Book - School of Science and Technology

Distribution for air-conditioning 42530º90ºAdjustment ofjet direction from30º to 90ºto horizontalSurface(a)(b)Figure 15.24 Typical desk-top outlets (Krantz): (a) Surface; (b) Section through linearLow level side-wall outletsSupply air panels are available for discharging usually cooled air, with a low supply-toroomtemperature differential, into a space at low level, from a side-wall or columnposition (Figure 15.25). The principle of design is to introduce a supply at low velocity,evenly distributed over the panel face and the effect on the air distribution in the space isthat of displacement ventilation. Pressure drop across the panel is 50±100 Pa and themaximum face velocity 0.5 m/s.Volume controlMeans for controlling as accurately as possible the volume of air issuing from each grille,diffuser or other inlet, are essential. Probably the commonest form of control is theopposed blade damper. For ceiling diffusers and the like, a variety of multi-louvredampers is available, or plain butterfly dampers in the ducts may be used. Any damperis a potential source of noise and its location and duty require careful consideration ifsuch is to be avoided. Where there is a risk of a noise problem due to excessive throttlingat the terminal, the major part of the regulation must be applied further upstream in thebranch ductwork, and where necessary the ductwork will need to be designed to suit.Selection of supply terminalsThe selection of the best number and size of inlet grilles, diffusers and the like, involves achoice to meet a number of variables at the same time:. Air velocities produced at head or foot level must not exceed those given at thebeginning of this chapter.. If the air entering is cooler than the room, as in an air-conditioning installation, therewill be a tendency to fall, best avoided by keeping a reasonable entering velocity withthe object of inducing air entrainment from the room; the air mixture then beingwarmer will have less tendency to fall.