VENTILATION, DUCTWORK AND GRILLES
If warm air is supplied at floor level in laminar flow into a cold room, the fresh air will rise and be extracted at ceiling level, in effect short circuiting the occupied zone and failing to achieve the desired effect.
Displacement ventilation is usually less
preferable where: Q Surplus heat is the main problem, rather than air quality
Q Ceiling heights are lower than approximately 2 – 3 metres
Q Disturbances to air flows are strong Q Contaminants are colder/denser than the ambient air
Q The REHVA Guidebook on Displacement Ventilation guidance, however, states that this approach has proved superior to mixed air distribution in restaurants, meeting rooms (offices) and classrooms. It is usually preferable in applications where:
Q Contaminants are warmer and/or lighter than the surrounding
Q Supply air is colder than the ambient air Q Room heights are more than 3 metres Q Large air flows need to be supplied in small rooms.
Design tips Key factors to consider include: Q Total cooling load: Displacement ventilation is typically recommended for cooling loads less than 40 W/m² for comfort, however individual applications must be investigated to ensure comfort is achieved.
Q Heating requirement: Displacement ventilation requires the supply air to be cooler than the room air. So, this approach is only suitable for cooling with a supply temperature range of -2K to -4K. As a general rule, heating is not recom- mended using displacement ventilation. The Trox FBA floor diffuser, however, incorporates a swirl unit which causes high induction and allows for heating, as long as the temperature differen- tial is kept to a minimum. In order to maximize the effectiveness of this heating, the FBAs should be evenly spread throughout the floor plate.
Q Room height: The height of the room is an im- portant factor since there can be a considerable amount of mixing in the region below the ceiling. This arises due to the interaction between up- ward and downward moving buoyant air flows. Hence, it is better to have high ceilings. Gener- ally, buoyancy-driven ventilation is less effective where ceiling heights are low, for example less than 2.5 m.
Q Adjacent zones: Displacement ventilation diffusers are ideal for spaces in which occupants move through the zone (transient). However, this
air movement strategy involves cool air moving along the floor in a stratified flow with a relative- ly constant depth (typical depth is about 200mm) with the maximum velocity in the stratified flow around l0% of this depth (approximately 20mm from the floor). So, an occupant sitting at a desk in the ‘non-comfort’ zone adjacent to the displacement ventilation diffuser, for example, could therefore sense cool air at ankle level. As a result, furniture layout and zone occupation need to be determined before the unit type is selected.
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Q Acoustic performance: It is important to consid- er that floor grilles/diffusers will be within the occupied zone and, most likely, will be much closer to any occupants than a ceiling/wall dif- fuser would be. Floor grilles/diffusers could also open up a sound path between rooms sharing the same floor void.
Trox is happy to assist customers with all aspects of system design, including sound attenuation, and can arrange demonstrations of air movement characteristics at its laboratory.
www.acr-news.com • October 2022 27
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