Book - School of Science and Technology

506 Fans and air treatment equipmentS 2E 2O 2E 0RE 1O 1 S 1(a)S 4O 2E 2RFigure 17.30 Performance of an air-to-air heat exchanger(b)Since temperature and enthalpy based calculations may produce results which aresignificantly different in terms of efficiency, it is important to examine critically any dataproduced by manufacturers in this respect. Where a heat exchanger has equal ability torecover both sensible and latent heat, as illustrated in Figure 17.30(b), the efficiencies inenthalpy terms become:Winter100 S 3 O 1 34:8 7:7ˆ 100 ˆ 76%R O 1 43:4 7:7Summer100 O 2 S 4 57:0 46:7ˆ 100 ˆ 76%O 2 R 57:0 43:4Some types of equipment have different characteristics with respect to sensible andlatent heat exchange. Taking the previous example and allowing for latent transfer to beat 50% efficiency, the winter and summer results in term of enthalpy would be 67 and58% respectively.Where supply and exhaust air quantities are not equal, the notional efficiency willchange pro rata to the mass ratio. That is, if the supply air quantity were twice that of theexhaust air, the efficiency would be about halved; in the converse case, the efficiencywould increase by about 25%; precise figures should be obtained from the manufacturer.Plate-type heat exchangersHaving no moving parts, this is probably the most simple type of equipment. The two airstreams are directed in cross- or counter-flow through a casing which is compartmented to