Book - School of Science and Technology

386 Air-conditioningoutput of the central plant correspondingly. Economies in overall operation in energyconsumption and in cost will thus result.If an adequate supply of outside air is to be maintained and problems of distributionwithin the conditioned space avoided, volume cannot be reduced beyond a certain level.Good practice suggests that minimum delivery should not be arranged to fall belowabout40% of the designed quantity. For different reasons, such a limitation may presentproblems to both internal and perimeter zones. At internal zones, if lights were individuallyswitched in each room, then the load in an unoccupied or sparsely occupied spacecould be greatly reduced, in consequence of which such rooms would be overcooled. Inthe case of perimeter zones, where conduction and solar gain form a high proportion ofthe design load, it may be necessary to introduce some level of reheat to augment capacitycontrol by volume reduction.A solution for both internal and perimeter zones where the reduced air supply quantityis required to be belowthat to produce satisfactory air distribution in the space, withoutcausing the cold supply air to dump into the occupied area, is to install variable geometrydiffusers: a volume flowreduction down to about 25% of the maximum is claimed for thistype of outlet. Figure 14.12(a) shows a typical example which operates to maintain the airvelocity at discharge to the space by changing the area of opening, thereby ensuringadequate diffusion of the supply air. A similar effect may be obtained using a two-sectionbypass type air diffuser in which the supply is divided into two passages, one of which iscontrolled at a constant volume to maintain the velocity of the air stream from thediffuser and hence satisfactory air distribution, as Figure 14.12(b).While most variable volume terminal units can be adapted to incorporate reheaters, therequired effect may equally well be achieved at perimeter zones via control of a constantvolume perimeter heating system or even space heating units such as hot water radiatorseither of which may, in any event, be required at windows to deal with down draughts.The difference in the space heating and cooling loads within internal and perimeterareas may require a two or three zone supply system, with the supply temperature of eachzone controlled to suit the particular characteristics of the area served. For single ormultizone systems, the zone temperatures may be controlled at a constant level, be variedto suit outside conditions (scheduled), or be varied in response to feedback from thecontrols at each terminal device to provide the optimum supply with a maximum operatingeconomy. Figure 14.13 shows a typical variable air volume system arrangement. Tomaintain the volume of outside air above the minimum requirement for occupant ventilation,it may be necessary to add air velocity sensors to the plant controls in the outside airMotorPlate dampermovementVariable volumesectionSelf-actuatingcounterbalancedflapdamperConstantvolumesection(a)(b)Figure 14.12 Variable geometry supply diffusers: (a) variable orifice (Wozair); (b) variable bypass (Trox)