Smoke Control in the Atrium - Report Example

Running header: Smoke control in the Atrium Student’s name: Instructor’s name: Subject code: Date of submission: Smoke control in the Atrium Introduction Over the years, smoke has come to be recognized as a major source of deaths in atrium fires. As such, a lot of research has been conducted in a bid to come up with effective smoke control systems in atrium which will not only ensure safety of property but more so the safety of the people who may be in the atrium in event of fire outbreaks. Owing to the research, natural smoke venting has come to be recognized as an effective method of preventing audience from smoke (Klote, 2002). In addition, a number of modern design approaches have also been developed for dealing with smoke in atrium. Sustainable smoke control systems in the atrium also help in minimizing and eliminating components that are found in conventional smoke control systems. As such, they are also deemed advantageous since they consume less power. This paper discusses the various systems and approaches available for dealing with smoke in atrium (NFPA92B, 2005). It also discusses design issues and challenges associated with smoke control in atrium. Smoke control systems for atrium Both natural and mechanical systems are used for smoke control in atrium. The systems include natural smoke filing systems, steady mechanical smoke exhaust systems, unsteady mechanical smoke exhaust systems, steady natural smoke venting systems as well as unsteady natural smoke venting systems. Airflow is also used in combination with these systems in smoke control although caution has to be taken since airflow can provide combustion air to the fire thus worsening the situation (Duda, 2004). These approaches have the main aim of preventing occupants from coming into contact with smoke by controlling smoke such that it descends only to a certain height during operation of the smoke control system. The predetermined height is usually between 6- 10 ft above the highest walking portion forming part of a required exit in the atrium. Atrium smoke control systems are also intended to provide tenability incase occupants come into contact with smoke (Klote, 2012). This implies ensuring visibility and limited exposure to heat, thermal radiations as well as toxic gases in a bid to save life. a) Natural smoke filling systems In this case, smoke is allowed to fill the atrium without any smoke removal. To ensure that occupants do not come into contact with smoke during evacuation, filling time has to be greater than the time required for evacuation for conventional smoke filling systems. However, this is not the case with tenability smoke filling systems since they allow occupants to come into contact with smoke as long as a tenable environment is maintained throughout the evacuation process and hence evacuation time need not be greater than the filling time. However, maintenance of a tenable environment is dependent on heat release rate of the design fire, the kinds of materials burnt as well as the location of the fire (Hadjisophocleous, 2008). b) Steady mechanical smoke exhaust systems This system uses mechanical exhausts designed to keep the bottom of the smoke layer at the predetermined height for the design fire. c) Unsteady mechanical smoke exhaust The system uses mechanical smoke exhaust although the exhaust’s flaw rate is less than that of mechanical exhaust and hence it slows the rate of descent of the smoke layer for some times hence allowing occupants to exit from the atrium. d) Steady natural venting systems The system uses non-powered smoke vents at the top of the atrium as opposed to exhaust fans commonly known as gravity venting since the smoke is vented as a result of buoyancy (Jackson, 2007). It is important that the smoke flow rate through the vents is maintained at a level where the bottom smoke layer is always at the predetermined height indefinitely. e) Unsteady natural venting systems Though similar to steady natural venting system, this system uses a smoke venting rate that slows the rate of smoke layer descent for a time to give time for safety exit of occupants from the atrium. Methods of analysis of atrium smoke control systems /calculations There are a number of methods used in analyzing smoke control systems in atrium including zone firing model, scale modeling, CFD modeling and algebraic equations. In order to come up with an effective smoke control system for atrium, some calculations are inevitable. They include smoke transport calculations, tenability calculations and evacuation calculations. Smoke transport calculations mainly deal with the rate at which the smoke diffuses into the atrium in order to determine how fast the smoke control system should be. Tenability calculations deal with the direct effects of the smoke to human life through heat and thermal radiation as well as toxic gases (NFPA92A, 1999). Smoke prevents people from seeing properly, they walk slowly and become disoriented which in turn prolong their exposure to smoke. As such it is important to analyze these effects in a bid to build a tenable environment throughout the evacuation process. Evacuation calculations on the other hand deal with establishment of pre-movement time as well as movement time (Daphine, 2002). Pre-movement time is