Laboratory Containment Offset and Pressure Direction

It is a question about the OFFSET between exhaust and supply, not either one. The air change rate sets a floor on ventilation, the hoods usually exceed it, and the larger of the two governs -- so on most chemical laboratories the hoods set the ventilation rate and the air change requirement is satisfied as a by-product. THE OFFSET IS SMALL AND THE FLOWS AROUND IT ARE LARGE, WHICH IS THE FRAGILITY. Holding a room negative takes a modest excess of exhaust over supply -- commonly a few percent or a fixed figure -- sitting inside total flows an order of magnitude bigger. A supply filter loading by ten percent moves more air than the entire offset, and the room reverses with nothing visibly wrong: no alarm, no noise, no change a person in the space would notice. That is why a pressure relationship is monitored rather than balanced once. THE VAV HOOD SWING IS LARGER THAN ANY DRIFT. A sash closing can take a hood's exhaust down by hundreds of cfm in seconds, and if the supply does not track it the room goes positive and pushes laboratory air into the corridor. The control system, not the balance, is what holds the relationship on a variable-volume laboratory -- and an arrangement where the general exhaust alone still exceeds the supply is the one that survives a control failure, which is worth knowing when choosing between them. AND THE DIRECTION IS A HAZARD DECISION RATHER THAN A PREFERENCE. Most chemical laboratories are negative to the corridor so that anything released stays in. Cleanrooms, some biological areas and pharmacy compounding spaces are positive so that nothing gets in. Getting it backwards is not an error of degree, and a space that is both -- a compounding pharmacy handling hazardous drugs -- needs an anteroom to reconcile the two. This computes the offset and screens it. It does not model the room's pressure differential in inches of water column, which depends on the leakage paths and is measured rather than calculated, size or evaluate the door undercut and transfer path -- that calculation carries a noise ceiling that limits it, and a tightly weatherstripped door starves the offset while the differential reads deep -- compare the air change rate against occupancy targets, which the air changes per hour calculation does, determine the required rate, offset or differential for any occupancy, size supply or exhaust equipment, or address the control sequence, monitoring and alarms a containment space needs. ANSI/AIHA Z9.5, ASHRAE 170 where it applies, the institution's chemical hygiene plan, and the design engineer govern.

air change airflow = volume x ACH / 60; the governing rate is the LARGER of that and total exhaust; the offset is exhaust minus supply, and a supply drift percentage is compared against it directly.

Room air balance on entered airflows. The pressure DIFFERENTIAL is not modelled, because it depends on the leakage paths and is measured rather than calculated.

Air change arithmetic and one difference.

Estimate. AHJ and licensed professional govern.

Field names used by the API: room_volume_ft3, required_ach, hood_exhaust_cfm, general_exhaust_cfm, supply_cfm, hood_sash_closed_cfm, supply_drift_pct, ach_airflow_cfm, total_exhaust_cfm, governing_cfm, offset_cfm, offset_pct, sash_swing_cfm, drift_cfm

• The differential is not modelled inches of water column depend on the leakage paths; measure itthe commissioning agent
• Airflows are entered measured at the terminal, not scheduledthe test and balance report
• The transfer path is elsewhere door undercut carries the noise ceiling that limits itthe door undercut transfer air calculation