Fire Styling in Houses: Vapor Dashboard Regularities - Essay Example

Fire Design in Buildings: Smoke Control systems [Name] [Professor Name] [Course] [Date] Abstract: Installation of a smoke control system can greatly improve the life safety of an apartment block. The major forms of smoke control systems vary in complexity, such as the natural smoke ventilation system, mechanical smoke ventilation system and pressurization systems. Despite their divergent application scenarios, their basic objectives are similar. This essay provides a basic understanding of the use of natural and mechanical smoke ventilation systems in common corridors of apartment blocks and how CFD modeling could potentially be used to model the performance of smoke control system. PART I The use of natural and mechanical smoke ventilation systems in common corridors of apartment blocks Introduction Installation of a smoke control system can greatly improve the life safety of a building. Although the smoke ventilation systems vary in forms and types, they are intended for similar objectives, such as keeping the common corridors of apartment blocks free from smoke, creating smoke free layers to assist the firefighting operations, delaying and preventing the spread of fire, reducing the thermal damage to the building in the event of fire and lastly, reducing the heat and smoke damage resulting from fire. There are several legislations and standards aiming at proper installation of these systems to ensure they meet the target objectives, from the Building Regulations for England and Wales (2000), as specified in Approved Document B (ADB) 2007. Natural and mechanical smoke ventilations systems are generally installed in common corridors of apartment blocks since the main escape route in apartment blocks are often via common corridors. Additionally, smoke can spread from one accommodation if a door is left open for a short period of time hence filling the corridors thus making evacuation difficult for the occupants (Chow & Steemers 2006). Overall both smoke control systems are critical for improving the conditions for easy evacuation and firefighting. The smoke control systems achieve this objective by limiting the toxicity, clouding and thermal exposure in the corridors as well as improving the conditions for firefighting. It is therefore the responsibility of the designer and architect of the smoke control system to ensure that the proposed systems supplement the building fire strategy. In addition the system should supply the proper level of fire safety Natural Ventilation ADB, while at the same time allowing the natural and mechanical ventilation to apartment corridors. This ascertains that the natural ventilation is the standard form of smoke control system while the mechanical ventilation system is an alternative (Kleivin 2003). Natural ventilation Natural ventilation has various benefits such as reliability, simplicity, low energy use and low noise. Nevertheless, its performance can sometimes be susceptible to wind effects. For natural shaft systems, there is a comparatively greater loss of floor space. The operation of natural ventilation is characterized by exploiting the thermal buoyancy and natural forces of wind to force airflow through the ventilator (Chow & Steemers 2006). This operation uses the driving force harnessed from the buoyancy of the hot smoke from the fire. Given that the forces of buoyancy can be comparatively smaller than the forces of the wind, the performance is significantly vulnerable to wind. Additionally, for natural ventilation to function effectively there should be an exhaust opening and an air inlet source. For a vent that is mounted on the wall, the vent serves to provide the inlet at the bottom of the exhaust. In any case, the inlet air can be supplied through an open stair door. In assistance, and to vent out smoke entering the stairs, a vent would have to be installed at the head of the stair. As ADB stipulates a building should have natural vent shafts, natural wall vents and vents located at the head of the stairs. Aspects to consider in designing natural smoke ventilation A primary factor that should be considered when designing smoke ventilation include the free open area windows and smoke shaft doors, once activated to force smoke out of the corridor. Without doubt, the further the ventilators, windows and doors can be opened, the greater the space allowing smoke to escape from the building. On condition that the free area is achieved, the designer can either use any form natural smoke ventilator. The typical choices would be side pivoting windows, ventilator or louvered vent. A major disadvantage of the freedom of using any type of natural ventilator, the vents could selected and situated so that they are highly vulnerable to wind effects that could blow the smoke back to the corridor (Chow & Steemers 2006). Designers should therefore focus on mitigating the wind effects during the selection and positioning of the natural smoke ventilators. In cases where a roof light is used as the opening vent (automatic), in compliance with AD-B section BS EN12101-2:2003, an opening angle of at least 140 degrees can be effective in mitigating unfavorable wind effects. Driving force for natural ventilators The driving force for natural ventilation results from the stack effect, or buoyancy resulting from wind-induced action of warm air. The stack pressure is