Book - School of Science and Technology

384 Air-conditioningSupply airOptional supply connectionsOptional reheaterInduction unit casingControlsHigh velocity primary airAir induced fromceiling voidFigure 14.9 Typical all-air induction boxAll-air induction systemsFor the particular case where air is returned to the central plant via a ceiling void and,further, where lighting fittings (luminaires) are arranged such that the bulk of the heatoutput (which may be as much as 80%) is transferred to this return air, an all-airinduction system may be used.With such an arrangement, conditioned air is ducted to induction boxes mounted in theceiling void, as shown in Figure 14.9. Each box incorporates damper assemblies or otherdevices which, under the control of a room thermostat, act to permit the conditioned airflowto induce a variable proportion of warm air from the ceiling void into the dischargestream. Reheat and consequent local control is thus achieved such that, with one type ofunit, the cooling capacity may be controlled down to about 45% of maximum. Thesubsequent introduction of either an automatic switching system, which will minimisethe period that heat gain from lighting is available, or the availability of more efficientlamps (both being changes which reduce the potential for reheat) would have a detrimentaleffect upon the operation of such systems. To counter such eventualities, reheaterbatteries are available as an optional feature and terminal units which vary the quantity ofthe primary air supply are also available.All-air variable volume systemsThe traditional approach to air-conditioning design placed, as a first principle, insistenceupon the concept of maintaining air discharge to the spaces served at constant volume.Load variations were catered for by adjustment to air temperature. This axiom arose, nodoubt, from the known sensitivity of building occupants to air movement and, furthermore,from the relative crudity of the air diffusion equipment then available.With the advent of terminal equipment not only more sophisticated but alsowith performance characteristics backed by adequate test data, circumstances have changed.The activities of BSRIA and of a variety of manufacturers in this area must be applauded.Hence the availability of potential for abandonment of the traditional approach.