Positive Pressure Systems

Positive pressure systems maintain building interior pressure above ambient atmospheric conditions through intentional supply-exhaust air imbalance, creating outward airflow that prevents unfiltered air infiltration. These systems suit applications requiring contamination exclusion, biosecurity protection, or environmental isolation from external conditions.

Fundamental Principles

Positive pressurization creates pressure differentials typically ranging from 0.005 to 0.020 inches water column (w.c.) relative to atmospheric pressure, sufficient to establish preferential airflow direction without excessive door opening forces or structural loading. The excess supply air volume exits through intentional relief paths or unintentional building leakage.

Airflow direction control represents the primary objective, preventing contaminated exterior air from entering through cracks, gaps, or door openings. When doors open, the positive differential drives interior air outward, preventing exterior air intrusion that could compromise interior environmental quality.

System design requires careful supply-exhaust balancing to maintain target pressure differentials across varying occupancy and door operation conditions. Automated pressure control systems modulate supply or exhaust airflow to maintain setpoints despite changing boundary conditions.

Application Requirements

High-efficiency particulate filtration typically precedes positive pressure spaces, removing airborne contaminants before air distribution into protected zones. HEPA filtration (99.97% efficient at 0.3 microns) proves common for cleanrooms and pharmaceutical facilities, while MERV 13-16 filtration suits agricultural biosecurity and sensitive manufacturing.

Airlock entry vestibules enhance contamination exclusion by creating intermediate pressure zones between exterior and protected interior spaces. The airlock maintains pressure above exterior ambient but potentially below the primary protected space, creating a graduated pressure cascade that minimizes contamination risk during personnel or material entry.

Personnel and material decontamination procedures complement positive pressurization, addressing contaminants that adhere to surfaces or clothing. Shower facilities, equipment cleaning stations, and material transfer protocols prevent contaminant introduction despite positive pressure protection.

Design Methodologies

Supply-exhaust imbalance calculations determine excess supply airflow required to maintain target pressure differentials. The required excess depends on building tightness, door opening frequency, and acceptable pressure deviation during transient conditions.

Building leakage characteristics govern required excess flow rates, with tighter construction requiring less excess to maintain pressure. Typical commercial construction requires 5 to 15 percent supply excess (relative to exhaust flow), while tight agricultural biosecurity facilities may require only 2 to 5 percent excess.

Door opening events temporarily reduce interior pressure as supply air rushes outward through the opened door. Recovery time depends on excess supply capacity and building volume, with design targeting pressure recovery within 10 to 30 seconds after door closure.

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