Book - School of Science and Technology

392 Air-conditioningin the wall in a manner similar to that shown in Figure 13.7. The hazards of dust and noisepollution to say nothing of unit overload due to wind pressure cannot be exaggerated.For a high proportion of the year, external conditions in the British Isles are such thatrooms on certain elevations of a building may require heating while others require cooling.It is thus desirable to provide units that are able to meet either demand at any time.Such an arrangement is known as a four-pipe system, heating flowand return and coolingflowand return pipes being connected to separate coils. A two-pipe system providesheated water to a coil in winter and chilled water to the same coil in summer and problemsarise in mid-season when some spaces require heating while others are calling for cooling:clearly both requirements cannot be satisfied simultaneously with this two-pipe arrangement.Such a system is suitable only where the period of climate change between summerand winter is short, with little or no mid-season (some parts of the USA have such aclimate), or where the internal loads are such as to require only local cooling.The chilled water flow to the units may be circulated at an elevated temperature, asdescribed later for air±water induction systems, eliminating the formation of condensateon the cooling coils and thus the need to pipe this from each unit to drain. Very basiccontrol of either individual units or groups may be effected by switching the fan motor(s)on and off. The preferred alternative for the climate of the British Isles, however, is tokeep the motor running and to control either the water temperature using automaticvalves as Figure 14.17 or the air side using mixing dampers as Figure 14.19.Air-side controlled fan-coil units have seen increasing popularity in recent years particularlywhen the units are to be mounted within ceiling voids in commercial offices. Theunits being controlled by a damper arrangement avoids the need for a large number ofsmall water control valves with their inherent reliability and maintenance issues. Installation,balancing and commissioning times can also be reduced. However, with both theheating and cooling coils operating at design water flow rates thermal insulation and airleakage between the hot and cold decks needs to be effective. This aspect is sometimes notPipesto aboveCoolingcoilDamperopenOutletDamperclosedHeatingcoilPlenumRoom airPipes frombelowCoolingFanDamperclosedHeatingDamperopenFigure 14.19 Typical vertical type fan-coil unit