6 Smoke Control Systems

6.1 Design Objectives The aim of a smoke control system is to remove heat and smoke from the compartment to meet the design objectives as determined by one of the options given in Section 4.1. The system is to provide tenable conditions within the compartment, or to contain smoke within a predetermined area and extract it to atmosphere. The design shall be based on calculation, and shall be based on the performance criteria given in the relevant sections below. For the purpose of this document two smoke control systems are considered: 1. SHEVS (Smoke and Heat Exhaust Ventilation Systems), which are designed to maintain tenable conditions within smoke or a clear layer beneath it 2. Jet Fan systems which are used to direct smoke to extract points or away from areas of the building, but which are not expected to maintain tenable conditions within the smoke. The system will be designed to maintain tenable conditions on escape routes when designing to aid means of escape (Section 4.1.2.2), which may require a clear layer. For other options (Sections 4.1.2.3, 4.1.2.4, 4.1.2.5) a smoke clear layer is likely to be needed, but alternatives may be possible. The performance criteria can be different for each option, for example, the minimum clear height can be different for occupant’s escape and for fire fighting, in view of the protective clothing and training available for fire-fighters. BS7346 Part 4 and BS 7974 give recommendations for smoke clear layers and tenability criteria where the SHEVS is to protect escape routes for employees and the public. Where the SHEVS is designed to provide a clear, smoke-free approach to the fire for fire fighters, the clear layer height shall be at least 1.75m. In the design of a smoke control system in loading bays and coach parks particular consideration should be given to the following: • The geometry of the building • Downstand beams • Obstruction by large vehicles • Height of the compartment • Achieving low level air movement • Interference with sprinkler activation and pattern • Excessive smoke temperatures affecting fan integrity 6.2 Design Criteria Notwithstanding the requirements for daily ventilation, on confirmation of a fire the main extract fans, where present, shall operate automatically to provide the required rate of extract within the compartment. It is important to ensure that the rate of extract is sufficient to meet the design objectives. For this purpose, the fire load used in the design calculations shall be compatible with the fire sizes outlined in this document, (see section 7.2.2), as a notional volumetric air 16 change rate may not be appropriate. If the SHEVS can meet the airflow requirements for vehicle exhaust emission control with a reduced volume flow rate compared to the requirements for smoke control, it might be used to fulfil both sets of requirements. The design of the SHEVS shall follow the smoke control guidance documents, with certain exceptions specific to loading bays and coach parks in a) to g): a) The design fire, whether steady-state or time dependent, shall be based on Section 7.2.2, or another justifiable alternative. b) Where the system is shared with any other ventilation or HVAC system in the building it must be designed to prevent the spread of smoke and flame to other areas of the building. c) To ensure through-flow of smoke and heat through the vents, inlet air will be needed at low level beneath the design smoke layer. Permanent openings or automatically opening inlets shall be provided. d) Inlets shall conform to the recommendations of BS 7346-4, and where the vehicle entrances and/or exits are required for the smoke control system the system shall ensure that any gates are automatically opened to provide exhaust or inlet. e) Automatically opening inlets for replacement air shall be large enough (if natural openings) or shall be sufficiently extensive and evenly distributed (if air is supplied by fans via ducts) to ensure that the airspeed in the incoming jets formed inside the inlets does not cause a recirculation of smoke. f) Inlet air provision shall be designed to ensure that it does not disturb the smoke layer. The maximum inlet air speed shall be 2 m/s, unless it can be demonstrated that higher velocities do not disturb the smoke layer. g) Additionally, the velocity of air on ramps and vehicle entrances shall not exceed 5 m/s in order to avoid impeding the escape of occupants of the building. Other ventilation systems shall shut down to prevent movement of smoke to unaffected areas of the building. 6.3 Natural SHEVS Ventilation will usually be by a mixture of permanently open vents or automatically opening vents. It is unlikely that manually operable vents will be appropriate. Care shall be taken to ensure that the relationship between ventilator area and aerodynamic performance is understood and considered in the design. The Approved Document B, the BS 7346 series and BS EN 12101-2 shall be consulted for calculation procedures and the methods of determining the performance of ventilators. 6.4 Recommendations Common to Mechanical Smoke Control Systems In all cases there shall be standby capacity of the main extract fans to maintain the design extract rate assuming failure of one fan. The system can be shared with that used for fume ventilation, although the extract rate may need to increase to deal with the larger volumes of smoke. The system shall extract only at high level when used to extract smoke, and any low level extract shall either shut down or operate to provide inlet air. The extract rate shall be achieved within the compartment. Therefore, the system shall be designed to allow for pressure losses within the extract system between the compartment and the fans and from the fans to outside. 17 Ducting, dampers and fans shall be rated in accordance with the recommendations given in the relevant section 9.2 of this guide, and at the temperatures specified as part of the design criteria. 