Smoke Control Systems - Case Study Example

Smoke control systems Name: Tutor: Course: Date: Table of Contents Table of Contents 2 List of Figures 4 List of Tables 4 1.0 Introduction 5 1.1Natural ventilation systems 5 1.1.1Description 5 Figure 1: Natural smoke control systems in apartment buildings 6 Figure 2b: Stairwell system 6 1.1.2 Operations of natural ventilation 7 Figure 3: Measured free area of open ventilators 8 1.1.3 Control requirements 8 1.1.4 Fire service controls in natural ventilation 9 1.2 Mechanical smoke ventilation systems 9 1.2.1 Description 9 Figure 4: Mechanical smoke extraction shafts 10 1.2.2 Design 10 Figure 5: Mechanical extract and natural inlet 11 Figure 6: Mechanical inlet and mechanical extract 12 1.2.3 Application 12 1.2.4 Requirements 13 1.2.5 Benefits 13 2.0 Computational Fluid Dynamics (CFD) modeling 13 2.1 Assumptions 14 2.2 Aim of modeling 14 2.3 Modeling elements 14 Table 1: CFD model parameters 15 2.4 Fire input and output parameters 16 Table 2: Fire input and output parameters 16 2.5 Scenarios 16 2.5.1 Scenario A 17 2.5.2 Scenario B 17 2.5.3 Scenario C 18 3.0 Recommendations 19 4.0 Conclusion 19 Reference list 21 Colt UK, 2011, Smoke control and environmental ventilation systems for multi-storey residential buildings, http://www.coltinfo.co.uk/smoke-control-residential-buildings.html 21 List of Figures Table of Contents 2 List of Figures 4 List of Tables 5 1.0 Introduction 6 1.1Natural ventilation systems 6 1.1.1Description 6 Figure 1: Natural smoke control systems in apartment buildings 7 Figure 2b: Stairwell system 7 1.1.2 Operations of natural ventilation 8 Figure 3: Measured free area of open ventilators 9 1.1.3 Control requirements 9 1.1.4 Fire service controls in natural ventilation 10 1.2 Mechanical smoke ventilation systems 10 1.2.1 Description 10 Figure 4: Mechanical smoke extraction shafts 11 1.2.2 Design 11 Figure 5: Mechanical extract and natural inlet 12 Figure 6: Mechanical inlet and mechanical extract 13 1.2.3 Application 13 1.2.4 Requirements 14 1.2.5 Benefits 14 2.0 Computational Fluid Dynamics (CFD) modeling 14 2.1 Assumptions 15 2.2 Aim of modeling 15 2.3 Modeling elements 15 Table 1: CFD model parameters 16 2.4 Fire input and output parameters 17 Table 2: Fire input and output parameters 17 2.5 Scenarios 17 2.5.1 Scenario A 18 2.5.2 Scenario B 18 2.5.3 Scenario C 19 3.0 Recommendations 20 4.0 Conclusion 20 Reference list 22 Colt UK, 2011, Smoke control and environmental ventilation systems for multi-storey residential buildings, http://www.coltinfo.co.uk/smoke-control-residential-buildings.html 22 List of Tables Table of Contents 2 List of Figures 4 List of Tables 5 1.0 Introduction 8 1.1Natural ventilation systems 8 1.1.1Description 8 Figure 1: Natural smoke control systems in apartment buildings 9 Figure 2b: Stairwell system 9 1.1.2 Operations of natural ventilation 10 Figure 3: Measured free area of open ventilators 11 1.1.3 Control requirements 11 1.1.4 Fire service controls in natural ventilation 12 1.2 Mechanical smoke ventilation systems 12 1.2.1 Description 12 Figure 4: Mechanical smoke extraction shafts 13 1.2.2 Design 13 Figure 5: Mechanical extract and natural inlet 14 Figure 6: Mechanical inlet and mechanical extract 15 1.2.3 Application 15 1.2.4 Requirements 16 1.2.5 Benefits 16 2.0 Computational Fluid Dynamics (CFD) modeling 16 2.1 Assumptions 17 2.2 Aim of modeling 17 2.3 Modeling elements 17 Table 1: CFD model parameters 18 2.4 Fire input and output parameters 19 Table 2: Fire input and output parameters 19 2.5 Scenarios 19 2.5.1 Scenario A 20 2.5.2 Scenario B 20 2.5.3 Scenario C 21 3.0 Recommendations 22 4.0 Conclusion 23 Reference list 24 Colt UK, 2011, Smoke control and environmental ventilation systems for multi-storey residential buildings, http://www.coltinfo.co.uk/smoke-control-residential-buildings.html 24 1.0 Introduction The aim of this report was to review the use of mechanical and natural smoke ventilation systems in common corridors of apartment blocks. In apartment blocks, smoke control systems are needed to protect the stairs where occupants will use to escape in the event of fire. Common corridors or lobbies to join the stairs in many multi-storey apartments. These stairs need to be free while conditions in the lobbies or corridors need to be improved. In order to improve conditions for fire fighting and escape, thermal exposure, toxicity, and obscuration needs improvement to fire and rescue activities. Architectural layout and space restrictions limit smoke control but Computational Fluid Dynamics (CFD) analysis and calculation provide performance based solutions (Magdanz, 2002). Smoke control systems provide smoke clearance and escape by rescue service and fire and aid any fire safety strategy in place. 