Ventilation
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Air movement principles for underfloor ventilation
Tim Tanner, product technical manager – air diffusers & attenuators, TROX UK, explores the benefits of underfloor ventilation
E
vents of the last few years have precipitated major changes in thinking when it comes to air conditioning, and HVAC as a whole. Firstly,
COVID-19 has brought air quality into sharp focus, as well as setting occupancy comfort as a priority, to properly welcome many back to offices after a period of working from home. More recently, the sharp spike in energy prices has brought new urgency to the process of improving environmental performance. Within the industry there has been a growing
interest in underfloor ventilation to address these priorities. The techniques are well established but often overlooked. Coanda effect: mixed air vs displacement ventilation The Coanda effect, named after Henri Coanda, is the tendency for a jet of fluid to attach to a nearby surface. When the jet is in free space it entrains and mixes with its surroundings, however if a surface is brought close to the jet, the entrainment and mixing is blocked. When this occurs, an area of low pressure is created between the jet and the surface, causing the jet to veer towards the surface and then attach to it. Mixed air distribution systems use the Coanda
Tim Tanner, product technical manager – air diffusers & attenuators, TROX UK
effect to achieve maximum occupancy comfort. In a typical mixed air distribution system, Coanda effect is exploited to cause the air discharged from the diffuser to move across the ceiling. See Figure 1. The throw distance along the ceiling
gives the air a chance to slow down, to ensure it is delivered into the occupied zone at a reduced velocity to avoid occupancy discomfort. A relatively high air velocity is required to, in a sense, ‘activate’ the Coanda effect and avoid draughts, or ‘dumping’ which will occur if the air enters the occupied zone too soon, causing high velocities in the occupied zone. This requirement for high velocity air supply, of course, has an impact on energy consumption. Underfloor ventilation systems, by contrast, employ
displacement air movement principles. Cool air is delivered at reduced velocity from the floor void, via specially-designed floor grilles. As the supply air is colder than the room air, it slowly pools across the room, rising towards extraction equipment when it comes into contact with a heat load, such as a room occupant. See Figure 2. This displacement air movement strategy can
Figure 1: Mixed air ventilation Figure 2: displacement-air-ventilation 10 November 2022
deliver valuable energy efficiency benefits. Aside from avoiding the high velocities necessary to exploit the Coanda effect, the air supplied to the space does not have to be reduced to as low a temperature as that required for mixed air distribution. In fact, for commercial premises, the temperature would typically be in the region of 19°C. In addition, it may be possible to reduce cooling loads as only the occupied zone needs to be supplied with conditioned air, which can be particularly beneficial in rooms with high ceilings. These lower cooling loads reduce the demand placed on chillers and other elements of the HVAC system, and there is also no longer a need for fans or motors within secondary terminal units such as fan coil units. There may also be increased opportunities for “free cooling”, wherein fresh air is utilised, for a large proportion of the year. An academic study in 2002 quantified potential energy savings of underfloor air
distribution as being between 5% and 35%. Underfloor displacement approaches can also
deliver air quality benefits. As the air pools along the floor, rising when it meets a heat source, it can take contaminates upwards and out through the extraction equipment in the ceiling, providing, of course, the contaminates are not heavier than the air. Further benefits come from ease of installation and maintenance (as there is no requirement to work at height) and flexibility for building owners/occupiers. As the floor grilles are typically installed into floor tiles (or designed as replacement 600 x 600 tiles, in the case of the TROX AFG), reconfiguration of spaces is made easier, without the need to reposition ductwork.
Design tips
Some factors to consider when designing underfloor ventilation systems include: ¡ Recommended total cooling loads are typically less than 40 W/m² for comfort. ¡ Displacement ventilation requires supply air to be cooler than room air. So, as a general rule, heating is not recommended using displacement ventilation. ¡ Displacement ventilation is ideal for high ceilings, but may be less effective where ceiling heights are less than 2.5 m. ¡ Consider placement of furniture/static seating to allow for lower temperature air at ankle level. ¡ Floor grilles/diffusers could potentially open up a sound path between rooms using the same floor void.
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