrows show leakage through closed-boundary gaps

The setpoint belongs to the facility design. Required pressure differences depend on room function, enclosure leakage, doors, exhaust and applicable requirements. More pressure is not automatically better: it can increase leakage, conditioning demand and door forces. Product protection and hazardous-material containment can require different airflow directions. Local exhaust enclosures and room pressure must be coordinated; a positive cleanroom does not by itself contain hazardous releases.

2. The air balance creates the pressure. For a simplified enclosure at steady state, incoming makeup air equals outgoing exhaust plus net outward leakage. Internal recirculation crosses neither side of that enclosure boundary, so it cancels from this balance. A real room may also have ducted supply, return and transfer flows; include every flow that crosses the chosen boundary. Merely recirculating more filtered air is not the same as adding net air to the enclosure.

Makeup air entering an enclosure balances exhaust and outward leakage while internal recirculation stays within the boundary Count air crossing the boundary Simplified steady-state enclosure balance makeupexhaustleakage recirculate makeup = exhaust + net outward leakage

Leakage is the outlet, not an endless accumulation. Once pressure stabilizes, air is not continuously piling up inside the room. Outflow balances inflow. Opening a larger path lets more air escape at a given pressure difference; if the supply cannot compensate, the pressure difference falls. For an idealized opening, flow scales roughly with opening area times the square root of pressure difference. Actual cracks, dampers and doors need an appropriate model rather than one universal leakage coefficient.

3. An open door changes the problem. A pressure difference maintained across small closed-door gaps can collapse when the opening becomes large. Door motion, people and temperature differences can produce local mixing even when the closed-door pressure reading was satisfactory. An airlock reduces direct connection between zones by controlling the opening sequence; it does not make every transit disturbance disappear. Recovery must be assessed for the operating conditions that matter.

An illustrative room pressure difference drops during door opening and recovers after closure A door opening is a disturbance Illustrative transient, not a measured recovery time door openrecovery pressure difference time → A recovered pressure does not prove particles have cleared

Measure the boundary you intend to protect. A differential-pressure sensor compares two locations. Wrong reference tubing, blocked ports, drift or a local air jet can mislead the reading. Record the sign convention and verify sensor performance. Trend door state, pressure and relevant ventilation status together so an operator can distinguish an expected transit event from a sustained loss of control.

An alarm starts diagnosis, not a blind fan-speed increase. Possible causes include changed exhaust, reduced supply, a door left open, a new leakage path or an instrumentation fault. Investigate under the facility's approved response procedure and assess exposed work where required. Pressure, installed filter integrity, airflow patterns and particle concentration answer different questions; none substitutes for all the others.

The practical takeaway: positive pressure steers leakage in the intended direction across a defined boundary. It works only as part of a qualified air balance, enclosure and operating procedure, including normal door use and credible disturbances.

Further reading: Terra Universal, Cleanroom HVAC Solutions and Positive/Negative Pressure Cleanroom Designs.

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