Book - School of Science and Technology

All-air systems 389HotairMixing bladeAir mixing sectionSoundattenuatorLow velocitymixed airdischargeColdairVolume flow regulatorActuatorFigure 14.15 Dual-duct mixing box (Trox)the dual-duct system where the mixing is transferred from the central plant to eitherindividual rooms or small groups of rooms having similar characteristics with respect toload variation.Use is made of two ducts, one conveying warm air and one conveying cool air, and eachroom contains a blender, or mixing box, so arranged with air valves or dampers that allwarm, all cool, or some mixture of both is delivered into the room. Figure 14.15 illustratessuch a unit, incorporated therein being a means for regulating the total air deliveryautomatically such that, regardless of variations in pressure in the system, each unitdelivers its correct air quantity. Referred to as `constant volume control', this facility isan essential part of such a system.Owing to the fact that air alone is employed, the air quantity necessary to carry thecooling and heating load is greater than that used in an air ± water system. Air deliveryrates with the dual-duct system are frequently of the order of 5 or 6 changes per hour,compared with the 1 1 ¤ 2 2 required for introduction of outside air. Owing to the considerablequantity of air in circulation throughout the building, dual-duct systems usuallyincorporate means for recirculation back to the main plant and this involves return airducts and shafts in some form.An advantage of the dual-duct system is that any room may be warmed or cooledaccording to need without zoning or any problem of change-over thermostats. Furthermore,core areas of a building, or rooms requiring high rates of ventilation, may equallybe served from the same system, no separate plants being necessary.Figure 14.16 shows the plant arrangement of a system in one form, though there arevariations of this using two fans, one for the cool duct, one for the warm duct.To avoid undue pressure differences in the duct system, due for instance to a greaternumber of the units taking warm air than cool air, static pressure control may beincorporated so as to relieve the constant pressure devices in the units of too great adifference of pressure, such as might otherwise occur under conditions where the greaterproportion of units are taking air from one duct than from both.Since dual-duct systems supply a constant air volume to the conditioned areas thisovercomes the potential problems arising from maintaining adequate quantities of outsideair and from providing satisfactory air distribution experienced with some variablevolume systems. The disadvantages of dual-duct systems are high energy use and theneed for large shafts and ceiling voids to accommodate the ductwork.In special circumstances, a dual-duct system may be provided with variable volumeterminal equipment to combine the best features of each system. With such an arrangement,