typical low in areas with low outdoor and indoor temperature differentials. For instance, when the indoor temperature is 23 degrees Celsius and the outdoor is 20 degrees Celsius would give 20 Pa in an apartment block building of 100 meters tall. In areas without local surface temperature distribution on the ground level, there are no upward warm air convective currents. Wind-induced action is the major driving natural ventilation in apartment blocks with no significant temperature and height differentials. Despite this, wind is a momentary phenomenon. The building has to be orientated and designed with regard to the area’s geographical features and records of wind action to provide data for assessing the natural indoor air flow. The design should as well be considered carefully with regard to the adjacent physical features of buildings (Kleivin 2003). Natural ventilation and static smoke exhaust systems Studies have identified smoke as a major concern in fire safety, specifically in high-rise apartment blocks. Fire from the affected apartments is in fact unlikely to heat up the while corridor space. Additionally, fire whirls are not triggered since there is no strong internal airflow. Basically, the driving force for smoke movement can be equated to those of the natural ventilation, namely wind induced action and stack effect. Stack effect is however insignificant when there is low outdoor and indoor temperature difference. On the other hand, wind-induced effects rely on the number of opening in the corridor, such as vents, door or windows. For instance, an air-conditioned or ventilated corridor in an apartment block, with doors isolated from the outside using air curtains experiences less wind-induced effects compared to when the smoke vent is opened. In areas that that have no restricted distribution of temperature to supply thermal conditions, buoyancy of warm air in natural ventilation wouldn’t be high before a fire occurs. Conversely, in the event of a fire, the hot air generated would rise due to buoyancy thus affecting the performance of static smoke exhaust system. Mechanical Smoke Ventilation System As recommended in AD-B, mechanical smoke ventilation may be used in corridors of apartment blocks as alternative to natural ventilation systems. Until recently, regulatory documents including British Standards and ADB specified smoke control systems installation for buildings for natural ventilations. The guidance of the use of mechanical smoke ventilation systems in residential corridors is provided for by the BS 9991:2011 code of practice. Using mechanical ventilation system has several advantages, including low susceptibility to wind action, specified rate of extraction, reduced cross section of shafts and determinable capacity to overcome system resistance. Practicalities in using of Mechanical Systems Work Mechanical ventilation systems typically operate as depressurization systems, as they are designed to extract heat and smoke from areas, hence depressurizing the space. Since the areas that surround the corridors, such as the stairs, have high pressures, air is likely to be retained from these sections towards the smoke shaft, hence preventing smoke from flowing into the staircase or the rooms of apartment from the room of origin. It is essential to provide extra inlet air to the areas being depressurized to prevent the area from being extremely depressurized. In case the area becomes depressurized, it can affect the smoke venting equipment. Additionally, smoke may be effectively extracted from the room where the fire originates. Across the doors, pressure differentials can also increase to a great extent, thus inhibiting the door from opening, or pulling open between the room he smoke is originating and the depressurized area. The composition of the inlet air for the ventilation system may be attained in a number of ways. This will however depend on the layout of the building. The makeup air for the system can however be attained through a natural inlet air through the AOV at the head of the stairs, natural air through the external air, mechanical inlet air through the shaft and a natural air through the shaft. Aspects to consider in using mechanical ventilation systems The requirements for using the mechanical system include the need to have a standby fan, maintained power supply, fire resistant wiring and temperature classified equipment. Internal pressures inside the building also have to be considered as well as restricted to enable the doors to be operable. It is also crucial that an air inlet be provided to the corridor to prevent the system from getting damaged and to check against extreme depressurization and pressurization, as this ensures that significant amounts of smoke are not eliminated from the apartment where the fire originates and the increased pressure differentials are eliminated, which would pull the escape doors open or render them inoperable. Additionally, designs should be located on one floor level that has been affected by fire, in which case, only the smoke vents on the affected floor and additional design critical vents should be open. The designers of the system should also keep from opening the ventilators of multiple floor levels, particularly where they are linked by a smoke shaft. This will prevent smoke from spreading to unaffected areas of the building, as well as minimize smoke extraction rate from the floor where