The Use of CFD Computer Models for Fire Safety Design in Buildings - Report Example

Natural and Mechanical Smoke Ventilations Name Lecturer Course Date Use of Natural and Mechanical Smoke Ventilations Introduction It is possible to have a fire outbreak in a block of flats endangering the lives of residents as well as possible loss of property worthy of millions. Once there is fire, the is a possibility of smoke and other gasses to enter into common corridors, escape routes or lobbies hampering the ability of occupants to escape. This may lead to chocking to death. According to Approved Document B, such building are required to have proper smoke ventilation system which extracts smoke to give occupants an opportunity to escape. A proper ventilation system ventilates the common corridor not only for occupants to escape but also easy accessibility by fire service men. Mechanical ventilation entails the use proper travel distance which complies with natural ventilation provided; it also recognises the use of vertical shafts and fans. The is also the use of automated opening ventilation to supply air to common corridors. This is usually at the top of escape routes. This section of the paper is going to detail the use of natural and mechanical ventilation system in a block of flats. Mechanical and Natural Ventilation as A Means of Smoke Control To make the common corridor of a block of flats to amenable means of escape various features need to be incorporated which include ventilation and interior travel ways, installing smoke and fire detection systems as well as installing fire fighting equipments. Ventilations will supply fresh air as well reduced effect of smoke in case there is fire in the building. Corridor ventilation as serves to supply fresh air and extract toxic gases thus they must be designed with fire and smoke behaviour in mind that is must be capable of eliminating smoke and heat during a fire. This makes ventilation essential to fire safety. Fire safety requires sufficient knowledge of ventilation-controlled fire as the presence of hot gases at the ceiling level radiates heat to combustible materials that can result to deadly flashover. Exposure can lead to dangerous conditions such as thermal injury, toxicity, asphyxiation, and obscuration. For this reason, a number of ventilation-related fire fighting technique is introduced such as horizontal and vertical ventilation for common corridor. Flats have enough air circulating within to enable fire burn substantially to produce smoke. This smoke becomes a danger to the occupants of the flat during escape. A small fire can generate gasses and smoke which are capable of affecting vision as well as choke an individual to death. This requires a well ventilated system which removes smoke from common corridors to enable proper escape and evacuation. One of the most important ventilation systems is smoke clearance ventilation system which eliminates smoke from the corridors. This are installed for the purpose of ventilating the corridors to remove smoke to facilitate evacuation as well as improve visibility during escape. This means that buildings should comply with approved document and British Standards 9991, BBS5588-6: 1991 which provides the size of ventilations that is required to be provided in a building. During design, certain conditions are taken into account before a natural ventilation system is decided. A flat that will have a mechanical ventilation system as well as water sprinkler systems will have a different ventilation size as compared to those without. According to Approved Document B when natural ventilation is provided, the basement should be closed, should have a minimum area of 1.5m2 where one dimension should be at least 0.85m. This means that the dimensions of the vent should be having a minimum dimension of 0.85 and should be2.5m above the ceiling of a building that is 2m away. It should also be constructed using incombustible materials and always should be at E30S side of the escape door. British standards also provide that ventilation should have a cross-sectional area of 3m2 to allow smoke to escape from the building. Doors should have a minimum free area of 1.5m2 to allow entry of natural air or elimination smoke and gasses during fire. A shaft should be provided to the lobby or balcony in order to allow smoke and air to flow free. A block of floods which have single stares and their escape exceeds 4.5m, automated opening ventilation system should be provided. This is a commendation made in Approved Document B in page 30. It recommends the same for lobbies and balcony if they are between the stairs and the corridor Automated ventilation is made possible by the use of the walls of the building that is having windows that open outward with a free area of 1.5m2. Again a shaft can be used instead of a window that is open outward and it should have an opening of 1m2. This will form a good smoke and gas control system for corridors and escape route of a block of flats with single stairs. When a block of flats have a number of stairs serving floors of the building, the maximum escape route should be 30m according Approved Document B. as in the case of single stair flat, both natural and mechanical ventilation should be provided in a form of a shaft above the corridor or a window that is open with cross-sectional area of 1.5m, the escape route distance is limited to 7.5m. The stairs for both a building with multiple and single stairs should be protected with a vent of 1m2 at the dead end of the stairs. However this opening depends on its location from the ground. If the building have floors that are above 11m or more than three stories, the common corridor should not connect to a stairs that goes to a car park. If it connects to car park, automated ventilation system is recommended. In case there is travel distance of 4.5m, a ventilator is not required. Approved Document B of UK Building Regulations aims to ensure the safety of the occupants and others who may be affected by a fire in closed environment . It should be noted that fire safety recommendations such as provisions for suitable means of escape, fire spread prevention, performance of mechanical ventilation, and others are ventilation related. For instance, means of escape requires routes free of smoke or toxic gases, fire spread prevention demand self-closing and fire resistant openings