Atrium smoke control systems and their associated mechanical exhaust rates and makeup air often come with a unique and challenging set of design constraints. Mechanical exhaust systems must be designed such that exhaust inlets are distributed and oriented appropriately to prevent plugholing. These design constraints associated with exhaust are generally manageable and can be overcome. Providing makeup air for the smoke exhaust system is often the most challenging constraint to overcome when designing an atrium smoke control system using mechanical exhaust.
Makeup air can be provided from natural (i.e. operable doors/windows) or mechanical sources (i.e. supply air fans). How and where makeup air is introduced can have a significant impact on the atrium’s architectural design and the performance of the smoke control system.
Newly constructed atria smoke control systems are subject to the requirements of the International Building Code or NFPA 92 (Standard for Smoke Control Systems, 2012 edition is used here) and must adhere to the makeup air limitations of NFPA 92, Section 4.4.4.1.4.
The prescriptive requirements of this standard place a significant constraint on makeup air velocity. Specifically, makeup air velocities are not allowed to exceed 200 ft/min where the air may come in contact with the plume (NFPA 92, 4.4.4.1.4). Makeup air velocities greater than 200ft/min can affect the fire, disrupt it’s plume and consequently produce greater volumes of smoke requiring removal. Identifying adequate area to introduce makeup air at velocities near 200 ft/min (approximately to 2.25 miles/hr.) can be challenging compared to locating exhaust inlets which will have an effective area that is an order of magnitude less than that needed for makeup air.
NFPA 92 does allow for higher makeup air velocities if supported by an engineering analysis, often through the use of a computer fire model. Computational Fluid Dynamic (CFD) fire models (such as the Fire Dynamic Simulator – FDS) simulate fire growth and smoke movement throughout an atrium and allow for the performance of the system to be evaluated based on the specific architecture of the atrium and the arrangement of exhaust and makeup air. Makeup air velocities greater than 200 ft/min are able to be simulated and evaluated to determine effects on the smoke development and performance of the smoke control system. Generally speaking, atrium smoke control systems that are evaluated using a CFD model can often support makeup air velocities in excess of 200 ft/min and thereby may allow for greater flexibility in the atrium’s design.
Atria smoke control system designs are rarely the same from project to project but using a CFD model often yields consistent advantages by allowing for increased design flexibility and cost savings. If you have an atrium that requires smoke control on your project, we’d be happy to discuss it further – please contact us at info@crcfire.com.