Engineering Manual o.. - HVAC.Amickracing

SMOKE MANAGEMENT FUNDAMENTALSThe door widths in Table 2 apply only for doors that arehinged at one side. For other arrangements, door sizes, or forhardware other than knobs (e.g., panic hardware), refer tocalculation procedures furnished in Design of Smoke ControlSystems for Buildings published by ASHRAE 3 .SMOKEBACKFLOWRELATIVELYLOW AIRVELOCITYSMOKEAIRFLOWis most commonly used to stop smoke movement through opendoorways and corridors. Figure 4 illustrates a system withrelatively high velocity to prevent backflow of smoke throughan open doorway. Figure 5 illustrates a system with relativelylow velocity which allows backflow of smoke. The magnitudeof the velocity of the airflow required to prevent backflowdepends on the energy release rate of the fire. Since this canvary, the velocity should be regulated to prevent oxygen frombeing fed to the fire. The fact that doors are sometimes leftopen during evacuation of a building, allowing smoke to flowthrough, should be taken into account in designing the smokecontrol system. This is done by designing and testing the systemwith one or more doors open.RELATIVELYHIGH AIRVELOCITYDILUTEDSMOKEM13024Fig. 4. High Air Velocity Preventing Backflowof Smoke Through an Open Doorway.PURGINGM13025Fig. 5. Low Air Velocity Allowing Backflowof Smoke through an Open Doorway.Because fires produce large quantities of smoke, purging cannotensure breathable air in a space while a fire is in progress. After afire, purging is necessary to allow firefighters to verify that thefire is totally extinguished. Traditionally, firefighters have openeddoors and windows to purge an area. Where this is not possible,the HVAC system can be designed to have a purge mode.The principle of dilution can be applied to zones where smokehas entered and is being purged. Purging dilutes the contaminatedair and can continue until the level of obscuration is reduced andthe space is reasonably safe to enter. The following equation allowsdetermining a concentration of contaminant in a compartmentafter purging for a given length of time: 3Where:C = C0 x e –atC = concentration of contaminant at time, tC0 = initial concentration of contaminanta = purging rate in number of air changes perminutet = time after doors close in minutese = constant, approximately 2.718Care must be taken in the use of this equation because of thenonuniformity of the smoke. Buoyancy is likely to cause greaterconcentration of smoke near the ceiling. Therefore,consideration of the locations of supply and exhaust registersis important to effective purging.CONTROL APPLICATIONSFigure 6 illustrates a smoke control system with detectors,an initiating panel, and a communications bus to an alarmprocessor and remote control panels in appropriate areas of thebuilding. A configuration similar to this will meet therequirements of UL 864, Standard for Control Units for Fire-Protective Signalling Systems, and comply with NFPA 92Arecommended practice for smoke control systems. The remotecontrol panels position dampers and operate fans to contain orexhaust smoke, depending on the requirements of the variousareas in the building. The system can have an operator’s controlconsole for the building personnel and an FSCS from which toview the status of and override the smoke control system. Thesystem requires a means of verifying operation, such asdifferential pressure or airflow proving devices, for each controlsequence. An uninterruptible power supply (UPS) is optionalbut recommended.178ENGINEERING MANUAL OF AUTOMATIC CONTROL