Use of Natural and Mechanical Smoke Ventilation Systems - Report Example

Usе оf nаturаl аnd mесhаniсаl smоkе vеntilаtiоn systеms in соmmоn соrridоrs оf араrtmеnt blосks Table of contents 1.0 Introduction 4 1.1 Natural ventilation 5 1.2 General Principles of mechanical ventilation 7 1.3 Mechanical Smoke Ventilation 8 2.0 Operation of the ventilation system 10 2.1 More effective ventilation system with more space saving 11 2.2 Shaft extended system 11 2.4 Extended Corridor System description 12 2.5 Scenario 2: Mechanical systems (pressurization) 13 References 15 List of Figures Part 1 Write a report (approximately 2,000 words - indicative limit) reviewing the use of natural and mechanical smoke ventilation systems in common corridors of apartment blocks. 1.0 Introduction There is a great need for smoke control in the escape routes in apartments as a way of increasing the chances of occupants making it out of the building when there is a fire incident. In order to ensure success of the design as well as the approval process, it is important for those involved in the designs to make sure that there is agreement and documentation with systems objective, any CFD analysis, the scenarios that are to be modeled, the criteria of modeling as well as the pass and fail criteria even before the start of the design (1). Of particular interest in the entire process with regards to the operating modes are the means of escape and the fire fighting operations. Those in charge of designing the smoke control systems the proposed system is complementary with the building fire strategy to ensure that the safety of those occupying the building is guaranteed. Some buildings may be adhering to the criteria that has been set in ADB and in such cases there will be no need of considering objectives and performance criteria, since the ventilation system will be deemed to satisfactory by simply meeting the standards set in ADB. In other cases such as the one involving our current building consideration of objectives and performance criteria for the system is important. Until very recently no guide had been given with to take care of such scenarios the only provision was that‘equivalence’ to the ADB prescriptive system was to be provided. Some works undertaken by Building Research Establishment (BRE) recently in support to the 2006 ADB edition has been instrumental in giving some insight regarding objectives and performance system given in ADB. The ADB 2006 edition lists the stairs, lobbies and corridors that open to stairs as the areas in a building that require provision of smoke ventilation. There are several methods that are applicable in investigating the performance just as it is expected for cases where there is alternative performance. One of the simplest method would involve hand calculation while the most sophisticated would involve the use CFD and zone models which are all sophisticated computer models. Each of these methods comes with their own benefits as well as limitations, with faster calculation being associated with faster calculation but a limitation being the degree of spatial and temporal resolution while of the others such as the CFD the reverse is true. 1.1 Natural ventilation In ADB natural ventilation is taken as the norm while mechanical ventilation is considered as being another alternative even though both the two methods are allowed as a solution to ventilation to common corridors/lobbies. There are numerous advantages associated with natural ventilation that include its simplicity, low noise, high reliability, low operation cost bearing in mind that no requirement of energy. The demerit of the natural ventilation is with regards to its sensitivity to effects of wind and where the design involves a natural shaft systems a lot of space is put to ‘waste’(2). In ADB there recommendations for natural wall vents, natural vent shafts and also vents located at the head of the stairs. The guides with regards to natural ventilation for any building will usually be found to be in conformation with what is given in ADB. In natural ventilation there is dependency on natural forces created by wind flow as well as forces that come as a result of thermal buoyancy to a means of ensuring air circulation through the ventilator. In a situation involving fires in a building, the target is utilization of the buoyancy of the hot smoke from the fire as the driving force. The performance of natural,ventilation may be affected considerably owing to the fact that the wind force in some circumstances may surpass the buoyancy forces created by the hot smoke. In natural the design should be such that there is provision for both inlet air as well as exhaust opening. For cases involving a vent mounted on a wall, generally the vent will be found to provide both the inlet at the bottom of vent while the exhaust will be at the top. Where this is not the case the air inlet may be provided via the door of the stairs upon it being opened. The vent need to be located at the head of the stair so as to ensure this design is effective. A key factor that needs to be put into consideration in the design of smoke and heat