Book - School of Science and Technology

646 Automatic controls and building management systemsSupplyairReturnairH 1T 5Exhaust airFanM 4TM 14Heating coilHumidifierBypassM 2CoolingcoilV 3 T 3 T 2V 1 H 2 T 1V 2M 3Outside airChilledwaterHotwaterFigure 22.27 Control principles of a central plant system with cooling coil by-passFan-coil systemsThe principal difference between those systems previously described and a fan-coil,arrangement is that where a ducted air supply is provided, this handles outside air onlyand not the full conditioned air quantity.Depending on the requirement for humidification in winter and dehumidification insummer the temperature of the outside air supplied to the space may be varied tominimise reheating at the fan coil unit (FCU). Controls will thus be as described for acentral plant system. Each FCU will be provided with cooler and reheater coils fitted withcontrol valves and these will be operated separately or, more usually, in sequence by athermostat in the fan-coil inlet.The loads can be controlled by varying the heating and cooling water through the coilsas shown in Figure 22.28 (waterside control) or by arranging the coils in parallel andvarying the airflow over them (airside control) as shown in Figure 22.29, valves in thewater connections which may be two-, three- or four-pipe. Manual speed adjustment andon/off control to the fan is often provided. Automatically controlled fan switching,however, may cause annoyance due to the change in noise level.Airside control units utilise one air control damper rather than water control valves. Assmall control valves are susceptible to blockages they may be chosen where there aremaintenance concerns, although they do incur greater running costs as the dampersgenerally cannot achieve the tight close off of the valves with the consequent wastagewith heat transfer between the heating and cooling coil.