Book - School of Science and Technology

372 VentilationLastly under this heading, the large diameter Punkah fans which were installed by thethousand in tropical countries before the days of air-conditioning, are being increasinglyused in a variety of buildings. They do no more than agitate the air at high level in theroom but, in so doing, movement is induced in the lower occupied area which producesevaporation from the skin and consequent cooling. The models used in the past werenoisy and ugly but modern versions are not only much quieter but greatly improved inappearance.Ventilation efficiencyTo conclude this chapter, it is appropriate to note the term ventilation efficiency which hasbeen used increasingly in recent years, as a result of the sensitivity of those non-smokerswho may be required to share spaces with those who smoke. This situation has been thesubject of considerable research and many published papers.*The term represents an attempt to quantify the rate at which contaminants are removedfrom a space and, in particular, from the occupied zone. No simple definition of the termexists but it may be expressed as the ratio between the average concentration of acontaminant within a space and that concentration present at the point of extract. Thisratio will depend upon many factors including the distribution, properties and period ofdischarge of the contaminant, the quality and rate of supply of replacement ventilation airand the pattern of air movement within the space.For contaminant emissions which are distributed, ventilation efficiency improves withthe rate of air supply and with those air movement patterns able to carry the contaminantto the extract positions quickly as may be produced by an effective piston displacementflow. Complete mixing of the air supply with room air produces no more than an averageventilation efficiency and any short circuits in room air flowpatterns, supply to exhaust,reduce that efficiency further.* Shandret, E. and Sandberg, M., `Air exchange and ventilation efficiency ± newaids for the ventilationindustry', Norsk VVS (Norway), 1985, 7, 527±34.Design and Performance of Mechanical Ventilation Systems ± Ventilation Effectiveness. BSRIAContract 7190A, 1987.