Book - School of Science and Technology

418 Air distribution(a)(b)Figure 15.13 Cone-type ceiling diffuserCone-type ceiling diffusersThe type of ceiling diffuser shown in Figure 15.13(a) has certain interesting characteristics.Figure 15.13(b) illustrates the kind of flow pattern to be expected from such a unitand it will be noted that entrainment air is drawn up in the centre immediately under thediffuser, being caught up in the nearly horizontal delivery from the unit itself. Due to thefact that air is discharged radially, the air stream velocity falls off rapidly as the distancefrom the centre increases, and hence this kind of unit may be used with temperaturedifferentials of up to 20 K.There is a great variety of makes of this type of ceiling diffuser, all under different tradenames and some with special features, such as:. An adjustable arrangement whereby the inner cones may be raised or lowered inrelation to the periphery, so causing a variation in the flow pattern to be achieved.Instead of the bulk of the air travelling horizontally, it is possible by this kind ofadjustment to make it discharge vertically downwards or at any intermediate flowpattern desired. This might be of advantage in cases where it is desirable to cause theair to descend quickly, such as in a hot kitchen, rather than that it should be dispersedand lost at high level. Terminals of the pattern may be provided with removable coresfor cleaning, which would be suitable for applications such as hospital operatingtheatres.. In another type, the unit is square instead of circular, this sometimes being necessary tomatch ceiling tiles, etc.. In other examples, the diffuser is flush with the ceiling instead of projecting.Perforated-face ceiling diffusersThis type, shown in Figure 15.14, has been developed from the early pan devices and hascharacteristics similar to the cone pattern: it is often used to blend in with perforatedacoustic-tile ceilings. The deflecting pan may be removed if a predominantly vertical airdistribution is required. Both the cone and perforated plate types may be fitted withinternal blanking pieces to limit the spread of air discharge.Multi-directional ceiling diffusersSuch units provide facilities for positive directional control of air discharge, the bladesbeing individually adjustable as illustrated in Figure 15.15. They are useful for applica-