6.5 Ducted Mechanical SHEVS For a mechanical SHEVS ducting shall be provided to extract smoke and heat from high level and be linked to at least two extract fans (duty/standby). 6.6 Jet Fans for Smoke Control 6.6.1 Design Principles The extract rate shall be calculated for the removal of the mass of mixed air and smoke impelled towards the exhaust intakes. The number of jet fans activated shall not cause the volume of air movement to be greater than the volume extracted by the main extract fans. All supporting calculations and justifications shall be fully documented. The effect of the system on all other areas of the building shall be considered. Areas, other than the one where the fire is located, should be kept substantially free of smoke. The position of the stairwells, means of escape corridors, and lobby doors, where present, shall be co-ordinated with jet fan locations and jet orientations to avoid exposing the doors to dynamic pressure effects which might cause smoke to enter the lobby, stairwell and/or corridors. The distribution of the jet fans shall be such that there are no stagnant areas in daily ventilation operational mode. The ventilation system shall be able to control the flow of smoke wherever the fire occurs within the compartment. The design objectives of the system shall be met even after failure of the jet fan closest to the fire. In large compartments it may be possible to design the system so that smoke is largely confined to within specific areas by the action of the jet fans. Designs based on the creation of these smoke control zones shall either: • Have physical partitions to create channels for the smoke and the induced air flow, thus separating neighbouring zones; or • Demonstrate, using a CFD model conforming to the Smoke Control Association Guide for CFD modelling of Car Parks, Loading Bays and Coach Parks, that smoke is contained within the zone boundaries, is channelled to the extract fans, and does not lead to untenable conditions being created due to significant quantities of smoke to circulate in other zones On detection of a fire, the main extract fans shall operate to provide the calculated rate of extract for a fire condition. After an appropriate delay, the jet fans shall activate in such numbers as necessary to direct the smoke efficiently towards the main extract points for a fire condition. 18 The delay is necessary to ensure that the escaping occupants are not compromised by the action of the jet fan system. Within the affected smoke zone, escaping occupants need to be able to walk to a clear storey exit such that they are not inhibited by the smoke and heat generated by the fire and moved by the fans operating during the initial escape period. The delay employed to achieve this outcome will depend on one or more factors, eg: • The size and geometry of the compartment • The number and size of smoke zones • The number and location of extract and jet fans • The numbers and type of occupants • The number and location of suitable exits The delay shall be based on a fire engineering methodology and show that the available safe escape time from the affected smoke zone is greater than the required safe escape time plus a suitable safety margin. The delay periods, if any, shall be confirmed in agreement with the approving authorities. The aerodynamic performance of the jet fan shall be tested in accordance with BS 848-10 or an appropriate European Standard. 6.6.2 Jet Fans to Protect Means of Escape The objective of the system is to provide protection of escape routes for occupants within the compartment on fire, to preserve tenable conditions on escape routes to either the exterior of the building or to a protected stairwell or corridor which leads to a final exit to a place of safety. Care shall be taken to ensure that routes for access to a point of escape are not compromised due to poor visibility. The following recommendations are made due to the potential for jet fan systems to cause smoke logging of areas downstream of the fire. • Where the compartment is divided into smoke zones there shall be a sufficient number of exit doors/escape routes unaffected by smoke for the estimated population in the compartment to evacuate safely with all exits in the extract direction in the affected smoke zone discounted • The design shall show that the available safe escape time from the affected smoke zone is greater than the required safe escape time plus a suitable safety margin. 6.6.3 Jet Fans to Aid Fire Fighter Access There shall be fire-fighter access (from the exterior, or from protected stairwells) to the compartment, positioned to allow fire-fighters to have at least one relatively clear approach route to any possible fire location. Information as to the zone in which the fire has occurred shall be automatically displayed at an appropriate location. A map of access points to the loading bay or coach park shall also be provided to enable fire fighters to assess the most appropriate route to approach the fire. The intention of the system is to aid fire fighters to locate the fire and approach to within a distance to allow fire hoses to be directed onto the fire. However, fires in loading bays and coach parks are potentially far larger than those in car parks. Unlike in car parks it may not possible to ensure that fire fighters can move through substantially smoke-free air when approaching the fire up to a distance of 10m from that fire. 19 Therefore, the acceptance criteria will need to be agreed with the local Fire Authority, recognising that the primary aim is to allow fire fighters to locate the fire and fight it more easily compared to a fire in a building without a smoke control system. 6.6.4 Jet Fans for Enlarged Compartments and Reduced Structural Fire Resistance The objective of the system is to limit the extent to which heat and smoke affect structure or other areas of the building. It is likely that a design to aid fire fighter access will also be part of the solution. The recommendations given in section 6.6.3 when designing to aid fire fighter access shall be followed.