The Various Aspects of Fire and the Built Environment - Term Paper Example

FIRE AND BUILT ENVIRONMENT Table of Contents Contents Contents 2 1. Introduction Fire is a process of combustion wherein heat, fuel, and oxygen interact. A fire in a built environment is different for an outdoor fire as it involves different phases and a number of factors influencing fire growth. Understanding fire in the built environments is critical, as safety of people occupying these places is extremely important. The following sections discuss the various aspects of fire and the built environment. These include the different characteristics of fire and the combustion process, features of the built environment and different enclosure fire models, fire behaviour in the built environment, the role of ventilation or enclosure openings in fire growth, fire prevention and control. 2. Fire and Built Environment 2.1 Characteristics of Fire and the Process of Combustion Fire is generally a process of combustion or interaction between heat, fuel, and oxygen or the “triangle of fire” (Perry, 2003, p.1). The chemical reaction between fuel and oxygen is responsible for starting the process of combustion thus no fire can occur if one of them is missing. This is the reason why fires are commonly put off with inert material like sand to deprive fire of oxygen or with water to remove heat from the triangle of fire (ibid, 2). The process of combustion is interplay between different kind of physical and chemical phenomena. The chemical aspect or the interaction of numerous chemical reactions resulting to combustion is the main phenomena followed by heat transfer, which is responsible for raising the temperature of the affected burning material. Heat is transferred by conduction, radiation, and diffusion resulting to a phenomenon known as turbulence. Mass transfer is the third aspect of combustion that can either motion of some or all gases involved in the flame or transferred by diffusion. The gas motion is influenced by either the gravitation field (buoyancy effect) or the flow of supplying the flame causing hot gases to rise and cooler gases to flow toward the flame. A lighted candle is good example of the occurrence of this complex series of phenomena. The flame in a lighted candle is often just above the wick, which by observation is in the gaseous state. On the other hand, the candle is a solid material melting and producing a pool of liquid at the base of the flame. The occurring chemical reactions is not limited to oxygen but extend to resulting atoms and radicals as evidence by the presence of slightly blue colour at the base of the flame which is emission of visible light CH radicals (Borghi et al, 1998, p.3). Fire produces heat and light from burning substances and the faster the oxygen interact with the substance, the hotter and brighter the fire (Frisch, 2002, p.8). Fuel is the actual material being consumed by a fire and any material that will burn is considered a fuel, which can be either solid, liquid, or gas. The composition and physical state of material determine their ability to burn, their rate of ignition and combustion. However, the combustion process actually occurs when the material is in the gas state and reacts with oxygen from the air. Under ordinary conditions, a solid material resists forces and retains its shape and size but expands and decomposes when heated. This decomposition is known as pyrolysis or the process of releasing molecules into the atmosphere, which will later react to oxygen molecules in the air slightly above the surface of the burning solid material (International Association of Fire Chiefs, 2004, p.126). Similarly, liquid fuels expand when heated and turn into gases when a specific level of temperature is reached. However, liquid fuels are one-step closer to combustion than solid as vaporization or release of molecules into the atmosphere occurs almost immediately when they are heated. Fire involving liquid fuels burns at the surface of the liquid. Gaseous fuels on the other hand needs little heat to burn as they are already in a state where combustion can immediately occur. The ratio between gaseous fuel and air determine the possibility of combustion thus when there is too much air but little fuel in the mixture, combustion will not occur. In the same way, too much fuel but little air will not lead to combustion (ibid, 127). By analysis, oxygen plays an important role in combustion, as most fuel will not burn without oxygen. Moreover, the amount of oxygen also determines the combustibility of fuel and the rate of the burning process. For instance, a fuel will not burn with too little oxygen but excessive oxygen can make fuel burn faster and hotter. Similarly, the amount of heat energy is important, as fire will not ignite if there is less heat. For instance, solid and liquid fuel requires a certain amount of heat to be able to release molecules into the atmosphere or turn into gaseous state. Gaseous fuel also requires a greater temperature to ignite. Once ignition occurs, heat energy will increase the temperature and hasten the combustion process until all fuel are consumed or oxygen supply is depleted (ibid, 127). Figure 1- The combustion process triangle The term “Flameover” in fire development in an enclosure refers to the flaming ignition of hot gases that are in different layer (thermal layering) of a room. Flameover often occur during the growth phase of the fire when hot gases are rising to the ceiling. Since