Book - School of Science and Technology

512 Fans and air treatment equipment. In extreme weather conditions, a small amount of preheat may be required to preventfreezing of condensed moisture since this could lead to damage to the equipment and anunacceptable resistance to air flow. Preheating may also be desirable for thermal wheelinstallations to avoid excessive moisture exchange in winter.. Heat exchangers which have small passages presented to air flow will soon becomeclogged with dirt unless pre-filters are provided: wash down may be required atintervals. Some manufacturers of thermal wheels claim that the reversal of air flowwhich occurs in normal operation acts to maintain cleanliness.In considering the relative economics of alternative methods, account must be taken ofannual mean rather than peakload efficiencies and of available means for control of heatexchange, since under certain outside conditions which normally occur during midseason,the maximum rate of heat transfer available may increase, not decrease, energyconsumption. As has been mentioned, the performance of a thermal wheel may be variedby speed change and that of a heat pipe unit by automation of the angle of tilt. The watercircuit of run-around coils may be fitted with motorised valves as required, but for plateheat exchangers an arrangement of face and by-pass dampers will be necessary.Resistance to air flow and consequent increases in fan power must be considered as mustthe energy consumption by auxiliaries, drive motors, pumps, etc. These, of course, are likelyto remain running even when the enthalpy of the supply and exhaust air streams is so closeas to lead to minimal interchange. Table 17.6 presents a summary comparing the performanceof the various types of equipment based upon the design conditions listed in the firstthree lines of Table 17.4, with supply and exhaust air quantities equal at a rate of 5000 litre/s.Indirect evaporative coolersA development in air-to-air heat exchanger techniques, using established principles, overcomesone of the disadvantages of direct evaporative cooling, namely that of considerableTable 17.6 Comparative performance of various types of air-to-air heat exchanger aEnergy reclaim efficiency (%)TemperatureEnthalpyType of equipment Make Winter Summer Winter SummerParallel plate metal A 62 62 40 26.5B 65 59 41 26C 57 53 37 20Glass tube C 61 57 40 23Thermal wheel A 74 74 48.5 30non-hygroscopic B 75 75 72 44C 74 74 52 37hygroscopic A ‡ C 74 74 74 74B 70 70 70 70Heat pipe C 64 64 43 29Run-around coilB 67 61 ± ±waterwater/glycol (25%) B 64 60 ± ±water/glycol (25%) C 55 50 35 30NoteFor the winter and summer design conditions listed in Table 17.4. Supply and exhaust air quantity ˆ 5 000 litre/s.