Use of Natural and Mechanical Smoke Ventilation Systems in Common Corridors of Apartment Blocks - Case Study Example

Name Course Task Date Part 1 Use of natural and mechanical smoke ventilation systems in common corridors of apartment blocks. Introduction Building ventilation is a very important consideration in fire safety design. The increase in heat and smoke due to fire will limit the operation of fire fighting and rescue services in a building. Thus ventilation should be provided through smoke shaft to aid in fire fighting. The main objective is to minimize the potential for contamination of stairways (BS 9999:2008). The minimum requirements for ventilation in a building are specified in British standards and can be applied depending on the occupancy level and the intended use. There two types of ventilation strategies in buildings which include natural ventilation and mechanical ventilation. BS5925 specify the requirements for natural ventilation in a building (BS5925:1991). The products of fire originating from the lower level floors tend to flow through stairways, which makes it for firefighters to access the building. By making available the outlets through which the smoke can escape, the problem will be reduced. Vents assist in improving the visibility and reducing the temperature, making it easy to perform rescue, search and fire fighting (BSI, 2008). According to ADB, it is not possible to keep corridors smoke free, except in pressurized systems. It is also important to protect the stairways as greater number of people will use it as compared to the corridors when fire occurs. Thus a more effective system design should be able to keep the stairways smoke free and should maintain tenable conditions for the travel distance along the corridors during the time for escape. The design may not work if the rooms’ doors are closed. Tenability is the basis for the design performance. To achieve this goal, the main focus should be maintaining the visibility, limiting the toxicity and limiting thermal radiation within the corridors. Smoke control systems are mainly installed within the escape routes in the building to improve the conditions for fire fighting and the means of escape. The objective is to reduce the obscurity, thermal exposure and toxicity along the routes of escape and improve the conditions for the rescue and fire fighting services. The smoke control systems basically complement the fire plan for the building and provide the right strategy for fire safety. The stairways should be designed to reflect smoke free conditions by maintaining the necessary ventilation flow between the stairs and the corridors. The performance criteria should be designed and approved, such that the CFD analysis, modeling and calculation meet the objectives of the systems. According to NFPA 92A smoke control systems does the following: Limit the flow of smoke from the compartment of origin Limit the smoke from escaping entering the means of exit, which minimize the tenability of the environment. Maintains tenable environment at the corridor to aid in rescue and fire fighting operation Protect life of the occupants Minimize damage to the property Natural ventilation Natural ventilation is ventilation that is provided by diffusion, thermal or wind effects through windows, doors and other openings in the building (Bonda & Sosnowchik, 2007). It is dependent on external wind to provide fresh air into the building. It makes use of thermal buoyancy and natural forces of wind to drive air through the ventilation systems. ADB recommends the use of natural vent shafts, natural wall vents at the head of the stairways. This application systems harness the buoyancy of hot air and smoke from the flame. But because the force is very small, the movement of smoke can be increased by the wind. Natural ventilation requires both the inlet and outlet for it to work effectively. A vent mounted on the wall has both the inlet source at the bottom and an outlet at the top of the vent. Or less the outlet will be provided by the door when opened. To help in any smoke movement in the stairs, a vent will be installed at the top of the stairways. ADB assumes that natural ventilation is main type of ventilation in a building, but allows mechanical ventilation as an alternative. Natural ventilation has a number of benefits, which include the following: It is simple and require less maintenance It is less noisy It is more reliable compared top mechanical ventilation It uses less energy Disadvantages Its performance is affected by wind effects which are not consistent Natural shaft systems require large floor space It is difficult to control the airflow Humid air entering the building may condense in wall cavities, thus compromising on the durability of the building. Corridors vents According to ADB The vent must be activated by smoke detectors in buildings with single stairways, but for multi-stairways buildings, manual activation may be used. But for both cases, the vent at the top of the stairways should be opened automatically. A vent should be constructed on the external wall of