Book - School of Science and Technology

368 VentilationTable 13.9 Air velocities through open sashes in fume cupboardsCategory ofcupboard useVelocity(m/s)Teaching 0.2±0.3Research 0.25±0.5Analytical 0.5±0.6Highly corrosive or toxic 0.5±0.75Radioactive* 0.5±2.0NotesSash openings are normally 750 mm to 1 m, velocities quoted are through open sash.* Grade of radioactivity determines velocity.for warmed, or conditioned, air make-up may be considerable, with a consequentheavy use of energy, if extraction from the cupboards continues whether they are inuse or not.The arrangement of the extract system might take the form of a fan connected to eachcupboard, discharging either individually to atmosphere or into a common dischargeduct. The former of these alternatives would produce a forest of unsightly terminals andthe latter, without suitable precautions, might lead to a situation of potential air flowimbalance in the common collecting duct and consequent recirculation to any inactivecupboard. Either solution would be prodigal of energy and of maintenance effort ifassured service were to be demanded of all the fans.A preferred solution would be to provide extraction using a duplicate set of centrifugalexhaust fans, sited in a convenient plant room, drawing from a commondischarge duct provided at the remote end with a dilution entry for outside air. Thecapacity of each fan would be somewhat in excess of the total extract requirement ofall the fume cupboards and the connection to the common duct from each individualcupboard would be fitted with a motorised damper, arranged for fast response variablecontrol. A closed loop control system would sense the air flow requirement related tothe sash position of the fume cupboards in use at any one time and adjust the positionof the dampers accordingly to maintain the recommended air velocities. At the sametime a constant efflux velocity is achieved at the discharge point to atmosphere. GuideSection B2 provides some additional basic information for laboratories with fumecupboards.As to replacement air, this would be provided from a suitable central plant through oneor more variable volume units which, via the control system, would modulate the quantityof supply air to match that extracted at any time. With this solution, optimum energyconsumption and convenient centralised maintenance would be provided.Exhaust of industrial fumesIn an industrial context, local extraction of fumes from benches is more effective andmore economic than an attempt to deal with them by treatment of the whole volume of aworkshop. For a welding bench, a volume of 200±300 litre/s needs to be removed anda convenient way of meeting this requirement is by use of a flexible tube supported froma wall mounted swivel as shown in Figure 13.19. The tube may be connected to anindividual fan or, where multiple benches exist, to a header duct and a central fan.