Article by: Natthawut Ruangtrakoon, Phd.
Department of Mechanical Engineering, Faculty of Engineering,
King Mongkut’s Institute of Technology Ladkrabang
Air conditioning system (A/C system), used for large commercial buildings and shopping malls mostly has water chiller, which is used to produce cool air. This cool air will be then distributed to a conditioned space. In Thailand, the cool air distribution system is typically distributed through air duct, which is located at plenum above the ceiling. This type of air distribution system is classified as overhead system.
A disadvantage of this system is that the cool air is not directly supplied to the occupied zone which its height is around 2 feet from the floor. Before the cool air coming to the occupants in occupied zone, it must firstly pass the upper zone of the room. Therefore, there is some energy loss from the supplied cool air that must be threat the high temperature air at the upper zone. Moreover, lighting system which located at the ceiling of the room is a major heat source or cooling load of A/C system.
In case of using overhead air distribution system to produce the comfort condition for the occupants, the temperature of supplied cool must be low enough to produce the heat transfer effect in all level of a room (from ceiling to occupied zone). The temperature and humidity must be controlled to meet the comfort zone for the occupants. Therefore, for overhead system, the conceptual design is that the temperature of whole room is designed to be almost identical (fully mixed air method). Additionally, the contaminants in the air which are floated and accumulated at the upper zone of a room will be also mixed with the supplied air and then supplied to an occupant. Hence, the new idea of air distribution system is developed to fix its disadvantage.
Underfloor cool air distribution system, UFADs, is different from the above conventional system. Figure 1.1 and 1.2, referred from [1], shows the difference perspective idea between the overhead and UFAD systems. The difference of these 2 systems is the installed location of the A/C system, which results in difference scheme of temperature distribution inside the room. As mentioned before, the temperature distribution of overhead system is uniform. As in Figure 1.1, temperature distribution of the room using A/C overhead system is uniform and maintained at 75°F (25°C).
On the other hand, the temperature distribution of the room using UFADs is divided into several zones as in Figure 1.2. In response to the purpose of human comfort, occupied zone is also controlled at 75°F (25°C). The schematic diagram of the zone separation generated by UFADs which the temperature distribution is divided into 4 zones is shown in Figure 2 [2].
The temperature in each zone is different. The air with the lowest temperature is supplied form the supply air plenum and the temperature will be increased with the height of the room in occupied and upper zones. The air with the highest temperature will be returned to the top zone of the room (return air plenum) for the purpose of air treatment process. This phenomenon causes the thermal stratification. The temperature of occupied zone is controlled at 75°F (25°C), but the temperature in the upper zone can be kept at higher level at 77°F. This operation is totally different from the conventional or overhead system.
In view of energy using in air conditioning process inside a room, the energy use in UFADs is saver than the overhead system by 25-50% [2]. Returning back to the Figure 1.1 and 1.2, if the controlled temperature of occupied zone is at 75°F (25°C), the temperature of supplied air in the case of overhead system is about 55°F (12.78°C). However, the temperature of supplied air of UFADs can be set higher than that around 10°F, or it can be set at 65°F (18.33°C). Hence, it is not surprising that the new commercial buildings will turn to use the underfloor air distribution system in the future.
Source:
[1] A. Bhatia, HVAC Overview of Underfloor Air Distribution (UFAD). Continuing Education and Development, Inc.
[2] Kai Zhang, Xiaosong Zhang, Shuhong Li, Xing Jin, Review of underfloor air distribution technology, Energy and buildings 85 (2014) 180-186.