Book - School of Science and Technology

524 Calculations for air-conditioning design15ºC DB12ºC WB6K DB riseRecirc/exhaust fan21ºC DB 14.8ºC WBInfiltration14ºC DB11.5ºC WBInlet fan15.52 kg/sDampersFilterExhaust air 4.82 kg/sRecirc air 10.7 kg/sAfter heaterCooling coilWinter preheaterFresh air 4.82 kg/s28ºC DB 18.5ºC WBMixture 15.52 kg/s23.2ºC DB 16.1º C WBFigure 18.5 Plant conditions when cooling (see example)Cooling coilsThe example above has assumed that the medium which is circulated through the coolingcoil is chilled water and that a suitable coil could be found to meet the required duty. Useof chilled water avoids cooling all the air down to a low dew point for dehumidificationpurposes and then, subsequently, reheating.Further, advantage is taken of the characteristic of such a coil such that moisture maybe deposited on the chilled surface without the bulk of the air being reduced to the sametemperature. This is achieved by keeping the surface at the lowest possible temperatureconsistent with absence of freezing. Had the coil been cooled by direct expansion of arefrigerant within it, a DX coil, the calculation would have been similar but the surfacetemperature of the coil would have been lower.In some circumstances, a coil might be arranged to have unchilled and recirculatedwater sprayed on to the surface in order to improve performance and to provide foradiabatic saturation of the air flow and means for humidification in winter. Concern as tohumidifier related illness has led to suspicion of sprayed coils (p. 500), since doubt existswhether plant owners and operators will ensure that the means provided to preventbacteriological contamination are maintained. Humidification by steam injection is thusoften preferred, with saturation efficiencies of up to 80%.Winter heating and humidifyingThe operation of the plant in winter might be with partial recirculation, as in summer, orwith 100% outside air. It might be arranged so as to be suitable for the latter method ofoperation in average spring and autumn weather when neither heating nor mechanicalcooling is required. In colder weather, recirculation might be used to promote economy inrunning cost. The latter arrangement is often made by providing an outside sensor tocontrol motorised dampers and vary the proportion of outside to recirculated air: this isoften referred to as an economy cycle. In order to avoid the complication of introducinganother variable, the two calculations which follow assume that heat losses through thebuilding fabric and those arising from infiltration are offset by a separate system such as