Book - School of Science and Technology

Traditional systems 379humidification is by means of steam injection. A humidifier would be provided only whenhumidification is required in winter.In an alternative version of a central plant system, the cooling coil may be connecteddirectly to the refrigerating plant and contain the refrigerating gas. On expansion of thegas in the evaporator, cooling takes place and hence this system is known as a directexpansion (DX) system. It is suitable for small to medium size plants. Humidificationcould be by means of a capillary washer or water spray into the air stream, but anonstorage type such as steam injection is preferred, due to the risk of Legionnaires'disease associated with types which incorporate a water pond to facilitate recirculationwithin the humidifier. For the same reason, an air cooled condenser could be used as ameans of rejecting unwanted heat from the refrigeration machine in place of the coolingtower shown.In Figure 14.3 it will be noted that there is a separate extract fan shown exhausting fromthe ceiling of the room. This would apply particularly in cases where smoking takes place,such as in a restaurant, to remove fumes which might otherwise collect in a pocket at highlevel. Sometimes this exhaust may be designed to remove the quantity equivalent to theoutside air intake, in which case the discharge shown to atmosphere from the return airfan would not be necessary.Motorised dampers are shown in the air intake, discharge and recirculation ducts toallowthe proportion of outside and recirculation air flowrates to be varied to effecteconomy in plant operation. Reduction in energy consumption may also be achieved bytransferring heat between the exhaust and intake air streams; this is not illustrated in thediagram but is described in Chapter 17.Where there is a number of rooms or floors in a building to be served, it is necessary toconsider means by which the varying heat gains in the different compartments may bedealt with. Some rooms may have solar gains, and others none; some may be crowded andothers empty; and some may contain heat-producing equipment. Variations in requirementsof this kind are the most common case with which air-conditioning has to deal andfor this a simple central system is unsuitable. The ideal, of course, would be a separatesystem for each room but this is rarely practicable unless the individual spaces are verylarge or important.Zoned systemsA building may be divided into a number of zones for air conditioning purposes. Thesubdivision could be dictated by spatial constraints, by the requirements for sub-letting orby the hours of use. Each zone may be served by a separate `central' plant, perhapslocated in a common plant room, or alternatively one plant located on each floor might bean appropriate arrangement. There is a number of ways in which a central plant, broadlyon the lines of that previously described, may be modified in design to serve a number ofgroups of rooms or zones. To take the simplest case, if such a plant served one large spaceof major importance and a subsidiary room having a different air supply temperaturerequirement, it would be possible to fit an additional small heater or cooler, or both, onthe branch air duct to the subsidiary room. Separate control of temperature would thus beavailable.High-rise buildings, say over 12 storeys, served from a central plant may require plantrooms at intermediate levels to reduce the air quantity conveyed in any one duct andthereby reducing the space taken by vertical service shafts. Similar vertical sub-divisionwould also be appropriate for closed and open piped distributions.