Book - School of Science and Technology

414 Air distributionExtract airSupply airSupply airPool tankExtract airBalance tankService ductIntegrated surround swimming poolFigure 15.8 Mixed system of air distributionComputer rooms may also be conditioned using a ceiling supply, normally through aperforated ceiling or an equivalent system, with the extract taken out via floor extracts orlow level side-wall grilles. The supply arrangement is described then under the heading ofperforated ceilings.An application of downward inlet with both downward and upward extract, suitablefor rooms of greater height, is shown in Figure 15.7(b). This is usually adopted wheresmoking occurs and it is necessary to provide some top extract to remove the smoke. Inthis case the top exhaust is discharged to atmosphere by a separate fan and the low levelextract constitutes the recirculated air. The low level extract also serves to ensure thatsatisfactory air movement is achieved at the occupied level, in cases where the room heightis over 4 m. Care must be taken when placing low level extract grilles close to areas wherepeople are seated: such grilles must be selected for a very low velocity through the freearea and be well spaced out so that excessive air movement will not occur.Mixed upward and downwardSuch a system is shown in Figure 15.8, illustrating a typical swimming pool hall application.It will be clear from the previous descriptions that the principle of the air distributionis, in effect, an upward system providing good mixing. Normally about 25% of the extractair quantity will be at low level also, the remainder being exhausted at high level.Mixed upward and lateralSuch a method has been used to describe a system where the air is introduced verticallyupwards from beneath a window. The flow pattern is vertical, or virtually so, up to ceilinglevel and then horizontal across part of the ceiling. Secondary room air is induced into theair stream producing a flow pattern as shown in Figure 15.9. This method of air