the lobby or the corridor with at least 1.5m2 free area. The location of the vent should be such that it is higher as much as practicable such as at the top of the door. The free area is obtained by multiplying the length of vent in the opposite direction to the fulcrum by the measured at ninety degrees to open window. Similar technique is used for the smoke shaft doors. The dimension of the free area of the ventilator is measured when the area is minimum when the airflow is at the right angle to the plane as shown below. An outward opening side or bottom vent should have a free area like as shown below. Free area = a x d ≥ 1.5m² (d = distance that the ventilators open perpendicular to the opened flap) The architectural designer can choose between ventilators or side or bottom pivoting window or louvred vent to achieve free area. On the other hand, vents can be located at a point where they are exposed to undesirable wind effects that will blow the smoke back into the stairs. Thus effects of wind should be mitigated when choosing the location and the type of vent, in spite of the absence of regulatory specification. According to BS EN12101-2, a roof light can be used to mitigate the undesirable wind effects by opening the vent with an opening angle of at least 1400. Smoke shafts The main factor to be considered when designing smoke ventilation in the corridors is to provide smoke shafts doors and an open space for windows that will allow the smoke to flow out. In addition, windows, doors and ventilation systems should be opened to allow smoke to escape from the building. ADB requires the following for an exhaust leading to the vertical smoke shaft. It should have a minimum cross section area of 1.5m2, at the level above other structures around. The free area of the vent between the shaft and the corridor or any internal location in the shaft should have a minimum of 1m2. The materials used to construct the smoke shaft should be non-combustible and vents should able to operate in the presence of smoke and fire, and it should have a minimum performance of E30Sa fire door. The shaft should have a minimum angle of inclination of at least 300. The vents should be able to open simultaneously when the smoke is detected in the lobby/corridor. Stair vents According to ADB the vent should have a minimum free area of 1m2 between the outside and the uppermost storey that should open automatically. Mechanical smoke ventilation Mechanical smoke ventilation is a type of ventilation that is powered by mechanical equipment like blowers and fans (Bonda & Sosnowchik, 2007). According to ADB, powered smoke ventilation is used as an alternative to natural ventilation. It is assumed that all the floors have reliable power systems. Advantages Less sensitive to wind Has specific extraction rates Has small shaft cross section Capable of overcoming system resistance It has greater control of the building ventilation and is not control by the surrounding environmental conditions In order to enable continuous supply of electrical power systems, the following should be performed. Power maintenance Use fire resistant wires Use standby fans The mechanical system is an alternative and equivalent to natural ventilation system as explained in ADB. Mechanical systems can provide excellent performance that can be used to provide extended travel distance in the corridors. Corridors subdivision doors will limit the travel distance through smoke as well as reduce the number of rooms that will require evacuation the fire fighters. The doors will assist the fire fighters in their work (Craighead, 2009). Minimum requirements The pressure inside the building should be regulated such that the doors can remain functional. For this reason the air is let in to the common areas to keep the systems working as well as prevent excessive increase or increase in pressure in the ventilated areas. Keeping the pressure constant will prevent the flow of smoke from apartment of fire origin to other apartments, and prevent pressure gradients that may interfere with the operation of the door (ADB). The designers should ensure that each floor will operate independently, such that the smoke vents on the apartment of origin or the staircase and the smoke shafts will open independently. A design that will open ventilators on multiple floors should be avoided, especially where the smoke shafts are connected. This is to prevent smoke from spreading to other parts of the building that has not been affected or slowing the rate of smoke removal from the affected floor. The materials used to construct the smoke shafts should be made of non-combustible materials and the vents should be fire resistance or its performance should not be affected by fire (ADB; Craighead, 2009). The systems can be activated by the smoke detection in the corridor, but other means of activation can also be used. The choice of system activation depends on the discussion by the stakeholders. When the system is activated, the smoke vents at the floor of fire origin, and the vents at head of the stair and at the top of the smoke shafts