exhaust ventilation (SHEV) is the free open area of windows and the smoke shaft doors upon their opening to facilitate smoke escaping. If the free area is achieved , it will be at the designers disposal to choose the preferred form of vent, with the louvred vent, ventilator or side pivoting window being considered to be the normal choices. With all this freedom of choice, the end result is that there can be selection and location of vents such that they will be highly susceptible to adverse effects of wind, with the possibility of wind being blown back into the corridor or lobby and into the stair which is the place that the occupants are to use to escape. It is upon the designers considering mitigating the wind effects when making a selection of vents and the point where they are to be located even though in ADB there are no regulatory requirement that have been provided. In a case where there is utilization of the roof light an automatic opening vent as described in BS EN12101-2:2003the a having 140° as the minimum opening angle will go a long way in mitigating the wind effects (3). In the case where we have an outward opening bottom or side hung corridor or lobby vent is being put into use the calculation of the free area will involve multiplying a and d where the former is the length of the vent that is in the opposite side of the pivot mechanism and the later is distance the ventilator is able to open with the measurements being taken 90° to the opened flap or window. The same approach is taken when it comes to smoke shaft doors, where there is use of the door height and the opening distance in the associated calculation. Most smoke shaft doors in the market do not have the ability of opening fully but some few companies such as SE Controls whose design allow the doors to open to the 90° maximum mark and in this there is provision of greater free area that ensures efficient smoke ventilation via the shaft. In compliance with ADB, the measurement of free area for the open ventilator is to be done in the plane area is at minimum and perpendicular to the direction where the air is flowing as can be seen in the figure. Figure 1.1 1.2 General Principles of mechanical ventilation Naturally there will be flow of air from high pressure areas to low pressure areas. If the pressure in the protected areas such as the corridors or lobby are increased beyond that in the affected apartment (or any other area of interest) then smoke spread into the escape route is preventable. Normally there will be pressurization of the section of escape route that is to be protected, even though theoretically depressurizing the apartment would serve the desired purposed, this second option is not practical. In any building, the building fabric serves the purpose of restricting smoke and air movement. In a theoretical situation where the building fabric is free of leakage pressure could be increased up to a certain point and this could then be maintained without any further pressurizing action need. However, in a practical situation buildings will be found to have some leakage points and this means that there will be need for continued blowing in order for the pressure differential to be maintained with the actual level or air requirement being dependant on the extent of leakage(4). This usually will be dependent on the number of doors that would allow leakage of air around the protected space, the area and the type of wall construction and any other opening in the protected space. The challenge is that upon opening the doors to protected space so as to be used by people who are escaping as well as the fire men who may want to access the apartment affected by fire, this would result to the leakage area being increased substantially and makes it challenging to have a significant positive pressure. This calls for designing a system that is robust enough that it is able to provide a minimal protection even when some doors are open, at the same time the system should be able to limit the pressure difference achieved when all doors are closed so that doors opening into pressurized space should not present a lot of difficulties opening as this will ensure those in apartments can open the doors with ease and make their way to safety. There is provision of guidelines by BS EN 12101-6 where performance achievable through pressure differential systems with focus on both conditions of closed doors and open doors with guide being given with regards to which doors are to be considered closed to facilitate calculations and coming up with the desired design. In EN 12101-6,2005 edition with regards to residential building, a system for protecting means of escape falls under Class A while Class B is that designed for assisting those involved in fire fighting. 