The design of the BSC exhaust system must consider the static pressure of the cabinet with dynamic filter loading over time. This static pressure value, generally assumed to be twice the initial pressure drop for the new (unloaded) HEPA filters, provides for a reasonable life of the HEPA filter(s). Therefore, the initial balance point for the exhaust is set at twice the initial pressure drop that is actually required. A VAV exhaust system can reduce energy consumption in contrast to exhaust systems that are balanced for the eventual loading of these filters, because a VAV system adjusts flow slowly over time to match actual filter loading.
The same exhaust air system can be used for laboratories, chemical fume hoods, and BSCs, according to the National Institutes of Health (NIH). This provides an energy-efficient, cost-effective installation of back-up exhaust fans. Exhaust for these cabinets may be singly vented or manifolded with other biological safety cabinets. However, when biological safety cabinets are ducted into manifolded constant-volume or VAV systems, the cabinets must be isolated from system airflow fluctuations and static pressure changes. [Simons, 1991; Crane, 1994]