cold gases are heavier, these gases fall when their temperature drops. However, convection currents during fire growth pulled them back to the fire. Once these gases reached their ignition point, it will ignite and all gases inside the room will be on fire. The flames from these gases can move at speeds ranging from 10 to 15 feet per second and raise room temperature rapidly. As mentioned above, gases rise are they are heated and form layers inside a room. This “thermal layering” is responsible for rising room temperature and occurrence of “backdraft” or the sudden explosion of hot oxygen-deprived fire gases. Backdraft only occurs in a closed environment or a room with limited ventilation to allow fresh air and oxygen. Combined with significant quantities of combustible gases heated about their ignition temperatures, explosive combustion or backdraft occur releasing force strong enough to cause injury or death. The signs and symptoms of impending backdraft include confine fire with considerable heat build-up, flame not visible from outside the building (indicating limited ventilation), smoke puffing out of the building, pressurized smoke, smoke-filled windows, turbulent smoke, and yellowish or sulphuric smoke (Fire Inspector, 2012, p.69). 2.2 Characteristics of Built Environment A built environment is generally a fabricated object and as far as fire is concern, this type of environment contains ignitable materials, combustible furniture, appliances, and others offering opportunity for fire. Statistics shows that almost 95% of life and property fire losses occur in the built environment more particularly residential fires (National Commission on Fire Prevention and Control, 1973, p.54). In terms of fire emergencies, built environments are complex and significantly influence by its content and fresh air supply (Beard & Carvel, 2005, p.324). Enclosure fires, the term commonly use to describe fire in built environment, enable two distinct homogenous zones or layers- the low cooler layer at ambient temperature and the upper hot layer at a uniform higher temperature. The concept of “zone” is based on physical phenomena observed in real enclosure fires that include other identifiable regions such as flaming zone, thermal plume, thin hotter gas layer, and the walls. It is commonly used to predict fire growth and its consequences of enclosure fire including multi-room buildings. More importantly, the zone model of enclosure fire recognizes the contribution of materials to fire growth and its impact to the environment (Rasbash et al, 2004, p.257). Figure 2- Two zone model of fire in enclosure (Troitzsch 2004) Another model for enclosure fires is CFD or Computational Fluid Dynamic models. Unlike the zone model, CFD divide the enclosure into a sub-volumes and use laws of mass, momentum, and energy conservation as shown below. Figure 3 - CFD sub-volumes (Troitzsch, 2004) CFD does not consider the physical and chemical processes occurring but instead use a number of different sub processes such as turbulence, radiation, soot modelling, pryrolysis, flame spread, and combustion modelling. CFD does not offer the same convenience as the zone model as it requires significant computational ability and expert knowledge of physics and chemistry, numerical methods, and computer science. Hand-Calculation Models is the third method of describing fire processes in an enclosure. This method is consist of solutions and empirical methods that can be used to calculate flame heights, mass flow rates, temperature, and velocities in fire plumes, time to sprinkler activation, room overpressure, and other related variables (ibid, p.43). Unlike outdoor fires, fire prevention and protection in the built environment often include considerations on compartmental construction, fire resistance, limitation of openings, partition and linings, roofs, vents, and escape routes. For instance, buildings are often constructed with spaces called compartments that are made up of fire-resistant walls, floors, and openings. The objective is to confine and control the movement of flames, smoke and heat. The purpose of fire resisting floors and wall in built environment is containment of the most severe and prolonged fire. Using the same logic, openings between compartments are often minimal and protected by fire resisting doors. To prevent a fire further from spreading, lining of walls and ceilings including roof are often built with non-combustible materials or with low flame spread ratings. Some building design includes vents to let hot gases and smoke through the roof thereby reducing temperature inside the enclosure (Mannan & Lees, 2005, p.289). Fire growth and spread in a built environment is often determined by building construction. For instance, flashover occurrence can be prevented if the material used in the lining of the walls and ceilings are not combustible materials. Fire growth in enclosure only occurs when there are enough flammable materials available as what had occurred in the Coconut Grove nightclub fire in Boston in 1942. The rapid growth of fire that was responsible for the deaths of almost 500 people was due to flammable decorations, inadequate exits, and overcrowding. Fire in built environment is different from outdoor fire such as forest and wild land fire because flame spread and both the building and its content can cause rapid fire growth. For instance, furnishings, interior finish, decorations, building construction elements can greatly influence the rate of fire growth and spread (Fire Inspector, 2012, p.69). 