Book - School of Science and Technology

Air-conditioning system controls 645A dry bulb thermostat or an enthalpy sensor is used to sense the outside air condition,the latter being used more generally nowadays in preference to a wet bulb thermostat.Modulating dampers in the recirculated, exhaust and outside air ducts are controlled toprovide the desired mixing ratio and, for reasons of energy economy, the plant mayoperate on full recirculation during building heat-up in winter.In summer, the dampers will normally be set to introduce a minimum outside airquantity and the cooling coil will be controlled to maintain the required dew-point forthe supply air and, during this season, moisture will be removed from the air for themajority of outside conditions. Rarely where very close control is required, and the energypenalty accepted, the final dry bulb temperature will be controlled by the after-heater.During mid-seasons, full outside air may be used to take advantage of the available `freecooling' potential which is available using air at outside temperature.In winter, the dampers may modulate to mix outside and recirculated air to providethe most economical ratio such that, whenever outside conditions permit, the mixture willprovide air at the desired supply temperature and thus eliminate the need for preheating.The humidifier will be operated as required to provide the correct moisture content inthe supply as required by the space, by sensing the humidity content of the return/exhaustair.Central system with cooling coil bypass (`face and bypass')This is a development of the previous system where, in order to achieve the dehumidificationrequired, whilst avoiding wasteful use of cooling and reheating in the event ofprolonged periods of less than peak load, a bypass may be introduced around the coolingcoil as shown in Figure 22.27. In this case, dampers in the recirculated air duct and in thebypass duct will operate together to allow the correct quantities onto and around thecooling coil.Zoned systemIn this case the central plant delivers air at a constant outlet condition and the zones arecontrolled individually.Terminal reheat systemIn this case, all or the bulk of the required reheating to achieve control of dry bulbtemperature is transferred from the central plant to individual rooms within the conditionedspace. Thus, the output of the central plant is controlled to meet the peak coolingload likely to occur at any one time in any individual room.Within each room, a thermostat will be arranged to control admission of heat to thereheater incorporated in the terminal unit. This reheater would usually take the form of ahot water coil provided with a control valve, but electric resistance heaters are sometimesused.This form of system is inherently wasteful of energy and is generally to be avoided.However it is sometimes possible that the amount of reheating can be acceptably minimisedby the use of a control system which controls the central plant under the dictatesof constantly interrogating the terminal load controllers to always achieve a supplycondition which minimises the wasteful reheat required at the terminals.