Natural and Mechanical Smoke Ventilation - Case Study Example

FV4201 Assignment Name: Course: Professor: Institution: Date: Part 1: Natural and mechanical smoke ventilation Introduction Building designs for fire safety depends upon a proper understanding of behavior of materials and systems involved in fire, the likely sources of fire, and the spread of fire. Smoke ventilation systems are required in buildings in order to keep escape routes clear of smoke, and allow occupants to evacuate the building. In addition, these systems reduce the amount of damage to the building as a result of fire. Multi-storey dwelling apartment blocks require smoke control systems, principally for the protection of common corridors and stairs. Common corridors into the stairs are normally the major routes of escape, and for this reason, the corridors opening onto stairs have to be adequately out of smoke with improved conditions. When a door into an apartment on fire is opened, a lot of smoke can fill up the corridor, making it difficult for occupants to escape. If the smoke finds its way to the stairs, it can make it difficult for occupants of other floors to escape, and also hamper efforts to deploy fire services. Ventilation systems have to be provided for each common corridor that opens onto a stair to prevent the spread of smoke to the stairs (The Building Regulations, 2006). For corridors, the distance between the fire doors is limited to 30m, while the distance of dead-end corridors is limited to 7.5m to ensure that occupants escaping the building travel over a limited distance through smoke. Figure 1(a): Corridors without dead-ends Figure 1(b): Corridors with dead-ends Key: D – Dwelling unit fd – fire door - Shaded region indicating the area where ventilation has to be provided There are two systems used in ventilation of buildings: natural and mechanical or powered ventilation systems. These systems have to comply with the provisions in Approved Document B (ADB) and BS 9991:2011. ADB allows the use of both natural and mechanical ventilation systems. However, a presumption is made that the natural ventilation systems is the norm, and that mechanical ventilation provides an alternative. 1.1 Natural ventelation systems Natural ventilation systems employ the use of natural forces of wind and natural thermal buoyancy to drive the smoke through ventilations. The systems also provide day-to-day ventilation, hence, serving a dual function. For effective operation of natural ventilation, there is need for both air inlet vent and an exhaust air opening. Vents mounted on the wall have inlet provided at the bottom and an exhaust provided at the top. Sometimes, air inlet can be provided through a stair door on opening the door. This requires a vent at the head of the stair to remove smoke that may enter to the stair. One important factor to take into account when designing for smoke ventilation and smoke & heat exhaust ventilation (SHEV) is the need for a free open area of smoke shaft doors and windows, when they are actuated to enable the smoke to escape. It is clear that wide opening windows, doors and ventilators provide a larger area that allows the smoke to flow out from the building. After achieving a free area, any form of vent can be used. Common choices of forms of vents include: a louvred vent, bottom pivoting widow/ventilator or side pivoting window/ventilator. Selection and location of the vents has to be done in a way that will limit susceptibility to adverse effects of wind that will blow the smoke backwards into the corridor and stairs. When a roof light is installed as an automatic opening vent (AOV) or natural flap ventilator, a minimum of 140o opening angle will be suitable in mitigation of adverse wind effects as provided in BS EN12101-2:2003. The figure below shows a window fitted with an AOV. The minimum opening angle is 60o. Figure 2: Chain actuated outward opening window used as both Automatic Opening Vent (AOV) and Opening Ventilator (OV) Smoke shaft doors that open to 90o provide a greater free area that can significantly improve ventilation through the vent. There are three fundamental approaches used in natural ventilation. These are: i. Buoyancy-driven stack ventilation ii. Wind-driven stack ventilation, and iii. Single-sided ventilation Buoyancy-driven stack ventilation Buoyancy ventilation depends on the differences in density to draw cooler air through lower ventilation openings and exhaust warmer air through the top ventilation openings. Below is a schematic diagram showing how the buoyancy-driven natural ventilation system works for a story dwelling apartment. Figure 2(a): Buoyancy-driven smoke ventilation A chimney is normally used in order to generate adequate buoyancy forces to drive the required airflow. As noted before, wind will always induce some pressure on the building that will act to drive the flow of air. A good design of a stack ventilation system will seek ways that can make full advantage of both wind and buoyancy forces. Wind-driven stack ventilation Cross ventilation driven by wind occurs on opposite sides of a closed building space. The depth of the floor plan of the building along the direction of airflow should be limited to efficiently remove smoke from the building by the driving force of wind. Sufficient airflow requires a minimal internal resistance and a significant difference in pressure between the outlet opening and the inlet. The figure below shows a schematic diagram of a wind-driven stack ventilation for a story building with several rooms. Figure 2(b): Wind-driven smoke ventilation Single-sided ventilation In this approach, the amount of airflow into the building is limited by single-sided smoke vents. The vents perform a dual function of air inlet and smoke outlet. Single-sided ventilation depends on the speed and direction of wind and the temperature differences between the outside and the inside of the building. It is also influenced by pressure differences and turbulence in the air. Turbulence is the main driving force of airflow, enhanced by buoyancy effect. Smoke buoyancy effect depends on the area of opening, temperature difference and distance between the ventilations. These driving forces are relatively smaller and variable. A schematic diagram of a single-sided approach is illustrated in the figure below: Figure 2(c): Single-sided smoke ventilation It is possible to combine the three basic approaches into a single ventilation system in order to meet a wide range of ventilation requirements. A notable