2.3 Fire Behaviour in the Built Environment Fire behaviour is the release of heat energy during combustion and characterized by its intensity, rate of spread at the fire front, flame type, and other related phenomena. Factors affecting fire behaviour include fuel load, fuel moisture, air temperature, relative humidity, and wind speed. In wild land fire, fuel load can be grass but in the built environment, this can be furniture, curtain, paper, and other combustible material available within the environment (Goodrich-Mahoney et al, 2008, p.784). 2.3.1 Fuel Load Fuel load is the total amount of combustible material within a fire area and directly associated with fire’s heat release rate (Smoke, 2009, p.269). Solid fuel fire development is different from liquid or gaseous fuel since a fire burning solid combustible material has different characteristics. Solid fuel produces rising hot gases and flame while fire spread is often influenced by convection. Fire with solid fuel often lead to downward spread of fire due to radiation and falling flaming materials. It is limited by the amount of fuel to burn and variations in the direction of fire spread. The length of exposure determines the intensity of the heat in solid-fuelled fire along with the total amount of material burned. It is entirely oxygen dependent (Fire Inspector, 2012, p.65). In contrast, liquid or gaseous fuel fires are dependent to the characteristics of the combustion of liquid and gaseous fuels. As solid fuels burn only when heated and converted to vapour, fuel and air must be present before liquid or gaseous fuel can ignite. Liquid or gaseous fuel need a strong ignition source and because they are often a mixture of compounds such as gasoline and others, the compound with lowest ignition temperature will ignite first (ibid, p.69). Gases and air will burn only in specific condition as they are subject to flammability limits. For instance, if there is too much fuel but not enough oxygen to support the combustion process, the mixture will not burn. In the same manner, gas or liquid fuels requires a certain amount of energy to ignite such as those coming from flames, electrical, static, frictional sparks, or hot surfaces. These ignition source must be capable of heating the vapour to its ignition temperature in the presence of air otherwise it will not ignite (ibid, p.70). 2.3.2 Fire and Air Flow Studies conducted about fire in the built environment suggest that the core difference between burning indoors and out is the presence and flow of air. According to Pyne (2001), fire creates gases and forces air to flow into and around the fire. In a room fire for instance, the fire will gradually die if no fresh air is entering the room. However, if there is enough oxygen and if the heat radiated inside the room is more than the ignition temperature of exposed surfaces, a flashover will occur. Similarly, if fresh air is passing through vents such as windows and doors, fire inside the room will continue to grow and follow the ventilation flow (p.105). At the start of the combustion process, the fire is assumed to have access to some amount of oxygen inside the enclosure. At this point, provided that its size and position is right, the ventilation openings serve as an exhaust for the hot gases effectively reducing thermal feedback and slowing fire growth. Only when the fire starved for oxygen that the size and shape of ventilations become important, as the rate of burning is now highly dependent on rate at which air can flow into the enclosure (Troitzsch, 2004, p.42). The amount of air that will enter the enclosure is proportional to the area and height of the opening or vent while the airflow is dependent on the ratio of inlet flow of air to the rate of discharge of products of combustion. According to Rasbash et al, (2004), the amount of combustion taking inside the enclosure in a ventilation-controlled fire is independent of fire load. Similarly, unburned volatiles with only ignite when they pass the openings and reach outside atmosphere. The heat being produced inside the enclosure is therefore equal to the sum of heat transferred to the inner surfaces, heat radiating from ventilation openings, and convected heat through openings by the flowing unburned gases or volatiles (p.113). Conducted experiments on the effect of airflow through openings shows that outside air is being pulled in the lower spaces of the enclosure due to the pumping affect of the fire plume air entrainment. The lower spaces in the enclosure allow air to pass through the openings to the fire plumes. In the upper layer however, hot products of combustion move out through the upper of the ventilation openings as indicated below (Smith & Harmathy, 1979, p.123). Figure 4 - Flow of hot gases and air in enclosure during a fire 2.3.3 Heat Release One important aspect of fire in built environment is the rate of heat release or RHR. According to Armer & O’Dell (1997), the rate of heat release is actually the source of temperature rise and directly responsible for gas and smoke spread in a room. For instance, during the growth phase, the fire starts small but if there is enough oxygen to sustain combustion, the fire size increase until it reaches the maximum. During a fuel-controlled fire, the rate of release is dependent on available fire load while limited by available oxygen or fresh air in a ventilation-controlled fire. For instance, if