1.1Natural ventilation systems Smoke control in high-rise apartment buildings involves two main requirements; fire fighting and means of escape. Both applications are engineered to use natural ventilation or pressurization to perform in emergency situations. 1.1.1Description Natural ventilation results from buoyancy forces that differ in density from ambient and smoky air gases owing to differences in temperature (Federation of Environmental Trade Associations, 2012, p.7). This ventilation type consists of two types in use in high-rise apartments. First, lobby and corridor ventilation provides natural ventilation because it allows for ventilators to be installed into the walls of corridors with the flexibility of automatic opening in the event of fire. It requires a combination of smoke shafts, and ducts and smoke dampers (see fig. 1) especially on enclosed lobbies and corridors. Second, stairwell ventilation is where ventilators are installed within the stairwell (see fig. 2) to create safe means of entry to fire fighters and safe exit route for occupants. In most residential flats, lobby and corridor ventilation as well as stairwell ventilation provide maximum smoke protection by working together (FETA, 2012). On the other hand, pressurization technique protects escape routes against entry of smoke since it maintains, at a higher level, pressure in escape routes that in adjacent spaces (Colt, 2011). It uses supply air which is air injected into the protected area and air release which consist of smoke and air released from the surrounding fire area. Figure 1: Natural smoke control systems in apartment buildings Figure 2a: Natural smoke ventilators Figure 2b: Stairwell system Source: Smoke control UK, 2011 From figure 2(a) and (b) above, natural ventilation makes use of natural wind forces and thermal buoyancy of thermals to ease flow of air into the ventilator and exit through the stairwell system. Buoyancy of hot smoke from the fire is the driving force. Buoyancy forces are small compared to wind forces meaning that performance of this system is affected by wind significantly. The benefits of natural ventilation are low energy use, low noise, reliability and simplicity (FETA, 2012). However, its performance is sensitive to the effect of wind while natural shaft systems may have relative loss in floor space. Natural ventilator systems usually recommend vents at the head of the stair, natural vent shafts and natural wall vents. 1.1.2 Operations of natural ventilation Effective operation of natural ventilation requires exhaust opening and source of inlet air. The vent provides exhaust, in a wall mounted vent, at the top and inlet at the bottom or stair door when it is opened (FETA, 2012). This means that any smoke entering the stair is let out of the vent at the stair head. It is a requirement that the stair and corridor joining the stair should be ventilated. Therefore, corridors or lobbies in apartment blocks must meet the following requirements; a) A minimum free area of 1.5m2 of the vent be located on each corridors’ external wall being ventilated b) High location of vents is advised especially the top edge that must be at the top of stair door c) Smoke detectors in corridors served should be fitted in vents in single stair apartment buildings. Manually, vents on multi-stair apartment buildings can also be fitted. Both apartments need head vents that automatically open with the vents. As shown in figure 2(a) above, the free area of smoke ventilators is measured at 900 to air flow. The cross-sectional area unobstructed is measured, at minimum in plane area, and perpendicular to the air flow direction. For vents, focus is on complying with regulations such as protection from falling, weathering and energy conservation. Figure 3: Measured free area of open ventilators Figure 3 above shows the bottom and side hung in the declared aerodynamic free area in accordance with BS EN 12101 – 2:2003 (Smoke and Heat Control Systems). Free area is defined as; a × d ≥ 1.5 m2. d = the ventilator distance that opens at 900 to the window opened or flap (FETA, 2012). Achieving the free area requires that designers freely uses any form of vent such as flap ventilators, side or bottom pivot window and louvred vents (FETA, 2012). Vents are selected and located, as a result of this freedom, to become exposed to adverse wind effects and likely to blow smoke back into the stair and the corridor. Natural ventilation designers need to mitigate effects of wind when locating and selecting vents notwithstanding the lack in regulatory requirements. The roof light in figure 2(a) works as automatic opening vent and minimum opening angle at 1400 to mitigate adverse wind effects. 