Book - School of Science and Technology

Special applications 367Firstly, the need for smoke control using mechanical ventilation systems should beconsidered as part of the overall fire safety strategy for the building, having first evaluatedthe implications on the design of the building of adopting a natural ventilation solution.The air tightness to be achieved with the escape route in terms of building fabricspecification also needs to be considered at an early stage.The objectives of the system are to control smoke to allowoccupants to escape throughprotected routes, to enable firefighting operations to proceed efficiently from a protectedshaft and lobby and in some cases protect valuable equipment. The mechanical smokecontrol system has to overcome the smoke movement forces of buoyancy, thermalexpansion, stack effect and wind pressure that arise in the event of a fire. The acceptabilityof any system depends ultimately on whether the necessary pressure differentials andairflowrates have been achieved when it is tested on completion.In essence, the Code designates stairswells and lift and other lobbies, with associatedcorridors in certain circumstances, as being areas in which air pressure should be maintainedat levels in excess of that which exists in surrounding accommodation zones. It isproposed that this situation should be achieved by the admission of an outside air supplyto the designated areas, via an independent plant or plants, such that a positive pressureof about 50 Pa, with respect to those surrounding areas, be maintained with all doorsclosed, a reduced pressure of 10 Pa is to be maintained when the main escape door to theoutside is open. A minimum air velocity of 0.75 m/s across an open door between thepressurised space and the accommodation area is also required. Detailed requirementsexist for different types of buildings, for example where the occupants could be sleeping,and for different fire strategies, where the evacuation of the building is phased. These areclassified A to E within the code of practice. With such large air volume flow ratesinvolved in the design of these systems it is essential to provide an air release path fromthe accommodation area to outside. This can be achieved using external wall vent, whichcan include automatically openable windows, vertical shafts or mechanical extract.Obviously, a high degree of reliability is required from smoke control systems and it isessential that stand-by equipment is provided including the electrical supplies to the fans.Regular testing and strict maintenance programmes are also required.The calculations necessary to determine both volume and pressure requirements tomeet the duty concerned are tedious and must take into account stack wind buoyancy andthermal expansion effects which may oppose the required air movement. Account mustalso be taken of the likely airtightness of the building fabric. It is, in consequence,appropriate to consider the magnitude of the margins which must be added to anysolution produced by the current calculation methods in order to take account of themany variables.Local fume extract systemsUnder this heading fall the familiar fume cupboards used in laboratories which, if they areto operate with any success, must be integrated with any inlet or extract ventilation systemserving the same space. In general terms, the air volume which is extracted from acupboard should be such that a face velocity of between 0.25 and 0.75 m/s is producedwith the sash fully open (Table 13.9) and a much higher level when the opening is reducedto the working height of between 25 and 50 mm.An interesting design problem arises in heavily serviced pharmaceutical laboratorieswhere as many as 25 or more fume cupboards may be required in a single room but arenot subjected to any predictable pattern of use. As will be appreciated, the requirement