Natural and Mechanical Smoke Ventilation Systems - Challenges and Strengths - Assignment Example

Design project Name Course Institution Date Part 1: Natural and mechanical smoke ventilation systems Introduction. Ventilation entails the movement of the outside air into a building or room, and distribution of the air in the construction. Ventilation of a building can be in three different forms, that is, natural, mechanical as well as hybrid (mixed -method) modes of ventilation. However, this document puts into consideration only the natural and mechanical modes of ventilation within the corridors of the apartment blocks (Faber et al. 2014). Integrating smoke ventilation systems within the buildings corridors enhances safety potentiality of the occupants residing in those apartment blocks. Smoke can be controlled either naturally or mechanically by use of vents. The natural and mechanical smoke ventilations aim at regulating the smoke in the apartments, provide the escape routes of the smoke, enhance fire fighters to work out the operation smoothly, and reducing both the loss of lives and properties. Despite the fact that the two smoke ventilation systems have similar functions, they are apply and operated differently (Cote, 2003). The operation of Mechanical System of Smoke Ventilation is done by heat and smoke extraction from the apartment area and consequently depressurizing the place. Natural system of smoke ventilation on the other hand involves the fire fighters to access entrance without smoke thus providing a way for heat and smoke from the structure building. The two methods of smoke control entail the resistance of the fire from damaging valuable resources in the buildings. Therefore the core purpose of this report is to elaborate the applicability both mechanical and natural smoke system ventilation Systems in big apartments. Natural Ventilation Natural ventilation is simpler, reliable, requires less energy, and its less noisy, therefore is more preferable and beneficial compared to mechanical ventilation. In its application, Natural Ventilation allows smoke to escape through the ordinary corridors of residence buildings. However, the practicability of system is determined by the winds and the innate beam. Natural ventilation functions by taking advantage of normal forces e.g. thermal buoyancy alongside wind to thrust air via the ventilator. What constrains the smoke is its ability to float in the air as soon as it parts the fire (Vedavarz et al 2007). The buoyancy drives are at times lesser compared to the influence of the wind, therefore, wind affects the performance of Natural Ventilation System greatly. For the effectiveness of the natural ventilation, an exhaust aperture enhances proper circulation of airflow is required (Cote, 2003). Challenges. Because of leakages found within the house structures, air should constantly be propelled into the apartment to maintain the pressure difference. Actually, the amount of air required greatly relies on the quantity of outflow intriguing the building. This profoundly depends on the total number of doors and or any other opening that is able to operate as getaways for air allowing the leakage in the region of the walls, the area and the walls nature of construction from the confined spaces (Building Regulations 2000, 2006). A difficulty arises in case the doors and the other opening within the secluded area are wide open. The surface area of seepage tremendously increases making it difficult to uphold the correct pressure balance. Therefore, the smoke ventilation system should offer protection even though the outlet openings e.g. doors and windows are open and the pressure difference is limited. However, if pressure is in excess when the doors are closed up, causes the door very difficult to open hence hindering the escape of the smoke in the protected areas. Strengths. Scheming smoke by means of natural ventilation systems is usually incredibly effectual for shielding people frustrated to get away from the fire, those waiting to be saved and fire brigadiers from the jeopardy of fires. In case of fire outbreak, the lower and the upper vents open automatically to supply cool air into the apartment and at the same time to pave out hot air and smoke (Cook 1993, pp. 10). This creates conducive conditions for both rescues and the fire brigadiers’ incoming the house. The plan of a protected and practicable smoke freshening system requires much specialization in designing automatic aperture regulator systems making it feasible for the fixed enclosure to be predetermined in some outer wall. Automatic Aperture Regulator systems are prepared with particulars for water executive which make certain that there is outflow and ooze outlet from the house structure. The structure is conked out to diminish the thrashing of heat and concentration (Ramachandran & Charters 2011). The installation of low-level inlet vents and high-level outlet vents suffices to be an example of the natural systems of smoke ventilation. In case there is a fire outbreak, the vents open automatically thus permitting the entry of cold air into the building and the exit of smoke and hot air out of the building. Consequently, it is apparent that the occupants find it easy to escape the building. Natural Smoke Ventilation Systems Installation. The installation of natural ventilation systems is done through 3 formats, they include; simple systems/ single-sided ventilation, networked control/ buoyancy-driven stack and centralised systems/ Wind-driven cross. Simple systems format is applied to diminutive buildings, networked control applies where the accessible room is not enough for the fixing of a central control system. The natural ventilation applies the theory of warm air rising above the cold air. The buildings