Natural and Mechanical Smoke Control Systems in Apartment Blocks - Coursework Example

Natural and mechanical smoke control systems in apartment blocks Name: Tutor: Course: Date: Table of contents Table of contents 2 List of Figures 3 1.0 Introduction 4 1.1 Natural smoke ventilation systems 4 1.1.1 Principle of operation 4 1.1.2 Lobby/corridor vents 5 Figure 1: Free area of smoke ventilators 5 Figure 2: Measured free area of the open ventilator 6 1.1.3 Smoke shafts 6 1.1.4 Advantages and disadvantages of natural smoke ventilation systems 7 1.2 Mechanical smoke ventilation systems 7 1.2.1 Principle of operation 7 1.2.2 Mechanical extract and natural inlet 8 Figure 3: Typical mechanical extract and natural inlet ventilation 9 1.2.3 Mechanical extract and inlet 9 Figure 4: Mechanical inlet and mechanical extract ventilation 10 1.2.4 Advantages and disadvantages of mechanical smoke control systems 10 Part 2: Computational Fluid Dynamics (CFD) Modeling 12 2.1 Introduction 12 2.2 Fire Scenario I: Natural ventilation case 12 Figure 5: Fire scenario I under natural ventilation 13 2.3 Scenario II: Mechanical systems 14 Figure 6: Fire scenario II under Mechanical systems 14 References 16 List of Figures Table of contents 2 List of Figures 3 1.0 Introduction 4 1.1 Natural smoke ventilation systems 4 1.1.1 Principle of operation 4 1.1.2 Lobby/corridor vents 5 Figure 1: Free area of smoke ventilators 5 Figure 2: Measured free area of the open ventilator 6 1.1.3 Smoke shafts 6 1.1.4 Advantages and disadvantages of natural smoke ventilation systems 7 1.2 Mechanical smoke ventilation systems 7 1.2.1 Principle of operation 7 1.2.2 Mechanical extract and natural inlet 8 Figure 3: Typical mechanical extract and natural inlet ventilation 9 1.2.3 Mechanical extract and inlet 9 Figure 4: Mechanical inlet and mechanical extract ventilation 10 1.2.4 Advantages and disadvantages of mechanical smoke control systems 10 Part 2: Computational Fluid Dynamics (CFD) Modeling 12 2.1 Introduction 12 2.2 Fire Scenario I: Natural ventilation case 12 Figure 5: Fire scenario I under natural ventilation 13 2.3 Scenario II: Mechanical systems 14 Figure 6: Fire scenario II under Mechanical systems 14 References 16 1.0 Introduction Smoke management schemes are in use in most multi-storey residential buildings to control smoke movement in common access areas. Common access areas include lift shafts, stairs, lobbies and corridors which are also the main escape routes. Smoke spreads quickly and fills apartment corridors making escape difficult for occupants. A significant amount of smoke enters through the apartment door, opened for escaping occupants, and quickly fills the lobby or corridors making escape difficult (Chen, 2009). Furthermore, fire and rescue services require clear lobbies and stairs as a bridgehead for their operations. For occupants in the higher or upper storeys, smoke entering the stairs will make their escape difficult. The life safety of a building is protected and improved with the incorporation of smoke control systems. In smoke management, a number of methods are appropriate in modifying the movement of smoke to benefit fire-fighters or dwellers (FETA, 2014). These include mechanical ventilation systems, natural smoke exhausts and pressurization systems. They create smoke free layer, keep escape routes free of smoke, prevent or delay flashover and the successive full development of fire. While mechanical systems extract smoke and heat from the apartment area, natural systems allows fire-fighters to access smoke free lobbies and stairs to control fire and evacuate occupants. This report reviews the use of natural and mechanical smoke ventilation systems in common corridors of apartment blocks. 1.1 Natural smoke ventilation systems 1.1.1 Principle of operation Natural ventilation works by harnessing thermal buoyancy and natural wind forces to drive airflow into the ventilator. The ventilation is caused by buoyancy forces that result from density and temperature difference in ambient and smoky air gas (Jiang & Chen, 2003). The buoyancy of hot smoke is the intended driving force of natural ventilation. Compared to wind forces, buoyancy forces are small making the performance of this smoke control system to be significantly affected by wind. A source of inlet air and exhaust opening are required for effective natural ventilation (Larsen, 2006). The vent, especially the wall mounted vent, provides inlet at the exhaust at the top and at the vent bottom (BRE, 2005). Alternatively when opened, the inlet air is provided through the stair door. The vent is needed at the stair head to vent any smoke entering the stair. According to ADB, the lobby/corridor and the stair should be ventilated. Natural ventilation mainly involves vents and shafts. Legislations and directions of Approved Document B (ADB) require that the length of dead-end corridor should be 7.5m and the distance between the fire doors and the common corridors should be 30m (The Building Regulations, 2006). If the apartments are sprinklered, BS 