1.1.3 Control requirements The minimum control requirements for corridor vents demand that smoke vents and design of critical vents like staircase and smoke shafts should remain open. Controls at single stair buildings need to be automatic and operate from smoke detectors at each storey. There should be simultaneous opening of vent at heat of stair, vent at smoke shaft head and vent of fire affected floor. Despite smoke being detected on floors apart from the fire floor, the vents on all floors must be closed. Smoke detectors in multi-storey building are not necessary and may be opened manually. This means that simple manual windows in multi-stair buildings should be used. Other additional issues require consideration for vents in the corridors such as; a) A limiting switch or other gadgets in each window to allow automatic opening b) Manual window opening by occupants for ventilations but may allow significant water entry during rains. c) Windows at the upper storeys pose challenges of achieving desired free area due to restricted opening to prevent falls. 1.1.4 Fire service controls in natural ventilation Motorized vents may be used and operated manually from local break glass switches. Local reset should be done after false alarm tests. Manual override switches can be provided at all the service entrants points. Besides, a central panel for fire-fighting is desirable as well as simplicity in fire service use. 1.2 Mechanical smoke ventilation systems Mechanical smoke ventilation systems are alternatives to natural ventilation systems. The rationale behind use of mechanical systems is that a shaft system can be used but any floor level can have their own dedicated powered systems (FETA, 2012). These powered or pressurization systems provide better protection since air supply systems prevents smoke entry and maintains positive staircase air pressure. A pressure relief or fan speed control is located at the staircase to avoid excess air pressure. 1.2.1 Description Mechanical ventilation systems prevent damage to the system by providing air inlet to communal area. It also prevents depressurization or excessive pressurization of ventilated areas. By doing so, it guarantees large smoke amounts from being drawn into the apartment from origin of the fire. In addition, differences in pressure happen so that escape doors are pulled open or cannot be operated. Source: secontrols.com Figure 4: Mechanical smoke extraction shafts In figure 4 above, mechanical shafts (about 0.6m2) use fan system to put out smoke from the fire ventilation area, and are not affected by pressures of external wind. Obstructions to duct airflow will also be less. It provides protection from the stairs using corridor opening OV (Opening ventilator) or 1.5m2 automatic stairwell ventilators (AOV) for all levels. 1.2.2 Design Based on a single floor level, mechanical systems are affected by fire and thus smoke vents at the fire floor always remain open. Ventilators on multiple floors remain closed especially when connected to a smoke shaft. This prevents smoke from spreading to unaffected parts of the apartment and reduces the extract for the floor that fire originates. All vents in the corridors are smoke or fire resistance performance equal to E30Sa fire door and the smoke shafts are constructed using non-combustible materials (FETA, 2012). When the system is activated, the smoke vents on the floor that the fire is originating will have vents at the head of the stairway and vents at the top of smoke shafts always open with all shafts running at design speeds. The system uses natural inlet and mechanical extract. A mechanical extraction system requires careful balancing since it has mechanical inlet that ensures shared open spaces are not entirely depressurized or pressurized in all the scenarios of fire. To enable all or some fans to be reversed, such systems are provided with a fire fighters switch. For example, a corridor that