Book - School of Science and Technology

382 Air-conditioningZone unitsSupplyTransferZonesupplyductRecirculation(a)Conditionedoutside airfrom mainplant(b)ZonereturnductSupplyDuctedrecirculationTypical floorFigure 14.6 Zoned system in multi-storey buildingescape as a return air path on the grounds that fire and/or smoke generated in any oneroom could be transferred into an escape route and lead to disruption and panic. It is now,in consequence, more usual to provide a quite independent return-air collection system,via duct branches from each room, back to the local zone plant, as shown in case (b).High velocity systemsTraditionally until the mid 1950s, air-conditioning systems were designed to operate withduct velocities of not much more than about 8±10 m/s and fan pressures of 0.5±1 kPa.With the advent of high-rise buildings and, concurrent with their introduction, demandsfor improved working environments coupled with less space availability for services therewas a requirement that tradition be overthrown. This situation led to a radical rethinkand to the introduction of a number of newapproaches to air-conditioning design usingduct velocities and fan pressures twice and more greater than those previously in use.Whilst the principal characteristic of the newgeneration of systems relates to the methodsadopted for distribution of conditioned air and exploits these to the full, the principlespreviously described in this chapter remain unchanged. As before, the conditioning mediummay be all air or air±water dependent upon a variety of circumstances. Figure 14.7 showscomparative space requirements for the alternative supply media. The lowvelocity extractducts associated with the respective supply air quantities will increase the space requirementsfurther in favour of the air±water systems. Added to which, the increase in space coolingloads arising from greater use of electronic equipment increases yet again the spatial benefitsArea ratio = 3:2All–air(supply & extract)Air–water(Supply, extract, heating & chilled water)Figure 14.7 Space requirements for all-air and air±water distribution arrangements