9991 permits a distance of 15m. 1.1.2 Lobby/corridor vents Natural ventilation uses automatic opening ventilators (AOVs), windows or opening ventilators (OVs). These systems evacuate smoke from areas where lobbies, corridors and common stairs are extended to the external walls (Evola & Popov, 2006). Vents are required to have a minimum area of 1.5m2 and be located on an external wall and as high as practicable as shown in fig. 1 below. They should be actuated with smoke detectors especially in single stair buildings and manually actuated in multi-stair buildings. The vent at the stair head should automatically open with the vents (BRE, 2005). Figure 1: Free area of smoke ventilators As shown in the figure above, a, according to BS EN 12101-2:2003 is the declared aerodynamic free area. On the other hand, b, is at right angles to the airflow direction and is measured in the plane as the unobstructed cross sectional area. In addition, the vents should comply with regulations relating to protection from falling, weathering and energy conservation. Common choices of vents are flap ventilators, side pivoting window or louvered vents (FETA, 2014). On the contrary, a range of these choices may make vents to be susceptible to wind effects which blow back smoke into stairs, lobbies or corridors (Chow, 2004). As such, a minimum opening angle of 1400 is permitted to mitigate adverse wind effects. The minimum free area of the open ventilator should be at right angle to the airflow direction as shown in the figure 2 below. Figure 2: Measured free area of the open ventilator As indicated in the figure 2 above, the free area = a × d ≥ 1.5m2, where, d, is the ventilator opening angle. This is measured at 900 to the opened window or flap. 1.1.3 Smoke shafts In enclosed lobbies and corridors, natural shafts with fire doors or dampers are used to evacuate smoke. ADB recommends use of natural shafts when closed at the base and should have a minimum area of 1.5m2 for its cross-section and minimum dimension of 0.85m in any direction. The shaft should extend 0.5m or more above any higher structure that is within 2m and should be constructed of non-combustible materials (Harrison & Miles, 2002). At an inclined angle (max 300), the shaft should be vertical and not exceeding 4m. Safety grilles in the shaft and any vent at the shaft top or into the shaft should have a minimum area of 1.0m2 (BRE, 2005). However, flaps that is hinged at the bottom and opens into the shaft increases the risk of failure when opened under gravity (detrimental mode). This implies that the completed assembly should be properly fire tested. Natural and mechanical smoke ventilation systems not only reduce heat and smoke damage but also reduce the thermal damage to the structural components of the building (FETA, 2014). Apartment blocks with over three floors or single stair must have smoke control and ventilation systems. 1.1.4 Advantages and disadvantages of natural smoke ventilation systems The advantages of natural shafts is that they are not only of low noise and costs but also do not require fans and rooftop plants. Natural smoke ventilation systems are cost-effective when used in common corridor has an external wall (Morgan, 1999). This is because they easily achieve compliance where existing windows are used. Owing to small amount of mechanical parts used, natural systems provide the ease of maintenance since they only involve actuators and fire doors. In multi-occupancy properties, this type of ventilation is ideal because it lack the required maintenance (BRE, 2005). Natural systems are appropriate where the systems may be vandalized and appropriate where maintenance budgets are limited. Conversely, natural smoke ventilators depend on the quantity of air outflow that intrigue the building. Although air is constantly propelled into the apartment to keep the pressure difference constant, this ventilation system relies on openings such as windows and doors (Klote et al., 2012). When wide open, the windows and doors in a secluded area may present challenges. Under increased surface area of seepage, the correct pressure balance becomes difficult to be maintained. Any excess pressure when the doors are closed makes it difficult for the door to open and this prevents smoke from escaping from the protected areas. 