that have natural form of ventilation uses this conceptual measure to install the vents in apartments. Natural ventilation installation takes the form of Wind-driven cross, buoyancy-driven stack or single-sided ventilation (Rock 2006). Wind-Driven Cross Ventilation. The exterior corridor walls ventilation installation in the building should have at least 1.5m2 vent. The position of the vent should be as elevated as achievable or staircases of the doors. It is significant to activate the vents in houses that have solitary staircases as in flats. However, the vent’s physical actuation is obligatory for multi-stair apartments. It is crucial that the vents located at the summit of the staircases should be opened automatically. It is also perceptible that at the time of fire outbreak, it is hard for the absconding inhabitants and fire-brigadiers to venture to open the vents manually. Therefore, it is important to automate them. Figure 1. Cross ventilation Buoyancy-Driven Stack Ventilation. This method is dependent on the density differences in drawing cool, outside air in at the lower ventilation openings as well as exhausting warm, interior air at the high ventilation openings. The Figure 2 is showing a schematic of stack ventilation for a building with many rooms. Chimneys or atria are often used in generating adequate buoyancy forces to attain the required flow. Nevertheless, a small quantity of wind will induce adequate pressure distribution on the corridors envelope that would also act in driving airflow (Mobley 2001). Certainly, wind impacts might well be more significant than buoyancy impacts in regard to the stack ventilation schemes; therefore, the successful design always seeks for the way out of making full advantage of both. Figure 2. Buoyancy-driven stack ventilation Single-Sided Ventilation. This form of ventilation usually applies to the single rooms and therefore offers local ventilation approach. The figure 3, below, is showing a schematic of single-sided ventilation within multi-room buildings. Figure 3. Single-Sided Ventilation. The ventilation airflows in this scenario are propelled by the room-scale buoyancy impacts, slight differences within the envelope wind pressure, and/or turbulence. As a result of this, driving forces for the single-sided ventilation have a tendency of being relatively small and exceedingly variable. In comparison to the different options, Single-sided ventilation avails the slightest attractive natural ventilation solutions but, on the other hand, an approach that serves individual offices (Cote 2003). Mechanical smoke ventilation. Mechanical/ powered smoke ventilation systems are alternative to the natural ventilation systems on the floorboards, entrance halls and hallway of buildings. The ventilation applies beam system basing on the mechanical ventilation procedure. The fundamental eminence of the system cover the decreased surface area of the shafts, its aptitude to surmount the opposition of the system, squat understanding of wind and exacting rates of withdrawal (Fitchen 1989). The indispensable desires of the powered ventilation system include sustenance of power, fire defy wiring, confidential temperature apparatus, and a reserved fan component. It is also very important to think about and maximize the interior pressures inside the accommodation to warranty the operability of the doors (Unforced 2015) The corridors of the apartment blocks can be mechanically ventilated by using fans as well as vents in exhausting the current basement air and bringing in fresh, outside air. This form of system might range from the placement of a tiny window fan in the opposite windows to technological as the installation of exhaust fans with ventilation pipes. Majority of homeowners prefer the mechanical smoke ventilation system to natural ventilation system due to the significant flexibility in addition to its automation (Ingason & Lönnermark 2014). Mechanical Smoke Ventilation System Installations. The most important plan principle for the mechanical smoke ventilation systems include: Natural and Mechanical principles (Butcher and Parnell, 1979). Natural Principle. Natural principle makes use of environmental settings and smoke’s tendency to rise in order to disperse smoke through the movement of air. The scheme involves mechanical extraction beams that supply the ordinary spaces partially or entire floor echelons on a building structure (Godish 2001). The closing stage of the shaft includes both the reserved and obligation fans that take out smoke from the ordinary spaces to the exterior (BRE 2005). Fresh air is driven in using the air “intake” vents and circulated to various apartments by use of fans and duct systems. This allows the heating and cooling system’s fans as well as the ducts to circulate the fresh air. The essence of linking to the return air ducts is that the outside air might have air conditioning or dehumidification prior to it is introduction into the home. However, since supply systems frequently bring in outside air, an apartment becomes slightly pressurized (Ingason & Lönnermark 2014). Mechanical Principle. Mechanical principle requires a power source to either extract smoke or to achieve pressure within a particular space (Butcher and Parnell, 1979). Carefully the system needs to be balanced to avoid pressurisation or depressurisation within the ordinary spaces. To achieve moderation of the pressure, some or all the fans are reversed to utilize the fire-fighting switches. Consequently, the classification employs convertible fans that carry out the responsibility of removing smoke and hot air from the regular space and supplying mechanical air bay. The fire recognition system manages the activities of the system. For example, when the smoke is detected, the fan nearest to the