Book - School of Science and Technology

Distribution for air-conditioning 429Figure 15.26 Air handling luminaires(d) Louvre fittingDust stainingThe streaks of dirt that appear sometimes at the point of air discharge from supplyterminals are caused either by dust in the room becoming entrained in the air stream,or by dirty supply air. The former is the most usual cause.Extract or return air grillesThe particular form of grille for extract is unimportant since air approach to a returnfitting is no aid to distribution. It may, for instance, be of egg-crate plastic within analuminium frame as Figure 15.27, or louvred, or any design giving the required free area.Dust collects on extract gratings on the outside, and close mesh or closely placed slats areundesirable as they quickly block up and impede the air flow.The sizing of extract and return grilles is normally based on limiting noise levels.Recommended maximum grille face velocities are given in Table 15.4, but for criticalapplications reference must be made to manufacturers' selection data.Where extraction takes place naturally from one space to another, it may be necessaryto use a light-trap grille to avoid direct vision. Transfer grilles may be fitted with firedampers where necessary. Figure 15.28 shows alternative fittings suitable for this purpose.A method of extracting room air which has been used with some success is illustrated inFigure 15.29. It has been claimed that heat loss and gain to the occupied space is muchFigure 15.27 Standard `egg-crate' register