1.2 Mechanical smoke ventilation systems 1.2.1 Principle of operation Mechanical ventilation systems acting as depressurization systems apply external energy to displace gases via a ventilator. They extract heat and smoke from an area and as a result, the space is depressurized. Given that surrounding areas like staircases will have a higher pressure, air in these areas is led to the smoke shaft (BRE, 2005). It helps to prevent smoke from flowing to the nearby areas or into the staircases. Additional inlet air is provided to the depressurized area to limit damage caused on the smoke venting equipment as a result of excessive depressurization. Smoke is actively removed from the fire room where the area is over depressurized (Black, 2010). Pressure differentials between the fire area and depressurized area and across the doors may become too great making the door harder to pull open or to simply open. Although they are alternatives to natural ventilation systems, mechanical smoke ventilation are based on the assumption of shaft systems being used. As well, any floor level should have its own dedicated mechanical system to remove smoke. During the design of residential buildings, British Standard 9991: 2011 provides guidance on fire safety using mechanical smoke shaft systems can be used. Compared to the equivalent performance of natural ventilation (1.5m²), mechanical systems achieves greater efficiency using shafts as small as 0.25m² (BRE, 2005). As a result, they provide benefits of substantial space saving and creates more saleable space within a property. Mechanical alternatives provide protection for staircases of the building and are suitable for buildings with common corridor extending over 7.5m travel distances (The Building Regulations, 2006). Mechanical systems mitigates non-compliance, clears smoke from common corridors of buildings and offer greater performance for means of escape. Mechanical smoke systems can have mechanical extract and inlet or mechanical extract and natural inlet as discussed below. 1.2.2 Mechanical extract and natural inlet These systems constitute mechanical extract shaft(s) that serve at least one common space on some or all floor levels. They automatically open vents that supplement the provision of natural air inlet to the outside. This can be either by way of a duct or shaft or directly. Typically, the mechanical extract fans can be directed to the outside or through shafts. While mechanical extract discharges directly to the outside, single mechanical extract requires the use of standby or duty fans to cushion from system failure as a result of fan failure (Barber, 2012). A typical mechanical extract is indicated in the layout below showing it combined use with natural inlet ventilation on common access corridors (Black, 2009). The one with two shafts is as shown in the figure below. Figure 3: Typical mechanical extract and natural inlet ventilation 1.2.3 Mechanical extract and inlet Mechanical air inlet and mechanical extract operate by means of reversible fans. In this design, a fire detection system controls the system and the fan closer to the initial detection point is selected to provide the air inlet. The means of escape mode with selected fans is facilitated by the smoke extract fan (Klote & Milke, 2002). Provided with mechanical inlet, a mechanical extraction system demands careful balancing so that there is not overly pressurization in the common access spaces. The extract fan should operate an appropriate temperature range unless a reversible system. Meanwhile, the inlet fans should not be temperature rated. This mechanical system, if well designed, provides a steady extraction rate across all the fire stages. An alternative to the system is to reflect the different fire stages by creating variable rate of extraction and the building requirements (Black, 2010). The transition is manual between fire fighting mode and the normal mode for operations with reversible fans. When undertaken by the designers, the decisions regarding the ventilation rates needs to reflect the specific building risks presented. The mechanical extract ventilation and mechanical inlet indicated in the layout shows smoke extraction from common access corridor as shown below. Figure 4: Mechanical inlet and mechanical extract ventilation With mechanical smoke ventilation systems, it is unlikely that they can be designed to maintain tenable corridor conditions that allow occupants to escape once firefighters get into work. They are also limited where doors, for any other reason, remain open to the flat of fire origin. 1.2.4 Advantages and disadvantages of mechanical smoke control systems Mechanical systems have overriding benefits because of low wind sensitivity and specified extraction rates. Furthermore, they have reduced shaft cross sections and possess known capability to overcome system resistances (Cote, 2003). Besides, they are more usable since they optimize space and does a lot of cost saving. In this case, mechanical systems have reduced ventilation shaft areas and removal of staircase. They are not affected by wind conditions hence efficient compared to a code complaint solution. When compare to the pressurization systems, mechanical systems are less complex, and is more user-friendly (BRE, 2005). Mechanical systems offer higher efficiencies, maximizes saleable space and achieves resultant design benefits. The system eliminates the need for secondary staircases and extended travel distances. On the contrary, mechanical systems have to be proven to be approved. For example, CFD modeling or calculations needs to be undertaken to demonstrate that the system works. A code compliant smoke control system in certain small buildings is much cost effective