is designed with a reversible fan provides mechanical air inlet. Figure 5: Mechanical extract and natural inlet In figure 5 above, fire detection system is in control so that the fan near to the original detection point is selected as the smoke extract fan. An override switch is also provided so that it allows the Fire fighters to switch both the fans to extraction mode. This is another inlet air source given the availability of the vent at the staircase head (FETA, 2012). The exact requirements determine the system design on proposed building layout and other arrangement such as inlet fans and dedicated extract (see fig. 6). At appropriate temperature range, the extract fan is specified to operate. Figure 6: Mechanical inlet and mechanical extract Unless a reversible system is used, mechanical system inlet fans do not necessarily have a temperature rating (FETA, 2012). The system design throughout all the stages of the fire provides a steady extraction rate. A variable rate of extraction is an alternative to the system provided to reflect the requirements of the building and the different stages during the fire. 1.2.3 Application Basic mechanical systems provide high performance and can be used for extended travel corridor distances. Care need to be taken when removing the subdivision doors at the corridor. By reducing the potential travel distance, it is most likely that the number of apartments that require evacuation will be limited. Nevertheless, when doors are removed there is a possibility that fire fighter safety is compromised (FETA, 2012). The location of inlet points and mechanical extract, irrespective of the need should safeguard any stair, while ensuring that layout lowers the probability of smoke and heat of fire from escaping to fire-fighters and occupants. The default position in a direction away from the stairs is by extracting smoke. The stair locations’ furthest point are the shafts located at the common access route. Mechanical shaft systems are expensive and should only be used on demand by building control, standards or regulations. The ventilation of shared entry spaces is the main role of mechanical extraction system. It has mechanical extraction shafts that serve at least one common space in some or all floor levels and discharges to the outside directly. The shaft also has a stand-by fan located at the end discharge to provide smoke ventilation from the communal area. The designer specifies the replacement of air in the powered system and the provision of air replacement ensures excess pressure does not occur at the closed door as it compromises means of escape. However, normal passive compartmentation should not be compromised in the design of powered systems at the source of inlet air (Colt UK, 2011). On the other hand, all the fire stages (fire-fighting and means of escape) involve mechanical extract where it provides a consistent extraction rate. It is also possible for the system to have a variable extraction rate that reflects the different stages at the occurrence of fire. For example, the system can have a boost facility that provides higher ventilation levels especially when fire-fighting activities begin. Moving from normal mode to boost mode should be manual. 1.2.4 Requirements These powered systems require temperature classified equipment, standby fan unit, fire resisting wiring, and temperature classified equipment and need for maintained power. 1.2.5 Benefits There are reduced shaft cross-sections, low wind sensitivity, specified extraction rates and known capability to overcome resistance of the system. 2.0 Computational Fluid Dynamics (CFD) modeling Computational Fluid Dynamics (CFD) is used to predict temperatures, smoke concentration and air velocities in an apartment block. Assumptions are made when using the CFD model and appropriate parameters. In CFD, physical spaces are divided into finite tiny cubes in which the fire source falls within a small number of cubes. CFD validates a smoke control strategy that complies with local fire codes and international best practice. Smoke is designed to flow in the story building through connecting voids into the roof reservoir where it is extracted. Smoke control systems provide added measure of protection in tall buildings with physically disabled persons and children. CFD ascertains the volume flow rate in order to maintain design conditions within corridors. Smoke control systems use equipment such as smoke detectors, dampers, ductwork and fans by creating pressure differentials. Buildings with storey over 18 high require fire-fighting access to be incorporated. The design is adopted to cope with the door entering the fire room and open to the lobby. 