such as doors while performance of mechanical ventilation demands effective smoke extraction. This is shown in the diagram below; Natural ventilations use pressure difference caused by wind and temperature in and out of the common corridor. However, due to wind suction forces, positioning of ventilation openings must be correct and able to support natural extraction of hot air from the common corridor. Sensor-controlled roof ventilations with opening actuators can be designed as a natural smoke and heat venting system. However, mechanical ventilation cost more and subject to equipment failure, utility service interruption, poor design, poor maintenance, and poor management. In contrast, properly installed and maintained natural ventilation can provide economical but with high ventilation rate, using natural forces. Smoke control is commonly considered as an important part of a fire safety strategy and it often coincide with the evaluation of the common corridor’s natural ventilation design. For instance, the air tightness that needs to be achieved within the escape may be affected by the common corridor’s natural ventilation. Mechanical smoke system are often designed with an objective of overcoming the smoke movement forces of buoyancy, thermal expansion, stack effect, and wind pressure during a fire thus any discrepancy in expected pressure differentials and airflow rates should be resolved. In fire safety strategy areas such as stairwells, lift, and lobbies are considered strategic locations where air pressure should be maintained at a level in excess of that of surrounding waiting zones. Naturally ventilations intended for smoke and heat extraction depends on the air provided by the openings in a certain area, influence of wind and size of the openings, and the timing of their opening. In contrast, mechanical ventilation system extracts air in controlled manner in order to allow a defined volume of heat and smoke to escape. Thus, to eliminate smoke from the common corridor there should be under platform exhaust system The size, height, area, and location of natural ventilation (doors, windows, cracks in wall and other openings), often dictate the maximum heat release rate of fire in a compartment (Carvel & Beard, 2005:234).Gravity vents is one example of natural ventilation that is commonly used in large warehouse. However, they are not intended to supply fresh air but facilitate smoke extraction during a fire. Gravity vents relies on the buoyancy of the hot products of combustion that usually rise towards the ceiling. Some warehouses used exhaust fans that are automatically activated in the event of fire but they require special protection to withstand heat. Natural ventilations use pressure difference caused by wind and temperature in and out of the building. However, due to wind suction forces, positioning of ventilation openings must be correct and able to support natural extraction of hot air from the building. Sensor-controlled roof ventilations with opening actuators can be designed as a natural smoke and heat venting system (Wurm, 2007:96). In some circumstances, airflow in a compartment fire is provided through mechanical means such as an HVAC and other air handling system. Compartment ventilation is not only intended for fresh air supply but smoke management during a fire. Advantages of mechanical ventilation over natural ventilation can be summarized as the ability to deliver the required flow rate, can easily integrate into air-condition system, can be directed and controlled to service critical areas, and can be placed anywhere with available electricity. However, mechanical ventilation cost more and subject to equipment failure, utility service interruption, poor design, poor maintenance, and poor management. In contrast, properly installed and maintained natural ventilation can provide economical but with high ventilation rate, using natural forces (Atkinson, 2009:10). Mechanical extract and natural air inlet plays an important role in keeping a steady air flow in a building. For instance, air extracted by mechanical extraction system causes a slight negative air pressure that trigger air infiltration from surrounding naturally ventilated spaces with inlet grills and open doors resulting to a certain rate of air exchange. According to Furness & Muckett (2007:155), these ventilation system must be capable of at least six complete air changes per hour otherwise the vapour in the surrounding spaces with reach its flammable limit. For this reason, it is very important to consider the use of mechanical ventilation particularly in situations where natural ventilation cannot control the concentration and volume of diluted air. Contrary to mechanical extract, mechanical ventilation systems that forces air into the area requiring dilution and venting to control flammable various must be provided with air outlet so vapours and other contaminants ventilate outside the building or into the open air. Naturally ventilations intended for smoke and heat extraction depends on the air provided by the openings in a certain area, influence of wind and size of the openings, and the timing of their opening. In contrast, mechanical ventilation system extracts air in controlled manner in order to allow a defined volume of heat and smoke to escape. Features to be incorporated to facilitate safe mass travel corridor It is essential to have effective, amenable and safe mass travel corridor which will ensure that there safe exit from a flat in case there is fire. British Standards and approved Document B for fire safety require buildings to have the means of escape, to have easy access to the building, the fire safety manual, and evacuation strategy. This is not possible by ensuring during construction the regulations made down are followed. More so, before constructing a house, the risk profile of resident is taken into consideration, that is, the escape or evacuation of a weak occupants is different from sleeping. In order to have a safe mass travel corridor various features need to be incorporated into the block of flats and they include the design and inclusion of ventilation, lighting of side works and interior travel ways, installation of fire and smoke detection. The installation of common corridor ventilations, constructing of fire and smoke detection equipments and provision of natural ventilation for circulation of