the fire originates. Using Mechanical ventilation systems in Apartment blocks ADM specifies that residential buildings with floors that are beyond 11 meters, typically more than 3 stories above the ground, should have each flat separated from the staircase by a corridor. Further, the document recommends that the corridors in such apartment blocks should have ventilation through a 1.5 square meters AOV direct to outside. Since the mechanical ventilation system extracts smoke and heat from the corridor is not dependent on the natural buoyancy of the smoke, more reduction of the shaft area is more practicable than that of the natural system. The area of the smoke shaft for mechanical ventilation system depends greatly on the size of the corridor, the extract rate of the fans, the size of fire in the room of origin and the size of the AOV opening into the shaft. Comparing natural and mechanical smoke ventilation There is a significant difference between the natural and the mechanical shaft areas. In cases where saving space in an apartment block is important, using a mechanical smoke ventilation saves floor space in corridors where is it is installed. Additionally, it should be noted that since the shaft is expected to pass through the floors of the compartment, it has to be fire rated, and since the mechanical smoke shaft is relatively smaller, it needs less fire rated construction, thus saving on the cost of installation. Compared to natural ventilation system, mechanical ventilation system can extract more smoke or heat from the corridor, thus ensuring that the corridors have more tenable conditions to allows the occupants of the building to evacuate safely, despite the length of the travel distances. PART II How CFD modeling could potentially be used to model the performance of smoke control system and hence justify the extended corridor arrangement Computational fluid dynamics (CFD) modeling is technique for real time analysis of the interaction of physical properties, which generate geometrically tailored solutions thus offering a great level of accuracy compared to hand calculations. Typically, it works by allowing a structure or an object to split into a computational mesh that contains hundreds of small volume cells (Jiru & Bitsuamlak 2010). The technique applies appropriately validated transport equations to each of the cells that can describe the transfer of quantities such as smoke or heat. In brief, it is a numerical modeling technique applied in simulating the condition associated with the behavior of fire or smoke in buildings. In compliance with the Building Regulations Approved Document-B Volume 2, improved ventilation system can be determined using reliable and valid calculation methodologies. The technique offers an insight into the movement of smoke through the corridors of apartment blocks and its impact on the protected areas like the staircase. Indeed, a number of studies have examined the direct application of CFD modeling, supplemented with experimental validation for designing natural smoke ventilations of building. The main approaches employed in such scenarios include whole-domain and domain decomposition CFD modeling. Before applying the CFD modelling, it is important to note that the outdoor microclimate conditions, include; the speed of wind, humidity and temperature and the surrounding aerodynamic. Factors such as the orientation and location of the building, the size and number of openings, the topography, determine the effectiveness of natural smoke ventilation. Due to these factors, it is essential that natural smoke ventilation designs be incorporated to the building in the early stages of the design processes. Towards this end, numerical modeling with CFD can be used to model the performance of smoke control system (Jiru & Bitsuamlak 2010). The main advantage of CFD modeling is that it offers information on the flow field variable with difficulty of controlling the experimental conditions, such as in full-scale experiments. The concept of domain decomposition CFD modeling technique (which this paper uses) was first introduced by Karabuchi et al (2008, 2009) as used in modeling natural ventilations. The technique involves three stages, namely outdoor flow field simulation, which is conducted for sealed buildings to design the airflow structure around the building. Next, the boundary conditions at the surface of the building, which are collected, and lastly, using the conditions of the collected boundary airflow simulation, which is conducted for the interior or the building. This approach has the advantage of not requiring the need for replicating the simulation of full flow field in case the positions of the openings are changed. Such changes can however be handled in the third stage through use of proper boundary conditions that have been collected in the second stage for the new position of the openings (Meroney 2009). The discharge of coefficient for an opening can be determined using the local dynamic similarity model (Karabuchi et al, 2004). It can further be used to evaluate the inflow or outflow rate of each of the opening. Karabuchi et al (2008) illustrated the validity of the domain decomposition model for cross-ventilation using a 15-storey building with wall windows. The conditions of the boundaries are imposed on the numerical domain to determine the internal flows using CFD. The flow rates of the opening for the