Book - School of Science and Technology

656 Automatic controls and building management systemsRoofplantroomLevel 2plantroomBasementplantroomBoilerplantAlarmindicationLiftsLightingSecurityFireOther system caninterface with the BEMS.The form of the interfacecan vary from simple voltfree pair to common BUScommunicationAlternatehead endLevel 1plantroomLower groundplantroomChillerplantMainhead endEntrance hall Control room /Fire command centreEngineers roomFigure 22.34 Typical building energy management system networkCentral intelligenceThe early systems had central intelligence, in which all signals were transmitted andreceived through a central processor.The vast majority of systems today have `distributed intelligence' with the head endproviding a management facility.Distributed intelligenceAs systems increased in size, the concept of having all communication with a centralprocessor led to congestion of the data flowing to and from a single location, and to theneed for excessive computing capacity. With the reducing cost of processing, a naturaldevelopment was to have as much as possible of the communication and data processingdealt with close to the plant. Intelligent outstations strategically located around thebuilding, with only limited communication with the central computer for monitoringand data gathering purposes, came into use.Commissioning of distributed intelligence systems is more flexible since plant can be setup to run from the outstations, the final commissioning of the central processor beingarranged to follow once outstations are operating satisfactorily. The central unit in adistributed system provides the same facilities as with the central intelligence alternative,including re-programming of the outstations.A further development, illustrated in Figure 22.35, is for individual items of plant suchas chillers, air handling units, boilers and terminal control units to be delivered to sitecomplete with pre-wired unitary controllers suitable for either stand-alone use or for fullintegration with a BEMS. This provides intelligent plant control which has been testedand proven before leaving the manufacturers' works and, in consequence, reduces sitewiring and control panel work.