example is the Queens Building at De Montfort University that has proven to be one of the most influential naturally ventilated buildings. The figure 2(d) below shows a schematic diagram of mixed global/local and wind/stack ventilation approach. Figure 2 (d): Mixed global/local and wind/stack smoke ventilation strategy In comparison to the other two ventilation approaches, single-sided ventilation provides the least attractive method and solution for natural ventilation. Most of the design considerations of natural ventilation in buildings are aimed at increasing the airflow in and out of the building. The benefits of natural ventilation include: low energy use and low noise, compatibility with day-lighting, a wide range of thermal comfort, improved environmental quality, reliability and simplicity. However, buoyancy forces can be significantly lower in comparison to wind forces. Thus, the performance of a natural ventilation system can be sensitive to adverse effects of wind. Also, natural shaft systems consume a relatively large floor space. 1.2 Mechanical Smoke Ventilation Systems (MSVS) According to ADB, a building used as a residential apartment with more than three storeys should have every flat separated from a common stair using a protected common corridor/lobby (The Building Regulations, 2006). It is also recommended that the common corridor be ventilated using a 1.5m2 AOV or natural smoke shaft that is connected to the outside of the building or, a pressure differential system. MSVS provides an alternative instead of using a pressure differential system of a natural smoke shaft. The ADB recommends that these ventilation systems be used as an alternative to the natural ventilation approaches. The design considerations for the systems are based a single floor being affected by the smoke and the use of a shaft system, although every floor can have its own powered smoke ventilation system. In case of fire within a given floor, only the vents on the affected floor and other additional design critical vents will be required to open. On a multiple floor building, designers normally avoid opening ventilators in order to prevent the spread of smoke to unaffected areas of the apartment that would otherwise reduce the rate of smoke removal from the building. The shafts are made of non-combustible materials while the vents to the corridors are recommended to have a smoke/fire resistance performance equivalent to that of E30S fire doors. The systems require maintained power supply, fire resistant wiring, temperature classified equipment, and stand-by fans. The pressure inside the building need also to be considered and be limited for the escape doors to remain operable. The communal area require an air inlet to prevent system damage and to ensure that there is no depressurization or excessive pressurization of the ventilated area. Preventing depressurization is essential to avoid drawing of large amounts of smoke from the apartment building that could lead to higher pressure differentials. In turn, higher pressure differentials could render the doors used as escape routes inoperable. How the MSVS work In case of a smoke in the common corridor, a mechanical smoke ventilation system is activated, opening the vents on the fire floor, the vents at the smoke shafts, and the vent at the head of the stair case. The fans also start running at a speed at which they are designed for. The system operates as a depressurizing system, where heat and smoke are extracted from the affected floor, thus forming a depressurized area. As a result of this, a higher pressure will built in other areas, such as the staircase. This will lead to air being drawn from these areas into the smoke shafts, hence, preventing the flow of smoke through the stairway and common corridors. To prevent excessive depressurization, it is required that an additional inlet be provided in the regions being depressurized. Excessive depressurization can cause damage to the venting equipment, and the smoke may be pulled from the source of fire. The air inlet for the mechanical systems can be achieved through natural or mechanical inlet. Natural inlet make-up include AOV at the head of the stairway, external air inlet and air inlet through shaft (Royal Institute of British Architects, 2005). Mechanical air inlet is via shaft. It may be possible to design mechanical systems with a higher performance that can allow longer travel distances in corridors. However, care should be taken when corridor sub-division doors are to be removed. In addition to limiting the distance of travel through the smoke, corridor sub-division doors may limit the apartments in need of evacuation by fire fighters, hence protecting the fire fighters (Colt International, 2015). Below is a schematic diagram of a vent smoke shaft system produced by uniVent and installed in hundreds of buildings in the UK. Figure 3: A mechanical vent smoke shaft system (Colt International Limited, 2015) Common corridors in most apartment buildings have no external linking walls, and may not have the potential to benefit from natural ventilation systems, making the common corridors/lobbies becoming stagnant areas. MSVS serving corridors/lobbies can be used continuously on a permanent basis, and can provide great benefits to the building and other additional services to the occupants of the apartment. Some of the benefits of mechanical smoke ventilation systems include: low sensitivity to wind, ability to overcome resistance, specified extraction rates, and reduced cross-section of the shaft (Smoke Control Services, 2011). MSVS are reliable in achieving the desired flow rate, irrespective of the effects of external factors, such as wind and ambient temperature. These makes MSVS to have a guaranteed performance and makes fire strategy to be more straightforward. The shaft area can be reduced since the system effectively pulls smoke from the corridor without depending on the buoyancy of the smoke. The problem with MSVS is the high installation and operational maintenance costs that may limit the systems performance. Another problem is that, the system may not work as expected, or normal operation may be interrupted as a result of poor design, interruption of utility service, equipment failure, or poor maintenance. Part 2: CFD modeling CFD modelling provides an insight for the possible movement behavior of smoke through protected areas and common corridors. It is often impractical to physically test the performance of a smoke control system before its