Book - School of Science and Technology

Air-water systems 393tested by manufacturers and individual tests may be required. Outside temperaturecompensation of the heating circuit is an energy efficient way of minimising this effect.Fan-coil systems are inherently flexible and are well suited to refurbishment projectssince any central air handling plant and duct distribution system will be relatively small insize. The cabinet type of unit, as Figure 14.19, has been designed specifically for applicationto existing buildings, the component parts being stacked vertically in a small plan area,space being available there also to enclose vertical water piping and a supply air duct.Air±water induction systemsAlthough there has been virtually no development in the design of this type of unit forsome years, it is appropriate, nevertheless, to outline the principles of their operation andapplication. As the name implies, the principle of induction is employed in this system as ameans to provide for an adequate air circulation within a conditioned room. Primary air,conditioned in a central plant, is supplied under pressure to terminal units, generallyplaced below the window with vertical discharge, each of which incorporates a series ofjets or nozzles as shown in Figure 14.20. The air induced from the room flows over thecooling or heating coils and the mixture of primary and induced air is delivered from agrille in the sill. The induction ratio is from three to one to six to one. The primary airsupply provides the quantity for ventilation purposes, and the means to humidify inwinter and deal with latent loads in summer.The coils are fed with circulating water which, in the so-called change-over system,is cooled in summer and warmed in winter, an arrangement more suited to sharplydefined seasons than the unpredictable long springs and autumns of the British climate.In Figure 14.21 (a) and (b), control is achieved by an arrangement of dampers, such thatPrimaryair supplyWatercirculationFlexible ductconnectionPiping tounit coilControlLint screenCoilInductionjetsStructuralinfillRecirculationgrilleFigure 14.20 Air-water induction unit (two pipe)