5 Smoke Clearance Systems

5.1 General Guidance The objective of the smoke clearance system is to: a. Help dilute the smoke and reduce the temperature during the course of a fire. b. Assist fire-fighters by providing ventilation to allow clearance of the smoke once the fire has been extinguished; This system is not specifically intended to maintain any area clear of smoke, nor to limit smoke density or temperature to within any specific limits or to assist means of escape. Therefore, there are no tenability criteria provided for this system. However, guidance on vent areas and extract rates is given in regulatory documents and summarised below. Given the anticipated large fires the system shall ideally be initiated as soon as practical after a fire is detected. However, it is possible that Jet Fan systems, if set in operation too early, might actually worsen conditions for means of escape by encouraging smoke circulation and descent of the smoke layer – see section 5.4. Other ventilation systems shall shut down to prevent movement of smoke to unaffected areas of the building. The discharge points for the smoke exhaust system shall be located such that they do not cause smoke to be recirculated into the building, spread to adjoining buildings, or adversely affect the means of escape. They shall be located so that inlet air is not drawn immediately into the extract system rather than smoke. The BS EN 12101 series provides advice on the performance of smoke control equipment. In particular, all fans intended to exhaust hot gases shall be tested in accordance with BS EN 12101 – 3 to verify their suitability for operating at 300o C for a period not less than 60 minutes (class F300). Where the main extract fans are located within the building, but outside the fire compartment which they serve, they shall be enclosed with elements of structure having a fire resistance at least equal to that required for compartmentation between the different parts of the building through which it passes. Ductwork, dampers, and fixings shall conform to Section 9.2. 5.2 Natural Ventilation for Smoke Clearance Guidance on smoke ventilation of car parks and basements recommends that 2.5% of the net floor area of the compartment must be available in openings directly to atmosphere, either in the walls and arranged to allow cross flow, or in the ceiling. Openings would normally need to be distributed evenly around the perimeter. Given the possibility of larger fires in service yards, loading bays and coach parks, the designer should give information on how the design objectives will be met. The design engineer should be able to justify that the available openings will be adequate to provide sufficient ventilation to meet the design objective. Compliance with Building Regulations guidance is one way. Another way in which this could be achieved is by the use of CFD modelling. (see section 11). 13 5.3 Ducted Mechanical Systems for Smoke Clearance This system would normally meet the requirements of the current building regulations by providing 6 air changes per hour under day to day ventilation conditions and 10 air changes per hour under fire conditions within each fire compartment served by that system. Sprinklers would normally be needed to ensure that the smoke temperatures do not cause failure of the system and impede fire growth. When deviating from Building Regulations guidance, the design engineer shall be able to justify that the alternative design will be adequate to provide sufficient ventilation to meet the design objective. One way in which this could be achieved is by the use of CFD modelling (see section 11). The system shall be independent from any other system (other than any other system providing normal ventilation to the compartment) and be designed to operate at 10 air changes per hour. Where part of a dual fume and smoke clearance system the extract points shall be arranged so that 50% of the exhaust capacity is at high level and 50% is at low level and evenly distributed over the whole compartment. The main extract system shall be designed to run in at least two parts, such that the total exhaust capacity does not fall below 50% of the rates required in the event of failure of any one part and shall be such that a fault or failure in one will not jeopardize the others. The system shall have a maintained power supply, designed to operate in the event of failure of the main power supply. The fans and associated control equipment shall be wired in protected circuits designed to ensure continued operation in the event of a fire. Care shall be taken to ensure that there are no stagnant areas in either daily ventilation or smoke ventilation operational mode. Provision shall be made for the supply of replacement air to the compartment. Care should be taken to ensure correct distribution of air inlets in relation to the extract point(s) to avoid short circuiting. The velocity of air along ramps and vehicle entrances shall not exceed 5 m/s in order to avoid impeding the escape of occupants from the building. 5.4 Jet Fans for Smoke Clearance When considering the use of a jet fan system, consideration should be given to how efficient the jet fans will be in achieving air movement at low level as well as high level due to the height of the space and the obstruction caused by larger vehicles. The main extract fans shall normally operate immediately on confirmation of a fire to provide the required extract rate. After an appropriate delay, the jet fans shall activate in such numbers as necessary to direct the smoke effectively towards the main extract points. 14 The delay in the activation of the jet fans may be necessary to ensure that escaping occupants are not compromised by the action of the jet fan system. The system shall be designed so that escaping occupants can walk to a 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, e.g.: • The size and geometry of the compartment; • The number and location of extract and jet fans; • The number and type of occupants; and • The number and location of suitable exits. The delay period, if any, shall be confirmed in agreement with the approving authorities. Consideration shall be given to the location of the means of escape when locating the position of the extract point(s). The position of 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. Care shall be taken to ensure that there are no stagnant areas in either daily ventilation or smoke ventilation operational mode. Adequate provision shall be made for the supply of replacement air. The velocity of air along ramps and vehicle entrances shall not exceed 5 m/s in order to avoid impeding the escape of occupants from the building. The system shall be independent from any other system (other than any system providing normal ventilation). The main extract system shall be designed to run in at least two parts, such that the total exhaust capacity does not fall below 50% of the rates set out in above in the event of failure of any one part and shall be such that a fault or failure in one will not jeopardize the other Each part of the main extract system and the jet fans shall have a maintained independent power supply, which will operate in the event of failure of the main power supply. The fans and associated control equipment shall be wired in protected circuits designed to ensure continued operation in the event of a fire (see Section 9).