which makes mechanical systems to be specific in use (Black, 2010). Mechanical Smoke Ventilation Systems can be misunderstood, especially by non-specialists. Given the need to include requirements for a secondary power, dampers and the supply of fans it makes the system more expensive in terms of installation costs and the initial kit. Part 2: Computational Fluid Dynamics (CFD) Modeling 2.1 Introduction CFD has been used in the recent past to predict and provide reliable information on the reactions, performance and development of fire in buildings. Evacuation procedures for fire safety have been provided for occupants and how to direct smoke from protected areas to the outside. Commercial and residential high-rise buildings have been fitted with vents and shafts to delay smoke and allow occupants in the higher floors some time to escape from the building. CFD is useful in predicting the indoor and outdoor airflows of the buildings based on experimental data of wind tunnels. Some studies have found that CFD simulations are accurate in airflow predictions even in buildings with complex treatment facades (Mohamed et al., 2013). As a result, predictions not possible with empirical models can be accurately made by accounting for the changes in ventilation performance. Wind direction and the size of the vent area to doorway affect the heat flow rate and smoke travel distance (Chen, 2009). On the other hand, the state of windows and doors as well as wind conditions outside the building affects the smoke speed and the rate of heat travel. Using CFD will help helps in validating ventilation rates and the wind pressure. In this case, CFD model was used to model the performance of smoke control systems in a 15-storey apartment block to be constructed in England or Wales. The fire scenarios taken are those under natural and mechanical ventilation conditions. It is assumed that the building will be made of concrete floors and block and pressurization fan is located at the ground floor which is 3m from the main entrance. All the room windows are open and the stairwell measures; Width (3.4m), Length (3.7m) and height (48.3m). The wind angles at 900 to the building. The fire scenarios are discussed below. 2.2 Fire Scenario I: Natural ventilation case In this scenario, fire begins in one the corners of the third flow of the left wing of the 15-storey apartment block. Since all the floor plans are similar in the building, the simulation will focus on the stairwell, vents and shafts of the third floor which has isolated doors. The direction of wind flow is from the windward side at 2m/s as shown in the diagram below. Figure 5: Fire scenario I under natural ventilation The combustion parameters considered under the CFD model were mainly heat release rates and smoke density. Other than that, some other factors considered were; Height of room ceiling: 3.4m Floor slab and ceiling thickness: Each 0.2m Smoke generation rate: 0.4kg/s Heat release rate (HRR) at source: 1.5kW/s External wind speed: 2m/s blowing at right angles to the windows The transient mode, from the CFD model, shows that the steady-state temperatures of the stairwell within the first 360s reaching 1700C. At this moment, the stairwell region and all the rooms in the third floor were full of smoke. However, the stairwell region and outside the fire room temperatures were reaching 1200C which posed a huge risk for occupants of the fourth floor and those above. The stairwell was full of smoke and most of the occupants in the upper floors managed to escape. The first to escape through the stairwell were occupants who were within 7.5m to the stairwell followed by those within 30m in the next 360s. Thereafter, it was difficult to control smoke and means of escape could not be guaranteed. In this scenario, stairwell temperatures reached 1600C making the fire environment untenable. HRR increased tremendously because the window areas decreased pressure loss. Similarly, the critical wind speed was exceeded by the average wind velocity at the doorway. 2.3 Scenario II: Mechanical systems In this case, fire occurs at the left wing of the 8th floor 28m to the stairwell and 2m to the common corridor. The windward windows were open while those on the leeward were partially open and blowing at 2m/s as shown in the figure below. Figure 6: Fire scenario II under Mechanical systems The prevailing conditions were; External wind speed: 2m/s Height of room ceiling: 3.4m Floor slab and ceiling thickness: 0.2m each Heat release rate (HRR) at source: 1.5kW/s Smoke generation rate: 0.4kg/s As opposed to the natural ventilation systems, a pressurization system connected to every floor was used in this case. The CFD results showed that smoke was controlled within the first 300s, keeping the stairwell out of smoke. Similarly, the fire temperatures were also controlled when the temperature was reaching 1000C. This means that the pressurization fan located at the opposite side of the stairwell was able to extract smoke and overcome the effect of wind. The