the size of vents in a room or compartment is too small to supply air then the rate of release is slow (p.92). Both ventilation and fuel-controlled fire can result to flashover or the transition stage to full fire development. This the equation below determine if the fire is fuel or ventilation controlled: Source: Armer & O'Dell (1997) The first term in this equation is associated with fuel-controlled fires while the second is for ventilation-controlled fires. The rate of heat release is considered essential in determining combustibility of a particular material used in constructing a structure and content of a room. Local fire intensity and subsequent development of a fire is influenced by the heat release rate of ignited materials (National Research Council, 1979, p.171). It is directly proportional to the generation of material vapours or volatile gases. According to Rasbash et al, (2004), the combustion of material vapours in real fire is not always complete and therefore unburned vapour is released near the visible flame similar to soot particles. The ratio between the heat of combustion and the overall heat of complete combustion or “combustion efficiency” of the material depends on its chemical composition, fire ventilation, and mixing of volatile gases during combustion. Combustion efficiency decreases with supply of fresh air thus in a free burning fires with unrestricted supply of oxygen, the pyrolysis rate as well as the energy release rate are affected by fuel alone. In this case, the main heating factor of the fuel is the flames of the burning item rather than fresh air from vents. Fire in a built environment such as a room or enclosure is therefore determined by the amount of available oxygen thus the rate of energy release of the fire is mostly affected by the inflow of air through openings or vents such as doors and windows (p.259). 2.3.4 Fuel-Controlled and Ventilation-Controlled Fire A fire in a built environment commonly has three distinct phases- growth, fully developed, and decay phase (Structural Engineering Institute, 2009, p.38). The growth phase is where the fire starts to grow from its point of origin and the temperature inside a built environment such as a compartment or a room start to increase. The fully developed phase occurs after flashover or when all combustible materials not built in the environment ignites due to high temperature, volatile gases, and oxygen. The duration of fire in the fully developed phase depends on fuel load and temperature inside the room thus when the fire already consume all available fuel, it begins to lose energy and temperature drops. This is the beginning of the decay phase and it will end when the fire dies and temperature reach normal room temperature (ibid, p.38). Typically, a fire begins as smouldering or flaming fire and its growth is dependent on the type and quantity of fuel or combustible content of the room and its point of origin relative to room geometry. For instance, fire started near the couch and draperies is likely to spread faster than a fire that started in an empty area. However, this is not necessarily the case in a fire started in near a wall since it will produce more hotter and rapid fire. Similarly, the rate of fire growth is related to the location of openings as flow of fresh air intensify fire growth (ibid, p.38). Depending on the availability and size of ventilation, a fuel-controlled fire will turn into a ventilation-controlled fire in a matter of minutes. Similarly, as fire enters the decay phase, a ventilation-controlled fire will return to a fuel-controlled fire. The size of a ventilation-controlled fire is dependent on the quantity of fresh air being supplied through vents in a particular room or compartment. In other words, lack of fresh air in a ventilation controlled fire will slow down combustion and burning rate of fire until it return to a fuel-controlled fire (ibid, p.40). A fuel-controlled fire typically burn at lower temperature than ventilation controlled fire (US Fire Administration, 1977, p.79). 2.4 Fire Growth and Control in a Built Environment Enclosure fire can start in a number of ways but generally, growth of such fire is determined by geometry, ventilation, fuel type, amount of fuel, and surface area. For instance, a fuel package burning in a smaller room is likely to grow faster due to high temperature and shorter smoke filling time, and more rapid feedback to the fuel. In contrast, the same amount of fuel burning in a large room will have lower gas temperatures, longer smoke filling time, less feedback to the fuel, and therefore slower fire growth as shown below (Troitzsch, 2004, p.40). Figure 5 - Fire in a smaller room (Troitzsch, 2004) As shown in Figure 4, compartment openings play an important role in fire growth. As the fire become oxygen starved at the later stage of combustion, these compartment openings will serves as both exhaust and inlet for fresh air. According to Fitzgerald (2004), small openings result in lower fire temperatures due to inefficient combustion but if less hot gases are released, heat remains in the room. Large openings on the other hand allow a more efficient combustion resulting to higher temperatures and rapid burning but with large amounts of hot gases released from the room (p.99). In a room, fire grows in stages and the first stage is called “free-burning stage” or fuel-controlled fire where fire propagates without any influence. The second stage is the “flashover” followed by a