should be opened and the fans starts to run at specified speed (Ching & Winkel, 2007). Computation of mechanical extraction of smoke in the 1st floor According to NFPA92B, the amount of smoke that can be removed using mechanical ventilation systems can be calculated using the formula below. Where mpl = mass flow of plume (kg/s) T0 = Absolute temperature for ambient air (K) ρ0 = the ambient air density (kg/m3) QC = 0.7Q Where Q is the heat release rate (KW), and QC is the convective heat release rate (Fu, Peng & Chen, 2014) Equipment installation The equipment to be used that meets the required performance should be chosen. All the components should be installed properly for the equipment to operate properly. The design plan will include the location, the type and the size of the equipment, as well as the power ratings and cable routing. The selection and the installation of the components should take care of the user safety, the environmental conditions, protection and ease of access. The components installed should be maintained. These include lubrication and cleaning. Access panels and doors should be constructed. Beams can be installed to assist in the repair and removal of components. The equipment should be installed in such a way that it does not discharge heat or smoke to the adjacent structures. In other words, the exhaust should not be directed at windows, walls or any part of the roofing that is combustible. The discharge outlet must be protected (Craighead, 2009). Testing Testing the ventilation system is the basic process to prove its performance as well as to set the systems to work. Since smoke control systems are the fundamental to life safety, especially as it aid in the rescue service, it is important that it is tested to prove its workability. Testing is also conducted in order to prove the compliance with the building regulatory requirements and for the project to be approved. Part 2 CFD modelling used to model the performance of the smoke control system The smoke propagation during the time of evacuation form the building can be predicted using CFD in order to prove that the smoke fumes will fill the evacuation routes during egress time. CFD is used to perform fluid flow and heat transfer analysis of fire scenario. The building under focus is a 15 storey building whose floor plan is shown below. Scenario 1 In this scenario, mechanical and natural ventilation systems are used. The building is ventilated through the windows and doors to the stairways. The pressure inside the building is maintained through mechanical ventilation systems, especially since the doors are opened in the opposite direction of the egress movement. This will help to keep the door open. The fire origin is located at the furthest location from the stairways. The smoke can spread the corridor to other parts of the building. In this scenario, it is assumed that the main combustible materials are seats and cables. The present in the room are assumed to be at higher risk. Scenario 2 In the second scenario, the fire starts near the middle of the building as shown below. It is assumed that fire risk is higher as the occupants in farthest location from the stairways are in greater danger of being trapped by the smoke which may flow into the corridor. Scenario 3 This scenario is similar to the second scenario except that the fire occurs of the opposite site of the stairways. The occupants are in danger of being trapped by the smoke in the corridor. For all the scenarios, axi-symmetric plume model is applied with volumetric fire source according to NFPA 92B. A fire design for smoke flow model using CFD modeling is characterized by t-square fire curve represented by: Where is the growth coefficient of fire The time taken before the fire services arrive is 300 seconds. Taking = 0.045kW/s2 for fast fire scenario defined by Hu (2006), the peak for heat release rate is 4.05 MW as shown in the figure below. HRR for the design fire The t-square fast fire adopted that has a peak of 4.05 MW is relevant for this fire scenario as it is based on real life experiment, although variation due to fluctuation has been ignored. Method Life safety in the building is determined by calculating the difference between the required theoretical time for evacuation, Required Safe Egress Time (RSET), and the time available for evacuation of the occupants, Available Safe Egress Time (ASET). The total evacuation time is the real time needed for all the occupants to be move to safety (Drysdale, 2011). Various factors which include recognition time, pre-movement time, response time, walking time, flow time and travel time used in evacuation are considered. Available Safe Egress Time (ASET) The definition of ASET as adopted by BS 7974 is the time from the fire ignition to the moment at which the tenable conditions in the building is reached due to heat, smoke, or toxic emissions. ASET provides the maximum exposure time to hazard from the fire which may be tolerated with no one being incapacitated. Therefore, all of the occupants in the building should be able to escape from the building before