Book - School of Science and Technology

Air-conditioning system controls 649reheating can be minimised by scheduling the supply air to the variable air volume (VAV)boxes. The optimum supply temperature can be determined if the individual VAV boxcontrollers are interrogated by the central plant controller.For perimeter zones, the necessary reheat may be controlled via any form of convectiveheating system that is installed if this can be arranged to provide adequate response.Modern control systems allow the perimeter heating and variable volume box controls tobe sequenced, room-by-room. Of particular importance is the supply of adequate quantitiesof outside air to each room. Unless additional controls are incorporated, the outsideair quantity will vary proportionally with the supply quantity, which may lead to inadequateoutside air provision and high running costs. To overcome this, an air velocitysensor is installed in the intake duct to control motorised dampers and ensure that theventilation air quantity is maintained at or above the minimum requirement.In Chapter 14 the concept of fan assisted control terminals was introduced as a meansto provide additional cooling to supplement that inherent in the air supply. In this case, avalve in the chilled water supply to the cooling coil, and in some parallel arrangements,the fan, is controlled by a thermostat sensing room temperature.It is also necessary to control the extract fan as a `slave' to the supply fan in order tomaintain the correct balance between the supply and extract air streams over the fulloperating range and to achieve maximum energy savings.Systems which are controlled to vary the volume of air delivered to rooms or zoneswhile the central plant volume remains constant do not offer the same facilities for energyconservation.Induction systemThere were two principal methods of control applied to induction systems, the `changeover'and the `non-changeover'. The former was more appropriate to climates havingdistinct seasons and need not concern us here. Two-, three- and four-pipe units wereavailable to provide progressively better availability for control of space conditions: thetwo-pipe system was the more common in Great Britain. However, induction unit systemsare seldom used these days; the preference being for either fan-coil or variable air volumesystems.Dual-duct systemFigure 22.30 illustrates the principle of the control system applying to a dual-duct systemwhere two fans are included, one for the warm duct and one for the cool.The temperature of the cool duct is varied according to the external temperature fromsay 7 C in summer to say 16 C in winter, or to a more limited schedule where the internalgains dominate the room loads. In summer it is cooled by the chilled water coil: in winterthe heater comes into use as necessary. The temperature of the warm duct is also variedaccording to the outside temperature from, say, 21 C in summer to, say, 38 C in winter.This form of system has historically been used where constant airflows and pressuresare required for reasons other than temperature control (e.g. hospitals and laboratories).In their constant volume role they are inherently wasteful of energy and should beavoided. However where the terminal device modulates the volume of both warm, coldand total volume, then the wastage is lessened. Both forms however require considerabledistribution space (see Chapter 14).