Book - School of Science and Technology

388 Air-conditioningDischargeto spaceReheater(optional)VAV damperPrimaryairsupplyPrimaryairsupplyVAVdamperFan–++FanCooling coilAir filterRecirculationfrom ceiling voidReheater/coolingcoil (optional)Discharge to spaceRecirculationfrom ceiling(a) Parallel flow(b) Series flowFigure 14.14 Fan-assisted variable volume terminal devicessupply temperatures, however, reduce the efficiency of the associated refrigerationmachines as explained in Chapter 19.Since the principle of operation with all VAV systems is to vary the quantity of primaryair supplied by the plant, it follows that the associated extract fan must respond to thechanges in supply volume to avoid under- or over-pressurisation of the building.Variable air volume systems are currently a popular choice because the associatedenergy consumption is lower than that of other equivalent systems. They are particularlysuited to buildings subjected to long periods of cooling load, and are easily adapted tochanges in office partition layouts. It is quite practical to incorporate constant volumeterminal boxes within a variable volume system in circumstances where, for example, it isdesirable to maintain a room at a positive pressure. In this application a terminal reheater,either an electric element type or a coil connected to a heating circuit, would normally beinstalled downstream of the terminal to provide temperature control of the space.The variable volume principle allows the ductwork to be sized to handle the air quantityrequired to offset the maximum simultaneous heat gain that could occur and not thesum of the quantities demanded by the peak loads in each of the individual spaces. Extensiveuse is made of computer analyses to establish the maximum simultaneous designconditions.All-air dual duct systemsThe multi-zone system previously described is arranged to mix, at the central plant,supplies of hot and cold air in such proportions as to meet load variations in buildingzones. An extension of this concept would be that an individual mixed-air duct wasprovided for each separate room in the building but this, on grounds of space alone,would not be practicable. The same degree of control may however be achieved by use of