42 www. refrigerationandaircon. co. za RACA Journal I February 2026
LABORATORIES technical
EXHAUST FLOW, FANS AND STACKS
By Eamonn Ryan
This is the second of a series of two instalments of insights from a SAIRAC Cape Town Centre technical talk.
… continued from part one in January.
The Cape Town Centre of SAIRAC recently hosted a technical session on the critical subject of controlling airborne pathogens. The technical talk was a joint presentation by Thabo Mngadi( AFMS mechanical engineer) and Cassandra-Anne Baxter( ALPA sales general manager).
In a standard configuration, the main exhaust duct includes bubble-type dampers, exhaust fans, a BIBO housing, and finally a stack that disperses clean air into the atmosphere. Exhaust velocity must be maintained between 15 – 20m / s, with stacks extending at least 3m above the highest roofline to prevent reentrainment of air.
Redundancy is built into the system by specifying two exhaust fans, ensuring uninterrupted operation if one fails. Before filter changes, both fans are shut down and the lab itself placed into shutdown mode.
DECONTAMINATION PROCEDURES Between the dampers and the BIBO sits a decontamination box, allowing fumigation of the housing before filters are changed.
Chemical agents such as vapourised hydrogen peroxide( VHP) or formaldehyde are circulated through the housing in a closed loop for up to two hours, neutralising any biological hazards before maintenance begins.
Alternative solutions include trolley-mounted decontamination units, which achieve the same result but with greater mobility. As Mngadi described, hoses and three-inch camlock fittings connect directly to the decon ports, creating a sealed loop that allows fumigant circulation without risk of escape.
Baxter expanded on the protocols, noting that international standards such as the BMBL( Biosafety in Microbiological and Biomedical Laboratories) require a secondary HEPA stage in BSL- 3 and BSL-4 labs. She outlined three principal decontamination methods:
• Chemical decontamination: most commonly with VHP, sometimes with formaldehyde. Aerosols are introduced and circulated until biological inactivation is confirmed
• Physical cleaning: external wipe-downs with alcohol or bleach. This method is discouraged since it may not reliably kill all pathogens
42 www. refrigerationandaircon. co. za RACA Journal I February 2026