occupants of the 8th floor were able to escape on time before the lapse of 360s which would have barred visibility. The pressurization fans and smoke shafts cleared the smoke within the common corridors and allowed for ease of access by the fire and rescue team to enter the floor. In this scenario, it implies that heat travel is slowed under wind driven conditions making smoke absorption easier. In the pressurized system, the apartment floor though under similar conditions to the natural ventilation systems had fans which were able to contain smoke within the floor and specifically in the area of source. Vents are located relative to the prevailing winds and it is evident that the vents in the windward direction are not easier to vent smoke as they face the wall and instead overcomes pressurization fans (Yim et al., 2009). It is notable that wind speeds of less than 2m/s may cause the pressurization fans to overrun. The preferred location is placing vents on the sidewall corners close to the windward facing to create negative pressure from the outside of the building. 2.4 Conclusion Safety and health of occupants in high-rise apartment blocks is dictated by the fire regulations on natural and mechanical smoke extraction systems. From the two fire scenarios, it is evident that smoke can be controlled regardless of the floor level, wind direction and fire position within the floor. The CFD model shows that, under similar conditions, pressurization or mechanical systems are more effective that natural ventilation systems. This is because the pressurizations systems were able to absorb all the smoke within the floor in the first 300s as opposed to the natural ventilation which reach 360s. Again, the wall temperature of the stairwell was kept at 1000C which allowed for ease extraction of smoke without overheating the fans. Meanwhile, fire-fighters easily gained access to the fire floor while occupants within the fire floor and those above found a means of egress. References Barber, N. (2012). Buildings and structures. London: Raintree. Building Regulations (2006). The Building Regulations 2000. Approved Document B: fire safety. S.1: TSO. Building Research Establishment (2005). Smoke ventilation of common access areas of flats and Maisonettes (BD2410) – Final Factual Report. Appendix A (Review). Available at: https://www.bre.co.uk/filelibrary/pdf/rpts/partb/Smoke_Ventilation_AppendixA.pdf Black, W.Z. (2009). Floor pressurization as a means of controlling smoke during a high-rise fire, Engineered Systems, 3: 46-49. Black, W.Z. (2010). COSMO-Software for designing smoke control systems in high-rise buildings. Fire Safety Journal, 45: 337-348. Chen, Q. (2009). Ventilation performance prediction for buildings: A method overview and recent applications. Building Environment, 44 (3): 848-858. Chow, W.K. (2004). Wind-induced indoor-air flow in a high-rise building adjacent to a vertical wall. Applied Energy, 77: 225-234. Cote, A. (2003). Operation of fire protection systems: a special edition of the Fire Protection Handbook. Quincy, Mass: National Fire Protection Association. Evola, G. & Popov, V. (2006). Computational analysis of wind driven natural ventilation in buildings, Energy Buildings, 38, 491-501 Federation of Environmental Trade Association (2014). Guidance on smoke control to common escape routes in Apartment Buildings (Flats and Maisonettes). Smoke Control Association. Harrison, R. & Miles, S. (2002). Smoke shafts protecting fire-fighting shafts: The performance and design. BRE Project Report No. 79204. Jiang, Y. & Chen, Q. (2003). Buoyancy-driven single-sided natural ventilation in buildings with large openings. International Journal of Heat Mass Transfer, 46: 973-988. Klote, J.H. & Milke, J.A. (2002). Principles of Smoke Management, ASHRAE, SFPE. Klote, J., Milke, J. & Turnbull, P. (2012). Handbook of smoke control engineering. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. Larsen, T.S. (2006). Natural Ventilation Driven by Wind and Temperature Difference. Aalborg University: Aalborg, Denmark. Morgan, H. (1999). Design methodologies for smoke and heat exhaust ventilation. Garston: CRC. Mohamed, M.F., King, S., Behnia, M. & Prasad, D. (2013). Coupled outdoor and indoor airflow prediction for buildings using computational fluid dynamics (CFD) Buildings, 3: 399-421 Yim, S.H., Fung, J.C., Lau, A.K. & Kot, S.C. (2009). Air ventilation impacts of the ‘wall effect’ resulting from the alignment of high-rise buildings. Atmospheric Environment, 43: 4982- 4994. Read More

The vent is needed at the stair head to vent any smoke entering the stair. According to ADB, the lobby/corridor and the stair should be ventilated. Natural ventilation mainly involves vents and shafts. Legislations and directions of Approved Document B (ADB) require that the length of dead-end corridor should be 7.5m and the distance between the fire doors and the common corridors should be 30m (The Building Regulations, 2006). If the apartments are sprinklered, BS 9991 permits a distance of 15m. 1.1.