fully developed ventilation controlled fire. At this stage, airflow rate through vents and smoke layer temperature determine the speed of fire growth (Quintiere, 1998, p.194). As mentioned earlier, fuel-controlled fire is a momentary stage in fire development as due to the increasing oxygen availability, it passed over to a ventilation-controlled fire (Prage & Rostek, 2006, p.9). Room temperature increases during fire growth phase thus surfaces and contents of the room start thermal decomposition and combustible materials will release volatile gases – pyrolysis. For instance, a fire in square shaped room will increase the temperature inside the room along with the rate of pyrolysis and concentration of volatile gases. When the temperature inside the room is high enough and the volume of volatile gases and oxygen is enough for ignition, flashover will occur because all the combustible materials inside the room will ignite. The flashover stage in fire development is characterized by 600 degrees centigrade temperature measured at the ceiling while 1,200 degrees centigrade or more in a fully developed stage (Structural Engineering Institute, 2009, p.39). In fire dynamics according to the International Investigators (2011), fire is interacting with its environment and to understand its behaviour one needs to understand aspects of fluid flows, heat transfer, ignition, flame spread, fuel packages, heat flux, and the difference between fuel-controlled and ventilation-controlled fires (p.29). Similarly, understanding the dynamics of fire in an enclosure require knowledge of the factors influencing fire development such as structure, room geometry, furnishings, and so on (Grimvall et al, 2009, p.227). In the same way, fire prevention and control should consider the above interaction in terms of structural designs limiting fire spread between buildings, limiting fire spread with buildings, facilitating escape and fire fighting, and limiting water damage (Mannan & Lees, 2005, p. 289). Built environments are usually used by people to perform certain functions thus building design should incorporate fire safety. According to Fitzgerald (2004), fire safety include not only fire dynamics but egress analysis, smoke control, detection, fire resistance, fire prevention, and risk management (p.79). This is because actual fire performance is highly dependent on critical factors such as physical layout, interactions between walls, floors, ceilings, fuel loads, and presence of other fire influencing factors such as vents, ducts, air-conditioning, and so on. Similarly, smoke and toxic gases are important hazards thus knowledge of their chemical composition and behaviour, movement, and physiological effects can greatly reduce possibility of injury or death. In terms of prevention, devices that can detect smoke or a particulate products of combustion such as carbon monoxide can greatly reduces chances of fire occurrence and poisoning as well as (US Fire Administration, 1973, p.135). Removal of oxygen and heat from the fire are often considered in fire protection and control along with reliability of water supply (Davis 1990, p.62). However, this approach is somewhat broad as fire prevention and control include protecting people and property. Using knowledge of enclosure fire dynamics, one would know that constructing with fire resisting materials has its advantage in terms of containment and escape. Similarly, strategically constructed vents or openings can greatly improve release of hot gases and smoke (Mannan & Lees, 2005, p.290). Analysis of enclosure fire development suggests that the main risk to life safety is the production of smoke and toxic gases thus such occurrence must be prevented. Another is the origin or location of the initial fire that may die out to lack of nearby combustible materials to ignite. It may also die down to lack of sufficient oxygen supply and may grow again if fresh air is introduced into the enclosure. In worse situation, a sudden introduction of large amount of air into an under-ventilated fire can led to deadly backdraught. Similarly, if the burning rates of fire reaches the maximum, its temperature is high enough to damage the building structure (Wang, p.204). Fire prevention and control therefore must be linked to the reality of fire behaviour, factors influencing this behaviour, and their consequences. 3. Conclusion Fire in the built environment is different from an outdoor fire considering the number of factors influencing a fire in an enclosure. The interaction between heat, fuel, and oxygen must be understood in order to prevent or control a fire. These include release of heat energy during combustion, rate of spread at the fire front, flame type, and factors affecting fire behaviour such as fuel load, fuel moisture, air temperature, relative humidity, air and ventilation. Understanding the characteristic of the built environment is also important such as the type of building, combustible furnishings, and others. It also includes familiarity with enclosure fire models such as zone and CFD in order to predict fire behaviour. The construction materials used in a particular built environment is essential to fire prevention and control such as fire resisting walls, doors, and others. Similarly, design of buildings must be evaluated in terms of fire safety and should include features such as compartmentation, detection and warning devices, and escape routes. 4. 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