the time taken for occupants to be ware of the smoke and hence initiate movement or evacuation process. Hence, when calculating evacuation time, one must give reasonable allowance for pre-movement time. Algebraic equations Atrium smoke control systems make use of a number of algebraic equations some of which are based on basic principles of engineering while others are empirical correlations based on research data. Common algebraic equations for smoke control systems in atrium include equations for natural venting, equations for the air velocity to prevent smoke backflow as well as equations for smoke filling. However, the algebraic equations are used in other models of smoke control including zone fire modeling and CFD modeling. Zone fire modeling Zone fire models are models which consider the fire compartment as being divided into two zones namely the smoke layer as well as the layer free of combustion products. The layers change in size depending on smoke flow (Walton, 2000). The temperature as well as concentration of contaminants changes throughout the smoke layer with the highest concentration being at the top. There is also a thin transition layer occurring between the smoke and the smoke free layer. These models are useful in designing of smoke control systems for atrium. CFD modeling The CFD model divides the atrium into a number of cells with the aid of computer programs to arrive at the governing equations for individual cells. The model is thus capable of producing realistic simulations. The model simulates the plume, smoke layer, transition zone as well as the ceiling jet. The models are also capable of simulating adverse effects of makeup air velocity during plume formation (Beverly, 2008). However, the models require high level of knowledge and are also time consuming sometimes requiring days to complete. Scale modeling These models give highly realistic simulations. The model involves conducting of fire tests on small model atrium and then converting the results to a full atrium scale. Designing of smoke control systems for atrium Design issues and design fires A design scenario for a smoke control system takes into consideration fire location, heat release rate, materials being burnt and weather. Designing a smoke control system should include a number of scenarios so that one can be assured that the system will function effectively. As such, design fire should be realistic and the analysis ought to include design fires located in the atrium as well as in the communicating spaces (Quintere, 2003). In this regard, a number of scenarios have been identified with configuration of smoke plumes that may occur within atrium. a) Axisymetric plume it results from fire occurring near the centre of atrium. Entrainment of air occurs over full plume height until it interfaces with a smoke layer that may have formed above. b) Wall plume – this is a plume generated by fire against a wall. c) Corner plume- this kind of a plume is generated by a fire located in the corner of the atrium. d) Window plume – this is a plume flowing from a window or doorway in an enclosed space (Murkowski, 2001). Design issues Atrium temperature The temperature of the air that is below the smoke layer rapidly approaches outdoor temperature for systems relying on mechanical smoke exhaust. This is caused by the large amounts of makeup air entering the atrium. As such, design analysis for such systems should use the outside design temperature for the ambient atrium temperature. As the temperature of the gases increase, its density decrease and hence leading to an increase in the volumetric flow rate needed for maintaining constant mass flow. This implies that atrium exhaust fans have to be sized to allow for maximum flow required for controlling smoke for the design conditions (Vincent, 2009). Therefore, smoke exhaust fans will need to be sized with an ambient atrium temperature that is equal to the outside design temperature. Minimum smoke layer depth This should be 20% of the floor –ceiling height except when the design models (e.g. CFD models) indicate otherwise. Upon the smoke plume reaching the ceiling, the smoke flows away from the impact point in a radial direction thus forming a ceiling jet. When this jet reaches a wall, the flow turns down and back under the ceiling jet (Lougheed, 2005). This ceiling jet is about 10% of the floor-ceiling height in depth same as the flow under the ceiling jet implying that the smoke layer depth should be 20% of the floor-ceiling height. Makeup air This is outdoor air which is supplied either by outside openings or mechanical fans. For systems having fan powered smoke exhaust, make up air has to be provided to enable the fans remove the design quantities of smoke and also to ensure the door opening force requirements are not exceeded. However, this make up air has to be provided far below the smoke layer interface so that the smoke layer is not disrupted. If make up air is provided using large openings such as vents, they should open automatically on activating the system (Hadjisophocleous, 2007). As such, the vents should provide 85-95% of the make up air while the rest is provided by leakage paths such as construction cracks. When the makeup air is provided by mechanical fans, it should be designed at 85-95% of the exhaust while the