internal flow calculations are estimated using the relations: Q = C Din AV H  Cpin ernal = (CPin + Cpout)/2 In the above formula, Q represents the cross-ventilation flow rate (m3/s), CDin represents the discharge coefficient, while A is represents the opening area (m2), VH represents the wind-speed at the height of the building, while Cpin and Cpout represents the pressure found at the opening locations of the sealed building at the event of full-external-flow CFD analysis (Meroney 2009). CDin , or the discharge coefficient, is estimated by the domain decomposition CFD modeling from the estimated tangential and internal flow pressures and external opening. However, for this scenario, the values are determined by their method to all equal 0.67. The other important opening inlet condition specified from the full-CFD calculations is the incident flow angle outside the upwind opening. The conditions of the boundary are enough to determine the flow field of the internal building using CFD. Given the pressure doesn’t have to be balanced between the inside of the several rooms, the subsequent iteration of the internal flow condition is not required (Stroup & Madrzykowski1995). Hence, when the domain decomposition modeling is applied to the 15-storey building, the results are compared to the wind tunnel measurements and the full CFD predictions of internal and external flows. For instance, when the internal building domain mesh consists of 51,200 hexagonal cells that are 0.25 centimeters wide on a side, with an inlet flow turbulent intensity of 10 percent and a length scale of 1 centimeters, the boundary conditions estimated from the full-CFD sealed apartment solution would be as shown below: Case Cpin CPinternal CDin Q(m3/s) Vin (m/s) (-)(deg) Building A 0.856 0.275 0.67 0.00293 3.5 20 Meroney (2009) suggests that when the domain decomposition CFD modeling calculations are compared to the measurements of the wind-tunnel of natural smoke ventilations, they produce accurate results that can support the design of the smoke ventilation systems. In conclusion, the above data can justify the extended corridor arrangement as it enables the corridor to be cleared of smoke more rapidly at the early and later stages of fire, when the natural smoke ventilation system is overwhelmed. For instance, CFD modeling could indicate that the corridor can be overwhelmed in less than 50 seconds, hence the need for an extended corridor arrangement. For instance, if the temperature is 50 degrees Celsius, at 25 seconds the smoke would reach almost half of the corridor, and at the end of the corridor in about 36 seconds. At 50 seconds, the flow would have reached the end of the corridor and a return wave would circulate to fill the corridor. The extended corridor arrangement would however allow safe escape conditions when the travel distance is extended by 3 or 4 meters in a single direction (Meroney 2009). Conclusion In conclusion, it is critical to install smoke control system to improve the life safety of a building. The major forms of smoke control systems vary in complexity, such as the natural smoke ventilation system, mechanical smoke ventilation system and pressurization systems. A primary factor that should be considered when designing smoke ventilation include the free open area windows and smoke shaft doors, once activated to force smoke out of the corridor. In modeling the performance of smoke control system, domain decomposition CFD modeling can be used since it offers information on the flow field variable with difficulty of controlling the experimental conditions, such as in full-scale experiments. References Chow, C & Steemers, K. 2006. Possible conflicts on smoke control in buildings with natural ventilation. Cambridge, University of Cambridge. (Online) Retrieved: Accessed 16 July 2013 Jiru, T. & Bitsuamlak, G. 2010. Advances in applications of CFD to natural ventilation. The Fifth International Symposium on Computational Wind Engineering (CWE2010) Chapel Hill, North Carolina, USA, May 23-27, 2010 Kurabuchi ,T., Ohba, M., Endo, T., Akamine, Y., Nakayama, F., 2004. Local Dynamic Similarity Model of CrossVentilation Part 1- Theoretical Framework. International Journal of Ventilation, 2(4), 371-382. Kurabuchi, T., Nonaka, T., Ohba, M., 2008. Domain Decomposition Technique Applied for Cross-Ventilation of Building. Proceedings of the 4th International Conference on Advances in Wind and Structures (AWAS'08), Jeju, Korea, May 29-31; 1129-1140. Kurabuchi, T., Ohba, M., Nonaka, T., 2009. Domain Decomposition Technique Applied to the Evaluation of CrossVentilation Performance of Opening Positions of a Building. International Journal of Ventilation 8(3). Kleivin, T. 2003. Natural Ventilation in Buildings: Architectural concepts, consequences and possibilities. (Online) Retrieved from: Accessed 16 July 2013 Meroney, R. 2009. CFD Prediction of Airflow in Buildings for Natural Ventilation. Paper Prepared for 11th Americas Conference on Wind Engineering June 22-26, 2009 San Juan, Puerto Rico Stroup, D. & Madrzykowski, D. 1995. Modeling Smoke Flow in Corridors. Paper prepared for International Conference on Fire Research and Engineering, September 10-15,1995. Orlando, FL Proceedings. Tanaka, T. 1999. “Performance Based Fire Safety Design Standards And FSE Tools For Compliance Verification.” International Journal on Engineering Performance-Based Fire Codes, Vo1. 1, No. 3, p.104-117 Read More

