Book - School of Science and Technology

All-air systems 383of the air±water system since the additional cooling load is handled by the water circuits; theair ducts being unaffected. There is, in consequence, a practical limit to the level of heat gainthat can be dealt with satisfactorily by an all-air system.To complement high velocity supply systems, where space for ductwork distribution islimited, consideration may be given to using a high velocity extract system, but this willimpose additional initial cost and subsequent running costs.All-air systemsSimple systemsThese, the most primitive of high velocity systems, differ from their traditional counterpartsby the form of the terminals used to overcome problems arising from noisegenerated in the air distribution system. To this end, a variety of single-duct air volumecontrol devices has been developed to provide for the transition between a high velocitydistribution system and air outlets local to the conditioned spaces. The terminal box, withoctopus distribution section illustrated in Figure 14.8, is typical of equipment producedfor this purpose. It consists, in principle, of an acoustically lined chamber provided with asophisticated air volume damper or `pressure reducing valve': such dampers are in someinstances fitted with self-actuated devices, others have powered actuators, arranged sothat they may be set to provide constant or variable output volume under conditions ofvarying input pressure. Such devices are a considerable aid in regulating air flowquantitiesduring the commissioning process.Simple all-air systems will provide adequate service in circumstances where the loadimposed is either constant or will vary in a uniform manner for the area served thusallowing temperature control by means of adjustment to temperature of the supply air atthe central plant.Ceiling diffuserHigh velocitysupplyFlexibleconnectionVolume control deviceOctopus sectionLow velocitybranchFigure 14.8 Single-duct high velocity terminal box