2.1 Assumptions a) Smoke temperature is less than 1200C or 600C in a moist environment with a constant flow rate b) Visibility is 10m for the large enclosure in the extension of travel distance c) The corridor returns to the normal smoke free environment within 2 minutes after the escape of the last occupant. d) Floors, walls and ceilings have constant ambient temperature but heat is still transferred by gases present to other surfaces. 2.2 Aim of modeling To predict and provide information, development and performance of smoke control systems in a 15-story residential apartment block 2.3 Modeling elements CFD modeling will involve key issues such as; escape duration upon detection of smoke, structural response and resistance to fire, heat radiation and space separation, fire characteristics, heat and smoke transport and smoke control effectiveness. Supposing the fire starts at the third floor of the 15-storey apartment block, the way forward is to determine the height of corridors and exit doors. The height of corridors is 3m in the third flow and all the other floors. Take single-leaf fire doors of measurements 2m×0.8m in a 200C ambient temperature. The fire is spreading from a 9m2 area within a 12m perimeter but from the centre of the third floor apartment. In the ‘simulated’ CFD model (see table 1), heat flux is 702kW/m2 means that total size of fire spreading in all directions is 6,318kW. Supposing a fast rate growth rate of 0.231kJ/s3, the properties for heat travel rate will be provided as follows (Carlsson, 2005, p.34); Table 1: CFD model parameters Property Assumed value Heat flux (Q) 6318kW Heat growth rate (α) 0.231kJ/s3 Travel time (t) travel distance (30m) / walking speed (1m/s) = 300 sec Travel distance (d) 30m from the fire exit Surface thickness 25mm Fire flat External façade (1.8m×1.2m); straight corridors and lobbies are modeled. Front door of fire flat ≤22m from the single stairway Corridor breadth 1.8m Staircase 15 storeys; singlecase for all floors Construction of spaces for treads and landings Smoke detection time 60 seconds Maximum predicted temperature in the fire plume 3000C The mass lost rate per unit area is calculated and measured at the cone heat flux of 702kW/m2. In the early ignition stages, about 5 minutes (300 seconds) were consistent. However, in the later stages the surface thickness of the boundary conditions. Reaction characteristics are based on heptane that starts the initial fire for both mass loss measurements. Contribution of vertical smoke shaft protects the staircase and common areas from high temperatures and non-visibility due to heavy smoke filling the corridors. Smoke visibility in the scenarios below at capped at 8m while temperature remains at 550C. 2.4 Fire input and output parameters CFD modeling is achieved by investigating four scenarios with location of vertical smoke shaft and location of fire flat (Carlson, 2005, p. 34). Each location of fire source is changed and modeled for five best and worst case scenarios of fire. Movement of smoke along the corridors are assessed and measured. It also allowed for vertical smoke evaluation to determine the effectiveness of mechanical shafts in input and output parameters as shown in the table 2 below. Table 2: Fire input and output parameters Input parameters Output parameters Node dimensions Smoke density table achieved through specified times and nodes Floor distances Pressure differences and mass flow rates throughout flow paths Inside and outside ambient temperature Simulated stochastic runs; shows mean and variances of smoke density Wind direction and speed Histogram; probability of smoke density Time take for fire room window to break Histogram; multiple stochastic runs Time taken for fire room door to burn down Specified nodes and times; smoke density plots The approach to smoke control in the 15 storey apartment block is provided in the three scenarios shown below. 