4) System Selection

4.1 Design Objectives The first part of the design is to identify the purpose of the smoke ventilation system. When the designer has followed building regulations guidance for compartmentation, fire resistance, means of escape and fire fighting access, then smoke clearance would normally provide an adequate standard of safety, supported by sprinklers where recommended. However, smoke control can form part of a fire engineered solution to assist means of escape, structural fire resistance, compartmentation or fire fighter access. Smoke control can also provide benefits for property protection, for example by aiding fire fighting. Therefore, the designer can choose one of the following design objectives of either smoke clearance or smoke control. 4.1.1 - Smoke Clearance Clearance of smoke during a fire and once the fire has been suppressed is the most commonly adopted form of smoke ventilation and is mainly aimed at the dilution and dispersal of smoke to aid fire fighting. It can also be used to return the building to normal use, by clearing smoke and heat after the fire has been suppressed. This can be achieved using natural cross ventilation, mechanical cross ventilation using conventional ducted mechanical ventilation, or mechanical cross ventilation using jet fans. 4.1.2 – Smoke Control 4.1.2.1 Design Guidance Documents Guidance on the design of smoke control systems can be found in the following documents: • BR 368 which covers a wide range of fire scenarios and building types • BS 7346-4 which contains performance recommendations and calculation procedures for steady state design • BS 7346-5 which contains performance recommendations and calculation procedures for time dependent design. BS 7974 describes the principles behind the fire engineering design, such as a comparison of available escape time with required escape time, and gives guidance on the tenability criteria that can be used when assessing the performance of smoke control systems. 4.1.2.2 To aid means of escape This type of system is aimed at protecting escape routes from the effects of smoke. The system is designed to delay smoke build up on escape routes, or to dilute smoke, or to provide a smoke-free path to a place of safety. The system shall maintain tenable conditions for occupants during the evacuation period. The design shall take into account smoke temperature, clear layer height, radiant heat flux, smoke toxicity, and visibility. It is likely that a “SHEVS” type system will be used to achieve the objectives of this application. As the occupants of loading bays are expected to be familiar with the area and relatively few in number, it may not be necessary to maintain a clear layer for 10 prolonged periods within the area. Instead a time dependent design may be appropriate. However, in publicly accessible coach parks a clear layer SHEVS may be needed. Jet fans may not be appropriate for this application as they will cause smoke logging downstream of the fire. There would normally be a short delay in the activation of the jet fans after detection of the fire or be activated by the fire brigade, to allow occupants to escape. However, in certain situations it may be possible to design the system to contain smoke to the area containing the fire and to maintain a proportion of the compartment relatively clear of smoke to allow occupants to reach exits unhindered by smoke and fire. 4.1.2.3 To Aid Fire Fighter Access The objective of the smoke control design is to aid fire fighters to more quickly locate and tackle a fire and carry out a search and rescue; this is achieved by creating and maintaining a relatively smoke-free route through the compartment for fire fighters to approach the fire. A steady state design SHEVS to maintain a clear layer or a jet fan ventilation system designed to achieve a clear approach for fire-fighters may be used. 4.1.2.4 Enlarged Compartments The purpose here would be to demonstrate that a larger compartment size does not compromise occupant’s escape or prevent effective fire fighting. It is unlikely that this can be achieved without a SHEVS or jet fan ventilation system. 4.1.2.5 Reduced Structural Fire Resistance The purpose here would be to demonstrate that smoke control contributes to removing heat from a compartment so that a lower period of structural fire resistance is possible. In all but the smallest loading bays or coach parks where the fire load is relatively small and well defined, it is likely that sprinklers would be needed as part of the solution. 4.2 System Types There are three system types available to ventilate loading bays or coach parks. These are: 4.2.1 Natural Ventilation This may be by permanent openings in walls or ceilings, by manually operable vents for use by fire fighters or automatically opening vents. 4.2.2 Ducted Mechanical System The traditional means of providing ventilation is to extract polluted air (exhaust fumes or smoke) through ductwork and discharging it to atmosphere. The ductwork is used to provide distribution of extract throughout the space. 4.2.3 Jet Fan Mechanical System The criteria used for the design of a jet fan ventilation system are the same as those used for a ducted system with the exception that the jet fans replace the ductwork to control the direction of the airflow. Air is directed either to extract fans that provide the required extract rate to vent smoke and heat to outside or, less commonly, to natural openings direct to outside. The principle behind the design is to induce sufficient air velocity within the fire zone to overcome the momentum of the smoke layer thus ensuring that the smoke will move in a 11 particular direction. The positioning and number of jet fans used must take into account the layout of the space and consideration must be given to a number of factors. Firstly, the fire size and height of rise of the smoke to the ceiling will influence the energy in the ceiling jet and therefore the velocity required to prevent backlayering. Secondly, a number of additional factors that will influence the performance of the fans, these include: • Floor to soffit height • Beams, downstands, columns, lighting cable trays, signage, and other physical obstructions • Vehicle size and positions • Other air currents/patterns in the space, created for example by incoming air from inlet openings • Building shape and stair cores Where a sprinkler system is to be installed, the location of the sprinkler heads and jet fans shall be co-ordinated to ensure that the effect of the jet fans on the spray pattern of the sprinklers is minimized. Care shall be taken to ensure that the number of jet fans activated will not induce the movement of a volume of air greater than that which the main extract fans are capable of extracting.