remaining air is provided by leakage paths in a bid to prevent positive pressurization of the atrium. The make up air should be 200fpm where it could come into contact with the plume so as to prevent deflection of the plume and disruption of the smoke layer. This is because deflection of the plume can lead to smoke control system failure. Wind Smoke control systems for atrium should be such that they minimize wind potential for resulting in velocities that are more than 200fpm making makeup air to come into contact with the plume. They should also prevent wind’s potential for resulting in smoke feedback from smoke vents into make up air (Sharon, 2006). As such, all makeup air openings should face one direction and mechanical fans should be used for smoke exhausts and makeup air to reduce the impact of wind. To prevent wind from carrying smoke from smoke exhausts to makeup air openings, the smoke vents and make up air openings should be placed far from each other so that the wind carries smoke away from makeup air inlets. As such, wind tunnel analysis should be carried out in a bid to evaluate wind’s potential for smoke feed back in the atrium. Stratification This is a situation where a hot layer usually 50 degrees forms under the atrium ceiling owing to solar radiation on the roof of atrium. This result in the formation of a stratified smoke layer under the hot air when the plume temperature is lower than that of the hot air layer thus preventing smoke from reaching ceiling mounted smoke detectors. When smoke stratification occurs, it is advisable to use projected smoke detectors. Control and operation of smoke control systems The smoke control systems should be automatically automated so as to protect occupants from smoke. Methods of automatic system activation include projected smoke detectors, heat detectors, sprinkler water flow and ceiling mounted smoke detectors (Patina, 2007). In addition, the system needs to reach full operation before the atrium conditions reach design conditions. In determining the time needed for full system operation, one needs to take into account the fire detection time as well as the HVAC system time of activation. In addition, manual systems for starting and stopping the smoke control system should be availed at a location which is close to the fire department. Difficulties for smoke control systems in atrium Apart from the issues raised above, a number of errors that could lead to system failure exists and hence should be avoided. Such errors can occur in design, construction and implementation of the system. These include; a) Inadequate theories – calculation methods are usually based on theories developed for idealized geometries. However, these can defer in real life situations (Simon, P2007). For instance spill plumes may rise past non straight spill edges or occurrence of plumes that are partially free and partially adhered. b) Unknown input data –although the building geometry is usually known when designing, there are many input parameters that are subjective. c) Poor communications-errors can occur in communication between system designers and installers as well as regulators that can lead to system failure. In addition, changes in atrium structure over time can be very significant as to render the smoke control system ineffective. d) Poor construction and installation –in most cases workers installing the system usually don’t know its use and at times they are under pressure to finish installation (Justa, 2010). As such a number of errors can occur including installing of funs backward, fitting equipments with its packaging materials or even leaving equipments such as wheelbarrows in the smoke vents hence blocking it. Such errors can lead to system failure or ineffectiveness. As such, there should be maximum cooperation between the system designers, installers and regulators in a bid to minimize chances of occurrence of these errors which could make an otherwise good smoke control system for atrium ineffective thus exposing the occupants to smoke (Janet, 2010). Conclusion Many of the deaths that occur in atrium fires are caused by inhaling of smoke. In addition, smoke reduces visibility of escape routes in addition to causing pain in the eyes and respiratory tract. Smoke may also contain toxic hazards of harmful gases such as carbon monoxide and hydrogen cyanide. Atrium contains large enclosed spaces and in a fire scenario, there is no physical separation thus enabling smoke to move uninterrupted to locations far from the source (Halleck, 2004). As such, controlling smoke production and movement in atrium is of paramount importance. Having a smoke control system in atrium will provide conditions for escape ensuring that the occupants do not come into contact with the smoke. However, to ensure effectiveness of such a system, appropriate calculation methods should be used. In addition, it is important that all stakeholders in the design and installation of a smoke control system ensure effective communication so as to avoid errors that may lead to system failure. References: Klote, J2002, Principles of smoke management, Atlanta, ASHRAE NFPA92B, 2005, Standard for smoke management systems in atria, malls and large areas, Quincy, National fire protection association. 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