Ventilation of Buildings - Report Example

Natural and mechanical smoke ventilation systems Introduction Incorporating smoke ventilation systems in corridors of apartment blocks has the potential of increasing the safety of the occupants of those apartments. Smoke control systems can be natural venting, pressurization or mechanical ventilation. The aim of all these three is to keep away smoke from the escape routes, to maintain smoke or keep it off the fire area so that it does not spread to other areas, to facilitate fire fighting operations and to safeguard life and reduce the loss of property. Both the natural and mechanical smoke ventilation systems have similar uses but they are used and operated in different ways (Awbi 2013, pp. 13). The Mechanical Smoke Ventilation System operates by extracting heat and smoke from a particular area and therefore depressurizes the place. On the other hand natural smoke ventilation systems maintain escape and fire brigade access routes free of smoke by providing a channel for heat and smoke out of the building. This implies that buildings remain fire resistant and that valuable assets cannot be damaged often. This report elaborates on the use of mechanical and natural smoke system ventilation Systems. Natural Ventilation Natural ventilation is always the norm in allowing smoke from fires to leave common corridors of apartment buildings. Natural ventilation is more beneficial especially because it is simpler, more reliable, uses low energy and produces lesser noise. Nevertheless, the performance of this system can be affected by the effects of wind and the natural shaft systems occupy a lot of space on the floor as well. Natural ventilation operates by capitalizing on natural forces of thermal buoyancy and wind to push air through the ventilator. What drives the smoke is its buoyancy as it comes out of the fire (Morgan, Ghosh and Garrad 1999, pp. 45). The buoyancy forces are sometimes smaller than the power of the wind. This means that the wind can affect the performance in a big way. In order for natural ventilation to function in an effective manner, there should be an exhaust opening and an inlet for air (Awbi 2013, pp. 56). Figure 1: Natural flow of air in and out For a vent mounted on the wall, the vent allows in air at the bottom and the air comes out through the top of the vent. Inlet air can also be allowed in through the stair door whenever it is open. In order to help in this and to get out all the smoke that comes in, a vent is provided at the top of the stair. An important thing to bear in mind is that when making the smoke and heat exhaust ventilation (SHEV) and the smoke ventilation, there should be a free open area for smoke shaft doors and the windows. These should be made available as an escape route for smoke. The more that ventilators, doors and windows are opened the more the space available get smoke out of the building. As long as the free space is there, any vent format can be used. A good choice can be having a louvered vent or having a side or bottom pivoting window. This freedom grants that one can easily select a vent and locate it is highly susceptible to strong effects by wind which can blow the smoke back into the corridor. With the right amount of pressure differential one can be sure that smoke coming from a particular apartment will not get into the stair under ordinary conditions (Wenting 2004, pp. 16). Naturally, air flows from a high pressure area to another area with low pressure. By raising the pressure in the areas that are protected or the escape routes above the pressure in the places where the fire can arise in the apartments, one can prevent from spreading into the corridors. This happens when the parts of the corridor that needs protection are pressurized. Even though the same effect can be achieved through depressurization of the departments, such an option is not practical. Figure 2: Smokes vents for natural ventilation systems In an apartment, the flow of air and smoke movement is hindered by the building fabric. If the fabric of the building has no leakage, it is possible to maintain a pressure differential with no other action once it has been created. However, since there is always leakage in buildings air must always be blown into the building so as to have the pressure differential maintained. The quantity of air needed will be determined by the amount of leakage taking place. This heavily relies on the amount of doors which allow for leakage around the walls as well as the area and the type of walls constructed and other open spaces that can act as escape routes for air from the protected spaces (Wenting 2004, pp. 34). A problem comes in when the doors opening to the protected area are open the area of leakage increases tremendously and this makes it hard to maintain a good amount of positive pressure. The smoke ventilation system should therefore be strong enough to offer the needed protection even when doors are opened while at the same time the pressure differentials that can be obtained with closed doors is limited. When the pressure is too much at a time when the doors are shut, makes it hard for the doors to open into the pressurized space and this prevents escape into the areas that are protected. Controlling