2.5 Scenarios 2.5.1 Scenario A Fire occurs at the left side of the fourth floor in the residential flat near the main corridor In Scenario A above, fire starts at the left wing of the fourth floor of the 15 storey apartment block with a width of 1.8m and 25m to the staircases. The smoke is moving to the right at 1/s because there are strong winds outside. The smoke detector is 25m away and so is the smoke shaft. This implies that it will take 25 seconds for the detector to sense the smoke and ignite the alarm. Occupants on the right wing reaction time will be 30 seconds and at running speed of 2m/s will take to escape at the mid staircases. Evacuation will also be facilitated at the smaller volume corridors in order to provide buffer for smoke at staircases and vents. 2.5.2 Scenario B Fire starts at the right end of the block adjacent to two corridors and the smoke shaft is located in the same corridor of the second floor. In scenario B above, fire starts 7m to the main corridor and the smoke is moving at 1m/s leftwards. It will take about 10seconds for the first smoke detector, and within the next 6 minutes the smoke will have reached the smoke shaft exit. In this case, the smoke is detected early and occupants in the leftwing and upper floors of the apartment block have ample time (10minutes) to escape through the central staircases and emergency stairs on the right. The smoke will be extinguished early enough because the mechanical system is next the fire source and so is the smoke detector. 2.5.3 Scenario C Fire location is near the means of escape (central stairs) in the 12th floor. The smoke detector is 5m away and it takes 5 seconds for the smoke to be detected. The fire alarm goes off and alerts all the occupants. In scenario C above, fire is near the exit points. The Smoke shaft will exit the smoke after 6 minutes since the travel distance is 35m. Occupants in the 11th floor downwards will escape downwards through the staircases. While those on the 12th floor will use the emergency exits. In this scenario, fire is easily extinguished since smoke vents are located just above the fire source. Since smoke is the greatest killer than fire itself, it is possible to save more than 98 percent of the occupants in the event of increased fire intensity. 3.0 Recommendations From the CFD modeling and 5 scenarios above, it is possible that smoke can be controlled by; a) Installing natural and mechanical smoke shafts at extreme ends of the corridor and at the staircases b) Boundary and surface conditions to be maintained in all areas of the building that enables evacuation to the central staircases c) Mechanical smoke shafts to be installed at the staircase and extreme ends of the building Simulation results permits smoke control system and demonstrates natural ventilation, vents, ducts and use of smoke detectors (Magdanz, 2002). This will ensure residents are notified for safe evacuations during the event of fire. 4.0 Conclusion Natural and mechanical ventilation systems are essential for allowing residents to escape before the outbreak of fire and smoke from filling the corridors before evacuation. Natural systems allow air to flow naturally through windows and corridors by way of vents and ducts. On the other hand, mechanical systems use pressurizations to reduce temperatures of the affected areas. CFD modeling allows for calculation and measurement to aid smoke control systems depending on boundary conditions and computational dynamics. Reference list Carlsson, 2005, Flame spread and fire growth-Modeling capabilities in various room configurations, Swedish defense research agency. www.foi.se/.../foir_1579.pdf Colt, 2011, Smoke control in apartment buildings, http://www.promet.co.il/image/users/198083/ftp/my_files/catalogs/colt/smoke/smoke%20in%20apartments.pdf?id=9021101 Colt UK, 2011, Smoke control and environmental ventilation systems for multi-storey residential buildings, http://www.coltinfo.co.uk/smoke-control-residential-buildings.html Federation of Environmental Trade Associations (FETA) 2012, Guidance on Smoke Control to Common Escape Routes in Apartment Buildings (Flats and Maisonettes), Smoke Control Association. https://www.feta.co.uk/uploaded_images/files/SCA%20Residential%20guide%20June%202012%20Revision%201.pdf Gubeau, N & Zhou, XX 2004, Evaluation of CFD to predict smoke movement in complex enclosed spaces, Health and Safety Laboratory, http://www.hse.gov.uk/research/rrpdf/rr255.pdf. Magdanz, 2002, An Overview to Designing Smoke-Control Systems, ASHRAE Journal, Vol. 2, pp. 32-37. bookstore.ashrae.biz/journal/download.php?file=MAGDANZ.pdf. Shevtec, 2012, Powered Smoke Shaft Ventilation System for a Residential Development, secontrols.com. http://www.ribaproductselector.com/Docs/7/22967/external/COL522967.pdf Read More