fresh air will make it more amenable. Common corridor ventilation is essential in ensuring the safety of users. The existing floor layouts of the block of flats need to provide a better environment for safe escape or evacuation, and at the same time, reduce the chances for fatalities. A fire exit is to be added to safety in case fire break out inside the building. Aside from creating an environment that would lessen the chance of fatalities, additional features for fire fighting are needed. There should be ideal heating and ventilation in the building to enable proper circulation of air and temperature. The door ways of a building should be labelled with proper colours or signs that indicate the escape routes in case of fire. It is required that a building should have a wide travel space which will enable both familiar and non-familiar occupants of a building to escape from fire. The materials used for construction should be environmentally friendly emitting minimum carbon and dust into the air. Dust from construction sites and construction materials can be reduced by the use of spray water with a principle aim of reducing air and water pollution in the construction area. The walls should be painted with paints that reduce chances of catching fire. The ceiling should be suspended to reduce the chances of catching fire this can contribute to the overall fire resistance of a floor and ceiling assembly. Part 2: CFD Modelling and Smoke control CFD modelling provides numerical analysis of air flow and distribution in rooms of buildings. It is a form of simulation which is employed at the design stage of a building in order to determine the number of escape route as well as ventilation sizes to be provided in a building. During design period, computer fluid dynamics model divides the building structure into cells and then carries out simulations addressing radiation, smoke, turbulence and flame spread. It examines the fire environment and interprets numerically the required spaces for safety evacuation(Advanced Smoke Group, 2012). CFD has many benefits it is used in simulating any planned structure such as increase in floor space and creation of safe evacuation escape routes. In the case at hand CFD modelling will be used to create high performance smoke control systems in terms of vents and escape stairwells. Using BR368, the vent should not exceed 10 percent of the building where the building is 64sq so 3.6m and state that we have tried in modelling with the natural vent but the escape route won’t be clear of smoke. These predict the mass flow rate due to a room of a building and this can be done for various rooms of the building and a graph is made whose slope of the line relating the mass flow rate gases with respect to the height of rise of the plume, can be used to design a ventilation. In CDF modelling, the stairs for the escape route will be ventilated and simulated as shown in the diagram below; In the diagram above it can be noted that the stairs has been protected and it is at the far end of either section of the building and it occupies the entire middle area which allows the use of natural ventilation system. It can be noted that the stairs can be accessed by occupants of that floor which is less than 15m. The stairs should connect with the escape route which is offer 1400mm constructed using fire resistant materials. The materials that will be used in the construction should have the ability to resist fire for more than 120minutes because of the height of the building. This is shown in the table below; The building should be installed with sprinklers because of its size. The design of atria will be done as shown below using CFD modelling; The modelling shows that smoke rises up and tends to move towards an opening which is provided in the compartment. This shows the impact of ventilation and the spread of smoke within an enclosure. The smoke has reached the ceiling of the compartment making the area around the fire invisible thus will begin to s descending from the ceiling and beginning to spread in the room. It should be noted that as time goes by smoke increases in terms of spread. This is associated with the transient behaviour of smoke. Smoke control is commonly considered as an important part of a fire safety strategy and it often coincide with the evaluation of the building’s natural ventilation design. For instance, the air tightness that needs to be achieved within the escape may be affected by the building’s natural ventilation. Mechanical smoke system are often designed with an objective of overcoming the smoke movement forces of buoyancy, thermal expansion, stack effect, and wind pressure during a fire thus any discrepancy in expected pressure differentials and airflow rates should be resolved (Oughton & Martin, 2012:1). In fire safety strategy areas such as stairwells, lift, and lobbies are considered strategic locations where air pressure should be maintained at a level in excess of that of surrounding accommodation zones. For this reason, there are fire safety strategies that actually allowed introduction of fresh air through mechanical vents in order to maintain a positive pressure of around 50pa (50 Pascals or 0.0072518869 psi or pounds per square inch). Similarly, minimum air velocity of about 0.75m/s is maintained through an open door between the pressurized area and the accommodation zone. Some of the most common strategy is installing an external wall vent such as openable windows, vertical shafts or mechanical extract (Oughton & Martin, 2012:1). References Advanced Smoke Group, 2012. Smoke Control Computational Fluid Dynamics Modelling. On Available < http://www.advancedsmoke.co.uk/smoke_control/smoke_control_cfd_modelling.aspx>[Accessed 24 July 2013]. Atkinson, J., 2009.Natural Ventilation for Infection Control in Health-Care Settings. London: Jones & Bartlett Publishers. Awbi, H.B, 2000. Air Distribution in Rooms: Ventilation for Health and Sustainable Environment. London: Elsevier. Carvel R. & Beard A, 2005.The Handbook of Tunnel Fire Safety, London: Thomas Telford, UK Chang, C., Banks, D., & Meroney., R. 2003. Computational Fluid Dynamics Simulation of the Progress of Fire Smoke in Large Space, Building Atria. Tamkang Journal of Science and Engineering, Vol. 6, No. 3, pp. 151-157 (2003) Hume, B., 2009. The Use of CFD Computer Models for Fire Safety Design in buildings Available: Read More