smoke using natural ventilation systems is normally very effective for protecting people trying to escape from fire, those who are waiting to be rescued as well as fire fighters from the danger of smoke and fires. As a matter of principle, both low level and high level out vents automatically open whenever there is fire so that cool air can flow into the building as hot air and smoke flow out (Cook 1993, pp. 10). This makes the conditions better for those escaping and the fire fighters entering the building. Where there is no ventilation the room can be filled with smoke and it is drawn down from the ceiling by the rise in convection temperatures increase causing bad flash over. The design of a safe and workable smoke ventilation system needs a lot of specialized involvement. Designing automatically opening valve systems makes it possible for the glazed frame to be fixed in any outside wall. Automatic Opening Valve systems are made with details for water management which ensures that there is leakage and seepage exits from the building. The frame is broken to reduce the loss of heat and condensation (Wood, and Salib 2012, pp. 32). Mechanical Smoke Ventilation Systems The mechanical Smoke Ventilation System works as a depressurization system. It removes heat and smoke from a particular space and in the process depressurizes the place. Because the areas around such as staircases could be having higher pressure, air moves from these places to the smoke shaft and in the process it prevents smoke from getting into the staircase and the areas around it. There is necessity for provision of additional inlet air to the place being depressurized so that the area does not suffer excessive depressurization (Cook 1993, pp. 67). If there is over depressurization in the area, smoke venting equipment could be damaged and the smoke will be actively removed from the fire room. There may also be an increase in pressure differentials between doors so that the door becomes too hard to open between the fire area and the depressurized area. Figure 3: Mechanical smoke ventilation The system’s make up inlet air can be obtained through several ways. This is majorly determined by the layout of the building. However, there are four main ways through which one can achieve the system’s make up air. These are; natural inlet through AOV at the top of the staircase, natural inlet through external air, natural inlet through shaft and mechanical inlet through shaft. Each way has its merits and demerits. Mechanical ventilation could only serve as an alternative to the natural system of ventilation (Morgan, Ghosh and Garrad 1999, pp. 28). A shaft can either be use or each of the floors in an apartment building can have its powered system. Mechanical systems are advantageous because of the low sensitivity to winds, specified rates of extraction, capability to overcome resistance to system and reduced cross sections of the shafts. Powered systems require that there be maintained power, equipment classified based on temperature, standby fans and wires that resist fire. Internal pressure in the building must be limited to maintain the operability of doors. Air inlet should be provided to the communal area so that the system is not damaged. It also ensures that the too much pressurization and depressurization of the area under ventilation is avoided. Figure 4: Fire and smoke ventilation systems Through the avoidance of too much pressurization or depressurization ensures that the huge quantities of smoke do not move from the place where the fire has originated (Niren 1993, pp. 88). It also helps to avoid high pressure differentials which can cause escape doors to be inoperable or to open. Designs of the system should consider the effect of fire on a single floor level. It is only the vents of smoke on the floor where the fire has originated and other important vents such as the head of staircase and smoke shafts should be opened. Those with the responsibility of designing systems should not open ventilators on many floor levels especially at places where there is a connection with a smoke shaft so that smoke does not flow to other parts of the building that are not affected by the fire. This can also reduce the rate of removal of the smoke from the floor on which the fire is. It is advisable that smoke shafts be made of materials that are not combustible. System activation is based on the agreement with approving authorities and other stakeholders that have an interest in it but the system can be activated whenever smoke has been detected on the common corridor. Whenever the system is activated, smoke vents on the floor of the apartment, vents at the head of the smoke shafts, and the vent at the head of the stairway must be opened and fans should be allowed to work at the speed they were designed to run at (Wenting 2004, pp. 29). Figure 5: Mechanical smoke ventilation controls Mechanical systems are made as an equivalent to typical natural systems of smoke ventilation. High performance systems can possibly be designed which can then be crucial in allowing for longer travel distances on the corridors. However, there should be a lot of care whenever corridor subdivisions are removed. The doors limit the potential distance for travel through the