smoke sensor serves as the smoke remove fan. The system too has an overrule knob that authorizes the Fire brigadiers to exchange all the fans into the removal method if the structure has a different air bay such as a stairway vent. With this type of ventilation system, equivalent amount of air are driven into and expelled out of the apartments. This is attained through the use of two fans—one that brings in fresh air as the other one sends interior air out. Figure 4. Balanced Ventilation System Assessment of smoke control systems. All the systems fundamentally seek to reliability hence controls smoke and by default minimize the hazards caused fire. As a result of this necessity, objective assessment should be done to both siege and reduce smoke and further provision of safest gateway routes. It is of great importance to have assessment on how vulnerable the system is to malfunctioning with the introduction of 100% external air throughout severe weather conditions. At least, these points of concern have to be talked about by the owner and the designing workforce so as to come to consensus on the approach they intend to deal with these issues. Documenting the procedures of the projects’ operating manuals as well commissioning reports is also of great importance. The aforementioned matters can also affect the duration of the functional and the required preparatory stages to pave way to the progress of testing without risking the equipment or construction resources and finishes (Burke 2008). The common assessment of smoke control system applies the concept of computer simulation in the modeling of a building fire, through the application of knowledge regarding physical laws on smoke flow. Computer simulation is advantageous due to its simplicity and reasonable prices in the testing results on the application of various systems and in diverse situations. Computer simulations are capable of working on statistical variability of the conditions of fire and the geometry of the construction and the environment (Fernando 2013). Part 2: How CFD modeling could potentially be used to model the performance of the smoke control system. The decision taken by the design team may have a considerable effect on energy utilization, indoor climatic performance, occupants’ health as well as their productivity. As a way of getting things right and avoiding costly post occupancy remedial treatments, designing teams are required to have successfully foretold air movements, distribution of temperature as well as the concentration of airborne noxious waste and smoke (Bre 2014). The CFD modeling can be of significance to the design team in designing a program that will be predicting airflow and highlighting the effects of the wind on building performance and residents comfort. Normally, smoke particles move in a similar pattern as air at equivalent temperatures, and this makes it possible to calculate the spread of smoke particles as it is with the calculation of the ordinary air movements and ventilation in areas with sufficient distance from locality of the fire that the smoke has significantly cooled. In locations that are closer to the fire, where the smoke has higher temperatures, there is an increase in smoke movements as a result of variance in temperature resulting to buoyancy becoming a point of concern (Bre 2014, Burke 2008). What can be modeled? By use of the well established computational systems such as CFD (computational fluid dynamics) the designing experts are capable of providing solutions in various aspects of the building and environmental Engineering design such as: distribution of temperatures, ventilation efficacy, thermal comfort, quality of air, electric and day lighting, fire and smoke movements, and occupant evacuation. Physical modeling might also be recommended that would test the building. In this case, within a controlled resourceful spot, simulation and monitoring of the performed in real life conditions (Ramachandran & Charters 2011). CFD modeling is of great significance in the sense that it provides adequate information and forecast in regard to the performance and reaction of the modeled subject. CFD modeling is also applicable in rationalization of a remedy in fire engineering (Fernando 2013). Assumptions and Input Parameters for the CFD Model The CFD model requires adequate details regarding the temperatures, location and smoke intensity of the fire in the fire partition. It is also essential to have details on the various cracks around windows and doors, closed and open doors and windows, and general environmental conditions. The CFD model is operational on the basis of the accessibility of information on spaces, layout and connections of the building between areas (Cote 2003). Basic modeling geometry/ settings • We are modeling fire that is situated on the 4th floor within the specified building of 15 floors. • The ceilings, Walls and floors are assumed to maintain constant ambient temperatures; however, this heat is transmitted to these surfaces from the gases present. We have also made an assumption of ambient temperature to be 20oC. • We have taken the height of lobby, corridors and for the flat where the fire takes place to be 2.5m. The height and width of the doors is taken as 2m and 0.8m respectively The fire • In regard to simulation, the ‘burner’ surface of 4m2in area, with 8m perimeter is considered to be our fire. • The rate of released heat is specified as 625kw/m2 . The total size of the fire in each model is 4m2 X625kw/m2 = 2500kW • It is assumed that the fire in the model has a medium growth rate, at 0.0117 kJ/s3. Since heat release rate is expressed as 𝑄= 𝛼𝑡2, making t the subject of the formula, t=Q/ (2𝛼). Since Q = heat release rate = 2500kW, α = 0.0117 kJ/s3, and t = time (seconds). Substituting