2 Lobby/corridor vents Natural ventilation uses automatic opening ventilators (AOVs), windows or opening ventilators (OVs). These systems evacuate smoke from areas where lobbies, corridors and common stairs are extended to the external walls (Evola & Popov, 2006). Vents are required to have a minimum area of 1.5m2 and be located on an external wall and as high as practicable as shown in fig. 1 below. They should be actuated with smoke detectors especially in single stair buildings and manually actuated in multi-stair buildings.

The vent at the stair head should automatically open with the vents (BRE, 2005). Figure 1: Free area of smoke ventilators As shown in the figure above, a, according to BS EN 12101-2:2003 is the declared aerodynamic free area. On the other hand, b, is at right angles to the airflow direction and is measured in the plane as the unobstructed cross sectional area. In addition, the vents should comply with regulations relating to protection from falling, weathering and energy conservation. Common choices of vents are flap ventilators, side pivoting window or louvered vents (FETA, 2014).

On the contrary, a range of these choices may make vents to be susceptible to wind effects which blow back smoke into stairs, lobbies or corridors (Chow, 2004). As such, a minimum opening angle of 1400 is permitted to mitigate adverse wind effects. The minimum free area of the open ventilator should be at right angle to the airflow direction as shown in the figure 2 below. Figure 2: Measured free area of the open ventilator As indicated in the figure 2 above, the free area = a × d ≥ 1.5m2, where, d, is the ventilator opening angle.

This is measured at 900 to the opened window or flap. 1.1.3 Smoke shafts In enclosed lobbies and corridors, natural shafts with fire doors or dampers are used to evacuate smoke. ADB recommends use of natural shafts when closed at the base and should have a minimum area of 1.5m2 for its cross-section and minimum dimension of 0.85m in any direction. The shaft should extend 0.5m or more above any higher structure that is within 2m and should be constructed of non-combustible materials (Harrison & Miles, 2002).

At an inclined angle (max 300), the shaft should be vertical and not exceeding 4m. Safety grilles in the shaft and any vent at the shaft top or into the shaft should have a minimum area of 1.0m2 (BRE, 2005). However, flaps that is hinged at the bottom and opens into the shaft increases the risk of failure when opened under gravity (detrimental mode). This implies that the completed assembly should be properly fire tested. Natural and mechanical smoke ventilation systems not only reduce heat and smoke damage but also reduce the thermal damage to the structural components of the building (FETA, 2014).

Apartment blocks with over three floors or single stair must have smoke control and ventilation systems. 1.1.4 Advantages and disadvantages of natural smoke ventilation systems The advantages of natural shafts is that they are not only of low noise and costs but also do not require fans and rooftop plants. Natural smoke ventilation systems are cost-effective when used in common corridor has an external wall (Morgan, 1999). This is because they easily achieve compliance where existing windows are used.

Owing to small amount of mechanical parts used, natural systems provide the ease of maintenance since they only involve actuators and fire doors. In multi-occupancy properties, this type of ventilation is ideal because it lack the required maintenance (BRE, 2005).

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