smoke, but they can also limit the apartments that need evacuation by firemen which give the fire fighters more protection. When these doors are removed, the safety of dire fighters may be compromised. Equipment and Installation Every piece of equipment should be selected to meet the system’s particular requirement performance. If all the system components are not properly installed, then system may fail to operate properly or it may fail to meet the performance targets that were set for it. A proper engineering plan with the required details should be made and factors such as equipment identification and location, size, power supply ratings, cause and effect summary and the sizes and routing of cables should be included (Morgan, Ghosh and Garrad 1999, pp. 32). During component selection and installation, environmental conditions, the safety of users, accessibility and protection must be put into consideration. All t he components that have been installed should be those that can be cleaned and maintained in the proper manner. There should be planned access so that routine maintenance tasks including cleaning and lubrication can be done. There should be appropriate provision of doors and access panels. To help in removing and repairing components, there should be installed beams and eyes for lifting. The installation of components should be done so that there is no discharge of heat and smoke into nearby structures. The exhaust discharges must not point at the windows and walls and protection should be made for any combustible sections of the roof close to the exhaust (Morgan, Ghosh and Garrad 1999, pp. 90). Comparison The mechanical ventilation system is advantageous because it optimizes on space and is more usable. It saves on cost because it has the potential for the removal of the staircase and areas of reduced ventilation shafts. It is better because it is not very complex. It is however, disadvantageous because for it to be approved, the system must be proven through CFD modeling or calculations. Non specialists may also not understand them. The systems are also not suitable for all buildings. On the other hand, natural ventilation is better because it consumes less energy. However, the system cannot provide thermal comfort in certain climates hence the need for back ups. Natural ventilation is also cheap and reliable. How CFD modeling could potentially be used to model the performance of the smoke control system The ability of the smoke control systems to remove smoke and heat from a building can be modeled using CFD modeling. The beginning point of modeling is the establishment of the computational domain of the model which is the limit of the area to be modeled. Primarily, one should consider the three dimensional domain and the domain boundaries which include the area of interest (Morgan, Ghosh and Garrad 1999, pp. 12). Selection of domain boundaries can also be affected by computational limits. CFD simulation can be too huge for the available computational resources. In such circumstances, focus should be placed on the important features that affect the flow of smoke and air and at the same time ensure that the influence of omitted factors will not affect the objective of or the outcome of CFD simulation (Niren 1993, pp. 45). Attached volumes may also affect the selection of domain boundaries. The volume where the interest is can be influenced or be linked to other volumes like the floors of the 15 storey building that are not adjacent to the floor of interest. Domain boundaries must be defined at places where there are minimal flows between attached volumes. Domain boundaries should be made in a manner that they don’t have an adverse effect on the simulated movement of smoke. Free of open boundaries must be placed far from the fire source because smoke can be lost to the computational domain while in reality the smoke could get back into the region affected by fire. Any smoke leaving the computational domain should do that at areas that are far from induced flows caused by fire, wind or mechanical means which could make it possible for the smoke to get back to the domain (Wood, and Salib 2012, pp. 56). Details in the Computational Domain It is important to establish the things that should be included and excluded from the model’s geometric representation. All the objects that can impact the flow of air or fire induced flows and the movement of smoke should be represented in the model. The list should have floors, ceilings and the walls of the structure, columns and beams and services (ADB 2000, pp. 14). Computational Mesh The computational domain should be divided into many smaller cells. The disposition and size of the cells should be made in consideration of the proper representation of geometric details. Where fire is involved, the flow that drives the movement of smoke should be properly resolved. Many mesh cells should be included to represent the dimensions of the plan. Enough detail should be included in order to capture rising hot gases. The area next to the ceiling should include a good amount of cell layers normal to the ceiling especially where a structured mesh is used. It is important to ensure that mechanically assisted or induced flows have proper representation. Adequate mesh cells