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Top1. Introduction
In the field of air conditioning, comfort cooling is always the focal point in designing a system, such that ventilation by complete mixing is very popular. The principle behind complete mixing, which is illustrated in Figure 1, is to dilute the contaminated air with the supply air through recirculation so that one achieves the same concentration as in the extract (Etheridge & Sandberg, 1996). Due to the emerging health issues as well as environmental concerns, indoor air quality and air quality, in general, became a central point of the policy discussion in the domain filed (Lode et al., 2016). In the Philippines, the passage and the implementation of the Clean Air Act compel engineers and industrialists to consider the provisions of the law, particularly in designing and selecting an air conditioning system.
Figure 1. Schematic diagram of mixing ventilation system
The displacement ventilation system is considered a viable option to address the concerns for clean air, aside from comfort cooling, especially in the workplace (Etheridge & Sandberg, 1996; Kosonen & Tan, 2004; Lin et al., 2005). The displacement ventilation system has been well studied in the domain literature (Liu et al., 2014; Wang et al., 2014; Schiavon et al., 2015; Mateus et al., 2015; Shan & Rim, 2018; Berlanga et al., 2018). A literature review of displacement ventilation was reported by Cho et al. (2005), and a European review context was put forward by Riffat et al. (2004). In this type of ventilation system, which is schematically shown in Figure 2, conditioned air is supplied at or near the floor level at low velocities (i.e., 0.5 m/s or less) directed to the occupied zone. Air spreads over the floor and then rises as it is heated by the heat sources that are in the room (e.g., occupants, computers). The exhaust outlets are located at or close to the ceiling through which the warm air is exhausted from the room. An upward convective flow in the form of the thermal plume is created, which brings heat and contaminants upward from the occupied zone, which is less dense than the surrounding air in the occupied zone. The air volume in plume increases as it rises due to the entrainment of the surrounding air. Where the airflow rate in the plume equals the supply airflow rate, a stratification level develops, creating two distinct zones. The lower zone, below the stratification level, has no recirculation flow and closely resembles an upward piston-like displacement flow. The upper zone has recirculation flow and accumulates excess heat and contaminants. The lower and upper zones can be described as clean and dirty zones, respectively. The height of the lower zone depends on the supply airflow rate and characteristics of heat sources and their distribution across the floor area. In a properly designed displacement ventilation system, the height of the lower zone is higher than the occupied zone so that the latter can be ventilated adequately. A stable, vertically stratified temperature field is essential for the displacement ventilation system to function properly (ASHRAE, 1997).
Contrary to conventional complete mixing ventilation, displacement ventilation is designed to minimize mixing of air within the occupied zone so that its environmental conditions are as close as possible to the conditions of the air supply, thereby providing a much cleaner air to the occupants. Thus, displacement ventilation is more efficient in removing the contaminants in the occupied area compared with the mixing ventilation (ASHRAE, 1997). Furthermore, with a displacement ventilation system, a higher temperature primary air compared to complete mixing is used as it is directly supplied to the occupied zone. The higher the efficiency attained in the displacement ventilation system against conventional mixing type can be considered as a reduction of the supply of fresh air without sacrificing air quality (Mathisen, 1989).
Figure 2. Schematic diagram of a displacement ventilation system