Book - School of Science and Technology

All-air systems 385In principle, the variable volume system may be considered as a refinement to thesimple all-air system whereby changes in local load conditions are catered for not byadjustment of the temperature of the conditioned air delivered, at constant volume, but byadjustment of the volume, at constant temperature. This effect may be achieved by meansof metering under thermostatic control of the air quantity delivered either to individualpositions of actual discharge, as shown in Figure 14.10, or to groups of such positions viaa terminal unit of the type illustrated in Figure 14.11.In such cases, a true variable volume arrangement is possible since the effect of reducedoutput at the terminal units or at the discharge positions may be sensed by a centralpressure controller, this being arranged to operate devices which reduce the volumeInsulatedsupply plenumPerforated baffleBellows(volume control)Acoustic liningAir supply diffuserFigure 14.10 Linear diffusers for variable volume (Carrier)High velocity primary airButterfly dampervolume controlAttenuator casingOptionalreheaterDampermotorVelocitysensorAir supplyFigure 14.11 Variable volume terminal unit (Wozair)