1.3 Mechanical Smoke Ventilation Mechanical smoke ventilation also know as powered smoke ventilation is an alternative that can be used in place of natural ventilation systems as per the recommendation in ADB. The system can be implemented by use of a shaft system or by each floor having a separate powered system. The mechanical system are: can achieve a specified rate of extraction, the system is less sensitive to wind, there is a known capability of overcoming system resistances and the required shaft cross section is relatively small. A powered system has its unique requirement including maintained power, there is need for temperature classified equipment, wiring used in the system is supposed to be fire resistant and stand by fans are also required. The internal pressure inside the building needs special consideration because it is important that the doors are remaining in operable condition. There is need for providing air inlet to the communal area so as damage to the system is prevented, in addition to ensuring that over pressurization and under pressurization of ventilated area is prevented. Excessive pressurization or depressurization are to be avoided so as a precaution against smoke being drawn the apartment where the fire is originating while high pressure differentials may result to the escape doors being inoperable or may be pulled open worsening the fire situation. Designs are supposed to be based on a scenario where a fire is affecting a single floor and thus only the smoke vents on the floor where the fire is originating and other design critical vents will be required to be open. The designers are to make sure ventilators are not opened on multiple floor levels more so in case where there is connection with a smoke shaft, so a to deter smoke spreading to unaffected areas of the building or the rate or smoke removal from the fire origin floor being affected. Materials that are used in the construction of the smoke shaft are supposed to be non-combustible and the vents opening into the corridors. Lobbies are supposed have fire/smoke resistance performance that is at least of equal magnitude as E30S fire door. Generally the system is to be activated when smoke is detected in the corridor/lobby. After the system is activated it is expected that smoke vents on fire floor, vents located at the top of smoke shafts and the as well as vent that are at the head of the stairway are supposed to open with the fans required to run at the design speed. For basic mechanical systems the design is such that its performance is equivalent to that offered by natural ventilation that is described in ADB. Designs offering higher performance in mechanical system are possible and when this is done it is possible to have extended longer travel distances in corridors but it requires caution if removing corridor subdivision doors is to be considered. The doors are useful in limiting the potential travel distance through smoke and also may also lower the number of apartment that need to be evacuated by the fire fighters and these is important in helping the fire fighters. Part 2 For the plan provided, and assuming a 15 storey apartment block with identical floor plan at each level, write a short report (approximately 1,000 words - indicative limit) detailing how CFD modelling could potentially be used to model the performance of the smoke control system and hence justify the extended corridor arrangement. It should be assumed that the apartment block is to be constructed in a major city in England or Wales. The report should propose appropriate scenarios for modelling (including justification), and propose reasonable criteria for acceptable performance. Please note you should not review CFD theory and no actual CFD modelling is required. 2.0 Operation of the ventilation system When dealing with one escape route, it would be expected that the system would be linked to an automatic system. When there is fire in one of the apartment and it spills in the corridors, the vents on the floor where there is fire at the top of the shaft and the top of the stair are expected to open at the same time while the vents on all the other floors where there is no fire are expected to remain closed. In the case where there are more than one stair it could be possible to have the ventilators being operated manually but the shaft ventilators and stair vent will be expected to happen simultaneously. For more convenience it is better to have a system that is fully automatic(5). In cases where smoke shafts are being used it will present some difficulty at the same time it require more space if natural ventilation is to be used. As an alternative system the mechanical system may be installed which may have performance that is equivalent to ADB shaft for the ventilation of the fire fighting lobby/corridor (6). The mechanical system will result to reduction in the area required for the shaft area by up to 60%. 2.1 More effective ventilation system with more space saving A much more efficient mechanical shaft can be used in high rise building which can serve satisfactorily in both escape or fire fighting stair core with only with the allowable shaft section being as low as 0.6m2 which is a 60% reduction in the floor space that is needed by ADB natural system(7). This means more space saving on each floor and it means more available space that can be used as lettable space translating to more profitability. With this design we have a design that performs better in comparison to the standard shaft as there is extraction at a defined rate meaning that no interference from external wind pressures. With such a system there is also there is reduced susceptibility to air being obstructed in the air duct. An automatic ventilation need to be included at the stairs and this usually will comprise of 1m2 AOV located at the head of the stairs. 