Supply air containing at least the minimum volume of outside air is filtered and conditioned to the required temperature and humidity. It is then delivered by the AHU to an underfloor plenum, traveling through a shorter distance of ductwork than for ceiling-based systems. Within the plenum, air flows freely and enters the workspace through diffusers at floor level or as part of the furniture or partitions. Because the air is supplied directly into the occupied zone, supply outlet temperatures are generally maintained no lower than in the range of 17-20°C (63-68°F) to avoid uncomfortably cool conditions for the nearby occupants. Under suitable climate conditions, this greatly increases the availability of outside air economizer use and also allows higher cooling coil temperatures to be used, if desired, thereby increasing chiller efficiencies.
Improved indoor air quality is expected by delivering the fresh supply air near the occupant at floor or desktop level, allowing an overall floor-to-ceiling air flow pattern to more efficiently remove contaminants from the occupied zone of the space. Under cooling conditions, an optimized strategy is to control supply outlets to allow mixing of supply air with room air only up to head height (6 ft [1.8 m]). Above this height, stratified and more polluted air is allowed to occur. The air that the occupant breathes will have a lower percentage of exhaust compared to conventional uniformly mixed systems. Another benefit of providing local air supply is that it improves air motion in the space and prevents the sensation of stagnant air conditions, often associated with poor air quality.Close Window