Natural ventilation Natural ventilation has various benefits such as reliability, simplicity, low energy use and low noise. Nevertheless, its performance can sometimes be susceptible to wind effects. For natural shaft systems, there is a comparatively greater loss of floor space. The operation of natural ventilation is characterized by exploiting the thermal buoyancy and natural forces of wind to force airflow through the ventilator (Chow & Steemers 2006). This operation uses the driving force harnessed from the buoyancy of the hot smoke from the fire.

Given that the forces of buoyancy can be comparatively smaller than the forces of the wind, the performance is significantly vulnerable to wind. Additionally, for natural ventilation to function effectively there should be an exhaust opening and an air inlet source. For a vent that is mounted on the wall, the vent serves to provide the inlet at the bottom of the exhaust. In any case, the inlet air can be supplied through an open stair door. In assistance, and to vent out smoke entering the stairs, a vent would have to be installed at the head of the stair.

As ADB stipulates a building should have natural vent shafts, natural wall vents and vents located at the head of the stairs. Aspects to consider in designing natural smoke ventilation A primary factor that should be considered when designing smoke ventilation include the free open area windows and smoke shaft doors, once activated to force smoke out of the corridor. Without doubt, the further the ventilators, windows and doors can be opened, the greater the space allowing smoke to escape from the building.

On condition that the free area is achieved, the designer can either use any form natural smoke ventilator. The typical choices would be side pivoting windows, ventilator or louvered vent. A major disadvantage of the freedom of using any type of natural ventilator, the vents could selected and situated so that they are highly vulnerable to wind effects that could blow the smoke back to the corridor (Chow & Steemers 2006). Designers should therefore focus on mitigating the wind effects during the selection and positioning of the natural smoke ventilators.

In cases where a roof light is used as the opening vent (automatic), in compliance with AD-B section BS EN12101-2:2003, an opening angle of at least 140 degrees can be effective in mitigating unfavorable wind effects. Driving force for natural ventilators The driving force for natural ventilation results from the stack effect, or buoyancy resulting from wind-induced action of warm air. The stack pressure is typical low in areas with low outdoor and indoor temperature differentials. For instance, when the indoor temperature is 23 degrees Celsius and the outdoor is 20 degrees Celsius would give 20 Pa in an apartment block building of 100 meters tall.

In areas without local surface temperature distribution on the ground level, there are no upward warm air convective currents. Wind-induced action is the major driving natural ventilation in apartment blocks with no significant temperature and height differentials. Despite this, wind is a momentary phenomenon. The building has to be orientated and designed with regard to the area’s geographical features and records of wind action to provide data for assessing the natural indoor air flow. The design should as well be considered carefully with regard to the adjacent physical features of buildings (Kleivin 2003).

Natural ventilation and static smoke exhaust systems Studies have identified smoke as a major concern in fire safety, specifically in high-rise apartment blocks. Fire from the affected apartments is in fact unlikely to heat up the while corridor space. Additionally, fire whirls are not triggered since there is no strong internal airflow. Basically, the driving force for smoke movement can be equated to those of the natural ventilation, namely wind induced action and stack effect. Stack effect is however insignificant when there is low outdoor and indoor temperature difference.

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