Modern Building Services

26 MODERN BUILDING SERVICES NOVEMBER 2022 FEATURE INDOOR AIR QUALITY U nderfloor ventilation typically employs a displacement air movement strategy, and the air quality benefits of this approach stem from the specific methods of supply and extraction. These are best understood by comparing displacement air movement methods with those of traditional mixed air distribution systems. With a mixed air distribution system, the air (for either heating or cooling) is delivered at a relatively high velocity from diffusers at the ceiling level. Having been delivered outside the occupied zone, the air moves along the ceiling due to coanda effect, reducing in velocity before entering the occupied zone. As long as the ceiling diffusers have been selected and positioned correctly, this high velocity air will not result in occupant discomfort from draughts, or ‘dumping’ of cool air (if the velocity is too low). See Figure 1. By contrast, an underfloor air distribution system employing a displacement air movement strategy will deliver cool air into the room from the floor void, through specially-designed floor grilles, at a lower velocity than that of the typical mixed air system. As the supply air is always cooler than the room air, it moves slowly across the room. When the cooler air comes into contact with a heat load, such as a room occupant, it rises towards the ceiling, where the system will typically include equipment for extraction. See Figure 2. This process can deliver air quality improvements because, as the air rises upwards it can take certain airborne contaminates with it, out of the occupied zone, for extraction at ceiling level. There are, of course, some factors to consider. Employing displacement air strategies, for example, may not be suitable for applications where the contaminants will be heavier than air. Sites such as laboratories/ science campuses and so on will also, of course, require specialist air management systems, such as TROX’s LabControl systems, that are purpose-designed for these applications. TimTanner , Product Technical Manager – Air Diffusers & Attenuators, at TROX UK, reviews the air quality advantages of underfloor ventilation employing floor grilles/diffusers and provides tips on system design. Air quality advantages of underfloor ventilation and tips for system design Figure 1. mixed-air-ventilation Figure 2. displacement-air-ventilation