Book - School of Science and Technology

Air-to-air heat exchangers 511From buildingInlet to plantExpansion valveEvaporatorCondenserCompressorTo exhaustOutside airFigure 17.35 Run-around coils using a refrigeration cycleLittle more needs to be added with regard to this type other than to emphasise that thesmall temperature differentials between either air stream and the working fluid will resultin deep coils (typically 6±8 rows) and high resistance to air flow with consequent penaltiesin fan power requirement. The overall efficiency, ignoring fan and pump power, some ofwhich will be recovered in winter but will be a penalty in summer, is not likely to be morethan 40±65% at best.Run-around coils (using refrigeration)If one considers an exhaust and a supply air duct, separate but not too distant, it isobvious that the evaporation and condensing elements of a refrigeration plant could befitted within the respective air streams, Figure 17.35. By such means, one of the disadvantagesof a water circulating system, i.e. small temperature differentials, could beovercome: in fact, using this `heat pump' principle (see Chapter 19), the supply airtemperature may be raised above that of the exhaust air. The energy required to drivethe compressor imposes a penalty but much of this would be recovered as heat to thesupply air stream.Many types of packaged air-conditioning plant designed for roof mounting incorporatenot only supply and exhaust fans but also air-cooled refrigeration equipment for summeruse. In some cases facilities are provided whereby air paths may be redirected during thewinter and some part of the refrigeration capacity used as a heat pump drawing energyfrom exhaust air.Systems comparedThe principal disadvantages of any air-to-air heat recovery system which does not makeuse of an intermediate fluid for heat transfer is, as previously explained, that two air ductswhich may be quite large must be brought together at the heat exchanger. Other problemsoccur also in analysis but these are amenable to technical rather than spatial solution:. Any heat exchange element which may at some periods of the year operate with wettedsurfaces must be provided with a condensation collection tray and drain facilities. It isimportant that the configuration of the exchange surfaces permits flow and collectionof moisture.