the values into the formula, we get t =462 𝑠𝑒𝑐𝑜𝑛𝑑𝑠 10% polyurethane soot yield was put into consideration as suitable value for the fire. Fire flat • The exterior facade for the flat has one window, which is 1.81m wide by 1m high. A lobby and straight corridors are modeled. • Throughout the building the front doors of the fire flat do not exceed 23.25m from a single stairway. Staircase • This 15-storey model has one staircase for all floors. Travel distances We have established that the furthest part of the building from exit is situated less than 45m from the fire exit terminal. Therefore, the distance to the exits in all scenarios is in accordance with the guidelines in Klote et al (2012) and ANSI (2007) as follows: Figure 3 Distance to travel between the extreme end of the floor and the exit. RSET – Travel Time Parameter explanation calculation Time Period (s) tdet Time to detection 60 ta Time to detection 0 tpre Pre-movement time 1st percentile +99th percentile of persons 30s + 60s 90 ttrav Travel time Travel distance (m)/ walking speed (m/s) 33 m / 1.2 m/s 27.5 Safety Margin 60 RSET 237.5s (3.95 min) Table 1.Required Input Parameters of the Model Possibilities for outputs of the simulation are given in Table 2 below. Table 2.Output Options Scenarios proposed for modeling Three scenarios have been put into consideration. The vertical smoke shaft and fire flat’s location have been altered in the three scenarios. By altering the two locations, gives room for effective modeling regarding the best and worst likely fire incidences. It also makes it possible for the measurement of the movement of smoke particles throughout the corridors towards the staircase. The altered location facilitates the evaluation of the vertical smoke shaft’s efficiency. Scenario 1. The two fire locations are situated at the two ends of the corridor and the smoke shaft almost sandwiched by the smoke shafts Scenario 2. Two smoke shifts on adjacent corridors and the fire location at the stair case locality Scenario 3. Smoke shaft at the fire flat are located at the far end corridor The common corridors were assumed to be straight, having a width of 1.7m and a length of 23.25m. While investigating the essence of the vertical smoke shafts in the protection of the staircase and various common areas, all the scenarios modeled are evaluated by use of criteria of 10m visibility and a 60oC temperature threshold (The Building Regulations 2000 2006). The results from the CFD evaluation are showing that the proposed smoke control system has the ability of allowing suitable situations to be sustained in majority of the buildings as evacuation progresses. This also offers favorable environment for firefighters to access the building. It also offers safety that is in compliance with the relevant measures. In conclusion, both mechanical and natural smoke ventilation mechanisms bring about favorable air for breathing by reducing the concentration of the smoke that originates from the building as well as expelling the smoke particles from the building. CFD modeling is of great significance since it provides adequate information and forecast on the performance and reaction of the modeled subject. CFD modeling is also applicable in rationalization of a solution in fire engineering. Reference List ANSI. (2007). American National Standard for the recirculation of air from industrial process exhaust systems. Fairfax, VA: The Association. Barber, N. (2012). Buildings and structures. London: Raintree. Beard, A. & Carvel, R. (2005). The handbook of tunnel fire safety. London: Thomas Telford. Bre, 2014, Computer modeling: microclimate performance assessment, available http://www.bre.co.uk/pdf/049.pdf Burke, R. (2008). Fire protection systems and response. Boca Raton: CRC Press. Cote, A. (2003). Operation of fire protection systems: a special edition of the Fire Protection Handbook. Quincy, Mass: National Fire Protection Association. Faber, O., Kell, J., Oughton, D. & Wilson, A. (2014). Faber & Kell's heating & air-conditioning of buildings. London: Routledge. Fernando, H. (2013). Handbook of environmental fluid dynamics. Boca Raton, FL: Taylor & Francis. Fitchen, J. (1989). Building construction before mechanization. Cambridge, Massachusetts: MIT Press. Godish, T. (2001). Indoor environmental quality. Boca Raton, Fla: Lewis Publishers. Haines, R. & Wilson, C. (2003). HVAC systems design handbook. New York: McGraw-Hill. Ingason, H., Li, Y. & Lönnermark, A. (2014). Tunnel fire dynamics. New York: Springer. Klote, J., Milke, J. & Turnbull, P. (2012). Handbook of smoke control engineering. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. Mobley, R. (2001). Plant engineer's handbook. Boston: Butterworth-Heinemann. Morgan, H. (1999). Design methodologies for smoke and heat exhaust ventilation. Garston: CRC. Ramachandran, G. & Charters, D. (2011). Quantitative risk assessment in fire safety. London New York: Spon Press. Rock, B. (2006). Ventilation for environmental tobacco smoke. Oxford Burlington, MA: Elsevier Butterworth-Heinemann. Snow, D. (1991). Plant Engineer's Reference Book. Burlington: Elsevier Science. The Building Regulations 2000. (2006). The Building Regulations 2000. Approved Document B: fire safety. S.l: TSO. Vedavarz, A., Kumar, S. & Hussain, M. (2007). HVAC handbook of heating, ventilation and air conditioning for design and implementation. New York: Industrial Press. Read More

By altering the two locations, gives room for effective modeling regarding the best and worst likely fire incidences. It also makes it possible for the measurement of the movement of smoke particles throughout the corridors towards the staircase. The altered location facilitates the evaluation of the vertical smoke shaft’s efficiency. 

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