2.2 Shaft extended system In the 15 storey apartment in quest it can be seen that the we have a total length of about 94.52m. with a corridor distance of 21237mm from the stairs towards the dead end while the distance between the two stairs is 29690mm. The extended corridor system that allows an extension of the travel distances in escape routes is required. This is a system that offers developers substantial space as well as cost saving benefits while safe evacuation route are provided as it allows the smoke to be cleared from the corridors at a much faster rate including the later stages of the fire when any other system could have well been overwhelmed. The extended corridor system provides safe escape conditions in the corridors even with2 or 3 fold extensions in one direction. With this system there is high level of smoke extraction through utilization of corridor at one end of the corridor as the extraction system while the other end of the corridor is to serve as air inlet. In our 15 storey apartment this system is to be adapted so as to accommodate the extended dead end that can be seen in the house. First, we have the vents that re between the two stairs and next to each of the stairs. If fire is in the dead end of the stairs then the three vents in the section should come into operation while the other side’s vents are expected remain closed. 2.4 Extended Corridor System description for the storey building In order to develop a near reality scenario there is need to have characterization (8). Scenario one is simulation of natural ventilation system where the layout is as seen in figure1.2 Fire combustion parameters are required give description of production rates, rate of heat release and the scenarios in the building (5). The parameters for the building are as follows: ceiling height of 3.4m; 0.2m floor slab sickness; 1.5kW/s source heat release rate; 0.4kg/s smoke generation rate and 2m/s external wind speed blowing perpendicular to the windows. Conduction CFD model in natural ventilation state in transient mode would result to a temperature rise of up to 1600C in the stairwell within 350seconds. The model would reveal all the rooms being filled with smoke and also the stairwell will also be filled with the smoke. The areas neighboring the fire room and the stairwell are to experience temperature rise in the region of 1000C and this means those in the immediate lower and above floors will be at danger. This clearly shows in natural ventilation the temperatures are to be untenable with 1600C temperature levels being reached. There is increased heat flow rate as a result of increased window area that would result to a decrease in loss of pressure. when there are prevailing wind conditions for the natural ventilation system, it will result to average doorway velocity going beyond the critical wind speed with the increased maximum flow rate going beyond 100% area ratio. Figure 1.2 2.5 Scenario 2: Mechanical systems (pressurization) For this case we consider fire occurring in corridor east wing. Here the sideward windows are open while the windows on the wind ward side are closed as can be seen from the figure 1.3. Here the parameters are as in scenario 1 ceiling height of 3.4m; 0.2m floor slab sickness; 1.5kW/s source heat release rate; 0.4kg/s smoke generation rate and 2m/s external wind speed blowing perpendicular to the windows. But in this scenario we have pressurization systems being connected to each floor as opposed to use of natural ventilation systems. For this scenario in the first 350s the smoke will be expected to be contained with temperatures being controlled not to go beyond 1000C. The fan that is cited in the opposite side of the stairwell is able to overcome the wind effects. This makes it possible for fire fighters to easily access the building for search and rescue. Figure 2.3 References 1. Fine R. A. (2011). Observations of CFCs and SF6 as ocean tracers. Ann Rev Mar Sci.; 3:173-95. 2. NFPA, (1985). Guide for Smoke and Heat Venting, NFPA 204M, National Fire Protection Association, Quincy,MA,. 3. Särdqvist, S., “Initial Fires: RHR, Smoke Production and CO Generation from Single Items and Room 4. Rodney C. (2004). Scientific American Inventions and Discoveries, p.351. John Wiley & Songs, Inc., New Jersey. ISBN 0-471-24410-4 5. Schlichting, H. (1955). Boundary Layer Theory, Pergamon Press, 1st edition, p. 48 6. Tewarson, A., “Generation of Heat and Chemical Compounds in Fires,” SFPE Handbook of Fire Protection Engineering, 2nd ed., National Fire Protection Association, Quincy, MA, 1995. 7. ADB(2006). The building Regulations 2000. Fire safety. 8. Colt. (2015) Smoke control and day to day ventilation systems for multi-storey residential buildings. COLT INTERNATIONAL LIMITED New Lane | Havant | Hampshire Read More