2.1 Mixing Systems and Physical Conditions
Mixing systems are the most common type of air distribution used in office spaces. When operated appropriately, these systems can effectively achieve their aim of providing relatively uniform air quality and thermal conditions within the entire office space (ASHRAE, 2001; Int-Hout, 1998; Spengler & Chen, 2000). In the present discussion, we focus on two aspects of mixing air distribution. Firstly, we discuss the effects of differences in mixing system characteristics. Following this, we address studies of mixing systems in open-plan offices.
Studies that examined differences between mixing system characteristics (i.e. Different types of mixing system, diffuser location and diffuser type) provided some evidence that these factors can influence airflow patterns, which in turn can affect ventilation efficiency, pollutant removal efficiency and thermal conditions (Bauman et al., 1991; 1992; Chen et al., 1992; Farant, Nguyen, Leduc, & Auger, 1991; Gan, 1995; Haghighat, Huo, Zhang, & Shaw, 1996; Haghighat et al., 1991; O'Donnell & Nguyen, 1990; Shaw, Zhang, Said, Vaculik, & Magee, 1993a; 1993b). However, there was no evidence that one particular mixing system configuration produced consistently superior IAQ or thermal environments, as compared to another. The main reason for this finding is that mixing systems are influenced by a number of extraneous factors that affect their performance in a given context. The physical conditions produced by any one mixing system configuration will be affected by factors such as the throw characteristics of diffusers, room geometry, supply air temperature, supply air velocity, room temperature, heat loads in the space, heating versus cooling operation, radiant heat from perimeter walls, pollutant sources, and the location of return grilles relative to supply outlets (ASHRAE, 2001; Chen et al., 1992; Farant et al., 1991; Gan, 1995). Therefore, the ‘best’ system in one situation might not be the most appropriate choice in another.
ASHRAE (2001) concluded that effective mixing air distribution is dependent on the appropriate selection and operation of systems for the given context. The main considerations in choosing mixing systems are to neutralise undesirable convection currents and avoid stagnant zones of air and short circuiting in the space. When the appropriate configuration is used, mixing systems are capable of providing a relatively uniform temperature, pollutant concentration and age of air within the office space (ASHRAE, 2001; Chen et al., 1992; Fisk, Faulkner, & Prill, 1991). In relation to the most commonly used type of mixing system (i.e. ceiling-mounted diffusers and returns), Int-Hout (1998) concluded that “…research has proved that with properly selected ceiling diffusers, excellent air distribution and ventilation mixing can be achieved with many types of diffusers and in many types of spaces…” (p.60). A more detailed discussion of the appropriate selection of mixing system configurations is provided by ASHRAE (2001), Int-Hout (1998), and Rock and Zhu (2002).
A few studies have examined the influence of open-plan workstations on the performance
of mixing systems (e.g. Bauman et al., 1991; 1992; Farant et al., 1991; Haghighat et al., 1996;
Shaw et al., 1993a; 1993b). These studies were all conducted using ceiling-mounted supply
diffusers and ceiling-mounted return grilles. Most of these studies found that partition height and
the presence of a gap at the bottom of partitions had minimal effects on air distribution patterns,
pollutant removal and ventilation efficiency. However, Farant et al. (1991) provided some
evidence of declining IAQ in situations where ceiling diffusers were located outside the
workstation and close to the return, resulting in short circuiting of the air supply. In addition,
Haghighat et al. (1996) noted that pollutant concentrations were highest in the workstation closest 办公室空调分配系统英文文献和中文翻译(4):http://www.751com.cn/fanyi/lunwen_31421.html