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VENTILATION, DUCTWORK AND GRILLES


Designing ventilation from the floor up


By Tim Tanner, product technical manager – air diffusers and attenuators, TROX UK


D


elivering air into the occupied space at floor level can provide valuable advantages in terms of air quality and energy efficiency, but a number of design factors need to be taken into consideration. This article reviews the benefits, and provides design tips for systems employing floor grilles/diffusers.


A number of the advantages of underfloor ventilation stem


from the displacement air movement strategy employed. In traditional mixed air distribution systems, air is delivered at relatively high velocity from diffusers at ceiling level. As it moves along the ceiling, due to coanda effect, the velocity of the air reduces before entering the occupied zone. See Figure 1. By contrast, underfloor air distribution systems deliver cool air at reduced velocity into the room from the floor void, through specially-designed floor grilles. 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 include equipment for extraction. See Figure 2.


Supplying air via floor grilles enables the air velocities to be


lower than for mixed air distribution systems, as there is no need to achieve coanda effect. In addition, the air supplied to the occupied zone does not have to be reduced to the lower temperatures necessary for mixed air distribution. In fact, for commercial premises, the temperature would typically be in the region of 19°C, just slightly cooler than the design temperature of the occupied zone.


As only the occupied zone needs to be supplied with conditioned air, overall cooling load can typically be lower, reducing the demand placed on chillers and other energy- consuming components across the HVAC system. This can be particularly beneficial in rooms with high ceilings. There may also be increased opportunities for “free cooling” (using fresh air) 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%. Displacement approaches can also deliver air quality benefits. As the air pools along the floor, rising when it meets a heat source (such as a person), it can take certain contaminates upwards, out of the occupied zone for ceiling level extract. There are, of course, a number of factors to consider in this regard. For example this technique may not be suitable for applications where the contaminates will be heavier than air.


26 October 2022 • www.acr-news.com


Further benefits arise from the fact that, depending on the building design, a raised floor can allow the floor void to be pressurised and act like a plenum, with the air being balanced at each diffuser. This can reduce the ductwork requirement. In addition, as the equipment is installed into the floor and floor void, the requirement to work at height is removed, reducing health and safety risk for contractors and service engineers. Lastly, as 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. Since there is little or no ductwork involved, the floor tiles can be easily rearranged, and the tiles incorporating diffusers can simply be moved to different locations to suit each new configuration. Given the frequency and cost of ‘churn’, this increased flexibility is extremely valuable to building owners and occupiers throughout the lifecycle of the equipment.


Suitable applications Firstly, establish whether a displacement ventilation approach will be appropriate for the specific application. Systems employing floor grilles and displacement air management are NOT always suitable for heating of spaces, depending on the room strategy. It is common with traditional displacement systems that the room is served by a few large units with the air entering the room in laminar flow.


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