Book - School of Science and Technology

General principles 415DischargeUnder sillunitReturn airSecondarycirculationFigure 15.9 Vertical air distribution from sill leveldistribution is in common use with induction unit, fan coil and reverse cycle heat pumpsystems, where the terminals are located at low level under windows. Fixed blade lineardiffusers are normally used and the flow may be vertical or angled towards or away fromthe window. Vertical throw is preferred as this avoids the risk of the air, when cool,`dumping' into the room. However, where there is a recess formed at the junction ofceiling and window, perhaps where there is a dropped ceiling, the air has to be directedaway from the window to avoid interruption of flow at the recess; an angle of 5 or 10 from the vertical is common. With upward flow over glass, care must be taken to ensurethat the air stream when cool will not cause the insolated glazing to crack under theconditions of thermal stress then created: the glass manufacturer should be consulted inthis respect.LateralThis arrangement may sometimes be necessary where dictates of planning preclude moreorthodox solutions. Air is introduced near the ceiling on one side of a long low room with asmooth flush ceiling, and is exhausted at the opposite end at the same level taking advantageof the `Coanda' effect which will be referred to in more detail later (see Figure 15.10). TheSupplyExtractEntrained currentsFigure 15.10 Lateral air distribution