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Another type of underfloor air conditioning is to make use of a plenum space between the structural concrete slab and the underside of a raised floor system to deliver conditioned air into the occupied zone through a variety of supply outlets or terminal units located at the floor level or as part of the furniture and partitions. High-induction swirl diffusers are sometimes used to quickly mix the supply air as it enters the occupied zone. Air is circulated in an upward motion similar to natural convection and is exhausted through return grilles in the ceiling or in the floor. This “ductless” system is highly flexible and has the potential to provide life-cycle cost savings and better space air quality for today’s modern offices. In recent years, with the growing needs of information technology and network equipment, the use of raised-floor construction has attracted much attention in Hong Kong and other cities. Combined with the underfloor air conditioning system, the raised-floor design can offer an effective solution for offices and commercial spaces. Some high-rise buildings in Hong Kong have used this plenum-based system for underfloor air distribution. To arrange for intake of outdoor air or primary air, either a central or a decentralized primary air handling system can be used (see Figures 1 and 2). To better integrate with the raised floor system, electrical power, telephone, data cable and other portions of the building’s infrastructure are located in the underfloor space. Plug-in electrical boxes, power/data outlet boxes and air diffusers are flush-mounted in the floor panels and can easily be moved to accommodate reconfiguration of the workspace or floor plan. The surface of the floor panels, usually designed in standard modules, can be made of a variety of materials, including carpet tiles, decorative concrete, linoleum, finished metal or wood composite.
4. POSSIBLE BENEFITS Bauman and Webster (2001) have reviewed and summarised the major benefits of underfloor air distribution as follows: · reduce life-cycle building costs by improving flexibility in reconfiguring building services and office layout; · improve thermal comfort, occupant satisfaction and productivity by providing inpidual comfort control; · improve ventilation efficiency, indoor air quality and health by delivering fresh air in the vicinity of building occupants; · reduce energy use from thermal stratification, reduced static pressures, and increased economizer operation; and · reduce floor-to-floor height in new construction by lowering the height of service plenums. The key benefits of an underfloor air conditioning system are occupant comfort, energy efficiency and space planning flexibility (Loudermilk, 1999). Occupants can adjust thediffusers to deliver air according to their inpidual temperature preferences. In addition, the upward flow of low-velocity air reduces lateral mixing compared to a conventional high-velocity HVAC system. By delivering air directly to each workspace, the HVAC system eliminates “dead zones” of air flow. The improved air circulation can further enhance comfort and reduce the concentration of air contaminants at the breathing level of seated occupants. The supply air of an underfloor system is typically delivered at a higher temperature (16-17ºC) than a conventional HVAC system (12-13ºC). This increases energy efficiency by extending the hours that the building can be “free-cooled” with 100% outside air. The lower static air pressures (25 Pa vs. 750 Pa) can deliver significant cost savings as a result of downsizing supply air fans. Under the low to intermediate supply air volumes, the performance of an underfloor air distribution system resembled that of displacement ventilation and may inherit some of the benefits of displacement systems like high ventilation effectiveness (Bauman, et al., 1995). Higher air circulation in the conditioned space during part loading conditions will also help to avoid stagnant air and ensure removal of air pollutants at part load. Other benefits of underfloor air distribution systems may include increased chiller efficiency and reduced cooling load requirements.