final installation, hence the need for CFD to mitigate smoke in the building right from the start. The CFD models incorporate the buoyancy effect on smoke, the effect of wind, and the combined effect of buoyancy and wind, and other ‘what if’ scenarios on the smoke control system. The system is assessed in the context of acceptable performance criteria under two main objectives; means of escape and fire services. First, the building plan with the 15 floors required is modelled using computer aided design (CAD) program to produce a 3D model. The 3D model produced is then run in a commercial CFD program that has been agreed upon by relevant authorities. The rest of the tasks are done and completed by the CFD program. After the building model has been done, a material is assigned to each volume and fluid. This process is followed by the determination of the boundary conditions. After determining all the boundary conditions, meshing is done. Meshing is a process used to divide the space into smaller sections, so that the movement of smoke can be observed in each section separately. Meshing is critical and has to be done carefully to ensure that the number of meshes is well set so as to obtain the desired results. To obtain the CFD model solution, the boundary conditions entered and the desired results are selected, then the required number of iterations determined. The time taken to completely clear the smoke from the common corridor and the staircases to allow a safe exit of the building occupants is also calculated. The effect of extraction fans on the stair door and the accumulation of smoke within the common corridor can be observed from the models. The temperature within the common corridor when the apartment block door is opened or closed is also measured at regular time intervals, to determine if the temperatures are ambient for the occupants to escape the building. Different Modelling Scenarios Three scenarios can be investigated by modeling for different locations of the smoke shaft and the flat of the fire source. The scenarios considered have been identified based on engineering judgment for the apartment block design, the functionality of the building, and fire safety analysis. Scenario No.1 The fire source is located near the central stairs in the 3rd floor used as escape route. The smoke detector is 8 meters away and this will take 8 seconds for the detector to ring a fire alarm, assuming that the smoke travels at a velocity of 1m/sec. It will take 8 seconds for the smoke detector to ring the fire alarm, and about 30 seconds for the smoke to travel to the smoke shaft. Occupants on the third and second floor can use the stairway to escape the building, while those from the fourth floor upwards can use emergency door exits since the stairway will be filled with smoke travelling to a smoke vent on the head of the stairway. Scenario No.2 The source of fire is from an apartment in the 10th floor on the left wing of the floor. The distance between the smoke detector and the apartment door is 25 meters, and it takes 25 seconds for the smoke to be detected when the apartment door is opened. Occupants from the apartment that is the source of fire and other adjacent apartments on the 10th floor can use an emergency rear stairway to escape the building since a better length of the common corridor will be filled with smoke. Some smoke will also flow up to the smoke vent on the head of the central stairway. Those in the 9th floor downwards can use the common corridor to the main entry via the central stairway. Scenario No. 3 In this scenario, there is fire in a corridor on the far right wing of the apartment on the 1st floor. The distance to the main corridor is 5m, and the smoke detector is 10 meters from the where the corridor meets the main common corridor. This will take about 15 seconds for the smoke to travel to the detector that rings a fire alarm. The smoke shaft is located where the apartment corridor joins the common corridor. The occupants of the apartment on the floor where the fire started can safely evacuate the building using the escape stairway and the common corridor. This is because of quick response by the smoke vent system due to close proximity to the source of fire that will ensure that the visibility of the corridor remains clear, and occupants can take minimum time to escape. Justification for CFD modelling CFD analysis must prove to the relevant authorities that the smoke ventilation system will perform adequately as expected and as early as during the design stage of the building. The results of the CFD provide solutions as to how the smoke is likely to move in the building apartment or areas in which it is likely to accumulate in case of fire. Also, smoke concentration in common corridors and stairways, or other areas that would be used by occupants to escape is also determined. Justification for the extended corridor arrangement In cases where the common corridor travel distance exceeds that recommended by ADB as shown in figure 1 (a) and (b), there is a need to perform a time-dependent and separate steady-state analyses representing each stage of a fire scenario. These time-dependent simulations are needed to determine how much time will be required in order to return tenable conditions in the corridor after exposure to smoke. However, extended travel distance can be taken care of by design of smoke control systems that can compensate for these extended lengths. Recommendations 1. Automatic stairwell ventilator should be installed at the head of both the central stair case and the escape stair case, with smoke detectors on the stairs of every floor to keep the stairs free of smoke. 2. The corridors in the apartments joining to the common corridor can utilize a smoke extractor system that is used to remove the smoke before it accumulates to beyond visibility. This is because of space constraints in these corridors. Conclusions A building design should permit maximum smoke ventilation and fire escape routes in case of a fire in any part of the building. It may be difficult to attain maximum natural smoke ventilation in apartment buildings, and therefore, the need for mechanical smoke ventilation systems, or combined mechanical and natural ventilation. The CFD models are applied in studying the effects of wind and buoyancy on the ventilation, and are important in assessing the performance of a smoke ventilation system prior to actual installation, and controlling the design conditions in a common corridor. References Read More