must be used to describe the dimensions of the inlet or fan in the plane where the air flows (Morgan, Ghosh and Garrad 1999, pp. 70). Several of them are needed and it should be ensured that changes in the mesh cells lying in the flow direction should not have an influence on the characteristics of flow. The guidance given by the developer of the property should be adhered to in order to ensure that the selection of mesh cell sizes remains in consistence with the modeling approach in use. Mesh cells should not be distorted in any way. Property developers should provide guidance on the level of distortion of mesh cells. An investigation should be done on how sensitive the results to the size of mesh cells are (Stationary Office, Great Britain 2007, pp. 61). Physical Sub-models The selected physical sub-models should define the equations that need to be modeled in the CFD modeling. The mechanisms that control the flow and should be noted includes combustion which produces the heat and the smoke, buoyancy since the heat from the fire causes buoyant flows hence influencing turbulence and turbulence (Morgan, Ghosh and Garrad 1999, pp. 112). Fans, inlets of air and obstructions to airflow create a turbulent air flow that has an influence on the exchange of heat with air, affects the mixing of air with fuel before combustion and also affects the movement of smoke and its dilution by way of mixing (Awbi 2013, pp. 42). Specification of the fire source and boundary conditions If there is a fire in the modeling process, the characteristics, size and location of the fire must be specified for CFD modeling. Modeling fire may not be required when dealing with ventilation for smoke clearance (Niren 1993, pp. 25). Any type of mechanism that happens to be external especially to the computational domain but which has an influence on the behavior of the flow in it should be represented. Examples include the initial flows present in the computational domain before the simulation, flows in and out via doors, openings, vents and windows, change of momentum and energy in simple representations of the mechanical systems, energy exchange on the walls in forms of heat, and sources of mass, energy and momentum. The results from the modeling exercise of the CFD are determined through boundary conditions. It is important for one to make the boundary conditions specific and to realize that they play an important role (Stationary Office, Great Britain 2007, 03). The extended corridor arrangement in CFD modeling is meant to help in the flow of air into the building and to ensure that the heat and smoke have enough space to travel through. Concentration of smoke and heat in a small corridor could easily lead to disastrous effects such as deaths since the smoke and heat have no space for escape. A spacious corridor ensures that air from the outside enters as much as possible (Royal College of Physicians of London Tobacco Advisory Group 2005, 33). Fire may start in the building as a result of faulty wiring, problems with home heating, clothes driers and faults in lighting. Conclusion Mechanical and natural smoke ventilation systems operate by facilitating the exit of smoke and heat and the entry of fresh air. In natural systems, air flows in naturally, as smoke and heat move out and are diluted by the incoming air. In the mechanical ventilation systems, the heat and smoke are forced to exit the room through a process of pressure reduction in the affected room. The smoke control system can be modeled using CFD modeling. A number of things should be observed such as computational domains, physical sub-models and the boundary conditions. Bibliography ADB. (2000). Approved Document B volume 2. United Kingdom: Communities and Local Government. Fire precautions in the design, construction and use of buildings: Code of practice for places of assembly. October 1991. Fire precautions in the design, construction and use of buildings: Code of practice for the incorporation of atria in buildings. November 1997. Fire precautions in the design, construction and use of buildings: Code of practice for means of escape for disabled people. May 1999. Code of practice for fire safety in the design, management and use of buildings. Fire detection and alarm systems for buildings: Code of practice for system design, installation, commissioning and maintenance of systems in non domestic premises. March 2013.  Morgan H.P., Ghosh, B.K and Garrad G. (1999). Design Methodologies for Smoke and Heat Exhaustion Ventilation. BRE Press. Awbi H.B. (2013). Ventilation of Buildings. Routledge. Cook, N.J. (1993). Wind Engineering. Thomas Telford. Niren L. N. (1993). Modeling of Indoor Air Quality and Exposure. Issue 1205. ASTM International. Royal College of Physicians of London. Tobacco Advisory Group (2005). Going Smoke-free. The Medical Case for Clean Air in the Home, at Work and in Public Places. Royal College of Physicians. Stationary Office, Great Britain 2007. The Building Regulations 2000: approved Document, B: Fire Safety Volume 2. TSO Shop. Wenting, D. (2004). Smoke Control Based on Solar Assisted Natural Ventilation System in an Atrium Building. Abebooks.  Wood, A., and Salib R. (2012). Natural Ventilation High Rise Buildings SALIB and WOOD. Routledge. Read More