Air quality advantages of underfloor ventilation and tips for system design
Tim Tanner, 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.
Underfloor 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.
For sites that are suited to a displacement air movement strategy, however, there are a number of valuable benefits associated with underfloor ventilation in addition to the air quality advantages already described. Firstly, depending on the application, a displacement ventilation strategy can be a valuable way of reducing energy consumption. In mixed air distribution systems, air needs to be supplied into the room at higher velocities in order to achieve the necessary coanda effect. Higher velocity of supply air, of course, has an impact on energy usage. By contrast, the lower velocities required for displacement air movement can deliver reductions in energy consumption/cost whilst also offering improved acoustics.
Furthermore, 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. In addition, it may be possible to reduce cooling loads as only the occupied zone needs to be supplied with conditioned air. This can be particularly beneficial in rooms with high ceilings. These lower cooling loads reduce the demand placed on chillers and other energy-consuming components across the HVAC system. Plus, there is no longer a need for fans/motors within secondary terminal units such as fan coil units. 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%.
Underfloor ventilation can also bring benefits during installation and maintenance, and can offer greater flexibility for building owners/occupiers. 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. Furthermore, as floor grilles are typically installed into floor tiles, 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. Depending on the building design, a raised floor may allow the floor void to be pressurised, to act like a plenum, with the air being balanced at each diffuser. This can mean that the requirement for ductwork is reduced.
System design tips
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. Therefore, before selection, ensure the project requirements are within achievable limits.
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 recommended using displacement ventilation. In order to maximize the effectiveness of this heating, the FBAs should be evenly spread throughout the floor plate.
Displacement ventilation can involve a considerable amount of mixing in the region below the ceiling, due to the interaction between upward and downward moving buoyant air flows. Therefore buoyancy-driven ventilation is best suited for high ceilings, and can be less effective where ceiling heights are less than 2.5 m.
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 relatively 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.
It is important to consider that floor grilles/diffusers will be within the occupied zone and are likely to be much closer to occupants than ceiling/wall diffusers would be. Floor grilles/diffusers could also open up a sound path between rooms sharing the same floor void.
1 R Brahme et al, ‘IAQ, Energy and Cost Implications of Underfloor Air Distribution Systems’, Proceedings: Indoor Air (2002), pp. 254-257