Book - School of Science and Technology

504 Fans and air treatment equipmentweek. Over an average winter season, a heat supply of about 280 GJ (equivalent to thatprovided by burning about 8 tonnes of oil) would be necessary to do no more than raisethe ventilation air supply to a degree or so less than room temperature. The associatedextract ventilation plant would then reject a similar air quantity, at room temperature orwarmer, backto outside.In order to overcome the nuisance of heat gain to rooms from lighting, modern practiceallows extracted air to pass over and through luminaires, see Figure 15.26, taking with itup to 70% or so of the electrical input thereto. In our hypothetical example, therefore, thetemperature of the air discharged might well be several degrees above that held in theoffice space proper ± say 22 or 23 C.For applications such as hospitals and similar buildings, which ventilate or air-conditionwithout recirculation, in order to avoid contamination, and swimming pools which maybe similarly served in order to reduce riskof condensation, the air quantity rejected is fargreater per m 2 of floor area. Further, in kitchens and industrial premises, where processheat gain may be high, air will be exhausted at temperatures much greater than thosequoted above.It will be obvious that great scope for energy conservation exists if the heat in exhaust aircan be reclaimed and applied, in part at least, as a source of energy to raise the temperatureof the outside air used in the parallel supply plants. These same comments apply of courseto economies to be achieved in cooling capacity during the summer months since thetemperature of air discharged from an air-conditioned building may then be less than thatof the outside ambient: it may, furthermore, carry less moisture. This aspect assumes moreimportance in climates which produce extremes of summer temperature.Available equipmentA variety of types of equipment is available for air-to-air exchange in ventilation or airconditioningplants. These fall under one of the following headings:. plate heat exchangers. glass tube heat exchangers. thermal wheels. `heat pipe' heat exchangers. run around coils with water circulation. run around coils using refrigeration.Of these six types, the first three effect heat exchange directly from air-to-air whereas theremainder employ an intermediate circulating medium.Efficiency of heat reclaimBefore describing the various types of equipment in more detail, the matter of theirefficiency in operation requires definition. The data normally quoted derive temperatureefficiency from the expression: ˆ t3 t 1100t 2 t 1where the supply and exhaust flow rates are equal, and t 1 is the temperature of theoutside air, t 2 is the temperature of the exhaust air from which heat is to be reclaimed,