Book - School of Science and Technology

All-air systems 387AirdischargeHeat recoverydeviceExtractfanDamperRecirculationdamperSteamhumidifierFanCoolerMixingchamber H – +Silencers+FilterPreheaterHeat recoverydeviceMotorFansuctionchamberHeaterSingle zone shown(A number of zonesmay be used)DamperFilter+Frost coilOutside airintakeFigure 14.13 Typical plant arrangement for a variable volume systemintake duct, to actuate the dampers in the outside air, exhaust air and recirculation ducts.This control must be arranged to override any damper controls provided to achieveeconomy of operation.The increased requirement for cooling, currently arising from the proliferation ofelectronic equipment in modern buildings, has led to the development of variable airvolume terminals which incorporate a means to provide additional capacity. This hasbeen achieved by introducing a secondary cooling coil in the terminal. Typically, arecirculation fan draws air from the ceiling void through a filter, and discharges thisacross the coil where it is cooled before being introduced into the space. Two types of fanassistedvariable air volume terminal devices have been developed; one has the secondaryfan in parallel with the primary air supply and the alternative is an in series configuration.Diagrammatic representations of these units are shown in Figure 14.14. The parallel flowtype is currently more popular on the grounds that the secondary fan runs only when thecooling load so demands, and thus uses less energy than the series flowterminal where thefan runs continuously. However, the variation in supply air quantity with the parallelarrangement could result in poor air distribution, and the intermittency of the fanoperation might cause more disturbance than the continuous running in the series flowunit.One advantage of the series flowarrangement is that, due to the inherent mixing ofprimary and recirculated air, the former may be delivered to the unit at a much lowertemperature, typically down to 8 C. In consequence, primary air volume and duct sizesmay be reduced, which may be an important consideration in refurbishment work. Lower