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Modern open-plan offices have increased the complexity of effectively distributing air. In addition to general performance issues, air distribution systems in these contexts need to be able to account for localised heat and pollution sources, and the possible influence of workstation furniture and partitions. Systems also need to be flexible, to respond to changes in open-plan workstation layouts (Bauman et al., 1991; Bauman & Webster, 2001; Faulkner, Fisk, & Sullivan, 1995; Hedge et al., 1993). As summarised by Bauman and Arens (1996):
In this review, we summarised the literature on air distribution systems and their effectson occupant environmental satisfaction.
1.1 Types of Air Distribution System
ASHRAE (2001) categorised air distribution systems into four types: mixing systems, displacement systems, localised (or task) systems, and unidirectional airflow systems.
Unidirectional airflow systems are used primarily for clean room environments, such as operating rooms, paint booths and computer rooms, and as such are not commonly employed in office buildings. In this review, therefore, we focused on the remaining three air distribution systems, each of which is briefly described below.
1.1.1 Mixing Air Distribution Systems
Mixing systems are the most common approach to air distribution in office spaces. These systems are designed to promote complete mixing of the supply air with room air, to maintain a desired set-point temperature and uniform air quality throughout the volume of air in the space. Conditioned air is discharged from outlets at relatively high velocities, to promote mixing, and may be warmer or cooler than room temperature, depending on heating or cooling requirements. The airstream from the diffusers mixes with room air by entrainment, which reduces the air velocity and equalises air temperature throughout the space. The aim of mixing systems is to provide a relatively uniform air velocity, temperature, humidity and air quality throughout the
space (ASHRAE, 2001; Bauman et al., 1991; Bauman & Webster, 2001; Spengler & Chen,
2000).
ASHRAE (2001) used Straub et al’s (1956; cited in ASHRAE, 2001) taxonomy to classify mixing systems into the following five types: outlets mounted on or near the ceiling with a horizontal air discharge; outlets mounted on or near the ceiling that project air vertically downwards; outlets mounted on or near the floor with vertical non-spreading jets; outlets mounted on or near the floor with vertical spreading jets; and outlets mounted on or near the floor with horizontal air discharge. In all cases, exhaust outlets are usually mounted at or near the ceiling. Historically, the most commonly used mixing system in office buildings is one that supplies air horizontally through diffusers evenly distributed across the ceiling, and returns air at ceiling level (Bauman & Webster, 2001; Hedge et al., 1993).
1.1.2 Displacement Air Distribution Systems
Displacement systems supply air at or near the floor and exhaust it at ceiling level. In contrast to mixing systems, displacement systems supply air at temperatures slightly lower than the desired room temperature, and at relatively low velocities (0.5 m/s or less). These characteristics cause the supply air to spread out at floor level and then to rise as it is warmed by heat sources (e.g. computers, occupants) in the space. The heat sources also create thermal plumes which lead to an upward convective flow of heat and contaminants from the occupied zone (ASHRAE, 2001; Jiang, Chen, & Moser, 1992; Spengler & Chen, 2000; Yuan et al., 1998).
Displacement systems have been used commonly in Scandinavia during the last twenty years, primarily to ventilate industrial facilities. More recently, the use of these systems has been expanded to office environments, although they are still more widespread in Europe than North America (Spengler & Chen, 2000).
1.1.3 Localised Air Distribution Systems
Localised air distribution systems are designed to provide air to specific locations; principally to each inpidual occupant in an office. Compared to mixing systems and displacement systems, localised air distribution uses a larger number of diffuser outlets that deliver air directly to each occupant. This can be achieved using either an underfloor (UF) system or a task/ambient conditioning (TAC) system (ASHRAE, 2001; Bauman & Arens, 1996; Bauman & Webster, 2001; Bauman, Zhang, Arens, & Benton, 1993; Spengler & Chen, 2000).