ASET is reached. ASET is calculated as, Where is the detection time, is the time for the onset of hazardous environmental conditions, and is the notification time (Great Britain, 2008). Required Safe Egress Time (RSET) RSET (also called escape time) is the time taken by the occupants to move to a safe place after fire ignition, i.e. the target time for complete evacuation and includes the time the occupants remain safe within the time spent in the building (Anson, 2008). BS 7974: 2002 is a comprehensive document which provides a systematic technique for calculating the escape time. The proposed formula for calculating RSET is shown below. Where: is the detection time, which is the time from the ignition to the detection time by automatic systems. This may vary depending on the fire scenario, the installed fire detection system and the ability of the occupants to detect fire (Muckett & Furness, 2007). Alarm Time,, is the time duration between detection and pre-movement time, , is the time from detection to the time the occupants move out of the building. It includes the time the occupants recognize the alarm to the time they respond to the alarm and begin to move out. is the travel time for the occupants from the building to the a safe place, which can be determined using evacuation software simulation. Travel time can be divided into walking time, queuing time, and flow time (Anson, 2008). Assumptions The design is based on the expected behavior of fire, which may start any part of the building before spreading to other parts. The occupants are alerted in time by alarm systems, speakers or shouting. The routes are protected in term of enclosure, and sufficient smoke control and alarms to provide warning about the existence of fire and provide them with a means of escape. The fire maintained heat release rate as it spread to the nearby objects such as furniture The building has both mechanical and natural ventilation system. The fire is growing at 4.05 MW The building consists of male and females of various ages and health status, which include the children and old people. Margin of safety This is the time difference between ASET and RSET. If the value of ASET is greater than RSET, the evacuation route will be considered suitable. The delay before beginning evacuation would expose the occupants to vulnerability. The behavior of the occupants during evacuation would affect their movement in case of fire, and the response is affected by the psychological and the physical state of the occupants at the time of fire recognition (Muckett & Furness, 2007). The timeline analysis is presented in the table below. Table: Timeline analysis Time (s) Details FDS+Evac time (s) 0 Fire starts in one of the rooms due to electrical faults. The occupants continue with their activities as they are not aware. The door is closed and the windows are open tdet = 900 660 The fire developed fully and reaches a max. heat release rate of 4,831MW 900 The smoke detector is triggered and the occupants are alerted 1020 The occupants moving to the exit treac =120 1022 1st occupant exit the building ttrav = 20 1040 The last occupant exit the building During the evacuation process, the air temperature and the visibility is maintained at the height of 2.1 m. Conclusion CFD model analysis of the building as described here is very significant in terms of fire, ventilation and smoke control systems in high rise building. It aids in creating a safe condition by exhausting the smoke and preventing the occurrence of losses as early as possible. Proper ventilation systems will aid in maintaining tenability for the travel distance along the corridors during the time of escape. The occupants will have enough time to exit the building before the beginning of untenable conditions because of temperature, visibility and poisonous gases such as carbon monoxide. References Anson, M. J. P. (2008). Handbook of Alternative Assets. Hoboken: John Wiley & Sons. Bonda, P., & Sosnowchik, K. (2007). Sustainable commercial interiors. Hoboken, N.J: John Wiley & Sons. Bradshaw, V. (2006). The Building Environment: Active and Passive Control Systems. Hoboken: John Wiley & Sons. BS 9999: 2008 Code of practice for fire safety in the design, management and use of buildings. (2008). British Standards. BS5925:1991: Ventilation principles and designing for natural ventilation. London, British Standards Institution. Ching, F. D. K., & Winkel, S. R. (2007). Building codes illustrated: A guide to understanding the 2006 International building code. Hoboken, N.J: Wiley. Craighead, G. (2009). High-rise security and fire life safety. Amsterdam: Butterworth-Heinemann/Elsevier. Fu, Q., Peng, B., & Chen, L. (January 01, 2014). Performance-based Smoke Prevention and Extraction System Design for an Exhibition Center. Procedia Engineering, Elsevier, 71, 544-551. Muckett M., Furness A., (2007). Introduction to Fire Safety Management, Routledge National Fire Protection Association. (2012). National fire codes: A compilation of NFPA codes, standards, recommended practices, and guides. Quincy, Mass: The Association. Read More