Book - School of Science and Technology

644 Automatic controls and building management systemsFrom this it may be possible to understand some of the problems concerned and how theymay be tackled. Other permutations and combinations of system and equipment may bebuilt up to suit any variety of circumstances.The main types of plant will be considered and reference to the diagrams will benecessary along with the following descriptions. For further information the reader isdirected to the CIBSE Guides Section B2 (2001) and H.Central system with cooling coilThe control arrangement in this case is illustrated in Figure 22.26 which shows a typicalarray of sensing devices, control valves and motorised dampers, including some of thedesired safety features, to provide full control over temperature and humidity in theconditioned space under all outside air conditions. This figure relates in principle to theexample air-conditioning calculation in Chapter 18 (p. 519) with humidification by steaminjection.In an all-air system, the quantities of outside and recirculated air, respectively, may bevaried in any combination between full outside air, with no recirculation, and a mixture inany ratio subject to the provision of the minimum quantity of outside air which will satisfythe ventilation requirements of the occupancy. The mixture ratio is controlled to providethe most economic level of plant operation as determined by the condition of the outsideair relative to the required condition of the supply air.FanSupplyairT 4V 2T 5M 3Heating coilRecirculated airHumidifier Cooling coilM 1M 2V 3 T 2V 1 H 2 T 1airReturnH 1T 3Exhaust airOutside airChilledwaterHotwaterFigure 22.26 Control principles of a central plant system with cooling coil