Book - School of Science and Technology

380 Air-conditioningAn extension of this principle, to serve two or more zones more or less equal in sizefrom a single plant, may be achieved by deleting the reheater of Figure 14.3; dividing thesupply fan outlet into the appropriate number of ducts and fitting a separate reheater toeach. Since, however, the output of the central plant cooling coil would have to bearranged to meet the demand of whichever zone requires the maximum cooling, extravagantuse of reheat would result and lead to uneconomic running costs.The plant illustrated in Figure 14.3 is arranged on the `draw-through' principle, thevarious components being on the suction side of the supply fan. It is, of course, possible toadapt this sequence to produce a `blow-through' arrangement with the fan moved to aposition immediately following the secondary filter. With such a re-disposition, what isknown as a multi-zone arrangement may be produced, as illustrated in Figure 14.4. Here,the fan discharge is directed through either a cooling coil or a heating coil into one orother of two plenum boxes; a cold deck or a hot deck. Each building zone is supplied withconditioned air via a separate duct which, at the plant, is connected to both plenum boxes.A system of interlinked dampers, per zone, is arranged such that a constant air supply isdelivered to each zone duct which may be all hot, all cold, or any mixture of the two asrequired to meet local demand. When providing a mixture, such a plant wastes energy andas a result is not in common use today.1 2 3 4Zones1 = All cold2 & 4 = All hot3 = MixtureSpindles todamper actuatorsHotdeckHeatingcoilColddeckCoolingcoilHCSectionFigure 14.4 Multi-zone plant showing damper arrangement