Book - School of Science and Technology

such an arrangement acts as `terminal recool' in winter, it is uneconomic in terms ofenergy wastage.A variation of the two-pipe induction system is the three-pipe, in which both warm andcool water are available at each unit, with a common return, and the control arrangementis so devised as to select from one or the other. Likewise, in the main system the return isdiverted either to the cooling plant or to the heating plant, according to the mean temperaturecondition. Such an arrangement suffers from problems related to hydraulic instabilitydue to the changes in water quantities flowing through the alternative paths (see p. 158).The preferred system for the British climate is the four-pipe system, two heating and twocooling, but it is correspondingly expensive. A unit having two coils, one for heating andone for cooling, is shown in Figure 14.21(d).Induction systems might be expected to be noisy, due to the high velocity air issuingfrom the jets, but the units have been developed with suitable acoustical treatment suchthat this disadvantage does not arise in practice.Morning preheating may be achieved by circulating heating water through the secondarycoils allowing the unit to function as a simple natural convector. This avoids the needto run the primary air fan and is therefore energy efficient.Figure 14.22 is a simplified diagram of an induction system showing the primaryconditioning plant, the primary ducting and the water circulation. The heat exchangershown is for warming the water circulated to the units, and this would be fed from a boileror other heat source. It will be noted that the chilled water supply to the coils of theinduction units is arranged to be in the form of a subsidiary circuit to that serving themain cooling coil of the central plant. Such a system has the advantage of providing adegree of free cooling when the outside air supply to the central plant is at low temperatureduring winter. Furthermore, since the flow to the room units is connected to the returnpipe from the cooling coil it has an elevated temperature and, as a result, this circuitarrangement provides an in-built protection against excessive condensation on the unitcoils such that local drain piping may therefore be dispensed with in most cases.To room unitsAir-water systems 395Chilledwater pumpHeatexchangerZone pumpChillerSilencerOutsideair intakeFrostcoilFilterPre heaterCooling coilFanMotorSteamhumidifierReheaterFigure 14.22 Typical plant arrangement for an induction system